Jiahui Xu

Zhaogang Dong

Mohamed Asbahi

Yiming Wu

Hao Wang

Liangliang Liang

Ray Jia

Hong Ng

Hailong Liu

Renaud A L Vallée

Joel K W Yang

Xiaogang Liu

Multiphoton Upconversion Enhanced by Deep-subwavelength Nearfield Confinement

Keywords: Upconversion, plasmonic nanostructures, self-assembly, nonlinear optical enhancement, deep subwavelength

Efficient generation of anti-Stokes emission within nanometric volumes enables the design of ultracompact, miniaturized photonic devices for a host of applications. Many subwavelength crystals, such as metal nanoparticles and two-dimensional layered semiconductors, have been coupled with plasmonic

nanostructures for augmented anti-Stokes luminescence through multiple-harmonic generation. However, their upconversion process remains inefficient due to their intrinsic low absorption coefficients. Here, we demonstrate on-chip, site-specific integration of lanthanide-activated nanocrystals within gold nanotrenches of sub-25-nm gaps via bottom-up self-assembly. The deterministic coupling of upconversion nanoparticles to subwavelength gap-plasmon modes enables amplified upconversion signals by four-to-five orders of magnitude and markedly boosted spontaneous emission rates. Numerical investigations reveal that the gap-mode nanocavity confines incident excitation radiation into nanometric photonic hotspots with extremely high field intensity, accelerating multiphoton upconversion processes.

The ability to design lateral gap plasmon modes for enhanced frequency conversion may hold potential to develop on-chip, background-free molecular sensors and low-threshold upconversion lasers. Plasmonic nanostructures that can direct and amplify light emission of nanoscale emitters provide suitable platforms for nonlinear optics with ultra-compact dimensions. [START_REF] Cai | Electrically Controlled Nonlinear Generation of Light with Plasmonics[END_REF][START_REF] Wang | Giant Photoluminescence Enhancement in Tungsten-Diselenide-Gold Plasmonic Hybrid Structures[END_REF][START_REF] Luo | Deterministic Coupling of Site-Controlled Quantum Emitters in Monolayer WSe 2 to Plasmonic Nanocavities[END_REF][START_REF] Fernandez-Bravo | Continuous-Wave Upconverting Lasing from Subwavelength Plasmons[END_REF] Deterministic integration of nonlinear quantum emitters with plasmonic circuits enables the design of chip-scale photonic devices for a host of applications in nanomedicine, photocatalysis, and background-free biosensing. [START_REF] Seyler | Electrical Control of Second-Harmonic Generation in a WSe 2 Monolayer Transistor[END_REF][START_REF] Das | Over 1000-Fold Enhancement of Upconversion Luminescence Using Water-Dispersible Metal-Insulator-Metal Nanostructures[END_REF][START_REF] Celebrano | Mode Matching in Multiresonant Plasmonic Nanoantennas for Enhanced Second Harmonic Generation[END_REF] For this purpose, controllable and selective positioning of efficient nonlinear emitters onto large-area, predetermined plasmonic circuitry is needed. Many state-of-the-art materials, such as gold and indium tin oxide nanoparticles and two-dimensional MoS 2 nanosheets, [START_REF] Wang | Giant Photoluminescence Enhancement in Tungsten-Diselenide-Gold Plasmonic Hybrid Structures[END_REF][START_REF] Dong | Second-Harmonic Generation from Sub-5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates[END_REF][START_REF] Aouani | Third-Harmonic-Upconversion Enhancement from a Single Semiconductor Nanoparticle Coupled to a Plasmonic Antenna[END_REF] have been exploited as nonlinear active platforms for plasmonic nano-device coupling. However, anti-Stokes luminescence of the given systems are often associated with high-order harmonic generation, in which high-power excitation is indispensable due to intrinsically low nonlinear absorption coefficients and thus low conversion efficiencies. Their practical applications are hindered by the need for coherent laser excitation and by the lack of controllability over large assemblies.

