Claudio A Gonza ´ Lez-Wevar

Steven L Chown

Simon Morley

Nestor Coria

Thomas Sauce

Elie Poulin

Out of Antarctica: quaternary colonization of sub-Antarctic Marion Island by the limpet genus Nacella (Patellogastropoda: Nacellidae)

Keywords: Biogeography, Southern Ocean, Antarctic Polar Front, Long-distance dispersal, Marion Island, Bayesian skyline plot

The distribution of the Southern Ocean near-shore marine benthic fauna is the consequence of major geologic, oceanographic, and climatic changes during the last 50 Ma. As a result, a main biogeographic principle in the Southern Ocean is the clear distinction of the Antarctic biota. The Antarctic Polar Front (APF) represents an important barrier between Antarctica and other sub-Ant-arctic provinces. However, the high degree of genetic affinity between populations of the Antarctic limpet Nacella concinna and its sub-Antarctic relative Nacella delesserti from Marion Island stands against this tenet. Here, we performed new phylogenetic reconstructions in Nacella with special emphasis on the relationship between N. concinna and N. delesserti. Similarly, we performed population-based analyses in N. concinna and N. delesserti to further understand the genetic legacy of the Quaternary glacial cycles. Phylogenetic reconstructions recognized N. concinna and N. delesserti as two closely but distinct monophyletic entities and therefore as valid evolutionary units. The cladogenetic process separating them occurred *0.35 Ma and is consistent with the origin of Marion Island (*0.45 Ma). Exceptional long-distance dispersal between provinces located inside and outside the APF, rather than revealing the permeability of the Antarctic Polar Front, seems to be related to latitudinal shift in the position of the APF during coldest periods of the Quater-nary. Diversity indices, neutrality tests, haplotype net-works, and demographic inference analysis showed that the demography of both species exhibits a clear signal of postglacial expansion.

Introduction

Waters of the Southern Ocean encircling Antarctica are moved around this continent by the strong eastward flow of the Antarctic Circumpolar Current (ACC) that is delimited by two main boundaries: the Antarctic Polar Front (APF) to the south and the sub-Antarctic Front (SAF) to the north [START_REF] Rintoul | The Antarctic circumpolar current system[END_REF][START_REF] Barker | Onset and role of the Antarctic circumpolar current[END_REF][START_REF] Rintoul | The southern ocean in the earth system[END_REF]. The positions of these fronts have major biogeographic conse-quences for the SO biota [START_REF] Lawver | Evolution of Cenozoic seaways in the circum-Antarctic region[END_REF][START_REF] Mackensen | Changing Southern Ocean palaeocirculation and effects on global climate[END_REF][START_REF] Greve | Nestedness of Southern Ocean island biotas: ecological perspectives on a biogeographical conundrum[END_REF][START_REF] Linse | Biodiversity and biogeography of Antarctic and sub-Antarctic mollusca[END_REF][START_REF] Griffiths | Towards a generalized biogeography of the Southern Ocean benthos[END_REF][START_REF] Pierrat | Comparative biogeography of echinoids, bivalves and gastropods from the Southern Ocean[END_REF]). On the one hand, the ACC constitutes a major oceanographic barrier for many invertebrate groups [START_REF] Shaw | Genetic structuring ofPatagonian toothfish populations in the Southwest Atlantic Ocean: the effect of the Antarctic Polar Front and deep-water troughs as barriers to genetic exchange[END_REF][START_REF] Gonza ´lez-Wevar | Molecular phylogeny and historical biogeography of Nacella (Patellogas-tropoda: Nacellidae) in the Southern Ocean[END_REF], 2012a;[START_REF] Poulin | Divergence between Antarctic and South American marine invertebrates: what molecules tell us about Scotia Arc geodynamics and the intensification of the Antarctic circumpolar current[END_REF]) and delimits Antarctic and sub-Antarctic provinces. On the other hand, the ACC can transport organisms between geographically distant sub-Antarctic areas of the Southern Ocean, espe-cially in those species with high dispersive potential such as marine invertebrates with planktotrophic larvae [START_REF] Page | More evidence of speciation and dispersal across Antarctic Polar Front through molecular systematics of Southern Ocean Limatula (Bivalvia: Limidae)[END_REF][START_REF] Díaz | Evolutionary pathways among shallow and deep-sea echinoids of the genus Sterechinus in the Southern Ocean[END_REF][START_REF] Mortimer | Mite dispersal among the Southern Ocean Islands and Antarctica before the last glacial maximum[END_REF], kelps [START_REF] Fraser | Kelp genes reveal effects of subantarctic sea during the last glacial maximum[END_REF][START_REF] Macaya | Genetic structure of the giant kelp Macrocystis pyrifera along the southeastern Pacific[END_REF], and kelp-dwelling organisms [START_REF] Nikula | Circumpolar dispersal by rafting in two subantarctic kelp-dwelling crusta-ceans[END_REF][START_REF] Cumming | Transoce-anic genetic similarities of kelp-associated sea slug populations: long-distance dispersal via rafting?[END_REF]. Recent biogeographic reviews highlighted both the high level of endemism of the Antarctic biota and the marked differentiation among sub-Antarctic, low Ant-arctic, and Antarctic/high Antarctic marine species [START_REF] Linse | Biodiversity and biogeography of Antarctic and sub-Antarctic mollusca[END_REF][START_REF] Griffiths | Towards a generalized biogeography of the Southern Ocean benthos[END_REF][START_REF] Terauds | Conservation biogeography of the Antarctic[END_REF]. Faunal relationships among provinces of the SO perfectly match with the flows of the two main oceanographic cur-rents, the ACC and the Antarctic Coastal currents [START_REF] Pierrat | Comparative biogeography of echinoids, bivalves and gastropods from the Southern Ocean[END_REF].

