Searching for Exoplanets Using a Microresonator Astrocomb.

Orbiting planets induce a weak radial velocity (RV) shift in the host star that provides a powerful method of planet detection. Importantly, the RV technique provides information about the exoplanet mass, which is unavailable with the complementary technique of transit photometry. However, RV detection of an Earth-like planet in the 'habitable zone'1 requires extreme spectroscopic precision that is only possible using a laser frequency comb (LFC)2. Conventional LFCs require complex filtering steps to be compatible with astronomical spectrographs, but a new chip-based microresonator device, the Kerr soliton microcomb3-8, is an ideal match for astronomical spectrograph resolution and can eliminate these filtering steps. Here, we demonstrate an atomic/molecular line-referenced soliton microcomb as a first in-the-field demonstration of microcombs for calibration of astronomical spectrographs. These devices can ultimately provide LFC systems that would occupy only a few cubic centimetres9,10, thereby greatly expanding implementation of these technologies into remote and mobile environments beyond the research lab.

On the properties of single-mode optical resonators.

We study the quality factor of single-mode optical whispering gallery mode resonators using finite element method simulations, with a particular focus on the photonic belt resonator geometry. We experimentally observe a large difference between the quality factors of TM and TE modes in such resonators. Examining radiative losses, we conclude that the TM fundamental mode of single-mode resonators can have geometry related radiative losses caused by mode hybridization and coupling that limits their achievable quality factor. However, TE modes are free from mode hybridization radiative losses. This leads to much higher achievable Q factors for TE modes, only limited by fabrication and material quality. We experimentally observed photonic belt resonator quality factors on the order of one billion for TE modes, higher than in any other single mode optical resonator of similar dimensions.

Properties of fluoride microresonators for mid-IR applications.

We study crystalline fluoride microresonators for mid-infrared (mid-IR) applications. Whispering gallery mode resonators were fabricated with BaF2, CaF2, and MgF2 crystals. The quality factors were measured at wavelengths of 1.56 and 4.58 µm. The impacts of post-fabrication processing, impurities, and surface water are highlighted. The mid-IR optical losses due to multiphoton absorption are measured. It is found that MgF2 resonators have a room temperature Q-factor of 8.3×106 at a wavelength of 4.58 µm, limited by multiphoton absorption.

Impact of cavity spectrum on span in microresonator frequency combs.

We experimentally study the factors that limit the span in frequency combs derived from the crystalline whispering gallery mode resonators. We observe that cavity dispersion is the key property that governs the parameters of the combs resulting from cascaded four wave mixing process. Two different regimes of comb generation are observed depending on the precise cavity dispersion behavior at the pump wavelength. In addition, the comb generation efficiency is found to be affected by the crossing of modes of different families. The influence of Raman lasing and its dependence on temperature is discussed.

Polarization conversion loss in birefringent crystalline resonators.

Whispering gallery modes in birefringent crystalline resonators are investigated. We experimentally investigate the XY-cut resonators made with LiNbO3, LiTaO3, and BBO and observe strong influence of the resonator's shape and birefringence on the quality factor of the extraordinary polarized modes. We show that extraordinary modes can have lower Q and even be suppressed owing to polarization conversion loss. The ordinary ray modes retain the high Q due to inhibited reflection phenomenon.

High-Q UV whispering gallery mode resonators made of angle-cut BBO crystals.

We report an investigation on angle-cut beta barium borate (BBO) whispering gallery mode (WGM) resonators operating in the ultra violet (UV) wavelength range. A quality (Q) factor of 1.5 × 10(8) has been demonstrated at 370 nm. New upper bounds for the absorption coefficients of BBO are obtained from the Q factor measurements. Moreover, polarization rotations of WGMs in the angle-cut birefringent resonators are observed and investigated. To the best of our knowledge, this is not only the first reported demonstration of an angle-cut WGM resonator but also the first reported high Q WGM resonator in the UV region.

Frequency comb from a microresonator with engineered spectrum.

