Momchil Minkov

Automated optimization of photonic crystal slab cavities

Thanks to their high quality factor, combined to the smallest modal volume, defect-cavities in photonic crystal slabs represent a promising, versatile tool for fundamental studies and applications in photonics. In paricular, the L3, H0, and H1 defects are the most popular and widespread cavity designs, due to their compactness, simplicity, and small mode volume. For these cavities, the current best optimal designs still result in Q-values of a few times 105 only, namely one order of magnitude below the bound set by fabrication imperfections and material absorption in silicon. Here, we use a genetic algorithm to find a global maximum of the quality factor of these designs, by varying the positions of few neighbouring holes. We consistently find Q-values above one million – one order of magnitude higher than previous designs. Furthermore, we study the effect of disorder on the optimal designs and conclude that a similar improvement is also expected experimentally in state-of-the-art systems.

Radiative coupling of quantum dots in photonic crystal structures

Reference EPFL-ARTICLE-188368doi:10.1103/PhysRevB.87.125306 URL: http://link.aps.org/doi/10.1103/PhysRevB.87.125306 Record created on 2013-09-04, modified on 2017-05-12

A compact, integrated silicon device for the generation of spectrally filtered, pair-correlated photons

The third-order non-linearity of silicon gives rise to a spontaneous four-wave mixing process in which correlated photon pairs are generated. Sources based on this effect can be used for quantum computation and cryptography, and can in principle be integrated with standard CMOS fabrication technology and components. However, one of the major challenges is the on-chip demultiplexing of the photons, and in particular the filtering of the pump power, which is many orders of magnitude larger than that of the signal and idler photons. Here, we propose a photonic crystal coupled-cavity system designed so that the coupling of the pump mode to the output channel is strictly zero due to symmetry. We further analyze this effect in presence of fabrication disorder and find that, even then, a pump suppression of close to 40 dB can be achieved in state-of-the art systems. Due to the small mode volumes and high quality factors, our system is also expected to have a generation efficiency much higher than in standard micro-ring systems. Those two considerations make a strong case for the integration of our proposed design in future on-chip quantum technologies.

Photonic crystal slab Laplace operator for image differentiation

Spatial differentiation is important in image-processing applications such as image sharpening and edge-based segmentation. In these applications, of particular importance is the Laplacian, the simplest isotropic derivative operator in two dimensions. Spatial differentiation can be implemented electronically. However, in applications requiring real-time and high-throughput image differentiation, conventional digital computations become challenging. Optical analog computing may overcome this challenge by offering high-throughput low-energy-consumption operations using compact devices. However, previous works on spatial differentiation with nanophotonic structures are restricted to either one-dimensional differentiation or reflection mode, whereas operating in the transmission mode is important because it is directly compatible with standard image processing/recognition systems. Here, we show that the Laplacian can be implemented in the transmission mode by a photonic crystal slab device. We theoretically derive the criteria for realizing the Laplacian using the guided resonances in a photonic crystal slab. Guided by these criteria, we show that the Laplacian can be implemented using a carefully designed photonic crystal slab with a non-trivial isotropic band structure near the Γ point. Our work points to new opportunities in optical analog computing as provided by nanophotonic structures.We propose an implementation of a Laplace differentiator based on a photonic crystal slab that operates at transmission mode. Such a device may facilitate nanophotonics-based optical analog computing for image processing.

Efficient continuous-wave nonlinear frequency conversion in high-Q gallium nitride photonic crystal cavities on silicon

We report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Optimized two-dimensional PhC cavities with augmented far-field coupling have been characterized with quality factors as high as 4.4 × 104, approaching the computed theoretical values. The strong enhancement in light confinement has enabled efficient SHG, achieving a normalized conversion efficiency of 2.4 × 10−3 W−1, as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation. The results herein validate the suitability of gallium nitride for integrated nonlinear optical processing.

Exact solution to the steady-state dynamics of a periodically modulated resonator

We provide an analytic solution to the coupled-mode equations describing the steady-state of a single periodically modulated optical resonator driven by a monochromatic input. The phenomenology of this system was qualitatively understood only in the adiabatic limit, i.e., for low modulation speed. However, both in and out of this regime, we find highly non-trivial effects for specific parameters of the modulation. For example, we show complete suppression of the transmission even with zero detuning between the input and the static resonator frequency. We also demonstrate the possibility for complete, lossless frequency conversion of the input into the sideband frequencies, as well as for optimizing the transmitted signal towards a given target temporal waveform. The analytic results are validated by first-principle simulations.

Photonic crystal slab cavity simultaneously optimized for ultra-high Q/V and vertical radiation coupling

We present a design for a two-dimensional photonic crystal slab cavity in which the electric field localization is due to an extra hole in the lattice, as opposed to the more standard procedure of removal of holes. This leads to a tighter field confinement and a mode volume that is several times smaller than that of conventionally used designs. Through small modifications of the holes around the cavity, we optimize the theoretical quality factor (Q) to an ultra-high value of 20.9 × 106 and furthermore illustrate the possibility for high coupling efficiency to free-space modes in the vertical direction, while keeping a high Q of 3.7 × 106.

Training of photonic neural networks through in situ backpropagation and gradient measurement

We provide a protocol for training photonic neural networks based on adjoint methods. The gradient of the network with respect to its tunable degrees of freedom is computed by physically backpropagating an optical error signal.

Low index contrast heterostructure photonic crystal cavities with high quality factors and vertical radiation coupling

We report here design and experimental demonstration of heterostructure photonic crystal cavities resonating near the Γ point with simultaneous strong lateral confinement and highly directional vertical radiation patterns. The lateral confinement is provided by a mode gap originating from a gradual modulation of the hole radii. High quality factor resonance is realized with a low index contrast between silicon nitride and quartz. The near surface-normal directional emission is preserved when the size of the core region is scaled down. The influence of the cavity size parameters on the resonant modes is also investigated theoretically and experimentally.