No abstract

Optical frequency combs are key to optical precision measurements. While most frequency combs operate in the near-infrared regime, many applications require combs at mid-infrared, visible or even ultra-violet wavelengths. Frequency combs can be transferred to other wavelengths via nonlinear optical processes, however, this becomes exceedingly challenging for high-repetition rate frequency combs. Here, it is demonstrated that a synchronously driven high-Q microresonator with a second-order optical nonlinearity can efficiently convert high-repetition rate near-infrared frequency combs to visible, ultraviolet and mid-infrared wavelengths providing new opportunities for microresonator and electro-optic combs in applications including molecular sensing, astronomy, and quantum optics.

We investigate the threshold of χ(2) frequency comb generation in lithium niobate whispering gallery microresonators theoretically and experimentally. When generating a frequency comb via second-harmonic excitation, also commonly known as second-harmonic generation, the threshold for the onset of cascaded second-order processes leading to a comb is found to be ∼85 µW. The second-harmonic generation efficiency up to this value is in excellent agreement with a previously known theoretical framework. This framework is extended here, showing that the onset of cascaded χ(2) processes and the maximum of the second-harmonic generation efficiency coincide. The model introduced here allows us to determine the frequency comb generation threshold analytically. Furthermore, we observe that the frequency distance between the comb lines is a function of the pump power. It changes from four free spectral ranges at the oscillation threshold to one free spectral range at 590 µW.

Adiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of 100 % down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. Frequency shifts of more than 5 GHz are achieved by applying just 20 V to 70-µm-thick crystal. Furthermore, we demonstrate that already with the same setup positive and a negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.Optical frequency conversion based on nonlinear optics in microresonators has been advanced over the last decades. Nonlinear frequency conversion is based on the nonlinear response of the material to light . 1 For example, frequency combs in microresonators made out of centrosymmetric materials 2 and tunable optical OPOs in non-centrosymmetric microresonators have been demonstrated. 3, 4 High conversion efficiencies require high intensities, as well as phase matching conditions need to be fulfilled . 5 Moreover, a pump threshold must be overcome for the most versatile conversion mechanism, optical parametric oscillation. An alternative optical conversion technique is the so-called adiabatic frequency conversion (AFC). Here, the frequency of light traveling in a resonator is shifted due to a change of the optical round-path length. One implementation is to change the refractive index of the material and to keep the geometrical path length constant. The frequency of light changes then accordingly to 6 ∆ν ν ≈ − ∆n n .(1)The change of the refractive index must happen in a time ∆t shorter than the propagation time t of the light in the material, i.e. before it gets lost by absorption or scattering. Since microresonators act as light traps, that can store light for many nanoseconds or even milliseconds, depending on their quality factor. Thus they are supposed to be well suited to realize AFC . 7 For AFC, no threshold has to be overcome. This frequency conversion scheme has been realized experimentally in several implementations, even down to a single photon level. 8,9 For example AFC was shown in photonic crystals , 10-12 in waveguides 9, 13-15 and in fiber grating cavities . 16 So far, two different attempts involve microresonators: Conduction-band electrons generated by laser pulses, inducing a change of the refractive index allow to shift a few hundred GHz to shorter wavelengths. 17 However, the ...

Light states relevant to the χ ( 2 ) comb generation in optical microresonators possess typically dual backgrounds for coupled first-harmonic (FH) and second-harmonic (SH) envelopes. Stability and/or instability of these backgrounds is crucial for realization of stable χ ( 2 ) combs and also for efficient SH generation. We explore the properties of the dual backgrounds and their instability for the cases of FH and SH pumping of the resonator. In contrast to optical parameteric oscillation, the instability is controlled by a fourth-degree characteristic equation for the increment. Coefficients of this equation depend not only on wavenumbers of the perturbations, pump power, and dispersion parameters, but also on FH-SH group velocity difference (temporal walk-off). Our results include characterization of the regions and conditions of stability for the FH and SH pumping cases and different spectral ranges.