Correlated photon pairs are a fundamental building block of quantum photonic systems. While pair sources have previously been integrated on silicon chips built using customized photonics manufacturing processes, these often take advantage of only a small fraction of the established techniques for microelectronics fabrication and have yet to be integrated in a process which also supports electronics. Here we report the first demonstration of quantumcorrelated photon pair generation in a device fabricated in an unmodified advanced (sub-100 nm) complementary metal-oxide-semiconductor (CMOS) process, alongside millions of working transistors. The microring resonator photon pair source is formed in the transistor layer structure, with the resonator core formed by the silicon layer typically used for the transistor body. With ultra-low continuous-wave on-chip pump powers ranging from 5 µW to 400 µW, we demonstrate pair generation rates between 165 Hz and 332 kHz using >80% efficient WSi superconducting nanowire single photon detectors. Coincidences-to-accidentals ratios consistently exceeding 40 were measured with a maximum of 55. In the process of characterizing this source we also accurately predict pair generation rates from the results of classical stimulated four-wave mixing measurements. This proof-of-principle device demonstrates the potential of commercial CMOS microelectronics as an advanced quantum photonics platform with capability of large volume, pristine process control, and where state-of-the-art high-speed digital circuits could interact with quantum photonic circuits.This manuscript describes work of the U.S. government and is not subject to copyright.

We propose and demonstrate localized mode coupling as a viable dispersion engineering technique for phasematched 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 (∼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. On-chip four-wave mixing-based devices have been the subject of much research lately for applications such as wavelength converters [1], octave spanning and phaselocked frequency combs [2, 3], and quantum-entangled biphoton sources [4]. Four-wave mixing (FWM) is a third-order nonlinear process arising from the χ (3) susceptibility where two pump photons are parametrically converted to a signal-idler pair while conserving energy and momentum. Resonant enhancement of both the pump and signal/idler modes greatly improves FWM efficiency [5]. Silicon has attracted interest as a nonlinear material due to a Kerr coefficient that is over 100 times that of silica [6] and the well developed fabrication of high quality factor (Q) optical resonators with small mode volumes.Aside from high Q and small modal volume, phase matching is critical to efficient resonant FWM. In the case of a degenerate pump, three interacting wavelengths are present (pump, signal and idler). Since cavity modes are intrinsically momentum matched [1], efficient FWM is achieved when the signal, idler, and pump are onresonance with the three modes. The problem of efficient FWM is then reduced to designing for photon energy matching, i.e. for resonant modes equally spaced in frequency. Adjacent longitudinal resonances of a single cavity, spaced by one free spectral range (FSR), are most commonly utilized for FWM due to device simplicity. Here, material and waveguide dispersion can lead to unequal FSRs and a serious reduction in FWM efficiency. Careful engineering of dispersion through resonator dimensions and pump wavelength is possible [1, 7] bu...

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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 st -and 3 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. Silicon photonics is beginning to enable complex on-chip optical networks comprising hundreds of devices. One emerging application is energy efficient, chip-scale photonic interconnects for CPU-to-memory communication [1]. With increasing device density and complexity in a planar photonic circuit, efficient waveguide crossings are indispensible in many network topologies [1]. Crossing designs based on adiabatic aperture widening are large and relatively lossy (0.3-1 dB) [2-4], while resonant designs permit low loss and crosstalk in a compact footprint, but have narrow bandwidth [5] (e.g. ∼ 4 nm [6]). Multilayer processes allow reduced scattering in crossing waveguides [7] or their complete isolation through vertical displacement [8], but they require multiple lithographic steps and/or material layers. Multimode-interference (MMI) based crossings [9][10][11][12][13][14], despite ostensibly multimode behavior, have a number of attractive features, with individual crossings down to 0.18 dB loss and 41 dB crosstalk [14].In this Letter, we describe ultra-low-loss waveguide crossing arrays based on a periodic multimode structure. Popović et al. [12] proposed an efficient approach to design a crossing array (Fig. 1) by constructing a low-loss Bloch wave in a matched periodic structure where the optical field synthesizes periodic focii that jump across gaps and avoid diffraction loss and scattering at the crossing points. This concept is reminiscent of periodic lens-array microwave beam guiding [15]. Microphotonic implementations use a minimum of modes to implement focusing physics, eliminate reflections, and introduce new degrees of freedom. In the first experimental demonstration of this concept [16], we showed record low waveguide-crossing loss of 0.04 dB/crossing (0.9%), equal to theoretical design efficiency [12]. Another recent paper [17] demonstrated similar crossing arrays based on our proposal in Ref. 12, achieving 0.14 dB loss, and introduced an improvement based on subwavelength patterning of the sidewalls, reducing the loss further to below 0.02 dB. To our knowledge, these two results represent respectively the lowest achieved crossing loss in CMOS-compatible photolith...

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We propose optimal designs for triply-resonant optical parametric oscillators (OPOs) based on degenerate four-wave mixing (FWM) in microcavities. We show that optimal designs in general call for different external coupling to pump and signal/idler resonances. We provide a number of normalized performance metrics including threshold pump power and maximum achievable conversion efficiency for OPOs with and without two-photon (TPA) and free-carrier absorption (FCA). We find that the maximum achievable conversion efficiency is bound to an upper limit by nonlinear and free-carrier losses independent of pump power, while linear losses only increase the pump power required to achieve a certain conversion efficiency. The results of this work suggest unique advantages in on-chip implementations that allow explicit engineering of resonances, mode field overlaps, dispersion, and wavelength-and mode-selective coupling. We provide universal design curves that yield optimum designs, and give example designs of microring-resonator-based OPOs in silicon at the wavelengths 1.55 μm (with TPA) and 2.3 μm (no TPA) as well as in silicon nitride (Si(3)N(4)) at 1.55 μm. For typical microcavity quality factor of 10(6), we show that the oscillation threshold in excitation bus can be well into the sub-mW regime for silicon microrings and a few mW for silicon nitride microrings. The conversion efficiency can be a few percent when pumped at 10 times of the threshold. Next, based on our results, we suggest a family of synthetic "photonic molecule"-like, coupled-cavity systems to implement optimum FWM, where structure design for control of resonant wavelengths can be separated from that of optimizing nonlinear conversion efficiency, and where furthermore pump, signal, and idler coupling to bus waveguides can be controlled independently, using interferometric cavity supermode coupling as an example. Finally, consideration of these complex geometries calls for a generalization of the nonlinear figure of merit (NFOM) as a metric for performance in nonlinear photonic systems, and shows different efficiencies for single and multi-cavity geometries, as well as for standing and traveling wave excitations.

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