True limpets of the genus Nacella (Patellogastropoda: Nacellidae) are dominant organisms of inter-and sub-tidal rocky ecosystems of the Southern Ocean. Nacella currently includes at least 11 nominal species distributed in different provinces of the Southern Ocean: South America, sub-Antarctic Islands, and Antarctica [START_REF] Powell | The Patellid limpets of the World (Patellidae)[END_REF]Valdovi-nos and Ru ¨ th 2005;Fig. 1). Species of Nacella are dioe-cious organisms with external fertilization, and their larval life span can extend for more than 2 months [START_REF] Bowden | Antarctic sessile marine benthos: colonisation and growth on artificial substrata over three years[END_REF]. At least eight nominal species were described in Patagonia [START_REF] Powell | The Patellid limpets of the World (Patellidae)[END_REF][START_REF] Valdovinos | Nacellidae limpets of southern South America: taxonomy and distribution[END_REF] but recent molecular and morphometric analyses (de Aranza-mendi et al. 2011;Gonza ´lez-Wevar et al. 2011a) indicate that only four of them represent valid evolutionary units (Gonza ´lez-Wevar et al. 2011a). One South American spe-cies, Nacella clypeater, expands its distribution up north to the Peruvian Province following the cold waters of the Humboldt Current System [START_REF] Powell | The Patellid limpets of the World (Patellidae)[END_REF][START_REF] Valdovinos | Nacellidae limpets of southern South America: taxonomy and distribution[END_REF]. Almost half of the species of the genus are endemic to one or two remote sub-Antarctic isolated islands including Kerguelen, Heard, Macquarie, Campbell, and Marion (Fig. 1). One of them, Nacella delesserti, is endemic to sub-Antarctic Marion Island, an extremely isolated volcanic island located north of the APF close to South Africa [START_REF] Bowden | Antarctic sessile marine benthos: colonisation and growth on artificial substrata over three years[END_REF][START_REF] Hall | Holocene glacial history of Antarctica and the sub-Antarctic islands[END_REF]Hall et al. 2011). Finally, the Antarctic limpet Nacella concinna is one of the dominant macroinvertebrate species of ice-free rocky ecosystems and is currently restricted to mari-time Antarctica (Antarctic Peninsula and associated islands) and peri-Antarctic islands including South Geor-gia, South Sandwich, and Bouvet (Bo ¨ lter et al. 2002).

Phylogenetic reconstructions based on mtDNA sequences recognized marked divergences among lineages of Nacella from Antarctica, sub-Antarctic Heard Island, and South America (Gonza ´lez-Wevar et al. 2010). However, a recent populationbased study in the Antarctic limpet including western Antarctic Peninsula populations recog-nized high levels of genetic identity ([99 %) with a single individual of N. delesserti from Marion Island (Gonza ´lez-Wevar et al. 2011b). Levels of genetic identity recorded between these nominal species fell within the range of intraspecific variation detected in nacellids [START_REF] Goldstien | Comparative phylogeog-raphy of coastal limpets across a marine disjunction in New Zealand[END_REF][START_REF] Bird | Contrasting phylogeography in three endemic Hawaiian limpets (Cellana spp.) with similar life histories[END_REF]) and Nacella species from the Magellanic Province [START_REF] De Aranzamendi | Different evolutionary histories in two sympatric limpets of the genus Nacella (Patellogastropoda) in the South-western Atlantic coast[END_REF]Gonza ´lez-Wevar et al. 2012b). At the same time, some ecologic features of N. concinna, including reproductive strategy, predation avoidance, population density, and growth rate, are similar to the ones described in N. delesserti [START_REF] Blankley | Ecology of the limpet Nacella delesserti (Phillipi) at Marion Island in the Sub-Antarctic Southern Ocean[END_REF]. Accordingly, the taxonomic status of N. concinna and N. delesserti is still unclear and the latter could just represent an eastward population of the Antarctic limpet. However, this statement is questionable consider-ing that few epipelagic species, with the exception of mammals and birds, are distributed both inside and outside the APF [START_REF] Dell | Antarctic Benthos[END_REF][START_REF] Aronson | Climate change and invasibility of the Antarctic benthos[END_REF].

In this study, we present new molecular reconstructions in Nacella using phylogenetic and phylogeographic approaches with special emphasis on the relationship between the Antarctic limpet and sub-Antarctic Marion Island N. delesserti. Using different DNA sources, we aim to examine the validity of these nominal species and thereby to determine whether the Antarctic Polar Front constitutes an efficient barrier between Antarctic and sub-Antarctic Nacella populations. At the same time, levels of genetic diversity and structure are expected to clarify the influence of contemporary climate change over benthic marine invertebrate populations of the Southern Ocean, especially in remote island endemic lineages. These results should help us to further understand the role of historic and recent processes in the distribution of the biota of the Southern Ocean.

Materials and methods

Sample collection

Specimens were collected between 2006 and 2012 from maritime Antarctica, South Georgia Island, and sub-Ant-arctic Marion Island. New N. concinna individuals were collected at Signy Island (60°43 0 S; 45°36 0 W: n = 16) during the summer of 2012. Additionally, we include 16 new individuals from South Georgia Island collected dur-ing the summer of 2006. New sequences from Signy and South Georgia islands were added to previous COI data-base in the species including populations from the South Shetland Islands (SSI), the Scotia Arc (SA), West-(WAP), and East-Antarctic Peninsula (EAP; Gonza ´lez-Wevar et al. 2011b, 2013). Individuals of N. delesserti were collected at Trypot beach (46°53 0 S; 37°52 0 E; n = 40) and Ships Cove (46°51 0 S; 37°50 0 E n = 24), Marion Island.

DNA preparation, PCR amplification, and sequence analyses Specimens were fixed in ethanol (95 %), and DNA was extracted from the mantle using a standard salting-out method [START_REF] Aljanabi | Universal and rapid salt-extraction of high quality genomic DNA for PCR-based techniques[END_REF]. For phylogenetic purposes, we amplified a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) gene using specific primers (Gonza ´lez-Wevar et al. 2010) and a fragment of the nuclear 28S rRNA gene with universal primers (Lit-tlewood et al. 2000;[START_REF] Williams | A molecular phylogeny of the Littorininae (Gastropoda: Littorinidae): unequal evolutionary rates, morphological parallelism, and biogeography of the Southern Ocean[END_REF]. New mito-chondrial sequences of N. delesserti and N. concinna were included in previous COI data matrix for the genus (Gon-za ´lez-Wevar et al. 2010), and it was necessary to amplify the nuclear 28S rRNA gene in all the analyzed Nacella species. In order to test the reciprocal monophyly of N. concinna and N. delesserti, a minimum of ten randomly selected sequences of the COI and five sequences of the 28S rRNA genes were included in the reconstructions, together with sequences of other Nacella species from South America and Heard Island (Gonza ´lez-Wevar et al. 2010). As previously stated for patellogastropods [START_REF] Koufopanou | A molecular phylogeny of Patellid limpet (Gastropoda: Patellidae) and its implications for their antitropical distribution[END_REF][START_REF] Nakano | Worldwide phylogeography of limpets of the order Patellogastropoda: molecular, morphological and palaeontological evidence[END_REF], sequences of Cellana were included as sister group and sequences of other patellogastropod genera were used as outgroups. Mitochondrial and nuclear sequences were assembled independently and edited using Geneious version 5.1.7 (Drummond et al. 2010), and multiple alignments were optimized by eye with Proseq version 3.2 [START_REF] Filatov | Processing and population genetic analysis of multigenic datasets with ProSeq3 software[END_REF]. Base composition and nucleotide sequences were analyzed with MEGA version 4 using a standard invertebrate amino acid coding table [START_REF] Tamura | MEGA4: molecular evolutionary genetics analysis (MEGA) software version 4.0[END_REF]. Similarly, we performed a DNA saturation analysis following [START_REF] Roe | Patterns of evolution of mitochondrial cytochrome c oxidase I and II DNA and implications for DNA barcoding[END_REF].

