Michele Paparella
email: michele.paparella@umontpellier.fr

Laura Monge-Bartolomé
email: laura.monge-bartolome@umontpellier.fr

Jean-Baptiste Rodriguez
email: jean-baptiste.rodriguez@cnrs.fr

Laurent Cerutti
email: laurent.cerutti@umontpellier.fr

Marco Grande
email: marco.grande@poliba.it

Liam O'faolain

Eric Tournié
email: eric.toumie@umontpellier.fr

/ Es

Jean Baptiste Rodriguez

Laura Monge Barlome

integration on Si platforms

Keywords: 111-V integration on Silicon, Silicon Photonics, Photonic Integrated Circuits

teaching and research institutions in France or abroad, or from public or private research centers.

INTRODUCTION

Silicon photonics has been widely used for telecom applications as well as for realization of optical interconnects, exploiting the miniaturization provided by Photonic Integrated Circuits (PICs). Nowadays, PICs can also be implemented for sensing purposes. Gases such as CH 4 , NH 3 or CO show fingerprint absorption lines in the wavelength region of 2-2.5 µm, with CH 4 displaying its second most important absorption line at 2.3 µm [ 1]. The increased use of silicon for PI Cs applications is mainly based on availability of cheap Si wafers, strong optical confinement and low losses in the wavelength range between 1.1 and ~ 6.5 µm [2]. However, the main bottleneck of silicon photonics is related to its indirect bandgap making it an inefficient light emitter. The best technology to get coherent light emission is based on III-V semiconductor laser diodes (LDs). In this work, we consider a GaSb-diode laser (DL) grown on on-axis (001) Si substrate butt-coupled with a passive waveguide. The cavity mirror is based on the definition of etched cavity ridge DLs [START_REF] Monge-Bartolome | Etched-cavity GaSb laser diodes on a MOVPE GaSb-on-Si template[END_REF]. Moreover, during epitaxy of the III-V laser on Si, poor quality III-V materials are deposited close to the passive waveguide interface and should be removed. Consequently, an unavoidable air gap(> 5 µm wide) separates the passive and active waveguides. 3D Finite Difference Time Domain (FDTD) simulations are performed to evaluate the transmitted power as function of this air gap between a GaSb-based DL and a passive waveguide made of SiN, Si, Germanium (Ge) and Chalcogenide (ChG). Efficient light coupling is achieved in the absence of a gap between the two waveguides. However, the introduction of an air gap drastically reduces the optical transmission. In order to improve the coupling performances, we propose alternative approaches such as filling the gap with high-index materials or using polymer-based optical waveguides.

n_ RESULTS

A_ Waveg11ide based on silicon technofogy

ln this section, we focu ot1 the light transmi sion analysis based on a butl-coup led geometry bet w een a Ga b-DL and a passive waveguide based on Si technology. Fi. r st, we analyze the i.rnpact of the air gap on the optical coupli.ng for a waveguide made of S • Si , Ge or ChG core surro und d by a Si0 2 cladding. The DL ridge as well as the pa ive waveguide i cho en to be JO µm wide, lo sless media and emi. sion wavelength fixed at À = 2. ). However, a-Si would not be easy to implement during the fabrication process as it wou ld requi.re the deposition of a thick conformai a-Si layer, and cou Id create horts on the laser facets. Among ail the other filling materials, a viable approach is to use BCB by spin coating, even if the coupling enhancement will be smaller_

B. Polymer based waveguide

Another option to improve light coupling i to use polymer-based waveguide that can be fabricat d usi.ng pi.n-coating and photolithograpby or electron beam lithography. In this approach, the air gap is suppressed as the polymer will match the end facet of the DL thanks to a uniform distribution. However, tbe DL is an obstacle to tbe uniform polymer spreading. We tlms assumed a slope configuration combined with a planar waveguide geometry (inset in Fig. 1 (c)). We studied the optical coupling for À= 2.3 µm i.nto a pa ive polymer waveguide made of cyclized transparent optical polymer (CYTO P ® ) a• Iower claddi.ng, Polymethyl Methacrylate (PMMA) as core and an air top cladding and at the sidewalls. We analyzed tl1e optical coupling as function of the PMMA slope length. Considering a lossless PMMA material, -60% of the light is transmitted (Fig. l(c)). This value is smaller thau the -80% i.n the ca e of SiN waveguide as previously di eus ed. However, the tran minanc tay alm st constant when varyi.ng tbe slope length. In order to account for a potential absorption contribution from the PMMA material, we introduced an extinction coefficient of 1 x 10-3 5 X 10-3 [START_REF] Zhang | complex refraclive indices measurements of polymers in infrared bands[END_REF] . The transmitted power reaches values close to 40% and 20% with 1 X 10-3 and 5 X 1 0-3 extinction coefficients, respectively, whic h shows tbat the coupling performance can rapid.ly deteriorate when the i.maginary part of the polymer complex refractive index is taken into accolltlt. -=-'""-c------c

  3 µm. The imulation result • poi.nt out au i.mportant coupling reduction wben the air gap is iDtroduced. Particularly, the transmittance decreases from -80 o/o clown to -20 o/o for a 5 µm gap width and stays approximately constant for wider gaps (Fig. ! (a))-These results outli.ne the in fluence of the air gap and the nece sity to reduce the tosses once the light propagates in the free-space region before to be coup) d i.nto a passive waveguide. We propo e to use materials uch as Benzocyc lobutente (BCB), SiN, ChG and amorphou Silicon (a-Si) to fill the gap i.n order to reduce the beam divergence and improve the coupling (Fig. 1 (b)). Considering a 5 µm wide gap, the results show a doubling of the optical coupling, from -20 o/o (Air gap) to -40 o/o (a-Si gap) (i.nset i.n Fig. l(b)

ill. 0

 0 CLUSIOWe have rnodeled the optical coup ling between 2.3 µm GaSb DL epitax..ially grown on on -ax..i silicon ( 00 1 ) and on chip passive waveguides pointing out the e:ffect of the unavoidable air gap on the light coupling and proposed different scenarios to improve the transmission.

.Fig. 1 .

 1 --.----=i=----...,; f---Transmittance as a function of air gap sizes for different pa sive core materials. The inset hows the electric field profile related to ibe fondamental TE optical mode coupled into the passive waveguide through an air gap 3 �rm wide. (b) Coupling tudy for a SiN waveguide: transmittance a a function of gap size fillecl with differeni materials. The inset outl.ine a transminance improvem nl as the refractive index value. in a gap 5 µm wide. is increased. (c) Transminance as function of the PMMA slope length. The inset shows the cross-section of the hybrid device with a passive waveguide made ofpolymeric materials.