Nonlinear wavelength conversion is essential for many classical and quantum pho-tonic applications. The underlying second-order nonlinear optical processes, however, generally exhibit limited spectral bandwidths that impact their application potential. Here we use a high-Q X-cut lithium niobate microdisk resonator to demonstrate both second-harmonic generation and spontaneous parametric down-conversion on chip. In particular, our lithium niobate microresonator, with its wide-range cyclic phase matching and rich optical mode structures, is able to achieve ultra-broadband spontaneous parametric down-conversion, with a bandwidth over 400 nm, inferred from recorded spectra of the down-converted photons. The produced biphoton pairs exhibit strong temporal correlation, with a coincidence-to-accidental ratio measured to be 43.1. Our device is promising for integrated quantum photonics where optical frequency could be used as a degree of freedom for signal processing.

We demonstrate a high-Q amorphous silicon carbide (a-SiC) microresonator with optical Q as high as 1.3 × 10(5). The high optical quality allows us to characterize the third-order nonlinear susceptibility of a-SiC. The Kerr nonlinearity is measured to be n2 = (5.9 ± 0.7) × 10(−15) cm(2)/W in the telecom band around 1550 nm. The strong Kerr nonlinearity and high optical quality render a-SiC microresonators a promising platform for integrated nonlinear photonics.

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We propose to selectively engineer a single cavity resonance to achieve frequency matching for optical parametric processes in high-Q microresonators. For this purpose, we demonstrate an approach, selective mode splitting (SMS), to precisely shift a targeted cavity resonance, while leaving other cavity modes intact. We apply SMS to achieve efficient parametric generation via four-wave mixing in high-Q silicon microresonators. The proposed approach is of great potential for broad applications in integrated nonlinear photonics.Optical parametric processes, including four-wave mixing (FWM), second-/third-harmonic generation (SHG/THG), parametric down-conversion (PDC), etc., have broad applications ranging from photonic signal processing 1,2 , tunable coherent radiation 3 , frequency metrology 4 , to quantum information processing 5 . In recent years, there has been increasing interest in optical parametric processes in high-Q micro-/nano-cavities, which exhibit great potential for dramatically enhancing parametric generation [6][7][8][9][10][11][12][13][14][15][16][17][18][19] . However, their efficiencies rely crucially on frequency matching among the interacting cavity modes, which is generally deteriorated by device dispersion. Frequency matching is particularly challenging in high-Q cavities, due to the narrow linewidths of cavity resonances.To date, a variety of methods have been proposed for frequency matching. For FWM, a χ (3) nonlinear process, current methods primarily focus on engineering groupvelocity dispersion of the devices 7,8,10,13,14,20 . For SHG, a χ (2) nonlinear process, intermodal dispersion or birefringence is generally employed to mitigate the frequency mismatch 11,12,[15][16][17][18][19] . These methods rely on manipulating material/waveguide dispersion of devices, which collectively shifts the resonance frequencies of all cavity modes by different extents.In this paper, we propose and demonstrate an effective approach for frequency matching that can be applied universally to optical parametric processes. Instead of modifying all cavity resonances via dispersion engineering, we directly shift a single cavity resonance to a desired frequency, while leaving other cavity modes intact. This is realized by splitting the targeted cavity mode at ω m into two modes at new frequencies ω m ± β m . By controlling the magnitude of splitting, one resonance can be shifted to coincide with the desired frequency ( Fig. 1(a)). We term this approach selective mode splitting (SMS).SMS can be applied to various χ (2) /χ (3) processes ( Fig. 1(b-d)). For example, degenerate FWM requires three interacting cavity modes to be equally spaced in a) Electronic mail: qiang.lin@rochester.edu. frequency, ω signal −ω pump = ω pump −ω idler , which is often not satisfied due to device dispersion. The problem can be solved by shifting one cavity mode to the desired frequency, thus enabling parametric generation ( Fig. 1(b)). SMS is particularly useful for nonlinear processes including SHG/THG and PDC ( Fig. 1(c)(d)), where ...