Abhinav Kumar Vinod

Stabilized chip-scale Kerr frequency comb via a high-Q reference photonic microresonator.

We stabilize a chip-scale Si3N4 phase-locked Kerr frequency comb via locking the pump laser to an independent stable high-Q reference microresonator and locking the comb spacing to an external microwave oscillator. In this comb, the pump laser shift induces negligible impact on the comb spacing change. This scheme is a step toward miniaturization of the stabilized Kerr comb system as the microresonator reference can potentially be integrated on-chip. Fractional instability of the optical harmonics of the stabilized comb is limited by the microwave oscillator used for a comb spacing lock below 1 s averaging time and coincides with the pump laser drift in the long term.

Quasi-phase-matched multispectral Kerr frequency comb

We demonstrate a new type of Kerr frequency comb with expanded parametric range, where the momentum conservation is ensured by azimuthal modulation of the cavity dispersion. A coherent multispectral frequency comb covering telecommunication O, C, L, and 2 μm bands is observed and characterized.

Enhanced interlayer neutral excitons and trions in trilayer van der Waals heterostructures

Vertically stacked van der Waals heterostructures constitute a promising platform for providing tailored band alignment with enhanced excitonic systems. Here, we report observations of neutral and charged interlayer excitons in trilayer WSe2–MoSe2–WSe2 van der Waals heterostructures and their dynamics. The addition of a WSe2 layer in the trilayer leads to significantly higher photoluminescence quantum yields and tunable spectral resonance compared to its bilayer heterostructures at cryogenic temperatures. The observed enhancement in the photoluminescence quantum yield is due to significantly larger electron–hole overlap and higher light absorbance in the trilayer heterostructure, supported via first-principles pseudopotential calculations based on spin-polarized density functional theory. We further uncover the temperature- and power-dependence, as well as time-resolved photoluminescence of the trilayer heterostructure interlayer neutral excitons and trions. Our study elucidates the prospects of manipulating light emission from interlayer excitons and designing atomic heterostructures from first-principles for optoelectronics.Optical physics: neutral and charged interlayer excitons in WSe 2 -MoSe 2 -WSe 2 heterostructuresTrilayer heterostructures have enhanced Coulomb interactions due to interlayer radiative recombination of neutral and charged excitons. A team led by Chee Wei Wong at the University of California, Los Angeles, fabricated type-II vertical van der Waals heterostructures consisting of a trilayer WSe2-MoSe2-WSe2 stack. Optimisation of the electronic band alignment by means of spin-polarized density functional theory allowed efficient interlayer radiative recombination. As a result, an order-of-magnitude increase of the photoluminescence quantum yield was obtained compared to bilayer heterostructures, and this was attributed to the larger electron-hole overlap and higher light absorbance in the trilayer stack, along with the formation of the neutral interlayer exciton. These results shed light to the underlying physics of light emission from interlayer excitons, and may pave the way to optimal design of van der Waals heterostructures with enhanced excitonic properties.

Dynamical modalities in Kerr frequency combs (Conference Presentation)

Recent advances in sub-wavelength nanoscale platforms have afforded the control of light from first principles, with impact to ultrafast sciences, optoelectronics and precision measurements. In this talk we will describe recent advances in chip-scale Kerr frequency comb oscillators, where we have achieved sub-100-fs mode-locking, stabilization down to a tooth-to-tooth relative frequency uncertainty of 50 mHz and 2.7×10^{−16}, and single-mode broadband frequency comb generation. Each of these are supported by linear and nonlinear numerical modeling. In this first chip-scale realization, coherent mode-locking is observed in the normal dispersion regime, verified by phase-resolved ultrafast spectroscopy at sub-100-attojoule sensitivities. The normal dispersion architecture uncovers the mode-locking mechanisms in Kerr frequency combs, matched with first-principles coupled-mode theory. In the second realization, we examine the noise limits in the full stabilization of chip-scale optical frequency combs. The microcomb’s two degrees of freedom, one of the comb lines and the native 18-GHz comb spacing, are simultaneously phase-locked to known optical and microwave references. Active comb spacing stabilization improves long-term stability by six orders of magnitude, reaching a record instrument-limited residual instability of 3.6 mHz per root tau. Comparing 46 nitride frequency comb lines with a benchmark fiber laser frequency comb, we demonstrate the unprecedented microcomb tooth-to-tooth relative frequency uncertainty down to 50 mHz and 2.7×10^{−16}. In the third realization, we report a novel design of Si3N4 microresonator in which single-mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The novel microresonator consists of uniform single-mode waveguides in the semi-circle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. With this microresonator, we demonstrate broadband phase-locked frequency combs. This supports the focus towards chip-scale precision spectroscopy, timing, coherent communications, and astronomical spectrography.

Internally phase stabilized Kerr comb

We demonstrate for the first time a phase stabilized Kerr comb without the use of external references or non-linear interferometry. Out-of-loop measurements confirm good coherence and stability across the comb, with measured optical frequency fractional instabilities of 5×10⁻¹¹/√τ