Phylogenetic reconstructions

Phylogenetic relationships were estimated using Maximum Parsimony (MP), Maximum Likelihood (ML), and Bayesian analyses (BA). Maximum Parsimony analyses were per-formed with PAUP* version 4.0b [START_REF] Swofford | PAUP*: phylogenetic analysis using parsimony (* and Other Methods)[END_REF] following [START_REF] Gonza ´lez-Wevar | Molecular phylogeny and historical biogeography of Nacella (Patellogas-tropoda: Nacellidae) in the Southern Ocean[END_REF]. Nucleotide substitution models for ML and BA analyses were selected separately for each gene using jmodeltest 2 [START_REF] Darriba | jModelTest 2: more models, new heuristics and parallel computing[END_REF] and Mrmodeltest version 2.3 (http://www.abc.se/*nylander/), respectively. The GTR ? I ? G (COI) and GTR ? G (28S rRNA) substitution models were used for ML analyses in PhyML [START_REF] Guindon | A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood[END_REF]. Tree node supports for MP and ML analyses were assessed through nonparametric bootstrap (BS) analysis with a full heuristic search option and 1,000 pseudoreplicates [START_REF] Felsenstein | Evolutionary trees from DNA sequences: a maximum likelihood approach[END_REF]. Bayesian analy-ses were used to estimate posterior probabilities (BPP) at each node using the Metropolis-coupled Markov Chain algorithm (MCMC) available in MrBayes version 3.1.2 [START_REF] Huelsenbeck | MRBAYES: bayesian inference of phylogenetic trees[END_REF]. Four chains were run twice in parallel for 5 9 10 7 generations, and trees were sampled every 1,000 generations until the average deviation of split frequencies was less than 0.01 [START_REF] Ronquist | MrBayes 3: bayesian phyloge-netic inference under mixed models[END_REF]. Bayesian posterior probabilities were esti-mated as the percentage of trees (after burning 5,000 trees) that showed a particular node. Posterior probabilities were summarized as a maximum clade credibility tree with Tre-eAnnotator version 1.6.1 (http://beast.bio.ed.ac.uk/TreeAn notator) and visualized using FigTree version 1.4 (http://tree. bio.ed.ac.uk/software/figtree).

Divergence time estimations

Divergence time estimations analyses were performed on the COI data set using a relaxed molecular clock analyses with an uncorrelated lognormal (ucln) model of molecular evolutionary rate heterogeneity and the GTR ? I ? G substitution model implemented in BEAST version 1.6.2 [START_REF] Drummond | Relaxed phylogenetics and dating with confidence[END_REF][START_REF] Drummond | Bayesian phylogenetics with BEAUti and the BEAST 1.7[END_REF][START_REF] Drummond | BEAST: bayesian evolutionary analysis by sampling trees[END_REF]. For calibration purposes, we incorporated age priors within the Nacella analyses including the most recent common ancestor (tmrca) for the Nacellidae; the age of the fossil record of Cellana ampla (38 Ma, SD 3.8 Ma; Lind-berg and Hickman 1986), the oldest N. concinna fossil from Cockburn Island (5 Ma, SD 0.5 Ma; Jonkers and Kelley 1998), and N. clypeater-like fossil from southern Peru (4.5 Ma, SD 0.45 Ma; DeVries 2009). Four chains were run twice for 3 9 10 7 generations, and trees were sampled every 1,000 generations. Majority rule consensus phylograms, as well as posterior probabilities of nodes, were estimated.

Genetic diversity and population structure in N. concinna and N. delesserti Levels of genetic polymorphisms were estimated in N. concinna and N. delesserti using standard diversity indices including the number of haplotypes (k), the number of segregating sites (S), haplotypic diversity (H), the average number of pairwise differences (P), and nucleotide diversity (p) for each locality and for each species using DnaSP, version 5.00.07 [START_REF] Librado | DnaSP v5: a software for comprehensive analysis of DNA polymorphism data[END_REF]. We performed neutrality statistical tests (Tajima's D and Fu's F S ) for each locality and species to measure whether data deviate from expectations under a neutral model. We estimated the levels of genetic differentiation between the analyzed localities following [START_REF] Pons | Measuring and testing genetic differentiation with ordered versus unordered alleles[END_REF] through mean pairwise differences (N ST ) and haplotype frequencies (G ST ) in Arlequin version 3.5 [START_REF] Excoffier | Arlequin (version 3.0): an integrated software package for population genetics data analysis[END_REF]. The statis-tical significance of genetic differences was estimated using permutation tests (20,000 iterations). The spatial analysis of molecular variance implemented in SAMOVA version 1 [START_REF] Dupanloup | A simulated annealing approach to define the genetic structure of population[END_REF]) was performed to esti-mate the number and composition of groups that were the most differentiated. The statistical significance of this test was evaluated with 20,000 random permutations.

Demographic inference in N. concinna and N. delesserti

We reconstructed genealogical relationships in N. concinna and N. delesserti using median-joining haplotype networks computed with Network, version 4.6.1 (http://fluxus-engi neering.com). For this purpose, we performed a unique reconstruction including the complete COI data set. At the same time, we estimated past population dynamics through time using a Bayesian skyline plot method implemented in BEAST, version 1.7 [START_REF] Drummond | BEAST: bayesian evolutionary analysis by sampling trees[END_REF]. Three independent Bayesian MCMC runs for 5 9 10 7 generation (sampled every 1,000 iterations) were con-ducted in each AMOVA detected group (maritime Ant-arctica, South Georgia Island, and Marion Island) using the GTR ? G?I model. For these analyses, we used a rough tenfold evolutionary rate correction for population-based analyses in nacellids (Gonza ´lez-Wevar et al. 2011b). Such a correction was implemented considering that a time dependency of molecular evolutionary rates has been described showing that short-term mutation rates (1-2 Myr) could be tenfolds higher than long-term substitution rates used for phylogenetic reconstructions [START_REF] Ho | Time dependency of molecular rate estimates and systematic overestimation of recent divergence times[END_REF][START_REF] Ho | Evidence for time dependency of molecular rates[END_REF][START_REF] Ho | Time-dependent rates of molecular evolution[END_REF]. Convergence between runs was confirmed with Tracer version 1.5, and the results for multiple runs were combined using LogCombiner version 1.4.7 (Drum-mond and Rambaut 2007). The median and corresponding credibility intervals of the Bayesian Skyline plot were depicted with Tracer.