Compared with the aforementioned materials, lanthanide-doped upconversion nanoparticles (UCNPs) allow more efficient frequency upconversion due to abundant, physically existing energy states of the lanthanide. [START_REF] Fan | Lifetime-Engineered NIR-II Nanoparticles Unlock Multiplexed in Vivo Imaging[END_REF][START_REF] Gai | Recent Progress in Rare Earth Micro/Nanocrystals: Soft Chemical Synthesis, Luminescent Properties, and Biomedical Applications[END_REF][START_REF] Liang | Energy Flux Manipulation in Upconversion Nanosystems[END_REF][START_REF] Nadort | Lanthanide Upconversion Luminescence at the Nanoscale: Fundamentals and Optical Properties[END_REF][START_REF] Haase | Upconverting Nanoparticles[END_REF][START_REF] Liu | Upconversion-Based Photodynamic Cancer Therapy[END_REF][START_REF] Marin | Upconverting Nanoparticle to Quantum Dot Förster Resonance Energy Transfer: Increasing the Efficiency through Donor Design[END_REF] UCNPs also possess extraordinary optical properties, including excellent photostability, large anti-Stokes shift, low pumping threshold, and broadly tunable multicolor emission. 17- 24 In recent years, the combination of UCNPs and metallic nanostructures have proven effective in enhancing luminescence intensity. [START_REF] Zhang | Large Enhancement of Upconversion Luminescence of NaYF 4 :Yb 3+ /Er 3+ Nanocrystal by 3D Plasmonic Nano-Antennas[END_REF][START_REF] Liu | Tunable Resonator-Upconverted Emission (TRUE) Color Printing and Applications in Optical Security[END_REF][START_REF] Lee | Upconversion Luminescence Enhancement in Plasmonic Architecture with Random Assembly of Metal Nanodomes[END_REF][START_REF] Park | Plasmon Enhancement of Luminescence Upconversion[END_REF][START_REF] Debasu | Allin-One Optical Heater-Thermometer Nanoplatform Operative from 300 to 2000 K Based on Er 3+ Emission and Blackbody Radiation[END_REF][START_REF] Bang | Asymmetric Nanocrescent Antenna on Upconversion Nanocrystal[END_REF][START_REF] Wang | Influence of Plasmonic Effect on the Upconversion Emission Characteristics of NaYF 4 Hexagonal Microrods[END_REF][START_REF] Gong | Upconversion Enhancement by a Dual-Resonance All-Dielectric Metasurface[END_REF][START_REF] Lu | Plasmon Enhancement Mechanism for the Upconversion Processes in NaYF 4 :Yb 3+ ,Er 3+ Nanoparticles: Maxwell versus Förster[END_REF][START_REF] Zhou | Observation of Considerable Upconversion Enhancement Induced by Cu 2-X S Plasmon Nanoparticles[END_REF][START_REF] Chen | Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF 4 :Ho Nanostructure for Direct Visualization of Electromagnetic Hotspots[END_REF] However, most of the luminescence improvements reported so far for such hybrid nanostructures only range from several to a thousand times in brightness, either due to the insufficient optical density of states within the localized 'hotspots' or the inefficient spatial and spectral overlaps between nanoparticles and optical resonators. Besides, the realization of large-scale assembly of UCNPs into well-defined plasmonic circuitry with single-particle resolution has not been achieved, imposing technical constraints on their on-chip integrated photonics. [START_REF] Wu | Controlled Patterning of Upconversion Nanocrystals through Capillary Force[END_REF] In principle, metallic nanogaps with ultrasmall mode volume are likely to generate electromagnetic hot spots with extreme near-field enhancement. [START_REF] Siegfried | Gap Plasmons and Near-Field Enhancement in Closely Packed Sub-10 nm Gap Resonators[END_REF][START_REF] Park | Radiative Control of Dark Excitons at Room Temperature by Nano-Optical Antenna-Tip Purcell Effect[END_REF][START_REF] Wu | Upconversion Superburst with Sub-2 s Lifetime[END_REF][START_REF] Groß | Near-Field Strong Coupling of Single Quantum Dots[END_REF][START_REF] Ojambati | Efficient Generation of Two-Photon Excited Phosphorescence from Molecules in Plasmonic Nanocavities[END_REF][START_REF] Kim | Dealloyed Intra-Nanogap Particles with Highly Robust, Quantifiable Surface-Enhanced Raman Scattering Signals for Biosensing and Bioimaging Applications[END_REF][START_REF] Nam | Plasmonic Nanogap-Enhanced Raman Scattering with Nanoparticles[END_REF] Especially, plasmon enhancement of multiphoton excitation should yield much larger enhancement relative to one-photon excitation, due to a nonlinear dependence of emission on local excitation intensity. [START_REF] Ojambati | Efficient Generation of Two-Photon Excited Phosphorescence from Molecules in Plasmonic Nanocavities[END_REF] Despite these challenges, we reasoned that the combination of template-stripped, smooth metallic nanotrenches with nanoparticle self-assembly may provide a feasible solution. [START_REF] Dong | Second-Harmonic Generation from Sub-5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates[END_REF][START_REF] Nagpal | Ultrasmooth Patterned Metals for Plasmonics and Metamaterials[END_REF][START_REF] Asbahi | Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution[END_REF] In contrast to plasmonic systems previously reported, our design not only provides scalable fabrication of deep subwavelength metallic structures with low surface roughness and minimized scattering loss, but also offers the possibility for site-specific positioning of UCNPs into nanoscale gaps where the largest field enhancement occurs. Additionally, the fundamental resonance modes of the structures can be systematically tailored to specific wavelengths, promoting excited-state population and radiative transitions.

Here, we experimentally demonstrate an efficient nonlinear platform comprising monolithically integrated UCNPs that self-assemble into gold nanotrenches with sub-25-nm gaps (Figure 1a,b). For an upconversion process to proceed, incident photons are primarily absorbed by sensitizers (such as Yb 3+ and Nd 3+ ) inside UCNPs and then transferred to adjacent activators (such as Er 3+ and Tm 3+ ), elevating the electrons in the activators to higher energy states. Subsequently, the electrons emit the accumulated photons and decay back to the ground states. When an UCNP is placed in the vicinity of a plasmonic structure, the strong local electric field induced by the surface plasmon resonance may enhance the photon flux of excitation, leading to improved energy harvesting by sensitizer ions. Concomitantly, the radiative decay rate of the activator would also be increased due to the enhanced local density of states, known as Purcell effect (Figure 1c). Here, single rows of UCNPs self-assembled into nanotrenches, where the resonant mode under 980-nm irradiation was strongly localized in the gap region with a maximum field enhancement (|E/E 0 |) of 50 (Figure 1d). Thus, UCNPs are likely to probe a much larger pumping photon flux to excite Yb 3+ ions from the ground state to the emitting state, leading to dramatically accelerated multiphoton processes and hence enhanced luminescence intensity. In addition, simulation results reveal that the plasmonic hotspot of the cavity is located between metal walls, a situation that further boosts luminescence due to low-level photothermal quenching.

