Compact lithium niobate microring resonators in the ultrahigh Q/V regime.

Lithium niobate (LN) is a promising material for future complex photonic-electronic circuits, with wide applications in such fields as communications, sensing, quantum optics, and computation. LN took a great stride toward compact photonic integrated circuits (PICs) with the development of partially etched LN on insulator (LNOI) waveguides. However, integration density is still limited for future highly compact PICs, owing to the partial etching nature of their waveguides. Here, we demonstrate a fully etched LN PIC platform, which, for the first time to our knowledge, simultaneously achieves ultralow propagation loss and compact circuit size. The tightly confined fully etched LN waveguides with smooth sidewalls allow us to bring the bending radius down to 20 µm (corresponding to 1 THz free spectral range). We have achieved compact high Q microring resonators with Q/V of 8.7 × 104 µm-3, almost one order of magnitude larger than previous demonstrations. The statistical mean propagation losses of our LN waveguides is 8.5 dB/m (corresponding to a mean Q factor of 4.9 × 106), even with a small bending radius of 40 µm. Our compact and ultralow-loss LN platform shows great potential in future miniaturized multifunctional integration systems. As complementary evidence to show the utility of our platform, we demonstrate soliton microcombs with an ultrahigh repetition rate of 500 GHz in LN.

Longitudinal piezoelectric resonant photoelastic modulator for efficient intensity modulation at megahertz frequencies.

Intensity modulators are an essential component in optics for controlling free-space beams. Many applications require the intensity of a free-space beam to be modulated at a single frequency, including wide-field lock-in detection for sensitive measurements, mode-locking in lasers, and phase-shift time-of-flight imaging (LiDAR). Here, we report a new type of single frequency intensity modulator that we refer to as a longitudinal piezoelectric resonant photoelastic modulator. The modulator consists of a thin lithium niobate wafer coated with transparent surface electrodes. One of the fundamental acoustic modes of the modulator is excited through the surface electrodes, confining an acoustic standing wave to the electrode region. The modulator is placed between optical polarizers; light propagating through the modulator and polarizers is intensity modulated with a wide acceptance angle and record breaking modulation efficiency in the megahertz frequency regime. As an illustration of the potential of our approach, we show that the proposed modulator can be integrated with a standard image sensor to effectively convert it into a time-of-flight imaging system.

Room-Temperature Mechanical Resonator with a Single Added or Subtracted Phonon.

A room-temperature mechanical oscillator undergoes thermal Brownian motion with an amplitude much larger than the amplitude associated with a single phonon of excitation. This motion can be read out and manipulated using laser light using a cavity-optomechanical approach. By performing a strong quantum measurement (i.e., counting single photons in the sidebands imparted on a laser), we herald the addition and subtraction of single phonons on the 300 K thermal motional state of a 4 GHz mechanical oscillator. To understand the resulting mechanical state, we implement a tomography scheme and observe highly non-Gaussian phase-space distributions. Using a maximum likelihood method, we infer the density matrix of the oscillator, and we confirm the counterintuitive doubling of the mean phonon number resulting from phonon addition and subtraction.

Time-of-flight imaging based on resonant photoelastic modulation: publisher's note.

This publisher's note corrects the Funding section in Appl. Opt.58, 2235 (2019)APOPAI0003-693510.1364/AO.58.002235.

Efficient bidirectional piezo-optomechanical transduction between microwave and optical frequency.

Efficient interconversion of both classical and quantum information between microwave and optical frequency is an important engineering challenge. The optomechanical approach with gigahertz-frequency mechanical devices has the potential to be extremely efficient due to the large optomechanical response of common materials, and the ability to localize mechanical energy into a micron-scale volume. However, existing demonstrations suffer from some combination of low optical quality factor, low electrical-to-mechanical transduction efficiency, and low optomechanical interaction rate. Here we demonstrate an on-chip piezo-optomechanical transducer that systematically addresses all these challenges to achieve nearly three orders of magnitude improvement in conversion efficiency over previous work. Our modulator demonstrates acousto-optic modulation with [Formula: see text] = 0.02 V. We show bidirectional conversion efficiency of [Formula: see text] with 3.3 µW  red-detuned optical pump, and [Formula: see text] with 323 µW blue-detuned pump. Further study of quantum transduction at millikelvin temperatures is required to understand how the efficiency and added noise are affected by reduced mechanical dissipation, thermal conductivity, and thermal capacity.

Cryogenic packaging of an optomechanical crystal.