Results

Within Nacella, COI sequences include 663 base pairs (bp) coding for 221 amino acids, and as expected when working with coding regions, no insertions/deletions were detected. A total of 123 positions (17.19 %) were vari-able, and most of them (92.1 %) were parsimoniously informative. 28S rRNA sequences include 840 bp, 49 positions were variable (5.83 %) and 85.7 % of them were parsimoniously informative. As previously estimated in Nacella (Gonza ´lez- Wevar et al. 2011aWevar et al. , b, 2012b[START_REF] Gonza ´lez-Wevar | Extinction and recolonization of maritime Antarctica in the limpet Nacella concinna (Strebel, 1908) during the last glacial cycle: toward a model of Quaternary biogeography in shallow Antarctic invertebrates[END_REF], COI sequences were A-T rich (65.7 %) compared to the mean G-C content (34.3 %). In contrast, 28S rRNA sequences were G-C-rich (64.1 %) compared to the mean T-A content (35.9 %).

Phylogenetic reconstructions and divergence time estimations in Nacella

All the reconstruction methods (MP, ML, and BA) and markers (COI and 28S rRNA) highly resolved the rela-tionship among patellogastropods, and no topologic incongruence was detected within Nacella (Fig. 2). The monophyly of Nacella and Cellana within the Nacellidae was highly supported. Within Nacella, mtDNA and nu-cDNA reconstructions recognized two main clades. The first one includes species from sub-Antarctic Islands (Heard and Marion) and the Antarctic limpet N. concinna. The second one encompasses the South American species (Patagonia and Central Chile) of the genus. High levels of genetic divergence were recorded among lineages belong-ing to Antarctica, South America, and sub-Antarctic Islands. The different reconstruction methods and markers used in this study recovered the reciprocal monophyly and the sister relationship between N. concinna and N. deles-serti; Fig. 2a,b). Divergence time estimations based on COI sequences suggest that the most common recent ancestor of the cur-rent lineages of Nacella occurred *12 Ma (14.5-10 Ma), after the Middle Miocene (Fig. 2a). Following this, a diversification of Nacella in different provinces of the SO (Antarctica, sub-Antarctic Islands, and South America) occurred at the end of the Miocene between 8.5 and 5.5 Ma (10-4.5 Ma). A second diversification of Nacella took place during the Quaternary and includes its radiation in Patagonia *0.8 Ma (1.3-0.6 Ma) and the colonization of Marion Island by N. delesserti with it subsequent separa-tion from its Antarctic relative *0.35 Ma (0.65-0.2 Ma; Fig. 2a).

Population genetic diversity and structure in N. concinna and N. delesserti Whole COI data set includes 307 individuals of N. con-cinna and 64 individuals of N. delesserti (Table 1). Levels of genetic polymorphism in the Antarctic limpet were low with 32 variable positions (3.61 %). Levels of genetic diversity in N. delesserti were much lower showing only four variable positions (0.6 %). At species level (Table 1), the number of polymorphic sites (S) in N. concinna varied between 13 (South Georgia Island) and 4 (Signy Island; Table 1), while in N. delesserti this index was either 3 (Trypot beach) or 2 (Ships Cove). The number of haplo-types (k) in the Antarctic limpet ranged between 12 (South Georgia Island) and 5 (South Bay and Signy Island) while in N. delesserti it was either 4 (Trypot beach) or 3 (Ships Cove). The average number of nucleotide differences (P) and the mean nucleotide diversity (p) were very low in N. delesserti, while these indices showed comparatively higher values for populations of N. concinna (Table 1). As expected working with different taxonomic units, pairwise comparison based on G ST and N ST recognized significant levels of genetic differentiation between N. concinna and N. delesserti populations. Within N. con-cinna, these analyses also detected significant genetic dif-ferentiation between South Georgia Island population and the rest of the analyzed localities from maritime Antarctica. No significant genetic structure was detected among mar-itime Antarctica populations, from Adelaide Island to Laurie Island, South Orkneys (Table 2). Similarly, no genetic differentiation was detected between the two pop-ulations of N. delesserti (Trypot beach and Ships Cove; Table 2). Population genetic structure based on G ST and N ST analyses was strongly supported by SAMOVA anal-yses recognizing three main groups: maritime Antarctica, South Georgia Island, and Marion Island. SAMOVA ana-lysis recognized a maximal difference accounting for 68.01 % of the total variance, while only 0.22 % was due to within-group variations among localities. Median-joining network comprises 29 haplotypes in N. concinna with a typical star-like topology and a short genealogy (Fig. 3). Only five haplotypes were recorded in N. delesserti showing again a star-like topology but a much shorter genealogy. Even when more than 350 individuals were included in the analyses, no shared haplotype was detected between N. concinna and N. delesserti. The dominant haplotype in N. concinna (H01) was present in 51.41 % of the individuals and was broadly distributed along the sampling sites, from Adelaide Island to South Georgia Island (Fig. 3). Despite its high frequency, H01 was only present in a 15.5 % of the sampled individuals from South Georgia Island. In fact, in contrast to the rest of As expected for star-like topologies, both general Taj-ima's and Fu's neutrality tests were negative and signifi-cant for the whole COI data set of each nominal species (Table 1). Sudden growth model analyses detected an older population expansion at South Georgia Island (*18 ka), while maritime Antarctica and Marion Island populations exhibited similar population expansions, 7.0 and 5.0 ka, respectively. Bayesian Skyline plot analyses recognized differences in the age of the trmca and in the timing of population expansion among South Georgia Island, mari-time Antarctica, and Marion Island populations (Fig. 4). The trmca for South Georgia Island was estimated as 25 Ka, while in maritime Antarctica and Marion Island pop-ulations, the trmca occurred about 6.5 and 4.5 ka, respec-tively. Similarly, the onset of population expansion in South Georgia occurred 14 ka while in maritime Antarctica and Marion Island, they took place *5 and *2 ka, respectively.