We first validated the feasibility of our strategy to achieve enormously enhanced upconversion luminescence. A template-stripping method based on patterned silicon substrates was used to create plasmonic nanotrenches (Figure 2a and Figure S1). [START_REF] Dong | Second-Harmonic Generation from Sub-5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates[END_REF] Compared to commonly used metallic nanostructures made by electron-beam lithography (EBL), the template-stripping strategy produces large-area metallic nanopatterns with reduced surface roughness, which decreases plasmon scattering and minimizes intrinsic energy losses (Figures S2 andS3). NaYF 4 :Yb/Er (18/2 mol%) nanoparticles of ~20 nm in diameter were synthesized for their known efficient upconversion process (Figure 1b and Figure S4). [START_REF] Gargas | Engineering Bright Sub-10-nm Upconverting Nanocrystals for Single-Molecule Imaging[END_REF][START_REF] Liu | Single Upconversion Nanoparticle Imaging at Sub-10 W cm -2 Irradiance[END_REF] Gold nanotrenches with sub-25 nm gaps were designed to ensure that each trench can only host a single row of UCNPs. The gap morphology and width were designed to match the particle size, thereby facilitating shape-selective immobilization of nanoparticles. [START_REF] Saboktakin | Plasmonic Enhancement of Nanophosphor Upconversion Luminescence in Au Nanohole Arrays[END_REF] Spreading-mediated self-assembly method was then employed to transfer nanoparticles from the solution to nanotrenches with the aid of a meniscus upon contact (Figure 2b and Figure S5). [START_REF] Asbahi | Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution[END_REF] Scanning electron microscopy (SEM) image indicates that UCNPs are well-positioned in nanoscale gaps, suggesting that deterministic coupling of UCNPs with plasmonic nanostructures is possible by combining top-down lithography and bottom-up self-assembly.

To exploit the maximum emission enhancement of the designed plasmonic platform, a series of gold nanotrenches with different pitch sizes were fabricated while keeping the gap width fixed to ensure the assembly of nanoparticles in well-defined locations (Figure 2a and Figure S6). Relative reflectance spectra of six nanoparticle-containing samples were measured with respect to the unpatterned gold film (Figure 2c and Figure S7). Plasmon resonances, signaled by the dips of the reflectance spectra, monotonically red-shifted with increasing pitch over a broad range of wavelengths, including both emission and excitation regimes. The enhanced absorption of incident energy fluence using metallic nanotrenches was attributed to cavity confinement, which is proportional to reduced reflectance: 1-R (R, reflectance). Reduced reflectance of nanoparticle emitter-containing nanotrenches with different sizes indicates that the absorption enhancement factor increases at a resonance wavelength near 980 nm (Figure 2d).

Luminescence investigation was performed using a custom-built optical microscope (Figure 2e). A continuous-wave 980-nm laser was focused on the samples with a lateral spot diameter of ∼15 μm. The intensity enhancement of the two main emission bands (green at 540 nm and red at 655 nm) of UCNPs was strongly dependent upon the pitch size of the nanotrench (Figure 2f). Notably, the red emission was significantly enhanced when the plasmon resonance mode of the hybrid structure approaches the 980-nm excitation band. We next measured the luminescence of UCNPs coupled to the gold nanotrench with a 300-nm pitch size and two control samples (UCNPs on a glass substrate and on a gold film) at an excitation power of 6 mW (Figure 3a). The luminescence intensity from UCNPs coupled to gold nanotrenches is substantially larger (> 10 2 -fold) than that of nanoparticles deposited on control samples (Figure S8). The emission enhancement can be visualized by luminescence imaging. As UCNPs were assembled within nanoscale gaps, we observed a sharp, uniform orange-to-red emission. In contrast, no apparent luminescence was observed for UCNPs deposited on the gold or glass substrate (Figure 3b). Owing to the limited resolution of our imaging system, upconversion emission from nanotrenches with pitch sizes smaller than 300 nm could not be optically resolved, while luminescence was discernable for samples with a pitch size larger than 600 nm (Figure 3c). The enhancement factor dependence of green and red emissions on different pitch sizes is summarized in Figure 3d. The maximum green emission intensity of UCNPs coupled to gold nanostructures is about 500 and 370 times higher than that of UCNPs deposited on an unpatterned gold film and glass slide, respectively, while the red emission intensity is enhanced by more than 7,000-and 2,000-fold (Table S1 andS2). We next performed power-dependent investigations of emission at 540 and 655 nm for UCNPs coupled to the nanocavity and two control substrates (Figure S10). Luminescence spectra with small deviations were collected from three random areas on each sample. We plotted the upconversion emission intensity as a function of the power density on a double-logarithmic scale (Figure 3e andg). Under weak excitation, the slopes of the green and red emission for samples deposited on the glass substrate were 1.31 and 1.53, respectively, while increasing to 1.34 and 1.85 on the flat gold substrate. For both control samples, the red emission features a larger slope than the green emission, suggesting a higher-order photon population process for the former. In contrast, for UCNPs located in plasmonic nanotrenches, the slopes of green and red emissions were reduced to 0.71 and 0.94, respectively, indicating early onset of pump saturation due to optical field enhancement at the absorption band. The power dependence on the emission enhancement factor also revealed that the effect of pump fluence on emission intensity under low-power excitation is more significant than under high-power excitation. Compared with samples on a glass substrate, green and red emissions were enhanced by a factor of 1,600 and 10,000, respectively, at low pumping power for nanocavity-coupled UCNPs.

Conversely, nanocavity-coupled UCNPs showed enhancement factors of up to 2,000 and 20,000 for green and red emissions, respectively, relative to samples on the gold substrate (Figure 3f and Figure S11).

It is important to note that a good match between the fundamental mode of the coupled UCNP-gold nanostructure and the wavelength of the pumping laser can amplify the excitation rate of UCNPs, while mode-matching of the plasmon resonance and the emission wavelength of UCNPs is likely to increase the radiative decay rate via the Purcell effect. [START_REF] Lu | Plasmon Enhancement Mechanism for the Upconversion Processes in NaYF 4 :Yb 3+ ,Er 3+ Nanoparticles: Maxwell versus Förster[END_REF] We attained higher EFs when the plasmon resonance was located spectrally closer to the excitation than the emission band of UCNPs. This result suggests that the excitation enhancement dominates the global luminescence enhancement, while the decay rate enhancement is a less prominent factor.

