arXiv subjects

Hong X. Tang

Publications and source records attributed to Hong X. Tang.

At least 145 records · Page 8Linked to original sources

arXiv2013

Waveguide integrated low noise NbTiN nanowire single-photon detectors with milli-Hz dark count rate

Superconducting nanowire single-photon detectors are an ideal match for integrated quantum photonic circuits due to their high detection efficiency for telecom wavelength photons. Quantum optical technology also requires single-photon detection with low dark count rate and high timing accuracy. Here we present very low noise superconducting nanowire single-photon detectors based on NbTiN thin films patterned directly on top of Si3N4 waveguides. We systematically investigate a large variety of detector designs and characterize their detection noise performance. Milli-Hz dark count rates are demonstrated over the entire operating range of the nanowire detectors which also feature low timing jitter. The ultra-low dark count rate, in combination with the high detection efficiency inherent to our traveling wave detector geometry, gives rise to a measured noise equivalent power at the 10^(-20) W/Hz^(1/2) level.

physics.optics↗

arXiv2013

Optical time domain reflectometry with low noise waveguide-coupled superconducting nanowire single-photon detectors

We demonstrate optical time domain reflectometry over 200 km of optical fiber using low-noise NbTiN superconducting single-photon detectors integrated with Si3N4 waveguides. Our small detector footprint enables high timing resolution of 50ps and a dark count rate of 3 Hz with unshielded fibers, allowing for identification of defects along the fiber over a dynamic range of 37.4 dB. Photons scattered and reflected back from the fiber under test can be detected in free-running mode without showing dead zones or other impairments often encountered in semiconductor photon-counting optical time domain reflectometers.

physics.optics↗

arXiv2013

Nonlinear optical effects of ultrahigh-Q silicon photonic nanocavities immersed in superfluid helium

Photonic nanocavities are a key component in many applications because of their capability of trapping and storing photons and enhancing interactions of light with various functional materials and structures. The maximal number of photons that can be stored in silicon photonic cavities is limited by the free-carrier and thermo-optic effects at room temperature. To reduce such effects, we performed the first experimental study of optical nonlinearities in ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid helium environment. At elevated input power, the cavity transmission spectra exhibit distinct blue-shifted bistability behavior when temperature crosses the liquid helium lambda point. At even lower temperatures, the spectra restore to symmetric Lorentzian shapes. Under this condition, we obtain a large stored intracavity photon number of about 40,000, which is limited ultimately by the local helium phase transition. These new discoveries are explained by theoretical calculations and numerical simulations.

physics.optics↗

arXiv2013

Matrix of integrated superconducting single-photon detectors with high timing resolution

We demonstrate a large grid of individually addressable superconducting single photon detectors on a single chip. Each detector element is fully integrated into an independent waveguide circuit with custom functionality at telecom wavelengths. High device density is achieved by fabricating the nanowire detectors in traveling wave geometry directly on top of silicon-on-insulator waveguides. Our superconducting single-photon detector matrix includes detector designs optimized for high detection efficiency, low dark count rate and high timing accuracy. As an example, we exploit the high timing resolution of a particularly short nanowire design to resolve individual photon round-trips in a cavity ring-down measurement of a silicon ring resonator.

quant-ph↗

arXiv2013

Casimir probe based upon metallized high Q SiN nanomembrane resonator

We present the instrumentation and measurement scheme of a new Casimir force probe that bridges Casimir force measurements at microscale and macroscale. A metallized high Q silicon nitride nanomembrane resonator is employed as a sensitive force probe. The high tensile stress present in the nanomembrane not only enhances the quality factor but also maintains high flatness over large area serving as the bottom electrode in a sphere-plane configuration. A fiber interferometer is used to readout the oscillation of the nanomembrane and a phase-locked loop scheme is applied to track the change of the resonance frequency. Because of the high quality factor of the nanomembrane and the high stability of the setup, a frequency resolution down to $2\times10^{-9}$ and a corresponding force gradient resolution of 3 $μ$N/m is achieved. Besides sensitive measurement of Casimir force, our measurement technique simultaneously offers Kelvin probe measurement capability that allows in situ imaging of the surface potentials.

cond-mat.mes-hall↗

arXiv2013

Cavity piezooptomechanics: piezoelectrically excited, optically transduced optomechanical resonators

We present a monolithic integrated aluminum nitride (AlN) optomechanical resonator in which the mechanical motion is actuated by piezoelectric force and the displacement is transduced by a high-Q optical cavity. The AlN optomechanical resonator is excited from a radio-frequency electrode via a small air gap to eliminate resonator-to-electrode loss. We observe the electrically excited mechanical motion at 47.3 MHz, 1.04 GHz, and 3.12 GHz, corresponding to the 1st, 2nd, and 4th radial-contour mode of the wheel resonator respectively. An equivalent circuit model is developed to describe the observed Fano-like resonance spectrum.