Demographic inference

Discussion

The evolution of the marine benthic fauna in the Southern Ocean is the result of historic and contemporary processes operating at different temporal and geographic scales since the Cenozoic [START_REF] Crame | An evolutionary perspective on marine faunal connection between southernmost South America and Antarc-tica[END_REF][START_REF] Linse | Biodiversity and biogeography of Antarctic and sub-Antarctic mollusca[END_REF][START_REF] Aronson | Climate change and invasibility of the Antarctic benthos[END_REF][START_REF] Fraser | Kelp genes reveal effects of subantarctic sea during the last glacial maximum[END_REF][START_REF] Griffiths | Towards a generalized biogeography of the Southern Ocean benthos[END_REF]Gon-za ´lez-Wevar et al. 2012a;[START_REF] Pierrat | Comparative biogeography of echinoids, bivalves and gastropods from the Southern Ocean[END_REF][START_REF] Strugnell | Persistent genetic signatures of historic climatic events in an Antarctic octopus[END_REF][START_REF] Poulin | Divergence between Antarctic and South American marine invertebrates: what molecules tell us about Scotia Arc geodynamics and the intensification of the Antarctic circumpolar current[END_REF]. Major geologic, climatic, and oceanographic changes explain the current distribution of families, genera, and species in different provinces of this region [START_REF] Clarke | The origin of the Southern Ocean marine fauna[END_REF][START_REF] Clarke | The Southern Ocean benthic fauna and climate change: a historical perspective [and discussion[END_REF][START_REF] Crame | An evolutionary perspective on marine faunal connection between southernmost South America and Antarc-tica[END_REF]. At the same time, major landscape changes associated with continental ice sheet advances and retreats during the Quaternary have shaped patterns of genetic diversity and structure of populations, particularly in those species restricted to continental shelves [START_REF] Allcock | Southern Ocean diversity: new paradigms from molecular ecology[END_REF][START_REF] Strugnell | Persistent genetic signatures of historic climatic events in an Antarctic octopus[END_REF][START_REF] Gonza ´lez-Wevar | Extinction and recolonization of maritime Antarctica in the limpet Nacella concinna (Strebel, 1908) during the last glacial cycle: toward a model of Quaternary biogeography in shallow Antarctic invertebrates[END_REF].

Biogeography in Nacella

In this study, new phylogenetic reconstructions based on mtDNA and nucDNA sequences recognized two main clades currently distributed at (1) South America and (2) Antarctica ? Sub-Antarctic Islands. The South American clade includes species from Central Chile (N. clypeater) and from Patagonia (Nacella deaurata, Nacella flammea, Nacella magellanica, and Nacella mytilina). A second clade includes species from Heard Island (Nacella kerg-uelensis and Nacella cf. macquariensis), the Antarctic limpet (N. concinna), and its closest relative N. delesserti from sub-Antarctic Marion Island. In this context, the reciprocal monophyly of both N. delesserti (Marion Island) and N. concinna (Antarctica), and their close affinity, was supported by different reconstruction methods and sources of DNA, and consequently, these nominal species unequivocally constitute different evolutionary units.

The origin and diversification of Nacella took place after the Middle Miocene, long after the physical separation of the continental landmasses or to the formation of the Antarctic Polar Front estimated between 41 Ma [START_REF] Scher | Timing and climatic consequences of the opening of Drake Passage[END_REF] and 24 Ma [START_REF] Pfuhl | Evidence for late Oligocene establishment of the Antarctic circumpolar current[END_REF]. This period represents a phase of major climatic and oceano-graphic changes in the Southern Ocean that triggered the initiation of polar subzero conditions in the region. Such changes are supported by a number of data sources, including the strengthening of the westerly winds and the intensification of the ACC, atmospheric d 18 O signatures [START_REF] Flower | The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling[END_REF], and by faunal composition changes at the Kerguelen Plateau [START_REF] Verducci | The Middle Miocene climatic transition in the Southern Ocean: evidence of paleoclimatic and hydrographic changes at Kergue-len plateau from planktonic foraminifers and stable isotopes[END_REF]) and at continental Antarctica [START_REF] Lewis | Mid-Miocene cooling and the extinction of tundra in continental Antarctica[END_REF]. A Late Miocene diversification of Nacella in Antarctic and sub-Antarctic is consistent with recent molecular studies in Southern Ocean fishes and marine invertebrates. For instance, molecular divergence between Antarctic and sub-Antarctic notothenioid genera indicate a separation that ranges between 9 Ma [START_REF] Bargelloni | Mitochondrial phylogeny of notothenioids: a molecular approach to Antarctic fish evolution and biogeography[END_REF] and 6.1 Ma [START_REF] Stankovic | Recent origin of sub-Antarctic notothenioids[END_REF]. Similarly, levels of genetic diver-gence between Antarctic and South American congeneric echinoderms [START_REF] Hunter | Evaluating connectivity in the brooding brittle star Astrotoma agassizii across the Drake Passage in the Southern Ocean[END_REF][START_REF] Janosik | Evolutionary history of Southern Ocean Odontaster sea star species (Odontasteridae: Asteroidea)[END_REF][START_REF] Díaz | Evolutionary pathways among shallow and deep-sea echinoids of the genus Sterechinus in the Southern Ocean[END_REF], mollusks [START_REF] Page | More evidence of speciation and dispersal across Antarctic Polar Front through molecular systematics of Southern Ocean Limatula (Bivalvia: Limidae)[END_REF][START_REF] Linse | Biodiversity and biogeography of Antarctic and sub-Antarctic mollusca[END_REF][START_REF] Wilson | Ocean barriers and glaciation: evidence for explosive radiation of mitochondrial lineages in the Antarctic sea slug Doris kerguelenensis (Mol-lusca, Nudibranchia)[END_REF]Gonza ´lez-Wevar et al. 2012a[START_REF] Poulin | Divergence between Antarctic and South American marine invertebrates: what molecules tell us about Scotia Arc geodynamics and the intensification of the Antarctic circumpolar current[END_REF], pycnogonids [START_REF] Krabbe | Cryptic mitochondrial lineages in the widespread pycnogonid Colossen-deis megalonyx Hoek, 1881 from Antarctic and Subantarctic waters[END_REF], and nemerteans [START_REF] Thornhill | Open-ocean barriers to dispersal: a test case with the Antarctic Polar Front and the ribbon worm Parborlasia corrugatus (Nemertea: Linei-dae)[END_REF] indicate that their respective separations occurred during the Late Miocene and Pliocene, no more than 8 Ma. The effective separation between Antarctic and sub-Antarctic lineages in different groups of marine benthic invertebrates during the Late Miocene may be a consequence of major oceanographic changes associated with the geodynamics of the Scotia Ridge. A recent study in the Central Scotia Sea recognized a remnant, now-submerged, volcanic arc that may have formed a barrier to deep eastward circulation until after the Mid-Miocene, approximately 11.6 Ma [START_REF] Dalziel | A potential barrier to deep Antarctic circumpolar flow until the late Miocene?[END_REF]. The development of a full ACC seems to have played an important role in the subsequent cryo-sphere expansion [START_REF] Lear | Cenozoic deep-sea temper-atures and global ice volumes from Mg/Ca in Benthic Foraminiferal Calcite[END_REF][START_REF] Zachos | Trends, rhythms, and aberrations in global climate 65 Ma to present[END_REF], as well as in the intensification of the ACC [START_REF] Flower | The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling[END_REF][START_REF] Shevenell | Middle Miocene Southern Ocean cooling and Antarctic cryosphere expansion[END_REF]. It is possible to envision that such changes generated the characteristic polar conditions of the Southern Ocean, with an Antarctic Polar Front acting as an effective barrier among provinces of the Southern Ocean.