It is also interesting to note that the red emission intensity was systematically more enhanced than the green one. The enhancement is strongly influenced by the number of photons involved in upconversion. Multiphoton paths exhibit higher power sensitivity than linear optical processes. [START_REF] Chen | Amplifying Excitation-Power Sensitivity of Photon Upconversion in a NaYbF 4 :Ho Nanostructure for Direct Visualization of Electromagnetic Hotspots[END_REF][START_REF] Liang | Upconversion Amplification through Dielectric Superlensing Modulation[END_REF][START_REF] Suyver | Anomalous Power Dependence of Sensitized Upconversion Luminescence[END_REF] In the lanthanide-mediated upconversion, long-lived intermediate excited states serve as energy reservoirs that sequentially populate higher emitting energy levels. The dependence of emission intensity, I, on excitation power density, P exc , can be expressed as I P n exc in the weak excitation limit, where n is the number of upconversion steps involved. [START_REF] Suyver | Anomalous Power Dependence of Sensitized Upconversion Luminescence[END_REF] In our study, Yb 3+ /Er 3+ co-doped UCNPs emit two peaks at 540 nm (green) and 655 nm (red) under 980-nm excitation. The green emission mainly results from two-photon excitation, while both two-photon and three-photon processes are mainly responsible for red upconversion emission (Figure 4a). [START_REF] Berry | Disputed Mechanism for NIR-to-Red Upconversion Luminescence in NaYF 4 :Yb 3+ ,Er 3+[END_REF][START_REF] Hossan | Explaining the Nanoscale Effect in the Upconversion Dynamics of β-NaYF 4 :Yb 3+ , Er 3+ Core and Core-Shell Nanocrystals[END_REF] To investigate the pump power sensitivity of the multiphoton upconversion in Yb 3+ /Er 3+ co-doped UCNPs, a population kinetic model involving two-and three-photon populations was numerically constructed (Figure 4b, Figure S12 and Table S3). Simulation results show that multiphoton paths exhibit higher excitation power sensitivity, confirming that significant luminescence enhancement of UCNPs likely occurs in plasmonic mode-mediated, higher-order photon population.

The increased local density of states within plasmon nanotrenches may lead to changes in the excited-state lifetime of emitters. The temporal decay investigation showed that UCNPs on the glass slide exhibit mono-exponential decay with a decay lifetime of 206 μs at 655 nm, while UCNPs on the gold film have a decay lifetime of 175 μs (Figure 4c). This slight reduction in the lifetime was attributed to surface quenching due to nonradiative energy transfer between UCNPs and the gold film. Intriguingly, when UCNPs were self-assembled in the plasmonic nanotrench, their luminescence lifetime drastically shortened to 56 μs, corresponding to a 3.7-fold increase in the upconversion emission rate. In this case, except for the quenching effect, we reason that the shortened lifetime is primarily attributed to increased radiative decay due to the Purcell effect.

To further probe the mechanism underlying luminescence enhancement, an excited-state absorption (ESA)-mediated upconversion process in a plasmonic nanocavity was numerically investigated based on a 3D-FDTD (Finite Difference Time Domain) method (Figure S13). 53,54 The nanopatterned substrate with an optimized pitch size of 300 nm was chosen, and the incident light was simulated as an x-polarized plane wave at 980 nm (Figure 4d ande). For ESA-mediated upconversion, two competing processes contribute to plasmon-induced excitation enhancement: the ground-state absorption improvement arising from the fourth power of near-field enhancement and the relaxation factor due to spontaneous transition from the intermediate state to the ground state. Simulated position-dependent results show significant absorption enhancement with a maximum factor exceeding 10 6 near the corner of the nanogap (Figure 4f and g), indicating an extremely confined gap-plasmon mode with a large local density of sates. The corresponding decay rate enhancements as a function of emitter positions located along xand zaxes revealed a plasmon-induced emission acceleration with maximum decay rate enhancement factors of more than 40 and 10 for red and green emission, respectively (Figure 4h andi). These numerical simulations suggest that the excitation enhancement dominates the nonlinear upconversion and the enhanced luminescence is accompanied by increased emission decay rate, which agrees well with experimental results.

In summary, we have demonstrated strong light-matter coupling by placing UCNPs into nanometric photonic hotspots of gold nanotrenches. This integrated nanoscale platform strongly confined incident excitation radiation within deep-subwavelength metallic nanogaps, leading to 10,000-fold and 3. Fabrication of gold nanotrenches (sub-20-nm gap) as plasmonic cavities for UCNPs. Briefly, hydrogen silsesquioxane resist was used to fabricate the mask for nanotrenches using electron beam lithography.

The silicon substrate was then partially etched using inductively coupled plasma, followed by removing the resist through reactive ion etching. Next, a gold film with a thickness of 150-nm was deposited on the pre-patterned silicon substrate by e-beam evaporation. A glass slide adhered to the gold film using optical adhesive glue. After curing, the gold substrate comprising two-dimensional nanotrenches was peeled off. Emission lifetime measurements at 655 nm for UCNPs embedded in plasmonic nanotrenches (yellow)

The

  UCNPs embedded in lateral gap-mode cavities with a high local density of states can probe a large pumping photon flux. The effective area for generating luminescence signals is the nanotrench region where the nanoparticles are trapped. To quantify the enhancement factors (EFs) for UCNPs coupled to plasmonic nanotrenches, emission intensities were normalized by effective area correction using the following formula (Figure 3c, Figure S9):  (1) where I gap is the luminescence intensity measured for UCNPs localized within nanotrenches, I gold and I glass are emission intensities measured for a monolayer of UCNPs deposited on a gold film or glass slide (Figure S8), and A gap is the effective area of plasmonic nanotrenches (See supplementary method).

FiguresFigure 1 .