cond-mat.mes-hall↗

arXiv2012

A superhigh-frequency optoelectromechanical system based on a slotted photonic crystal cavity

We develop an all-integrated optoelectromechanical system that operates in the superhigh frequency band. This system is based on an ultrahigh-Q slotted photonic crystal (PhC) nanocavity formed by two PhC membranes, one of which is patterned with electrode and capacitively driven. The strong simultaneous electromechanical and optomechanical interactions yield efficient electrical excitation and sensitive optical transduction of the bulk acoustic modes of the PhC membrane. These modes are identified up to a frequency of 4.20 GHz, with their mechanical Q factors ranging from 240 to 1,730. Directly linking signals in microwave and optical domains, such optoelectromechanical systems will find applications in microwave photonics in addition to those that utilize the electromechanical and optomechanical interactions separately.

physics.optics↗

arXiv2012

Aluminum nitride as a new material for chip-scale optomechanics and nonlinear optics

Silicon photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, silicon is limited to wavelengths above 1100 nm and does not allow device operation in the visible spectrum range where low noise lasers are conveniently available. The narrow band gap of silicon also makes silicon optomechanical devices susceptible to strong two-photon absorption and free carrier absorption, which often introduce strong thermal effect that limit the devices' stability and cooling performance. Further, silicon also does not provide the desired lowest order optical nonlinearity for interfacing with other active electrical components on a chip. On the other hand, aluminum nitride (AlN) is a wideband semiconductor widely used in micromechanical resonators due to its low mechanical loss and high electromechanical coupling strength. Here we report the development of AlN-on-silicon platform for low loss, wideband optical guiding, as well as its use for achieving simultaneous high optical quality and mechanical quality optomechanical devices. Exploiting AlN's inherent second order nonlinearity we further demonstrate electro-optic modulation and efficient second-harmonic generation in AlN photonic circuits. Our results suggest that low cost AlN-on-silicon photonic circuits are excellent substitutes for CMOS-compatible photonic circuits for building new functional optomechanical devices that are free from carrier effects.

cond-mat.mtrl-sci↗

arXiv2012

Casimir Force and In Situ Surface Potential Measurements on Nanomembranes

We present Casimir force measurements in a sphere-plate configuration that consists of a high quality nanomembrane resonator and a millimeter sized gold coated sphere. The nanomembrane is fabricated from stoichiometric silicon nitride metallized with gold. A Kelvin probe method is used in situ to image the surface potentials to minimize the distance-dependent residual force. Resonance-enhanced frequency-domain measurements of the nanomembrane motion allow for very high resolution measurements of the Casimir force gradient (down to a force gradient sensitivity of 3 uN/m). Using this technique, the Casimir force in the range of 100 nm to 2 um is accurately measured. Experimental data thus obtained indicate that the device system in the measured range is best described with the Drude model.

quant-ph↗

arXiv2012

A 1.16-μm-radius disk cavity in a sunflower-type circular photonic crystal with ultrahigh quality factor

We present a 1.16-\mum-radius disk cavity with ultrahigh quality (Q) factor by embedding the disk into a sunflower-type circular photonic crystal (CPC). The band gap of the CPC reduces the bending loss of the whispering-gallery mode of the disk, leading to a simulated Q of 10^7, at least one order of magnitude higher than a bare disk of the same size. The design is experimentally verified with a record high loaded Q of 7.4 \times 10^5 measured from an optimized device fabricated on a silicon-on-insulator substrate.

physics.optics↗

arXiv2012

Dispersive coupling and optimization of femtogram L3-nanobeam optomechanical cavities

We present the design of a femtogram L3-nanobeam photonic crystal cavity for optomechanical studies. Two symmetric nanobeams are created by placing three air slots in a silicon photonic crystal slab where three holes are removed. The optical quality factor (Q) is optimized up to 52,000. The nanobeams' mechanical frequencies are higher than 600 MHz due to their femtogram effective modal masses. The optical and mechanical modes are dispersively coupled with a vacuum optomechanical coupling rate g0/2pi exceeding 200 kHz. The anchor-loss-limited mechanical Q of the differential beam mode is evaluated to be greater than 10,000 for structures with ideally symmetric beams. The influence of variations on the air slot width and position is also investigated. The devices can be used as ultrasensitive sensors of mass, force, and displacement.

physics.optics↗

arXiv2012

High Q micro-ring resonators fabricated from polycrystalline aluminum nitride films for near infrared and visible photonics

