arXiv subjects

Neelesh Kumar Vij

Publications and source records attributed to Neelesh Kumar Vij.

5 recordsLinked to original sources

arXiv2026

Optimization of circular cavities via guided-mode expansion method based inverse design

Spin-photon interfaces, realized by coupling optically active spin systems to photonic cavities, are essential for quantum networking and quantum information processing. Implementing such an interface for polarization-encoded photons requires a cavity that supports arbitrary polarization, provides efficient optical access through its far-field mode, and maintains sufficiently high quality factors to enable high cooperativity with the system's optical transitions. However, inherent trade-offs between the Q-factor and far-field emission mode make the simultaneous optimization of these parameters toward the realization of spin-photon interfaces challenging. In this work, we implement a gradient-based inverse-design framework using guided-mode expansion with automatic differentiation to obtain the geometrical features of a circular ring cavity that supports arbitrary polarization while simultaneously optimizing the cavity quality factor and far-field mode profile. The resulting optimized non-periodic cavity achieves a quality factor of approximately $9,000$, about an order-of-magnitude higher than that of a periodic ("bullseye") cavity while preserving a Gaussian-like far-field emission pattern. Furthermore, by varying the cavity geometry within a $\pm 6$ nm fabrication tolerance, we demonstrate the robustness of the design against fabrication errors and identify the innermost ring width and central disk radius as the parameters with the greatest impact on the quality factor and far-field mode. These results establish guided mode expansion-based inverse design as a powerful and computationally efficient approach for developing high-cooperativity spin-photon interfaces for quantum photonic applications.

quant-ph↗

arXiv2025

Inverse-Designed Photonic Crystal Cavities with Controllable Far-Field Numerical Aperture

Photonic crystal cavities confine light to subwavelength volumes, enabling strong light-matter interactions for applications in low-power photonics, opto-electronics, nonlinear optics, and quantum information. These applications demand cavities that combine high quality factors, low mode volumes, and high coupling efficiencies. However, optimizing across these metrics requires exploring a large design space, motivating the use of inverse design strategies. Previous inverse design efforts targeted high quality factors and low mode volumes, sacrificing the coupling efficiency or lacking the ability to precisely control the far-field radiation pattern. In this work, we present an inverse design framework that simultaneously optimizes cavity quality factor and far-field numerical aperture, both specified as design targets. Using this method, we design L3 photonic crystal cavities, with different far-field numerical apertures, in the visible wavelength and fabricate them in silicon nitride. Photoluminescence measurements confirm experimental control of the far-field numerical aperture and reveal a 28-fold and 3.9-fold improvement in the coupling efficiency and quality factor respectively when compared to the standard L3 cavity. Disorder analysis further shows that the designs retain significant performance despite nanofabrication imperfections. Our work demonstrates a versatile inverse design framework for multi-objective optimization of photonic crystal cavities to attain high quality factors and coupling efficiency.

physics.optics↗

arXiv2024

Coherent spectroscopy of a single Mn-doped InGaAs quantum dot

Doping a self-assembled InGaAs/GaAs quantum dot (QD) with a single Mn atom, a magnetic acceptor impurity, provides a quantum system with discrete energy levels and original spin-dependent optical selection rules, which thus has large potential in quantum photonics, notably as a source of multi-entangled photons. To investigate this potential further, we perform coherent optical spectroscopy under continuous wave excitation of the 3-level V-like system formed in such a Mn-doped QD when charged by a single hole. In spite of a large inhomogeneous broadening of the optical transitions, we demonstrate Autler-Townes splitting both by resonant Raman scattering and by probe absorption spectroscopy for different saturation powers. Analysing these data with a comprehensive model based on optical Bloch equations, we show evidence for quantum interference within the V-like system and assess the pure dephasing rate between the corresponding spin states.

cond-mat.mes-hall↗

arXiv2023

Cavity enhanced emission from a silicon T center

Silicon T centers present the promising possibility to generate optically active spin qubits in an all-silicon device. However, these color centers exhibit long excited state lifetimes and a low Debye-Waller factor, making them dim emitters with low efficiency into the zero-phonon line. Nanophotonic cavities can solve this problem by enhancing radiative emission into the zero-phonon line through the Purcell effect. In this work we demonstrate cavity-enhanced emission from a single T center in a nanophotonic cavity. We achieve a two-orders of magnitude increase in brightness of the zero-phonon line relative to waveguide-coupled emitters, a 23% collection efficiency from emitter to fiber, and an overall emission efficiency into the zero-phonon line of 63.4%. We also observe a lifetime enhancement of 5, corresponding to a Purcell factor exceeding 18 when correcting for the emission to the phonon sideband. These results pave the way towards efficient spin-photon interfaces in silicon photonics.

physics.optics↗

arXiv2022

Multi-parameter Optimization for Ground-state Cooling of Mechanical Mode using Quantum Dots

Cooling a mechanical mode to its motional ground state opens up avenues for both scientific and technological advancements in the field of quantum meteorology and information processing. We propose a multi-parameter optimization scheme for ground-state cooling of a mechanical mode using quantum dots. Applying the master equation approach, we formulate the optimization scheme over a broad range of system parameters including detunings, decay rates, pumping rates, and coupling strengths. We implement the optimization scheme on two major types of semiconductor quantum dot systems: colloidal and epitaxial quantum dots. These systems span a broad range of mechanical mode frequencies, coupling rates, and decay rates. Our optimization scheme lowers the steady-state phonon number in all cases by several orders of magnitude. We also calculate the net cooling rate by estimating the phonon decay rate and show that the optimized system parameters also result in efficient cooling. The proposed optimization scheme can be readily extended to other driven systems coupled to a mechanical mode.

quant-ph↗