 1 Figures

Lastly,

  UCNPs from the solution self-assembled into nanoscale gaps through capillary force. (b) Left panel: Scanning electron microscopy (SEM) image of UCNPs embedded in square arrays. Scale bar: 200 nm. Right panel: transmission electron microscopy (TEM) image of the as-synthesized NaYF 4 :Yb/Er nanoparticles (scale bar: 50 nm) and high-resolution TEM image of a single nanoparticle (scale bar: 5 nm). (c) Proposed gap plasmon-coupled upconversion processes. Simplified energy level diagram depicting interactions between gap-plasmon modes and quantum energy levels of UCNPs. Either the accelerated decay or the enhanced upconversion population can be controlled by adjusting the spectral overlap between the localized surface plasmon resonance (LSPR) and the upconversion excitation or emission wavelength. (d) Simulated lateral plasmonic gap-mode with a maximum field enhancement of ∼50.

Figure 2 .

 2 Figure 2. Spectral and optical characteristics of hybrid nanostructures. (a) SEM images of template-

Figure 3 .

 3 Figure 3. Luminescence enhancement of UCNPs coupled to plasmonic nanotrenches. (a, b) Upconversion

Figure 4 .

 4 Figure 4. Theoretical investigations of the gap-plasmon upconversion acceleration. (a) Energy level

ACKNOWLEDGMENT This work is supported by the National Key & Program of China (No. 2018YFA0902600), the National Natural Science Foundation of China (21635002, 21771135, 21871071), the Ministry of Education, Singapore (MOE2017-T2-2-110), the Agency for Science, Technology and Research (A*STAR) (Grant NO. A1883c0011 and A1983c0038), the Agency for Science, Technology and Research (A*STAR) SERC Pharos Project (Grant NO. 1527300025) and National Research Foundation, the Prime Minister's Office of Singapore under its NRF Investigatorship Programme (Award No. NRF-NRFI05-2019-0003).

We also thank P. Lu, Z. Mu, D. Ding for technical assistance and helpful discussion.

AUTHOR INFORMATION

Corresponding Author

*E-mail: chmlx@nus.edu.sg joel_yang@sutd.edu.sg

Notes

The authors declare no competing financial interest. and deposited on a glass slide (blue) or gold film (black). (d, e) Schematic and cross-sectional electricfield distributions of the UCNP-coupled gap nanocavity (xz plane). The depth and width of the gap are slightly modified to keep the resonant wavelength close to 980 nm. (f, g) The fourth power of local Efield-induced absorption enhancement factors (black) and relaxation factor (brown) as a function of x position and z position within the gap. (h, i) Normalized radiative decay rates at 540 nm (blue) and 655 nm (red) for a dipole emitter located at different x position and z position within the gap. Decay rates are averaged by inserting a dipole in three orthogonal directions.

TOC

On-chip, site-specific confinement of lanthanide-activated nanocrystals within gold nanotrenches of sub-25 nm is achieved via bottom-up self-assembly. This integrated nanoscale platform exhibits 10,000-fold and 3.7-fold enhancement in luminescence intensity and emission rate, respectively.


Thibault Martin

Michael A Weston

Andreas Ravache

Eric Vidal

Martin Thibault

Flight-initiation response

Keywords: behaviour, disturbance, escape, learning, seabirds

published or not. The documents may come   L'archive ouverte pluridisciplinaire

ones [START_REF] Tershy | The importance of islands for the protection of biological and linguistic diversity[END_REF]. The increasing use of, and visits to, these isolated territories by people (especially tourists) enables the investigation of how biodiversity reacts to evolutionarily novel pressures [START_REF] Hall | An island biogeographical approach to island tourism and biodiversity: an exploratory study of the Caribbean and Pacific Islands[END_REF]. At the same time, development of integrated conservation strategies is required to prevent biodiversity changes and losses on islands associated with human presence [START_REF] Tershy | The importance of islands for the protection of biological and linguistic diversity[END_REF]. A key challenge for environmental managers is to meet sometimes competing demands to balance human uses and natural values so they coexist [START_REF] Saarinen | Wilderness use, conservation and tourism: what do we protect and for and from whom?[END_REF]. Humans can impart a variety of changes to wildlife, such as introductions of invasive species, but they can also have an indirect effect associated with their mere presence (i.e. through disturbance, the disruption of normal behaviour or physiology; [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF]. This disruption occurs when wildlife responds to people as if they are a threat associated with predators, i.e. animals escape to avoid being preyed upon [START_REF] Frid | Human-caused disturbance stimuli as a form of predation risk[END_REF]. The escape decision of animals facing approaching humans then depends on the animal's cost of escaping (body size, reproductive status) and on context (habitat openness, urbanization, resource availability) [START_REF] Samia | Increased tolerance to humans among disturbed wildlife[END_REF]. The respective influences of cost, context and experience have still to be clarified.

Seabirds generally rely on isolated, 'low-disturbance' areas to reproduce. They are one of the most threatened animal groups at a global scale and their risk of extinction is increasing faster than any other group of birds [START_REF] Xu | Decline of recent seabirds inferred from a composite 1000year record of population dynamics[END_REF]. They are a priority for managers of protected oceanic and coastal areas, both as indicators of the quality of marine environments and as major targets of restoration, conservation and management programmes [START_REF] Thibault | Seabirds: sentinels beyond the oceans[END_REF]. Human visitation to breeding sites constitutes one of the major pressures on seabird populations and managing these activities is a key challenge for their conservation (United Nations 2017).

Measuring the indirect effects of anthropogenic disturbances on animals' life history traits (weight, physiology) and demography (survival, reproductive success) appears to be the most common approach (Cianchetti-Benedetti et al. 2018). However, direct behavioural studies can provide a better measure of sensitivity of some species or groups of individuals to disturbance (Barros et al. 2016). Flight-initiation distance (FID) when approaching by walking, boat, or with a flying object can index the sensitivity of birds to disturbance [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF]. This method has the advantage of being non-invasive, minimizing the effects of different observers, and being inexpensive and easily transferable to conservation tools such as setbacks [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF][START_REF] Guay | Observer effects occur when estimating alert but not flightinitiation distances[END_REF].

