Selective excitation of high-order modes in two-dimensional cavity resonator integrated grating filters.

The selective spatial mode excitation of a bi-dimensional grating-coupled micro-cavity called a cavity resonator integrated grating filter (CRIGF) is reported using an incident beam shaped to reproduce the theoretical emission profiles of the device in one and subsequently two dimensions. In both cases, the selective excitation of modes up to order 10 (per direction) is confirmed by responses exhibiting one (respectively two) spectrally narrowband resonance(s) with a good extinction of the other modes, the latter being shown to depend on the parity and order(s) of the involved modes. These results pave the way toward the demonstration of multi-wavelength spatially selective reflectors or fiber-to-waveguide couplers. Also, subject to an appropriate choice of the materials constituting the CRIGF, this work can be extended to obtain mode-selectable laser emission or nonlinear frequency conversion.

Critical coupling in cavity-resonant integrated-grating filters (CRIGFs).

We experimentally demonstrate critical coupling in miniature grating-coupled resonators known as cavity-resonant integrated-grating filters (CRIGFs). Using previously proposed asymmetric grating coupler designs for non-linear CRIGFs, and introducing a dedicated variant of a coupled-modes theory model to estimate physical properties out of the measured reflection and transmission characteristics of these resonators, we demonstrate fine control over the in-and out-coupling rate to the resonator while keeping constant both the internal losses and the resonant wavelength. Furthermore, the critical coupling condition is also observed to coincide with the maximum enhancement of the second harmonic generation signal.

Extreme enhancement of the quality (Q)-factor and mode field intensity in cavity-resonator gratings.

In this paper, dielectric Cavity-Resonant Integrated-Grating Filters (CRIGFs) are numerically optimized to achieve extremely high-quality factors, by optimizing the cavity in/out-coupling rate and by introducing apodizing mode-matching sections to reduce scattering losses. Q-factors ranging between 0.1 and 50 million are obtained and two different domains are distinguished, as a function of the perturbation parameter which controls the cavity in/out-coupling rate. When the cavity coupling Q-factor is lower than the Q-factor of the uncoupled Fabry-Perot cavity, corresponding to the over-coupling regime, the reflectivity response exhibits a high resonance maximum. On the contrary, in the under-coupling regime the resonant reflectivity maximum is much weaker since the scattering losses of the uncoupled cavity dominate. Between these two domains, the so-called critical coupling condition leads to very strong field enhancement inside the device, reaching up to 104 times the incident field amplitude. This theoretical work paves the way towards the practical implementation of CRIGFs with much higher Q-factors than currently demonstrated, potentially reaching performance on a par with other resonators such as photonic crystal cavities or whispering gallery mode resonators. These results can serve to optimize the design of narrow-band planar grating filters, particularly for application in non-linear optics.

Cavity-resonator integrated bi-atom grating coupler for enhanced second-harmonic generation.

We report on the design of cavity-resonator integrated grating couplers for second-harmonic generation. The key point is that the base pattern of our grating coupler (GC) is made of two ridges with different widths (bi-atom). Thus, we reach extremely high Q-factors (above 105) with structures whose fabrication is not challenging, since the bi-atom base pattern is close to that of the surrounded distributed Bragg reflectors (DBR). Yet, the parameters of the structure have to be chosen cautiously to reduce the transition losses between each section (GC, DBR). We numerically demonstrate conversion efficiencies η of several tenths per Watt, even doubled when we include a phase-matching grating within the structuration. Such efficiencies are comparable to those obtained with waveguides and nano-resonators.

Cavity resonator-integrated guided-mode resonance filters with on-chip electro- and thermo-optic tuning.

Cavity resonator grating filters (CRIGFs) integrated on lithium niobate on insulator (LNOI) with electrical tuning elements are reported. The resonance wavelength of the filters is in the 780 nm range. Integrated thermo-optical tuning range of 2.5 nm is measured using integrated resistors, whilst a 0.7 nm electro-optical tuning range using capacitive metallic pads is achieved with ±400V drive voltage.

GaAs membrane PhC lasers threshold reduction using AlGaAs barriers and improved processing.

Active suspended membranes are an ideal test-bench for experimenting with novel laser geometries and principles. We show that adding thin AlGaAs barrier near the top and bottom Air/GaAs interfaces of the membrane significantly reduces the carriers non-radiative recombinations and decreases the threshold of test photonic crystal test lasers. We review the existing literature on photonic crystal membrane fabrication and propose an overview of the significant defects that can be induced by each fabrication step. Finally we propose a complete processing scheme that overcome most of these defects.

Broadband high-contrast visible optical switches based on a spin-crossover material.

Visible optical switches embedding a spin-crossover thin film and exploiting the frustrated total internal reflection operation principle are studied and optimized numerically with a view to obtain broadband high-contrast devices. A practical implementation using uncoated SF11 prisms embedding a 1-µm-thick layer of iron-triazolyl-borate complex as the thermo-active phase-change material is shown to support p-polarized modulation with contrast in excess of 90% over a spectral bandwidth greater than 270 nm and over an angular acceptance bandwidth of 0.45°, surpassing the performance achievable with optically resonant devices.