There is an increasing need to simplify optical coupling techniques for low-temperature integrated photonics experiments. Various promising and scalable photonic packaging techniques have been under development, but few methods compatible with low-temperature operation have been reported. Here, we demonstrate 25% coupling efficiency from an optical fiber to a silicon optomechanical crystal at 7 mK in a dilution refrigerator without in-situ optical alignment at cryogenic temperatures. Our coupling scheme uses angle-polished fibers glued to the surface of the chip. The technique paves the way for scalable integration of optical technologies at low temperatures, circumventing the need for optical alignment in a highly constrained cryogenic environment. The technique is broadly applicable to studies of low-temperature optical physics and to emerging quantum photonic technologies.

Resolving the energy levels of a nanomechanical oscillator.

The quantum nature of an oscillating mechanical object is anything but apparent. The coherent states that describe the classical motion of a mechanical oscillator do not have a well defined energy, but are quantum superpositions of equally spaced energy eigenstates. Revealing this quantized structure is only possible with an apparatus that measures energy with a precision greater than the energy of a single phonon. One way to achieve this sensitivity is by engineering a strong but nonresonant interaction between the oscillator and an atom. In a system with sufficient quantum coherence, this interaction allows one to distinguish different energy eigenstates using resolvable differences in the atom's transition frequency. For photons, such dispersive measurements have been performed in cavity1,2 and circuit quantum electrodynamics3. Here we report an experiment in which an artificial atom senses the motional energy of a driven nanomechanical oscillator with sufficient sensitivity to resolve the quantization of its energy. To realize this, we build a hybrid platform that integrates nanomechanical piezoelectric resonators with a microwave superconducting qubit on the same chip. We excite phonons with resonant pulses and probe the resulting excitation spectrum of the qubit to observe phonon-number-dependent frequency shifts that are about five times larger than the qubit linewidth. Our result demonstrates a fully integrated platform for quantum acoustics that combines large couplings, considerable coherence times and excellent control over the mechanical mode structure. With modest experimental improvements, we expect that our approach will enable quantum nondemolition measurements of phonons4 and will lead to quantum sensors and information-processing approaches5 that use chip-scale nanomechanical devices.

Time-of-flight imaging based on resonant photoelastic modulation.

A time-of-flight imaging system is proposed and its working principle demonstrated. To realize this system, a new device, a free-space optical mixer, is designed and fabricated. A scene is illuminated (flashed) with a megahertz-level amplitude-modulated light source, and the reflected light from the scene is collected by a receiver. The receiver consists of the free-space optical mixer, comprising a photoelastic modulator sandwiched between polarizers, placed in front of a standard complementary metal-oxide-semiconductor (CMOS) image sensor. This free-space optical mixer downconverts the megahertz-level amplitude modulation frequencies into the temporal bandwidth of the image sensor. A full-scale extension of the demonstrated system will be able to measure phases and Doppler shifts for the beat tones and use signal processing techniques to estimate the distance and velocity of each point in the illuminated scene with high accuracy.

Optomechanical antennas for on-chip beam-steering.

Rapid and low-power control over the direction of a radiating light field is a major challenge in photonics and a key enabling technology for emerging sensors and free-space communication links. Current approaches based on bulky motorized components are limited by their high cost and power consumption, while on-chip optical phased arrays face challenges in scaling and programmability. Here, we propose a solid-state approach to beam-steering using optomechanical antennas. We combine recent progress in simultaneous control of optical and mechanical waves with remarkable advances in on-chip optical phased arrays to enable low-power and full two-dimensional beam-steering of monochromatic light. We present a design of a silicon photonic system made of photonic-phononic waveguides that achieves 44° field of view with 880 resolvable spots by sweeping the mechanical wavelength with about a milliwatt of mechanical power. Using mechanical waves as nonreciprocal, active gratings allows us to quickly reconfigure the beam direction, beam shape, and the number of beams. It also enables us to distinguish between light that we send and receive.

Analysis of enhanced stimulated Brillouin scattering in silicon slot waveguides.

Stimulated Brillouin scattering has attracted renewed interest with the promise of highly tailorable integration into the silicon photonics platform. However, significant Brillouin amplification in silicon waveguides has yet to be shown. In an effort to engineer a structure with large photon-phonon coupling, we analyzed both forward and backward Brillouin scattering in high-index-contrast silicon slot waveguides. The calculations predict that gradient forces enhance the Brillouin gain in narrow slots. We estimate a currently feasible gain of about 10(5) W(-1) m(-1) in horizontal slot waveguides, which is an order of magnitude larger than in a stand-alone silicon wire. Such efficient coupling could enable a host of Brillouin technologies on a mass-producible silicon chip.