We explored the behavioural reaction of a breeding seabird species to repeated visits at a study site on remote islets in the Bampton-Chesterfield-Bellona archipelago, a newly classified reserve in the Coral Sea Natural Park. We studied Brown Noddies Anous stolidus which nest in low vegetation on the edges of islets. The The increasing use of, and visits to, isolated territories by people (especially tourists) enables the investigation of how biodiversity reacts to evolutionarily novel pressures. We explored the behavioural reaction of a breeding seabird species, the Brown Noddy Anous stolidus,to our repeated visits at two study sites in the Chesterfield Islands, a newly classified reserve in the Coral Sea Natural Park. Repeated measures of flight-initiation distances (FIDs) at three sites and over time suggest that human visitations induced both a spatial phenotypical sorting or learning of individuals and a temporal habituation. In light of the novel 'Sit and Defend' mode of avian nest defence, the study finally provides the first dataset of FIDs for this species, and highlights management opportunities that could arise from the replication of such an approach to other breeding seabirds and therefore the conservation of remote island ecosystems.

The remoteness of oceanic islands has allowed for the evolution and maintenance of rich and endemic biodiversity, isolated from many pressures including anthropogenic species is considered to be very sensitive to pressures from human visits, native predators and invasive species [START_REF] Burger | Defensive aggression in terns: effect of species, density, and isolation[END_REF][START_REF] Megyesi | Brown Noddy chick predation by Great Frigatebirds in the northwestern Hawaiian Islands[END_REF], Pandolfi & Bretagnolle 2002[START_REF] Angel | A Review of the Impacts of Introduced Rodents on the Islands of Tristan da Cunha and Gough[END_REF]. We measured FID of nesting Brown Noddies at three sites during our first visits to account for a potential heterogeneity in the spatial distribution of escape behaviour that could be due to variation in the prevailing human environment. We then repeated these measures at the site most exposed to humans over 2 weeks to test whether a group of birds was capable of a behavioural habituation to our presence. Results are discussed considering the available knowledge of anti-predator behaviours in birds, and the ecology and management of the study species.

METHODS

Study system

Fieldwork was conducted on two islets within the Chesterfield Islands, a V-shaped group of islets within the Bampton-Chesterfield-Bellona Archipelago. They lie in the middle of the Coral Sea (19°05 0 -21°50 0 S, 158°15 0 -159°35 0 E), more than 500 km west of the northern edge of New Caledonia's main island. Reefs and islets are distributed within an area of c. 24 000 km 2 and consist of two main plateaux which are classified as a natural reserve (IUCN Category 2). None of these islets is inhabited by either humans or mammalian predators but whaling and guano digging activities occurred there in the late 19th century [START_REF] Borsa | Breeding avifauna of the Chesterfield Islands, Coral Sea: current population sizes, trends, and threats[END_REF]. We worked on two islets within the southern Chesterfield plateau: 'Loop' and 'Mouillage 1' (out of four), the latter being classified as an integral reserve since August 2018 (Gouvernement de la Nouvelle Cal edonie 2018). Managers visit Loop on a regular basis (yearly or twice a year) to work at the meteorological station and conduct biodiversity monitoring, which could lead to a behavioural response. This is not the case on Mouillage 1. Moreover, there is one appropriate landing area along Loop's coast which favours visitations by boaters and fishermen, unlike for Mouillage 1. At the time of our study, the two islets hosted similar seabird communities: three species of Boobies, two of Noddies, two of Frigate-bird species, Wedge-tailed Shearwaters Ardenna pacifica, and about 100 breeding Sooty Terns Onychoprion fuscatus. Thereby, the soundscapes were similar across our sampling locations.

Brown Noddy is a widespread tropical seabird, present in the Pacific, Indian and Atlantic Oceans as well as in the Caribbean. It is abundant both globally (c. 180 000-1 100 000 individuals; IUCN 2019) and on the Chesterfield Islands (c. 4300 breeding pairs; DAM/ SPE 2017). Population trends are considered to be stable so that the species is considered of Least Concern (IUCN 2019). At our study sites, the species nests at the boundary between the beach sand and the herbaceous layer. The species is colonial, individuals practise shallow plunge diving, and some may mob predators [START_REF] Wilson | Breeding biology of Brown Noddies Anous stolidus at their southern-most breeding site, Gough Island, in comparison to other sites[END_REF]). Adults lay one egg either on a platform-shaped nest made of twigs or directly on the sand. Within the colony, birds nest close to each other with distance between nests varying from a dozen centimetres to a few metres.

Sampling

Six data replicates were collected on 'Loop' and two on 'Mouillage 1' between 10 and 22 April 2019. 'Loop' islet was divided into two parts according to the human regimen experienced by the birds. The 'high-use zone' is where the boat landing area is located. During the study period, it was visited daily by five people walking along the beach to access a colony of Wedge-tailed Shearwaters for a total of 19 visits. The 'low-use zone' of the islet was visited only twice for other reasons than the measurement of FIDs, five times in total.

