Accurate ray-tracing optical model for coded aperture spectral snapshot imagers.

The image formation in coded aperture spectral imagers is key information for processing the acquired compress data, and the optical system design and calibration of these instruments require great care. We propose an analytical model for CASSI systems that builds upon ray-tracing equations of each optical component. The model takes into account optical distortions, sampling effects, and optical misalignments, and allows accurate modeling and fast calibration. Numerical comparisons with a simpler model usually exploited in the literature are provided, and an experimental validation is presented.

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.

Spectrally-shaped illumination for improved optical inspection of lateral III-V-semiconductor oxidation.

We report an hyperspectral imaging microscopy system based on a spectrally-shaped illumination and its use to offer an enhanced in-situ inspection of a technological process that is critical in Vertical-Cavity Surface-Emitting Laser (VCSEL) manufacturing, the lateral III-V-semiconductor oxidation (AlOx). The implemented illumination source exploits a digital micromirror device (DMD) to arbitrarily tailor its emission spectrum. When combined to an imager, this source is shown to provide an additional ability to detect minute surface reflectance contrasts on any VCSEL or AlOx-based photonic structure and, in turn, to offer improved in-situ inspection of the oxide aperture shapes and dimensions down to the best-achievable optical resolution. The demonstrated technique is very versatile and could be readily extended to the real-time monitoring of oxidation or other semiconductor technological processes as soon as they rely on a real-time yet accurate measurement of spatio-spectral (reflectance) maps.

Fast reconstruction of hyperspectral images from coded acquisitions using a separability assumption: erratum.

We present an erratum to our publication [Opt. Express30(5), 8174 (2022)10.1364/OE.448893] correcting a numerical value without affecting the results and conclusions of the original publication.

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.

Fast reconstruction of hyperspectral images from coded acquisitions using a separability assumption.

We present a fast reconstruction algorithm for hyperspectral images, utilizing a small amount of data without the need for any training. The method is implemented with a dual disperser hyperspectral imager and makes use of spatial-spectral correlations by a so-called separability assumption that assumes that the image is made of regions of homogenous spectra. The reconstruction algorithm is simple and ready-to-use and does not require any prior knowledge of the scene. A simple proof-of-principle experiment is performed, demonstrating that only a small number of acquisitions are required, and the resulting compressed data-cube is reconstructed near instantaneously.

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.

Optimized coded aperture for frugal hyperspectral image recovery using a dual-disperser system.

We present a novel acquisition scheme based on a dual-disperser architecture, which can reconstruct a hyperspectral datacube using many times fewer acquisitions than spectral bands. The reconstruction algorithm follows a quadratic regularization approach, based on the assumption that adjacent pixels in the scene share similar spectra, and, if they do not, this corresponds to an edge that is detectable on the panchromatic image. A digital micro-mirror device applies reconfigurable spectral-spatial filtering to the scene for each acquisition, and the filtering code is optimized considering the physical properties of the system. The algorithm is tested on simple multi-spectral scenes with 110 wavelength bands and is able to accurately reconstruct the hyperspectral datacube using only 10 acquisitions.

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.

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.

Quantum state measurement using coherent transients.

We present the principle and experimental demonstration of time resolved quantum state holography. The quantum state of an excited state interacting with an ultrashort chirped laser pulse is measured during this interaction. This has been obtained by manipulating coherent transients created by the interaction of femtosecond shaped pulses and rubidium atoms.

Femtosecond spectral electric field reconstruction using coherent transients.

We have implemented a new approach for measuring the time-dependent intensity and phase of ultrashort optical pulses. It is based on the interaction between shaped pulses and atoms, leading to coherent transients.