Scanning probe microscopy of thermally excited mechanical modes of an optical microcavity

The resonant buildup of light within optical microcavities elevates the radiation pressure which mediates coupling of optical modes to the mechanical modes of a microcavity. Above a certain threshold pump power, regenerative mechanical oscillation occurs causing oscillation of certain mechanical eigenmodes. Here, we present a methodology to spatially image the micro-mechanical resonances of a toroid microcavity using a scanning probe technique. The method relies on recording the induced frequency shift of the mechanical eigenmode when in contact with a scanning probe tip. The method is passive in nature and achieves a sensitivity sufficient to spatially resolve the vibrational mode pattern associated with the thermally agitated displacement at room temperature. The recorded mechanical mode patterns are in good qualitative agreement with the theoretical strain fields as obtained by finite element simulations

Theoretical and experimental study of radiation pressure-induced mechanical oscillations (parametric instability) in optical microcavities

Radiation pressure can couple the mechanical modes of an optical cavity structure to its optical modes, leading to parametric oscillation instability. This regime is characterized by regenerative oscillation of the mechanical cavity eigenmodes. Here, we present the first observation of this effect with a detailed theoretical and experimental analysis of these oscillations in ultra-high-Q microtoroids. Embodied within a microscale, chip-based device, this mechanism can benefit both research into macroscale quantum mechanical phenomena and improve the understanding of the mechanism within the context of laser interferometer gravitational-wave observatory (LIGO). It also suggests that new technologies are possible that will leverage the phenomenon within photonics

Characterization and scanning probe spectroscopy of radiation-pressure induced mechanical oscillation of a microcavity

Microcavities can enter a regime where radiation pressure causes oscillation of mechanical cavity eigenmodes. We present a detailed experimental and theoretical understanding of this effect, and report direct scanning probe spectroscopy of the micro-mechanical modes

Coupling ideality of integrated planar high-Q microresonators

Chipscale microresonators with integrated planar optical waveguides are useful building blocks for linear, nonlinear and quantum optical devices. Loss reduction through improving fabrication processes has resulted in several integrated micro resonator platforms attaining quality (Q) factors of several millions. However only few studies have investigated design-dependent losses, especially with regard to the resonator coupling section. Here we investigate design-dependent parasitic losses, described by the coupling ideality, of the commonly employed microresonator design consisting of a microring resonator waveguide side-coupled to a straight bus waveguide. By systematic characterization of multi-mode high-Q silicon nitride microresonator devices, we show that this design can suffer from low coupling ideality. By performing full 3D simulations to numerically investigate the resonator to bus waveguide coupling, we identify the coupling to higher-order bus waveguide modes as the dominant origin of parasitic losses which lead to the low coupling ideality. Using suitably designed bus waveguides, parasitic losses are mitigated, and a nearly unity ideality and strong overcoupling (i.e. a ratio of external coupling to internal resonator loss rate > 9) are demonstrated. Moreover we find that different resonator modes can exchange power through the coupler, which therefore constitutes a mechanism that induces modal coupling, a phenomenon known to distort resonator dispersion properties. Our results demonstrate the potential for significant performance improvements of integrated planar microresonators, achievable by optimized coupler designs.Comment: 8 pages, 3 figures, 1 tabl

Photonic clocks, Raman lasers, and Biosensors on Silicon

Micro-resonators on silicon having Q factors as high as 500 million are described, and used to demonstrate radio-frequency mechanical oscillators, micro-Raman and parametric sources with sub-100 microwatt thresholds, visible sources, as well as high-sensitivity, biological detectors

Analysis of radiation-pressure induced mechanical oscillation of an optical microcavity

The theoretical work of V.B. Braginsky predicted that radiation pressure can couple the mechanical, mirror-eigenmodes of a Fabry-Perot resonator to it's optical modes, leading to a parametric oscillation instability. This regime is characterized by regenerative mechanical oscillation of the mechanical mirror eigenmodes. We have recently observed the excitation of mechanical modes in an ultra-high-Q optical microcavity. Here, we present a detailed experimental analysis of this effect and demonstrate that radiation pressure is the excitation mechanism of the observed mechanical oscillations

Large second harmonic generation enhancement in SiN waveguides by all-optically induced quasi phase matching

Integrated waveguides exhibiting efficient second-order nonlinearities are crucial to obtain compact and low power optical signal processing devices. Silicon nitride (SiN) has shown second harmonic generation (SHG) capabilities in resonant structures and single-pass devices leveraging intermodal phase matching, which is defined by waveguide design. Lithium niobate allows compensating for the phase mismatch using periodically poled waveguides, however the latter are not reconfigurable and remain difficult to integrate with SiN or silicon (Si) circuits. Here we show the all-optical enhancement of SHG in SiN waveguides by more than 30 dB. We demonstrate that a Watt-level laser causes a periodic modification of the waveguide second-order susceptibility. The resulting second order nonlinear grating has a periodicity allowing for quasi phase matching (QPM) between the pump and SH mode. Moreover, changing the pump wavelength or polarization updates the period, relaxing phase matching constraints imposed by the waveguide geometry. We show that the grating is long term inscribed in the waveguides, and we estimate a second order nonlinearity of the order of 0.3 pm/V, while a maximum conversion efficiency (CE) of 1.8x10-6 W-1 cm-2 is reached

Probing the loss origins of ultra-smooth Si3N4\mathrm{Si_3N_4} integrated photonic waveguides

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion (GVD) are important to nonlinear photonic devices such as soliton microcombs. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication induced surface roughness. Therefore, microresonators with ultra-high Q factors are, to date, only attainable in polished bulk crystalline, or chemically etched silica based devices, that pose however challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride ($\mathrm{Si_3N_4}$) waveguides with unprecedentedly smooth sidewalls and tight confinement with record low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable \emph{mean} microresonator Q factors larger than $5\times10^6$ for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques we differentiate the scattering and absorption loss contributions, and reveal metal impurity related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time they are identified, and play a tangible role, in absorption of integrated microresonators. Taken together, our work provides new insights in the origins of propagation losses in $\mathrm{Si_3N_4}$ waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high Q regime