FIDs were measured during three sessions only on the 'isolated' side of Loop islet, to reduce the disturbance of our visitations. Mouillage 1 was visited and sampled only twice. The number of breeding pairs was systematically censused at each site. FIDs were recorded from haphazardly selected individuals seated on nests, i.e. incubating eggs or pre-laying, to reduce a potential bias associated with the initial activity of individuals. We measured FID as the horizontal distance between the observer and the nest when the targeted individual flew away [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF]. We chose FIDs because: (1) alert distance (AD) is more dependent on the observer [START_REF] Guay | Observer effects occur when estimating alert but not flightinitiation distances[END_REF]) and (2) AD and FID were close and difficult to separate in breeding Brown Noddies. A single observer initially walked along the swash zone to avoid disruption of focal birds. When an individual was selected, the observer walked perpendicular to this line to face the bird and minimize variation in starting distances (which strongly influences FID if not controlled; [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF] and the disruption of neighbouring individuals. As the geomorphology of the two islets was slightly different, Loop being very flat compared with Mouillage 1 with its central mound, we prioritized the sampling of individuals nesting at eye height relative to a standing human in the swash zone. Starting from the swash zone allowed for a consistent starting distance (%12 m). When two nesting adults were very close (i.e. <10 cm), only one was sampled. All measurements were taken with a laser rangefinder (Makita LD060P). We recorded over 30 FIDs per sampling session and site to allow for statistical reliability while minimizing the risk of measuring the same individuals twice during a single session. were observed' [START_REF] Barter | Human disturbance to colonially breeding seabirds and guidelines for visitor management on islands off the coast of Western Australia[END_REF]). Lancelin Island is popular for ecotourism and recreational activities. Located only 800 m from the mainland, it has been and still is frequently visited by people [START_REF] Barter | Human disturbance to colonially breeding seabirds and guidelines for visitor management on islands off the coast of Western Australia[END_REF]). The FIDs of Brown Noddies on Lancelin Island broadly correspond to the FIDs we report at Loop's high-use zone after 6 days of daily visitations. The highest mean FID we measured (4.4 AE 1.8 m) was on the isolated Mouillage 1 islet, suggesting a relatively weak terrestrial predator avoidance strategy. Indeed, this FID is especially low given those known for other species of lighter (e.g. Common Tern Sturna hirundo 29.6 AE 5.9 m) or heavier body masses (e.g. Great Cormorant Phalocrocorax carbo 32.3 AE 20.6 m) and because open habitats such as those we studied are associated with longer FIDs [START_REF] Blumstein | Flight-initiation distance in birds is dependent on intruder starting distance[END_REF][START_REF] Blumstein | Developing an evolutionary ecology of fear: how life history and natural history traits affect disturbance tolerance in birds[END_REF][START_REF] Barros | Escaping from Predators: An Integrative View of Escape Decisions[END_REF][START_REF] Samia | Increased tolerance to humans among disturbed wildlife[END_REF]. We acknowledge that the FIDs we report are of breeding birds and that FIDs of non-breeding birds may not be comparable [START_REF] Weston | Human disturbance[END_REF]. The relatively short FIDs we report may at least partly be due to 'island tameness', the reduction of escape responsiveness with evolutionary isolation from predators [START_REF] Blumstein | The loss of antipredator behaviour following isolation on islands[END_REF]. Short FIDs may also indicate that leaving the nest is costly. Air temperatures were high during sampling (27-28 °C) and absent birds risked their eggs overheating. Whatever the drivers of the short FIDs we describe, they are likely to accentuate the vulnerability of adult Brown Noddies to novel predators.

While human regimens are rarely quantified (Weston 2019), we observed slightly lower FIDs at one site that experiences greater exposure to humans. This difference was not significant and should thus be confirmed by further sampling, but it is consistent with the patterns expected as a result of learning [START_REF] Blumstein | Habituation and sensitization: new thoughts about old ideas[END_REF], phenotypic sorting of personalities [START_REF] Møller | Transgenerational consequences of human visitation[END_REF] or both. We also report a slight, but rapid decrease in FIDs with increasing exposure to humans over the timescale of days. This can probably be attributed to learning [START_REF] Blumstein | Habituation and sensitization: new thoughts about old ideas[END_REF][START_REF] Møller | Transgenerational consequences of human visitation[END_REF]), as it is unlikely that individuals sampled in the same nests changed over our study period. In only 6 days, the mean FID measured on the same group of individuals dropped from 3.6 AE 1.3 to 1.8 AE 0.6 m (i.e. c. 50%), before stabilizing between 0.4 and 2.7 m afterward (1.9 AE 0.6 m). This would suggest a behavioural habituation to our repeated visits. Individuals could quickly learn from harmless visits and modify their escape behaviour to lower possible negative effects on their reproductive outputs [START_REF] Barros | Escaping from Predators: An Integrative View of Escape Decisions[END_REF][START_REF] Blumstein | Habituation and sensitization: new thoughts about old ideas[END_REF]. Of course, a behavioural habituation stricto sensu could only have been measured over time by repeatedly sampling the same known individuals. Banding was not possible in our study context and may have imparted a specific learning event on individuals; however, we tried repeatedly to target the same nests at each site and consider that our results at a group scale reflect a generalized behavioural shift among individuals. Examples of M. Thibault et al.

Analyses

To examine whether FIDs varied across a historical gradient of human useage, we tested for potential differences in mean FIDs during the first visit to each site using a one-way analysis of variance (ANOVA) and Tukey posthoc tests. To examine whether Noddies adjusted their FIDs to repeated expose to humans, we focused on the part of Loop islet visited every day. We ran a generalized linear model (GLM) with a gamma distribution and inverse-link function, using the package 'lme4' v1.1.21 and checking graphically for the suitable error structures [START_REF] Bates | Fitting linear mixed-effects models using lme4[END_REF]. We used FIDs as a response variable and the days since the start of the experiment as a discrete predictor. Bootstrapped confidence intervals were set at the 98% confidence level. Confidence intervals were calculated from 500 parametric bootstrap simulations, i.e. calculating the effect estimate from 500 resampled datasets to evaluate its uncertainty. Data were compiled and analysed using R v3.4.0 (R Core Team 2017). All results are presented as mean AE standard deviation.

RESULTS

We counted 778 breeding pairs on Loop islet and 71 breeding pairs on Mouillage 1 (68 and 100%, respectively), with adults sitting on nests and brooding one egg. The remaining adults were pre-laying and no chicks were observed. Among the 455 measurements collected, 198 were collected on the 'disturbed' side of Loop islet, 159 on the 'isolated' side and 98 on Mouillage 1 Islet.