Marion Island colonization

The close relationship between the Antarctic limpet and Marion Island N. delesserti recorded in this study contests one important paradigm of the Southern Ocean biogeogra-phy, the historic isolation of the Antarctic continent, and therefore points toward a potential permeability of the APF during the Quaternary. According to [START_REF] Barnes | Incursion and excursion of Antarctic biota: past, present and future[END_REF] on short ecologic timescales, many natural dispersal processes including airborne, oceanic eddy, rafting, and hitchhiking on migrants permit the passage to and from Antarctica. Simi-larly, humans have become influential by directly trans-porting organisms and indirectly by increasing the survival and establishment of exogenous taxa via climate change [START_REF] Barnes | Incursion and excursion of Antarctic biota: past, present and future[END_REF]. Therefore, at macro-and mesoscales, the flow of the ACC is complex in direction and velocity and rapid turbulent flow create abundant eddies that make the APF porous to transport plankton both northwards or southwards [START_REF] Froneman | Biogeographical structure of the microphytoplankton assemblages in the region of the subtropical convergence and across a warmcore eddy during austral winter[END_REF]. In context, the finding of Antarctic krill in Chilean fjords [START_REF] Antezana | Plankton of Southern Chilean fjords: trends and linkages[END_REF] and the presence of non-Antarctic benthic crustaceans larvae at King George Island, South Shetland Islands, Antarctic Peninsula [START_REF] Thatje | First record of anomuran and brachyuran larvae (Crustacea: Decapoda) from Antarctic waters[END_REF][START_REF] Thatje | Antarctic reptant decapods: more than a myth?[END_REF][START_REF] Thatje | Challenging the cold: crabs reconquer the Antarctic[END_REF] are evidence of oceanographic connec-tivity across the APF probably mediated by eddies [START_REF] Clarke | How isolated is Antarctica?[END_REF]. However, in the particular case of Nacella, the absence of shared haplotypes between Antarctica and Mar-ion Island constitutes strong evidence toward a lack of contemporary gene flow across the APF. Accordingly, it seems like the APF presently constitutes an impassable barrier for Nacella larvae and/or adults between these areas of the Southern Ocean. Moreover, reciprocal monophyly of N. concinna and N. delesserti recorded with different sources of DNA (COI and 28S rRNA) suggests a prolonged time of independent evolution [START_REF] Hudson | Mathematical consequences of the genealogical species concept[END_REF][START_REF] De Queiroz | Species concepts and species delimitation[END_REF]. In this context, the cladogenetic process separating both species agrees with the known geologic age of Marion Island (*0.45 Ma; [START_REF] Chown | Biogeography of Southern Ocean Islands: species-areas relationships, human impacts, and conservation[END_REF]). Hence, our results could be explained by a transient permeability of the APF during the Quaternary, allowing the colonization of this newly formed island. After this, APF became again an effi-cient barrier that prevented further gene flow between pop-ulations from Antarctica and Marion Island, a process that marked the onset of their genetic divergence. As an alter-native explanation for these results, the colonization of Marion Island by Nacella, rather than be related to the per-meability of the APF, could be associated with latitudinal shifts of this oceanographic front. The boundaries of the APF are dynamic in space and time, and its position has varied during Quaternary glacial cycles [START_REF] Barnes | Incursion and excursion of Antarctic biota: past, present and future[END_REF][START_REF] Fraser | Kelp genes reveal effects of subantarctic sea during the last glacial maximum[END_REF]. Mapping of diatom mat deposits in the Atlantic provides evidence of a glacial-interglacial front migration of up 6° of latitude during the Early/Mid-Pleistocene. The Mid-Pleistocene transition marks a stepwise minimum 7° northward migration sustained for about 450 ka until an abrupt southward return to its modern position [START_REF] Kemp | Migration of the Antarctic Polar Front through the mid-Pleistocene transition: evidence and climatic implications[END_REF]. Similarly, [START_REF] Gersonde | Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG last glacial maximum: a circum-Antarctic view based on siliceous microfossil records[END_REF] recorded a northward expansion of Antarctic cold waters by 5°-10° in latitude and a relatively small displacement of the subtropical front during the Last Glacial Maximum. Under this scenario, it is possible to envision that Marion Island could have been located within the APF at times during the Quaternary, allowing long-distance dispersal events from Antarctic to islands that are currently located in the sub-Antarctic realm. For this, Southern Ocean islands including South Georgia, South Sandwich, and Bouvet may have played a key role as bridges between Antarctica and Marion Island through a stepping-stone model of colonization.