We demonstrate that by varying the ratio between the linewidth and dispersion of a whispering gallery mode resonator we are able to control the number N of free spectral ranges separating the first generated comb sidebands from the pump. We observed combs with N = 19 and N = 1. For the comb with N = 1 we have achieved a span of over 200 nm using a 0.4 mm MgF2 resonator pumped with 50 mW at 1560 nm. This pump power is a factor of 10 lower than previously reported for combs with comparable bandwidth.

Phonon laser action in a tunable two-level system.

The phonon analog of an optical laser has long been a subject of interest. We demonstrate a compound microcavity system, coupled to a radio-frequency mechanical mode, that operates in close analogy to a two-level laser system. An inversion produces gain, causing phonon laser action above a pump power threshold of around 7 microW. The device features a continuously tunable gain spectrum to selectively amplify mechanical modes from radio frequency to microwave rates. Viewed as a Brillouin process, the system accesses a regime in which the phonon plays what has traditionally been the role of the Stokes wave. For this reason, it should also be possible to controllably switch between phonon and photon laser regimes. Cooling of the mechanical mode is also possible.

Thermal instability of a compound resonator.

We investigate the thermal and Kerr nonlinearity in a system of two optically-coupled silica microtoroid resonators experimentally and theoretically. A model for two coupled oscillators describing nonlinear resonance curves is developed. Stability of the static solutions is analyzed. It is shown that thermal nonlinearity is responsible for driving the eigenfrequencies of the two resonators apart, making the normal modes of the system unstable as the pump power grows. The red-detuned normal mode becomes unstable for certain pumping powers.

Generation of optical frequency combs with a CaF2 resonator.

We demonstrate optical frequency combs using the fluorite whispering gallery mode resonator as a nonlinear Kerr medium. Two regimes of generation are observed, giving the record low repetition rate of 13 GHz, equal to the cavity's free spectral range (FSR) or high repetition rates of multiples of cavity FSR. An intermediate regime was also observed. Raman lasing spectrum similar to modulation instability in fibers was observed for the first time to the best of our knowledge.

Brillouin lasing with a CaF2 whispering gallery mode resonator.

Stimulated Brillouin scattering with both pump and Stokes beams in resonance with whispering gallery modes of an ultrahigh Q calcium fluoride resonator is demonstrated for the first time. The resonator is pumped with 1064 nm light and has 3 muW Brillouin lasing threshold. The scattering is observed due to the unique morphology of the resonator reducing the phase mismatch between the optical modes and the hypersound wave.

Ultralow-threshold Raman lasing with CaF2 resonators.

We demonstrate efficient Raman lasing with CaF2 whispering-gallery-mode resonators. Continuous-wave emission threshold is shown to be possible below 1 microW with a 5mm cavity, which is to our knowledge orders of magnitude lower than in any other Raman source. Low-threshold lasing is made possible by the ultrahigh optical quality factor of the cavity, of the order of Q=5x10(10). Stokes components of up to the fifth order were observed at a pump power of 160 microW, and up to the eighth order at 1 mW. A lasing threshold of 15 microW was also observed in a 100 microm CaF2 microcavity. Potential applications are discussed.

On the fundamental limits of Q factor of crystalline dielectric resonators.

The temperature dependence of the processes which fundamentally limit optical quality factor of ideal crystalline whispering gallery mode resonators is investigated. The example of CaF(2) is used to show that spontaneous Raman scattering is the main limitation of the quality factor at low temperatures. Stimulated Raman scattering is also shown to be important at any temperature. We experimentally demonstrate nonlinear absorption due to stimulated Raman scattering in a real cavity at room temperature and theoretically derive Raman gain of CaF(2). We conclude that optical storage times in excess of one second could be achieved in millimeter sized cavities.

Morphology-dependent photonic circuit elements.

We theoretically propose and experimentally demonstrate the design of a novel one-dimensional ringlike macroscopic optical circuit element. The similarity between morphologies of an optical planar waveguide and a whispering-gallery axially symmetric solid-state resonator is used.