FIDs were slightly higher on Mouillage 1 than on Loop islet and within Loop islet slightly higher in the low-use zone than the high-use zone (Fig. 1a). However, these differences were not significant (mean difference = 1.08, adjusted P = 0.082). FIDs decreased with our repeated visitations to the high-use zone of Loop islet (Fig. 1b). The effect of time as a proxy of repeated visits on measured FID was significant (estimate: À5.64; 98% CI: À7.03 to À4.24). McFadden's pseudo R 2 for this model was 0.28.

DISCUSSION

This study reports two main results: (1) FIDs were generally short and (2) frequent human visits to a remote island affected the behaviour of breeding seabirds over a short timescale. The lower mean FIDs observed at Loop islet's high-use zone need to be confirmed.

We provide the first dataset on the FIDs of Brown Noddies approached by a single observer on a remote islet. Previous work conducted on Lancelin Island (Western Australia) estimated this distance to be 'within 2 m of target-breeding birds before flight initiation responses behavioural habituation patterns of birds at such short timescales have apparently hitherto been undocumented.

In the context of remote islands free of mammalian predators, weak escape behaviour reflected by short FIDs could instead reflect strong defence behaviours [START_REF] Megyesi | Brown Noddy chick predation by Great Frigatebirds in the northwestern Hawaiian Islands[END_REF]. Although egg predators exist in our study system (e.g. Banded Rails Gallirallus philippensis feed on eggs of Brown Booby Sula melanogaster; M. Thibault unpubl. obs.) predators of adult Noddies may be absent or rare. This may have led to a 'Sit and Defend' strategy. In the absence of predators of adults, with high-density breeding, the greater risk for groundnesting birds might be the loss of their breeding outputs. 'Boldness' (i.e. short FIDs) may effectively encourage depredation of nearby eggs defended by 'shyer' adults. The reaction toward novel encounters such as humans would then be guided by a trade-off between fear and protection of eggs/chicks. Such a strategy would also engender competition between nearby breeders in terms of their FIDs, possibly even selection, which may contribute to differences in FIDs between localities. A Sit and Defend strategy probably involves careful assessment of the risks to the adult associated with an approaching threat and predicts a rapid reduction in FID with exposure to a benign stimulus. We report results consistent with these ideas. In this study, humans were non-lethal agents, and birds exposed to people rapidly reduced their FIDs. Additionally, site-scale differences in FIDs were observed.

Management and research implications

Habituation of a group, as suggested by our results, is not universally benign [START_REF] Baudains | Habituation and habitat changes can moderate the impacts of human disturbance on shorebird breeding performance[END_REF]. Such results call for a deeper understanding of the relationship between two habituation-like processes that could have different impacts on a disturbed population, namely learning and phenotypic sorting. Habituation should be tested in regard to different stimuli and at different intensities and time scales because it could help to explain the resilience of some species to human disturbance. Similarly, the phenotypic sorting potentially induced by frequent human visitations could underpin insidious processes influencing bird communities such as a decline in the local population of individuals that are more sensitive to disturbance (Mart ın et al. 2015). Here we were able to test the effect of a high frequency of visitations over a short period, suggesting that a habituation could be possible. However, tourism for example could also take the form of a low frequency of visitations but over longer periods. Past local studies have already suggested that long-term human visits on islands of the southern lagoon of New Caledonia could be associated with lower reproductive success in Brown Noddies (Pandolfi & Bretagnolle 2002).

Understanding the behavioural response of island fauna is critical in an era of increasing demand for naturebased tourism in remote places [START_REF] Booth | The value of species rarity in biodiversity recreation: a birdwatching example[END_REF]. Efforts to evaluate the vulnerability of island seabirds to indirect (human) and direct (habitat changes, invasive species, pollution) pressures from frequent human visits will greatly assist management of these remote ecosystems. Recent calls were launched to warn scientists and environment managers against this forthcoming threat to remote tropical island ecosystems [START_REF] Vidal | Underpin tourism regulation with data[END_REF]. In these small and complex systems, uncontrolled changes occurring in the terrestrial compartments could have negative cascading effects on the surrounding marine environment [START_REF] Lorrain | Seabirds supply nitrogen to reef-building corals on remote Pacific islets[END_REF][START_REF] Graham | Seabirds enhance coral reef productivity and functioning in the absence of invasive rats[END_REF]. Thus, we believe that further studies should focus on: (1) multi-species assessments of the responses to human disturbances, (2) linking behavioural responses of individuals to fitness and demography, and (3) understanding the cascading effects on the island-reef continuum. Producing metrics such as FIDs for an ensemble of species and habitats could usefully assist managers in mitigating the impacts of ongoing changes in the use of these systems of very high conservation value [START_REF] Weston | A review of flight-initiation distances and their application to managing disturbance to Australian birds[END_REF].

The study was conducted as a side work of the BIOPELA-GOS programme, a collaborative programme jointly conducted by the Institute for Research and Development and the South Pacific Community. The campaign 'Puffins-Chesterfield', which this study is part of, was authorized by the decree No. 2019-809/GNC of 2 April 2019. Access to sample sites, data collection and use are presented in the convention No. 05/2019 of 3 April 2019. The programme benefits from the financial support of the BEST 2.0 programme. We thank the government of New Caledonia for giving access to their ship Amborella.Thanks to the ship crew for providing important logistical support onsite. Thanks to F. Brescia from IAC for sharing part of the field material. M.A.W. was supported by the Beach Ecology and Conservation Hub (Venus Bay).

Figure 1 .

 1 Figure 1. (a) Flight-initiation distances (FIDs) measured on Brown Noddies during the first visit at three sites. (b) Decrease in FIDs measured for breeding Brown Noddies over repeated approaches during a 2-week sampling session on Loop islet's high-use zone. The solid line represents the fitted generalized linear model with associated standard error. The dashed line represents the cumulative number of past human visits over time. [Colour figure can be viewed at wileyonlinelibrary.com]