Population-based analyses in N. concinna and N. delesserti In general terms, levels of genetic diversity in Antarctic and sub-Antarctic populations of Nacella are lower than those found in temperate patellogastropods [START_REF] Nakano | Systematic revision of Patelloida pygmaea (Dunker, 1860) (Gastropoda: Lottiidae), with a description of a new species[END_REF] and in N. magellanica from Patagonia [START_REF] De Aranzamendi | Limpets of the genus Nacella (Patellogastropoda) from the Southwestern Atlantic: species identification based on molecular data[END_REF][START_REF] De Aranzamendi | Different evolutionary histories in two sympatric limpets of the genus Nacella (Patellogastropoda) in the South-western Atlantic coast[END_REF]Gonza ´lez-Wevar et al. 2011a, 2012b). Similar patterns of genetic diversity have been recorded in several Antarctic marine invertebrates including Chorismus antarcticus [START_REF] Raupach | Genetic homogeneity and circum-Antarctic distribution of two benthic shrimp species of the Southern Ocean, Chorismus antarc-ticus and Nematocarcinus lanceopes[END_REF], Sterechinus neumayeri [START_REF] Díaz | Evolutionary pathways among shallow and deep-sea echinoids of the genus Sterechinus in the Southern Ocean[END_REF], and Parbolasia corrugatus [START_REF] Thornhill | Open-ocean barriers to dispersal: a test case with the Antarctic Polar Front and the ribbon worm Parborlasia corrugatus (Nemertea: Linei-dae)[END_REF]. These studies evidence the strong influence of Quaternary glaciations over the demographic dynamics in different phyla of the Southern Ocean [START_REF] Convey | Exploring biological constraints on the glacial history of Antarctica[END_REF][START_REF] Fraser | Kelp genes reveal effects of subantarctic sea during the last glacial maximum[END_REF][START_REF] Clarke | Evolutionary dynamics at high latitudes: speciation and extinction in polar marine faunas[END_REF][START_REF] Allcock | Southern Ocean diversity: new paradigms from molecular ecology[END_REF]Gonza ´lez-Wevar et al. 2012a). Major reduction in shallow rocky habitats during glacial periods must have generated considerable reduction in population sizes, especially in shallow benthic invertebrates, and thus generated a marked decline in haplotype diversity; a process that has been usually explained by bottleneck effects or by founder events fol-lowed by population expansion [START_REF] Slatkin | Pairwise comparisons of mitochondrial DNA sequences in stable and exponentially growing popula-tions[END_REF]. Patterns of mismatch distribution and Bayesian Skyline plot in N. concinna recognized an older demographic his-tory in South Georgia Island populations. As previously stated (Gonza ´lez-Wevar et al. 2013), the high proportion of private haplotypes detected in South Georgia and its high genetic diversity compared to maritime Antarctic popula-tions supports the hypothesis that this island acted as a glacial refugia for the species during the Last Glacial Maximum. After this, the Antarctic limpet would have recolonized maritime Antarctica through a strong founder effect following the deglaciation process [START_REF] Gonza ´lez-Wevar | Extinction and recolonization of maritime Antarctica in the limpet Nacella concinna (Strebel, 1908) during the last glacial cycle: toward a model of Quaternary biogeography in shallow Antarctic invertebrates[END_REF]. In Marion Island, N. delesserti exhibited extremely low level of genetic diversity compared to N. concinna from South Georgia and maritime Antarctica, even compared to the southermost populations from western Antarctic Pen-insula. Haplotype diversity, star-like topologies, short genealogies, significant negative neutrality indices, and demographic inference analyses recorded in Marion Island populations suggest the occurrence of a dramatic bottle-neck event during the last glacial period followed by a postglacial population expansion during the last 5 ka. Such decrease in population size may be the result of strong glacial impact on this small, steep, and isolated island characterized by short and narrow seashore. In fact, glacial reconstructions in Marion Island suggest that this island was strongly impacted and almost entirely covered by ice during the Last Glacial Maximum [START_REF] Hall | Rapid deglaciation as an initiator of volcanic activity: an hypothesis[END_REF][START_REF] Hall | Holocene glacial history of Antarctica and the sub-Antarctic islands[END_REF]Hall and Meiklejohn 2011;Hall et al. 2011). This study provides evidence of Quaternary (*0.35 ka) colonization of Marion Island, a newly formed sub-Ant-arctic island, by an Antarctic benthic species and its sub-sequent differentiation. It also highlights the importance of historic and contemporary climatic/oceanographic processes in explaining the current distribution of genea-logical lineages in different provinces of the Southern Ocean. Alternation of glacial and interglacial periods of the Quaternary is responsible for the current patterns of genetic diversity and structure observed in Antarctic and Sub-Antarctic species of Nacella. Similar demographic trajec-tories detected in N. concinna from maritime Antarctica and N. delesserti from Marion Island are evidence of the strong effect of past glacial events, and especially of the LGM on the demography and distribution of polar benthic invertebrates with shallow and narrow bathymetric ranges.

Table 1

 1

	Diversity indices and neutrality tests for Nacella	Locality	N	K	H	S	P	p	Tajima's D	Fu's F S
	concinna and Nacella delesserti
	n number of sampled individuals,
	k number of haplotypes, S
	polymorphic sites,
	H haplotype diversity, P
	average number of nucleotide
	difference, p nucleotide
	diversity * p \ 0.05,
	** p \ 0.01, *** p \ 0.001.
	WAP Western Antarctic
	Peninsula, EAP Eastern
	Antarctic Peninsula, SSI South
	Shetland Islands, SOI South
	Orkney Islands, SGI South
	Georgia Island

Table 2 G

 2 ST (below diagonal and N ST (above diagonal) pairwise comparisons for populations of Nacella concinna and Nacella delesserti Rothera Station; S.B. South Bay; A.B. Admiralty Bay; F.B. Fildes Bay; C.B. Covadonga Bay; E.I. Elephant Island; J.R.I. James Ross Island; L.I. Laurie Island; S.I. Signy Island; S.I.G. South Georgia Island; T.B. Trypot Beach, Marion Island; S.C. Ships Cove, Marion Island. Statistical significant differences 100,000 iterations (p \ 0.05) are marked in bold circle indicating the site of collection. The size of each haplotype is proportional to its frequency in the whole sampling effort maritime Antarctica populations, not one dominant, but four medium frequency haplotypes were detected in South Georgia Island. The dominant haplotype (H31) in N. del-esserti was present in 76.98 % of the analyzed individuals and was found at both localities in Marion Island.

	Localities R.S.	B.S.	A.B.	F.B.	C.B.	E.I.	J.R.I.	S.O.I.	S.I.	S.G.I.	T.B.	S.C.
	R.S.	****	0.011	0.020	0.007	0.000	0.014	0.033	0.024	0.047	0.125	0.849	0.833
	B.S.	0.000	****	0.011	0.001	0.006	0.000	0.050	0.007	0.031	0.120	0.829	0.810
							0.000	0.000	****	0.000	0.111	0.854	0.840
	S.I.	0.005	0.012	0.012	0.006	0.001	0.001	0.009	0.000	****	0.121	0.862	0.850
	S.G.I.	0.148	0.121	0.223	0.189	0.108	0.105	0.114	0.152	0.160	****	0.683	0.642
	T.B.	0.526	0.495	0.596	0.551	0.473	0.479	0.477	0.532	0.546	0.385	****	0.000
	S.C.	0.498	0.465	0.580	0.529	0.440	0.447	0.445	0.505	0.519	0.353	0.000	****
	B.S.

Acknowledgments INACH Office project G_04_11, PhD thesis projects INACH B_01-07, Conicyt 24090009 and postdoctorate fel-lowship 3120075 (C.G-W), projects INACH 02-02 & 13-05 and Ecos C06B02 (E.P). Institute of Ecology and Biodiversity (IEB, Universidad de Chile) ICM-P05-002 & PFB-023-CONICYT. Census of Antarctic Marine Life (CAML), EBA-SCAR and PROSUL-Brazil for encour-aging and supporting Antarctic research in Evolution. Simon Morley was funded by the British Antarctic Survey (BAS). Program JR230 of the British Antarctic Survey (T.S). This work was supported by South African National Research Foundation Grant SNA2011110700005. Collections on Marion Island were made with the assistance of Aleks Terauds, Jennifer Lee, and Susana Clusella-Trullas.


RIDGES PASSIFS EN D ' Onde

Silicium Poreux

Fabien Cassio

Luiz Poffo

Nathalie Lorrain

Parastesh Pirasteh

Jonathan Lemaitre

Mohammed Guendouz

FILTRAGE DES MODES DE POLARISATION DE LA LUMIERE DANS DES GUIDES

Keywords: silicium poreux, optique intégrée, polarisation, filtrage

Le silicium poreux est un matériau nanostructuré utilisé en optique intégrée. Nous montrons l'impact de ses caractéristiques (morphologie et structuration) sur la polarisation dans le domaine spectral du proche infra-rouge. Nous mettons en évidence un filtrage des modes de polarisation pour différentes longueurs de guides d'onde en silicium poreux.

