In this work, we perform numerical studies of two photonic crystal membrane microcavities, a short line-defect L5 cavity with relatively low quality (Q) factor and a longer L9 cavity with high Q. We compute the cavity Q factor and the resonance wavelength λ of the fundamental M1 mode in the two structures using five state-of- the-art computational methods. We study the convergence and the associated numerical uncertainty of Q and λ with respect to the relevant computational parameters for each method. Convergence is not obtained for all the methods, indicating that some are more suitable than others for analyzing photonic crystal line defect cavities.
In this communication, we report on the design, fabrication, and testing of Silicon Nitride on Insulator (SiNOI) and Aluminum-Gallium-Arsenide (AlGaAs) on silicon-on-insulator (SOI) nonlinear photonic circuits for continuum generation in Silicon (Si) photonics. As recently demonstrated, the generation of frequency continua and supercontinua can be used to overcome the intrinsic limitations of nowadays silicon photonics notably concerning the heterogeneous integration of III-V on SOI lasers for datacom and telecom applications. By using the Kerr nonlinearity of monolithic silicon nitride and heterointegrated GaAs-based alloys on SOI, the generation of tens or even hundreds of new optical frequencies can be obtained in dispersion tailored waveguides, thus providing an all-optical alternative to the heterointegration of hundreds of standalone III-V on Si lasers. In our work, we present paths to energy-efficient continua generation on silicon photonics circuits. Notably, we demonstrate spectral broadening covering the full C-band via Kerrbased self-phase modulation in SiNOI nanowires featuring full process compatibility with Si photonic devices. Moreover, AlGaAs waveguides are heterointegrated on SOI in order to dramatically reduce (x1/10) thresholds in optical parametric oscillation and in the power required for supercontinuum generation under pulsed pumping. The manufacturing techniques allowing the monolithic co-integration of nonlinear functionalities on existing CMOS-compatible Si photonics for both active and passive components will be shown. Experimental evidence based on self-phase modulation show SiNOI and AlGaAs nanowires capable of generating wide-spanning frequency continua in the C-Band. This will pave the way for low-threshold power-efficient Kerr-based comb- and continuum- sources featuring compatibility with Si photonic integrated circuits (Si-PICs).
Utilizing the inverse design engineering method of topology optimization, we have realized high-performing all-silicon ultra-compact polarization beam splitters. We show that the device footprint of the polarization beam splitter can be as compact as ~2 μm<sup>2</sup> while performing experimentally with a polarization splitting loss lower than ~0.82 dB and an extinction ratio larger than ~15 dB in the C-band. We investigate the device performance as a function of the device length and find a lower length above which the performance only increases incrementally. Imposing a minimum feature size constraint in the optimization is shown to affect the performance negatively and reveals the necessity for light to scatter on a sub-wavelength scale to obtain functionalities in compact photonic devices.
Space division multiplexing (SDM) is currently widely investigated in order to provide enhanced capacity thanks to the utilization of space as a new degree of multiplexing freedom in both optical fiber communication and on-chip interconnects. Basic components allowing the processing of spatial modes are critical for SDM applications. Here we present such building blocks implemented on the silicon-on-insulator (SOI) platform. These include fabrication tolerant wideband (de)multiplexers, ultra-compact mode converters and (de)multiplexers designed by topology optimization, and mode filters using one-dimensional (1D) photonic crystal silicon waveguides. We furthermore use the fabricated devices to demonstrate on-chip point-to-point mode division multiplexing transmission, and all-optical signal processing by mode-selective wavelength conversion. Finally, we report an efficient silicon photonic integrated circuit mode (de)multiplexer for few-mode fibers (FMFs).
We have designed and for the first time experimentally verified a topology optimized mode (de)multiplexer, which
demultiplexes the fundamental and the first order mode of a double mode photonic wire to two separate single mode
waveguides (and multiplexes vice versa). The device has a footprint of ~4.4 μm x ~2.8 μm and was fabricated for
different design resolutions and design threshold values to verify the robustness of the structure to fabrication tolerances.
The multiplexing functionality was confirmed by recording mode profiles using an infrared camera and vertical grating
couplers. All structures were experimentally found to maintain functionality throughout a 100 nm wavelength range
limited by available laser sources and insertion losses were generally lower than 1.3 dB. The cross talk was around -12
dB and the extinction ratio was measured to be better than 8 dB.