Design of mesoscopic self-collimating photonic crystals under oblique incidence.

Mesoscopic Photonic Crystals (MPhCs) are composed of alternating natural or artificial materials with compensating spatial dispersion. In their simplest form, as presented here, MPhCs are composed by the periodic repetition of a MPhC supercell made of a short slab of bulk material and a short slab of Photonic Crystal (PhCs). Therefore, MPhCs present a multiscale periodicity with a subwavelength periodicity within each PhC slab and with a few-wavelength periodicity for its supercell. Thanks to this mesoscopic structure, MPhCs allow the self-collimation of light, through a mechanism called mesoscopic self-collimation (MSC), along both directions of high symmetry and directions oblique with respect to the MPhCs slab interfaces. Here, we propose a new design method useful for conceiving MPhCs that allow MSC under oblique incidence, avoiding in-plane scattering and ensuring propagation via purely guided modes, without out-of-plane radiation losses. In addition, the proposed method allows a systematic search for optimal MSC structures, which also simultaneously satisfy the impedance matching condition at MPhC interfaces, thus reducing the effect of multiple reflections between bulk-PhC interfaces. The proposed design method has the advantage of an extreme analytical simplicity and it allows direct design of oblique-incidence MPhC structures. Its accuracy is validated through Finite Difference Time Domain simulations and the MSC performances of the designed structures are evaluated, in terms of angular direction, beam waist, overall transmittance, and through discussion of a Figure of Merit that accounts for residual beam curvature. This simple yet powerful method can pave the way for the design of advanced MSC-based photonic interconnects and circuits that are immune to crosstalk and out-of-plane losses.

Mesoscopic self-collimation along arbitrary directions and below the light line.

Self-collimation (SC) and mesoscopic self-collimation (MSC) have been successfully demonstrated along the directions of high symmetry of photonic crystals. Indeed, wide angular acceptances are obtained only in these directions which offer extremely flat isofrequencies. In this article, we numerically demonstrate that mesoscopic self-collimation with large angular acceptance can be achieved along arbitrary directions that are not of high symmetry. In particular, we propose a simple method that allows to easily find all the non-trivial collimation directions and corresponding frequencies. Thanks to the double periodicity of the mesoscopic crystal, these solutions can be effectively tailored in terms of direction and frequency. Moreover, non-trivial MSC solutions can be found well below the light cone. These MSC features open up the possibility of designing complex systems by combining different configurations, such as high reflection (HR) or anti reflection (AR) ones, or active materials.

Second-harmonic-generation enhancement in cavity resonator integrated grating filters.

We demonstrate numerically and experimentally second-harmonic generation (SHG) in a cavity resonator integrated grating filter (CRIGF, a planar cavity resonator made of Bragg grating reflectors) around 1550 nm. SHG is modeled numerically for several different systems, including a thin plane layer of LiNbO3 without and with a grating coupler to excite a waveguide mode. We demonstrate that when the waveguide mode is confined to a CRIGF, designed to work with focused incident beams, the SHG power is increased more than 30 times, compared to the case of a single grating coupler used with an almost collimated pump beam.

Low-loss buried AlGaAs/AlOx waveguides using a quasi-planar process.

In this paper, we demonstrate that buried oxide-confined waveguides can be formed using a lateral oxidation process carried out through a discrete set of small-diameter via-holes instead of the conventional scheme where the oxidation starts from the edges of etched mesas. The via-hole oxidation is shown to lead to straight waveguides with smooth oxide/semiconductor interfaces and whose propagation losses are similar to one obtained using the standard process but with the advantage of maintaining a quasi-planar wafer surface. It thereby paves the way towards a simplification of the fabrication of III-V-semiconductor-oxide photonic devices.

Design of angularly tolerant zero-contrast grating filters for pixelated filtering in the mid-IR range.

Zero-contrast gratings (ZCG) can be used to implement narrow bandpass transmission filters. However, they suffer from poor angular tolerance, which hinders their use in pixelated applications. Combining ZCG with double-corrugation grating, we increase the resonance width and angular tolerance of the filter by more than 1 order of magnitude. Filters tunable around 4.6 µm with more than 90% transmission and compatibility with 140 µm pixel size are demonstrated.

Mesoscopic self-collimation and slow light in all-positive index layered photonic crystals.

We demonstrate a mesoscopic self-collimation effect in photonic crystal superlattices consisting of a periodic set of all-positive index 2D photonic crystal and homogeneous layers. We develop an electromagnetic theory showing that diffraction-free beams are observed when the curvature of the optical dispersion relation is properly compensated for. This approach allows us to combine slow-light regime together with self-collimation in photonic crystal superlattices presenting an extremely low filling ratio in air.