INTRODUCTION

Le silicium poreux (SiP) est un matériau structuré dont le procédé de fabrication [START_REF] Zhang | Porous Silicon Formation and Electropolishing of Silicon by Anodic Polarization in HF Solution[END_REF] permet de contrôler la porosité (et donc l'indice de réfraction), la taille des pores et l'épaisseur des couches poreuses [START_REF] Jane | Porous silicon biosensors on the advance[END_REF][START_REF] Arshavsky-Graham | Porous silicon-based photonic biosensors: current status and emerging applications[END_REF][START_REF] Stefano | Porous silicon optical biosensors: still a promise or a failure?[END_REF]. Peu d'études traitent de l'impact de la structuration du SiP sur la polarisation de la lumière. Des pertes optiques plus faibles pour les modes transverses magnétiques (TM) que pour les modes transverses électriques (TE) ont été mesurées dans des guides d'onde planaires multimodes en SiP qui pourraient être liées à la morphologie du SiP [START_REF] Charrier | Effect of interface roughness and polarization on the optical losses of porous silicon-based waveguides[END_REF]. Une autre étude concernant des guides d'onde actifs a permis de mettre en évidence que les modes de polarisation (TE et TM) influaient sur le guidage de la lumière [START_REF] Escobar | Light polarization in active photonic waveguides of porous silicon[END_REF]. Une corrélation entre la photoluminescence visible et le degré de polarisation a été déterminée dans des structures optiques SiP excitées par une lumière polarisée, mais indépendamment de la morphologie [START_REF] Andrianov | Anomalous photoluminescence polarization of porous silicon[END_REF][START_REF] Koyama | Polarization retention in the visible photoluminescence of porous silicon[END_REF]. Dans cet article, nous présentons l'effet de la polarisation du signal d'entrée sur la propagation de la lumière dans des guides d'onde ridges passifs en SiP.

CONDITIONS EXPERIMENTALES

Les guides d'onde ridges en SiP, représentés en Fig. 1(a), ont été fabriqués par un procédé de photolithographie standard à partir de quatre couches consécutives de SiP, préparées par anodisation électrochimique, à une température ambiante maintenue à 18 °C. Le substrat de silicium utilisé est fortement dopé de type P, avec une orientation cristalline (100) et une résistivité de 5 mΩ.cm. Dans ces conditions, une morphologie mésoporeuse colonnaire est obtenue [START_REF] Zhang | Porous silicon: morphology and formation mechanisms[END_REF]. Deux fines couches (appelées couches barrières) à faible porosité ont été nécessaires pour le procédé de photolithographie afin d'empêcher l'infiltration de la résine à l'intérieur des couches de guidage et de confinement. Les porosités des couches de guidage et de confinement ont été choisies pour obtenir des indices de réfraction respectivement de 1,57 et 1,41.

L'étalonnage de la polarisation a été effectué à l'aide du banc optique présenté sur la Fig. 1(b). Le signal optique d'entrée provenant d'une source laser accordable et à large bande de 1400 à 1625 nm (Yenista Tunics T100S-HP) est injecté dans l'échantillon de SiP à l'aide d'une fibre à maintien de polarisation microlentillée. La polarisation d'entrée est réglée à l'aide d'un rotateur de fibre monté sur un banc de micro-alignement qui permet d'injecter dans un guide d'onde ridge en SiP soit le mode TE, soit le mode TM, soit une distribution égale ou non des deux polarisations. La modification de la polarisation induite par la microlentille est considérée comme négligeable [START_REF] Thual | Micro-lens on polarization maintaining fibre for coupling with 1.55 µm quantum dot devices[END_REF]. Le signal optique de sortie est focalisé vers un polarimètre, servant également de puissance-mètre, grâce à un objectif de microscope.

RESULTATS

La Fig. 2 présente la puissance en sortie pour différentes longueurs de guides d'onde ridges en SiP pour trois configurations de polarisation d'entrée différentes : linéaire verticale (TM), linéaire à 45° (50 % TM/50 % TE) et linéaire horizontale (TE). La longueur des guides d'onde a été réduite par clivage et les tranches des échantillons ont été polies afin d'optimiser le couplage en entrée et en sortie du guide. Nous pouvons remarquer la présence de deux zones distinctes, notées A et B sur la figure. En exploitant les courbes de tendance des trois configurations dans la zone A, les pertes de propagation sont de l'ordre de 18 dB/cm pour les polarisations linéaires verticale ™ et à 45° (50 % TM/50 % TE). Pour la polarisation linéaire horizontale (TE) la puissance détectée en sortie correspond au bruit du détecteur, autrement dit, aucun signal n'est détecté. L'atténuation de 3 dB entre les ordonnées à l'origine des deux premières configurations illustre l'extinction complète de la composante horizontale de la polarisation (mode TE). Dans la zone B, en deçà d'une longueur de 2,3 mm, propre aux conditions de l'expérience, la composante horizontale de la polarisation (mode TE) n'est pas totalement atténuée.

CONCLUSION

Dans cet article, nous avons présenté la caractérisation optique de guides d'onde ridges en SiP. Les couches de SiP ont été réalisées par anodisation électrochimique à partir d'un substrat de silicium fortement dopé de type P orienté (100), donnant une morphologie mésoporeuse colonnaire des pores. Nous avons mis en évidence un filtrage de 116 dB/cm de la polarisation TE du signal d'entrée que nous avons attribué à la morphologie du SiP. Ce phénomène pourrait trouver un intérêt dans les dispositifs d'optique intégrée pour réduire les pertes de propagation dues à un mauvais contrôle de la polarisation. Il serait par conséquent nécessaire d'injecter un signal optique contrôlé en polarisation dans la couche de guidage du SiP, afin de réduire les pertes optiques déjà élevées dans ce matériau qui sont inhérentes à sa nature poreuse.

Fig. 1 :

 1 Fig. 1 : Représentation schématique de guides d'onde ridges en SiP (a) et banc optique utilisé pour la caractérisation optique en fonction de la polarisation d'entrée.

Fig. 2 :

 2 Fig. 2 : Évolution de la puissance de sortie pour différentes longueurs de guides pour les trois configurations étudiées : TM, 50 % TM/50 % TE et TE. L'exploitation des courbes de tendance dans la zone B ont permis d'estimer des pertes de propagation de l'ordre de 18 dB/cm, 34 dB/cm et 116 dB/cm pour les polarisations linéaires verticale (TM), à 45° (50 % TM/50 % TE) et horizontale (TE) respectivement. Ces résultats montrent une atténuation 6,5 fois supérieure du mode TE par rapport au mode TM.

Remerciements : Ces travaux de thèse sont financés par la région Bretagne et Lannion Trégor Communauté. Les travaux de fabrication et de caractérisation des guides d'onde ridges ont été réalisés au sein de la plateforme technologique, le CCLO, de l'institut Foton.