Stephane Calvez

We present a generalized constraint-based optimization approach for photonic integrated circuit (PIC) device design, relying on a sparse exploration of the geometrical parameter space primarily driven by propagation loss mechanisms, and applied consistently to both linear and nonlinear devices. To that effect, an analytical model for propagation loss is developed, accounting for scattering contributions from both horizontal and vertical waveguide interfaces. This model is then applied in combination with effective refractive index computations by the finite element method (FEM) to perform the optimization of waveguide geometries for various applications, including electro-optic (EO) modulators, second-harmonic generation (SHG), and four-wave mixing (FWM). The results demonstrate that the optimal waveguide geometry arises from a trade-off between surface roughness distribution, waveguide geometry, and application-specific requirements.

This study investigates the use of Standard Cleaning 1 (SC-1), a routine treatment in silicon micro-/nano-fabrication, as a step in the processing of photonic structures and waveguides on thin films of lithium niobate on insulator (LNOI). We investigate the use of SC-1 for surface preparation and as a cleaning method to remove residual byproducts after ICP etching, evaluating its impact on etching dynamics and the quality of the LiNbO ₃ thin film. Our results demonstrate that SC-1 cleaning removes a surface layer of LiNbO ₃ characterized by higher oxygen vacancies. These oxygen vacancies accelerate the etching process and lead to faster under-etching of Si ₃ N ₄ capping layers on LiNbO ₃ , causing pattern delamination, particularly at sub-micron scales where under-etching is more detrimental. Finally, we show that carrying out a SC-1 treatment preliminary to Si ₃ N ₄ capping layer deposition enables the fabrication of Si ₃ N ₄ gratings with sub-100 nm width on LiNbO ₃ thin films.

We demonstrate that the deposition of organic thin films whose thickness is spatially shaped with sub-100-µm lateral resolution can be achieved by evaporation through a reusable flux-controlling micrometric-hole sieve that is separated from the to-be-coated substrate. This approach offers a practical way to grayscale-pattern organic materials without exposing them to detrimental ambient air or chemical solutions. As such, it facilitates the fabrication of long-lasting organic microstructured devices such as the reported spatially-varying absorptive color filters based here on the organic semiconductor (4,4'-Bis[4-(di-p-tolylamino)-styryl]biphenyl (DPAVBi)). Additionally, the ability to readily make multiple thickness-coated samples offers a material-sparing yet convenient way to measure the properties of the evaporated material, a feature exploited hereafter to extract the optical dielectric constants of DPAVBi over the visible and near-infrared range. This work and its extensions to inorganic compounds and freeform shaping pave the way for a new generation of devices that should find applications in light-matter coupling studies, hyperspectral imaging, structural coloring, or optical systems exploiting freeform optics.

This paper demonstrates the fabrication of near-infrared transparent electrodes customized for specific light polarization. We achieve this by employing semiconductor deep subwavelength monolithic high-contrast gratings (MHCG) integrating metal stripes. Through the combination of GaAs-based MHCG with TiAu or Au stripes within a deep sub-wavelength grating, polarization selective transmission exceeding 80% over a wide spectral range is achieved for structures designed to operate around a wavelength of 940 nm for either TE or TM excitation. At maximum 85% transmission is reached which relates to 122% transmission with respect to the transmission through plain GaAs-air interface. Additionally, these structures exhibit low sheet resistance (1.4 Ω sq − 1 ). Hybrid metal-stripes/MHCG grating (MetalMHCG) enables record-breaking polarized light transmission and electrical conductivity simultaneously. Our work represents the first demonstration of MetalMHCG in the near-infrared range, thanks to an advanced nanofabrication process of this sub-λ hybrid semiconductor/metal grating, showing optical and electrical properties not seen in alternative transparent conductive electrode designs. The proposed approach has the potential to enhance electrical injection uniformity in optoelectronic devices such as LEDs, semiconductor lasers and photodetectors.

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.

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.

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.

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.

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.