Tunable coupled-mode dispersion compensation and its application to on-chip resonant four-wave mixing

We propose and demonstrate localized mode coupling as a viable dispersion engineering technique for phase-matched resonant four-wave mixing (FWM). We demonstrate a dual-cavity resonant structure that employs coupling-induced frequency splitting at one of three resonances to compensate for cavity dispersion, enabling phase-matching. Coupling strength is controlled by thermal tuning of one cavity enabling active control of the resonant frequency-matching. In a fabricated silicon microresonator, we show an 8 dB enhancement of seeded FWM efficiency over the non-compensated state. The measured four-wave mixing has a peak wavelength conversion efficiency of -37.9 dB across a free spectral range (FSR) of 3.334 THz ($\sim$27 nm). Enabled by strong counteraction of dispersion, this FSR is, to our knowledge, the largest in silicon to demonstrate FWM to date. This form of mode-coupling-based, active dispersion compensation can be beneficial for many FWM-based devices including wavelength converters, parametric amplifiers, and widely detuned correlated photon-pair sources. Apart from compensating intrinsic dispersion, the proposed mechanism can alternatively be utilized in an otherwise dispersionless resonator to counteract the detuning effect of self- and cross-phase modulation on the pump resonance during FWM, thereby addressing a fundamental issue in the performance of light sources such as broadband optical frequency combs

Ultra-low-loss CMOS-Compatible Waveguide Crossing Arrays Based on Multimode Bloch Waves and Imaginary Coupling

We experimentally demonstrate broadband waveguide crossing arrays showing ultra low loss down to $0.04\,$dB/crossing ($0.9\%$), matching theory, and crosstalk suppression over $35\,$dB, in a CMOS-compatible geometry. The principle of operation is the tailored excitation of a low-loss spatial Bloch wave formed by matching the periodicity of the crossing array to the difference in propagation constants of the 1$^\text{st}$- and 3$^\text{rd}$-order TE-like modes of a multimode silicon waveguide. Radiative scattering at the crossing points acts like a periodic imaginary-permittivity perturbation that couples two supermodes, which results in imaginary (radiative) propagation-constant splitting and gives rise to a low-loss, unidirectional breathing Bloch wave. This type of crossing array provides a robust implementation of a key component enabling dense photonic integration

Wide-band On-chip Four-Wave Mixing via Coupled Cavity Dispersion Compensation

Abstract: We demonstrate a dual-cavity resonant structure that employs frequency splitting at one of three resonances to structurally compensate dispersion. We show seeded four-wave mixing across the largest free spectral range to our knowledge of 26nm. On-chip four-wave mixing (FWM) has received much attention recently for applications from wavelength conversion [1] to quantum photonic circuits In this paper, we propose and demonstrate FWM in a dispersion compensating device consisting of two coupled resonators referred to as the 'primary' and 'auxiliary' cavities with different FSRs as illustrated i

Ultra-low-loss Waveguide Crossing Arrays Based on Imaginary Coupling of Multimode Bloch Waves

Abstract: We experimentally demonstrate ultra-low-loss waveguide crossing arrays showing loss down to 0.04 dB/crossing. They rely on a low loss, focusing Bloch wave that is stabilized by radiative scattering, via a radiative form of coupling. Introduction Silicon photonic circuits promise to enable energy efficient chip-scale photonic interconnects for CPU-to-memory communication. As device density and complexity increase in a planar photonic circuit, efficient waveguide crossings become essential. For some network topologies, the number of waveguide crossings required rises quickly and tolerable levels of loss and crosstalk per crossing accordingly drop to very small limits. A multitude of work has considered crossing designs [1-9], including resonant [1], MMI-like In previous work, Popovic et al. proposed an efficient approach to design a crossing array by constructing a low loss Bloch wave in a matched periodic structure, that avoids contacts, and using non-adiabatic, tapered excitation structures to efficiently excite these low-loss Bloch waves In this paper, we experimentally demonstrate this concept showing near-theoretical performance with loss as low as 0.04 dB/crossing (1%/crossing), and under 0.1 dB measured over a wide wavelength range exceeding 100 nm

Broadband Linear-Dichroic Photodetector in a Black Phosphorus Vertical p-n Junction

The ability to detect light over a broad spectral range is central for practical optoelectronic applications, and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here we demonstrate a linear-dichroic broadband photodetector with layered black phosphorus transistors, using the strong intrinsic linear dichroism arising from the in-plane optical anisotropy with respect to the atom-buckled direction, which is polarization sensitive over a broad bandwidth from 400 nm to 3750 nm. Especially, a perpendicular build-in electric field induced by gating in black phosphorus transistors can spatially separate the photo-generated electrons and holes in the channel, effectively reducing their recombination rate, and thus enhancing the efficiency and performance for linear dichroism photodetection. This provides new functionality using anisotropic layered black phosphorus, thereby enabling novel optical and optoelectronic device applications.Comment: 18 pages, 5 figures in Nature Nanotechnology 201

Integrated nonlinear photonics based on coupled-cavity resonator systems

Efficient nonlinear optical devices are designed and demonstrated in "photonic molecule''-like coupled-cavity resonator systems on a semiconductor chip. A coupled-cavity resonator may be designed to support distributed supermodes, and to allow independent control of the resonant frequency and linewidth of each supermode. Such control allows reduction of dispersion without compromising effective nonlinearity in the resonator, as well as the design of anisotropic output coupling or radiation that allows optimized nonlinear functions. Therefore this resonator manifests itself as a favorable platform for building nonlinear devices including optical parametric wavelength converters and oscillators based on four-wave mixing that call for different couplings to the signal, pump and idler modes. A physical model based on coupled-mode theory describes all relevant linear and nonlinear processes in triply-resonant microcavities, and a generalization of the usual nonlinear figure of merit is proposed to evaluate the effects of distributed supermodes on nonlinear conversion efficiency in such devices. Experimental work is presented that demonstrates coupled cavity devices for wavelength conversion in crystalline silicon, where two-photon absorption sets conversion efficiency limitations. In addition, an investigation of deposition conditions of hydrogenated amorphous silicon is described where amorphous silicon allows for a higher nonlinear figure of merit than crystalline silicon, promising increased performance in such devices. More generally, mode interference and coupling in coupled-cavity resonators, as a unique degree of freedom in integrated optics, is explored through designs of linear devices including efficient optical filters, wavelength converters, and modulators