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Shape optimization approaches to inverse design offer low-dimensional, physically-guided parameterizations of structures by representing them as combinations of shape primitives. However, on discretized rectilinear simulation grids, computing the gradient of a user objective via the adjoint variables method requires a sum reduction of the forward/adjoint field solutions and the Jacobian of the simulation material distribution with respect to the structural shape parameters. These shape parameters often perturb large or global parts of the simulation grid resulting in many non-zero Jacobian entries, which are typically computed by finite-difference in practice. Consequently, the gradient calculation can be non-trivial. In this work we propose to accelerate the gradient calculation by invoking automatic differentiation (AutoDiff) in instantiations of structural material distributions. In doing so, we develop extensible differentiable mappings from shape parameters to shape primitives and differentiable effective logic operations (denoted AutoDiffGeo). These AutoDiffGeo definitions may introduce some additional discretization error into the field solutions because they relax notions of sub-pixel smoothing along shape boundaries. However, we show that some mappings (e.g. simple cuboids) can achieve zero error with respect to volumetric averaging strategies. We demonstrate AutoDiff enhanced shape optimization using three integrated photonic examples: a multi-etch blazed grating coupler, a non-adiabatic waveguide transition taper, and a polarization-splitting grating coupler. We find accelerations of the gradient calculation by AutoDiff relative to finite-difference often exceed 50x, resulting in total wall time accelerations of 4x or more on the same hardware with little or no compromise to final device performance. Our code is available open source at https://github.com/smhooten/emopt, Comment: 29 pages, 15 figures

Thermal tuning of the optical refractive index in the waveguides to control light phase accumulation is essential in photonic integrated systems and applications. In silicon photonics, microheaters are mainly realized by metal wires or highly doped silicon lines placed at a safe distance (1um) from the waveguide to avoid considerable optical loss. However, this poses a significant limitation for heating efficiency because of the excessive free-carrier loss when a heater is brought closer to the optical path. In this work, we present a new concept of using optically transparent 2D semiconductors (e.g. MoS2) for realizing highly efficient waveguide integrated heaters operating at telecom wavelengths. We demonstrate that a single-layer MoS2 heater with negligible optical absorption in the infrared can be placed in close proximity (only 30nm) to the waveguide and show the best-reported heating efficiency of 15 mW per FSR without sacrificing the optical insertion loss. The heater response time is 25us, limited by Au 1L-MoS2 Schottky contact. Both the efficiency and response time can be further significantly improved by realizing 2D MoS2 heaters with ohmic contacts. Our work shows clear advantages of employing 2D semiconductors for heaters applications and paves the way for developing novel energy-efficient, lossless 2D heaters for on-chip photonic integrated circuits.

The dark exciton state in semiconductor quantum dots constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses with enhanced photon extraction, to optically prepare and readout the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding state, which are identified deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of $(0.82\pm0.01)\,$ns corresponding to a fine-structure splitting of $(5.0\pm0.7)\,\mu$eV between its eigenstates $\left|\uparrow\Uparrow\pm\downarrow\Downarrow\right\rangle$. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory., Comment: 5 pages, 3 figures

We use semiconductor quantum dots, "artificial atoms," to implement a scheme for deterministic generation of long strings of entangled photons in a cluster state, an important resource for quantum information processing. We demonstrate a prototype device which produces strings of a few hundred photons in which the entanglement persists over 5 sequential photons. The implementation follows a proposal by Lindner and Rudolph (Phys. Rev. Lett. 2009) which suggested periodic timed excitation of a precessing electron spin as a mechanism for entangling the electron spin with the polarization of the sequentially emitted photons. In our realization, the entangling qubit is a quantum dot confined dark exciton. By performing full quantum process tomography, we obtain the process map which fully characterizes the evolution of the system, containing the dark exciton and n photons after n applications of the periodic excitations. Our implementation may greatly reduce the resources needed for quantum information processing., Comment: 11 pages, 4 figures; supporting material is attached at the end of the main paper

Interatomic coulombic decay (ICD) is a relaxation process induced by electronic correlation. In this work we study the ICD process in a two coupled Quantum wells (QWs) nano-structure. We study a simple one-dimensional effective potential using experimental parameters of the semiconductor QW layers i.e. using the single band effective-mass approximation . In our calculations we consider the discontinuity of the effective mass of the electron in each of the QW layers. We control the ICD lifetime by changing the distance between the two wells. The expected overall trend is a decrease of ICD lifetime with a decrease in the distance between the wells. We show that the distance can be tuned such that the emitted ICD electron is trapped in a meta-stable state in the continuum i.e. a one electron resonance state. This causes the life time of the ICD to be an order of magnitude smaller even in very long distances, and improves the efficiency of the ICD. For the ICD to be dominant decay mechanism it must prevail over all other possible competitive decay processes. We have found that the lifetime of the ICD is on the timescale of picoseconds. Therefore, based on our results we can design an experiment that will observe the ICD phenomenon in QWs nano-structure for the first time. This work can lead to designing a wavelength sensitive detector which is efficient even in low intensities.