We demonstrate wideband integrated photonic circuits in sputter-deposited aluminum nitride (AlN) thin films. At both near-infrared and visible wavelengths, we achieve low propagation loss in integrated waveguides and realize high-quality optical resonators. In the telecoms C-band (1520-1580 nm), we obtain the highest optical Q factor of 440,000. Critical coupled devices show extinction ratio above 30 dB. For visible wavelengths (around 770 nm), intrinsic quality factors in excess of 30,000 is demonstrated. Our work illustrates the potential of AlN as a low loss material for wideband optical applications.

physics.optics↗

arXiv2012

Integrated high frequency aluminum nitride optomechanical resonators

Aluminum nitride (AlN) has been widely used in microeletromechanical resonators for its excellent electromechanical properties. Here we demonstrate the use of AlN as an optomechanical material that simultaneously offer low optical and mechanical loss. Integrated AlN microring resonators in the shape of suspended rings exhibit high optical quality factor (Q) with loaded Q up to 125,000. Optomechanical transduction of the Brownian motion of a GHz contour mode yields a displacement sensitivity of 6.2\times10^(-18)m/Hz^(1/2) in ambient air.

cond-mat.mes-hall↗

arXiv2012

High-Q silicon optomechanical microdisk resonators at gigahertz frequencies

We report disk-shaped silicon optomechanical resonators with frequency up to 1.75 GHz in the ultrahigh frequency band. Optical transduction of the thermal motion of the disks' in-plane vibrational modes yields a displacement sensitivity of 4.1 \times 10^(-17) m/Hz^(1/2). Due to the reduced clamping loss, these disk resonators possess high mechanical quality factors (Q), with the highest value of 4370 for the 1.47 GHz mode measured in ambient air. Numerical simulation on the modal frequency and mechanical Q for disks of varying undercut shows modal coupling and suggests a realistic pedestal size to achieve the highest possible Q.

physics.optics↗

arXiv2012

Frequency and phase noise of ultra-high Q silicon nitride nanomechanical resonators

We describe the measurement and modeling of amplitude noise and phase noise in ultra-high Q nanomechanical resonators made from stoichiometric silicon nitride. With quality factors exceeding 2 million, the resonators' noise performance is studied with high precision. We find that the amplitude noise can be well described by the thermomechanical model, however, the resonators exhibit sizable extra phase noise due to their intrinsic frequency fluctuations. We develop a method to extract the resonator frequency fluctuation of a driven resonator and obtain a noise spectrum with dependence, which could be attributed to defect motion with broadly distributed relaxation times.

cond-mat.mes-hall↗

arXiv2012

Femtogram Doubly Clamped Nanomechanical Resonators Embedded in a High-Q Two-Dimensional Photonic Crystal Nanocavity

We demonstrate a new optomechanical device system which allows highly efficient transduction of femtogram nanobeam resonators. Doubly clamped nanomechanical resonators with mass as small as 25 fg are embedded in a high-finesse two-dimensional photonic crystal nanocavity. Optical transduction of the fundamental flexural mode around 1 GHz was performed at room temperature and ambient conditions, with an observed displacement sensitivity of 0.94 fm/Hz^(1/2). Comparison of measurements from symmetric and asymmetric double-beam devices reveals hybridization of the mechanical modes where the structural symmetry is shown to be the key to obtain a high mechanical quality factor. Our novel configuration opens the way for a new category of "NEMS-in-cavity" devices based on optomechanical interaction at the nanoscale.

physics.optics↗

arXiv2011

Dynamic manipulation of mechanical resonators in the high amplitude regime through optical backaction

Cavity optomechanics enables active manipulation of mechanical resonators through backaction cooling and amplification. This ability to control mechanical motion with retarded optical forces has recently spurred a race towards realizing a mechanical resonator in its quantum ground state. Here, instead of quenching optomechanical motion, we demonstrate high amplitude operation of nanomechanical resonators by utilizing a highly efficient phonon generation process. In this regime, the nanomechanical resonators gain sufficient energy from the optical field to overcome the large energy barrier of a double well potential, leading to nanomechanical slow-down and zero frequency singularity, as predicted by early theories . Besides fundamental studies and interests in parametric amplification of small forces, optomechanical backaction is also projected to open new windows for studying discrete mechanical states, and to foster applications. Here we realize a non-volatile mechanical memory element, in which bits are written and reset via optomechanical backaction by controlling the mechanical damping across the barrier. Our study casts a new perspective on the energy dynamics in coupled mechanical resonator - cavity systems and enables novel functional devices that utilize the principles of cavity optomechanics.

physics.optics↗

arXiv2011

GHz optomechanical resonators with high mechanical Q factor in air

We demonstrate wheel-shaped silicon optomechanical resonators for resonant operation in ambient air. The high finesse of optical whispering gallery modes (loaded optical Q factor above 500,000) allows for efficient transduction of the wheel resonator's mechanical radial contour modes of frequency up to 1.35 GHz with high mechanical Q factor around 4,000 in air.

physics.optics↗