A narrow band (3dB bandwidth <2nm) transmission notch filter based on polarisation conversion within a photonic
crystal waveguide is demonstrated. Signal contrast between quasi- TE and TM eigenstates reaching 40dB is achieved.
Further, multiple resonant wavelength coupling between the two eigenstates is also observed. These offer a novel
alternative approach to sensing and biodiagnostics compared to previous use of the band edge of a photonic crystal
Within the last years, interest in photonic wires and photonic crystals grew due to their demonstrated ability
of controlling light propagation and characteristics. One of the limitations of such devices is due to the induced
roughness during the fabrication process. Generally, an increase in roughness leads to loss increase thus limiting
the propagation length and postponing the commercialization of such structures. In this paper we present a
new algorithm for measuring the sidewall roughness of our devices based on atomic force microscope (AFM)
approach. Using this algorithm, the roughness can be quantified and thus actions in decreasing it can be taken
improving the device's performance.
We review our work done for topology optimization of passive photonic crystal component parts for broadband and
wavelength dependent operations. We show examples of low-loss topology-optimized bends and splitters optimized for
broadband transmission and demonstrate the applicability of topology optimization for designing slow-light and/or
wavelength selective component parts. We also present how the dispersion of light in the slow-light regime of photonic
crystal waveguides can be tailored to obtain filter functionalities in passive devices and/or to obtain <i>semi</i>-slow light
having a group velocity in the range ~(c<sub>0</sub>/15 - c<sub>0</sub>/100); vanishing, positive, or negative group velocity dispersion (GVD);
and low-loss propagation in a practical ~5-15 nm bandwidth.
We report thorough investigations of photonic crystal waveguide properties in the slow light regime. The transmission and the group index near the cutoff wavelengths oscillate in phase in close analogy with the 1D photonic crystal behavior. The influence of having a finite number of periods in the photonic crystal waveguide is addressed to explain the spiky character of both the transmission and group index spectra. The profile of the slow-light modes is stretched out into the first and second rows of the holes closest to the waveguide channel. One of our strategies to ameliorate the design of photonic crystal devices is to engineer the radii of holes in these rows. A topology optimization approach is also utilized to make further improvements. The results of the numerical simulations and the optical characterization of fabricated devices such as straight waveguides with bends and couplers are presented. A nice match is found between theory and experiment.
Very low propagation losses in straight planar photonic crystal waveguides have previously been reported. A next natural step is to add functionality to the photonic crystal waveguides and create ultra compact optical components. We have designed and fabricated such structures in a silicon-on-insulator material. The photonic crystal is defined by holes with diameter 250 nm arranged in a triangular lattice having lattice constant 400 nm. Leaving out single rows of holes creates the planar photonic crystal waveguides. Different types of couplers and splitters, as well as 60, 90 and 120 degree bends have been realized. We have designed and fabricated components displaying more than 200 nm of useful bandwidth around 1550 nm. Design strategies to enhance the performance include systematic variation of design parameters using finite-difference time-domain simulations and inverse design methods such as topology optimization. We have also investigated a new device concept for coarse wavelength division de-multiplexing based on planar photonic crystal waveguides. The filtering of the wavelength channels has been realized by shifting the cut-off frequency of the fundamental photonic band gap mode in consecutive sections of the waveguide. Preliminary investigations show that this concept allows coarse de-multiplexing to take place, but that optimization is required in order to reduce cross talk between adjacent channels and to increase the overall transmission. In this work the design, fabrication and performance of these planar photonic crystal waveguide components are reviewed and discussed.
We report on direct numerical calculations and experimental measurements of the group-index dispersion in a photonic crystal waveguide fabricated in silicon-on-insulator material. The photonic crystal is defined by a triangular arrangement of holes and the waveguide is carved out by introducing a one-row line defect. Both the numerical and experimental methods are based on the time of flight approach for an optical pulse. An increase of the group index by approximately 45 times (from 4 to 155) has been observed when approaching the cutoff of the fundamental photonic bandgap mode. Numerical 2D and 3D simulations of pulse dynamics in the waveguide made by the time-domain method shows excellent agreement with measured data in most of the band. These group index values in a photonic crystal waveguide are to the best of our knowledge the largest numbers reported so far by direct tracking of pulse propagation.