[Show abstract][Hide abstract] ABSTRACT: The synthesis of ultra-long lived acoustic phonons in a variety of materials
and device geometries could enable a range of new coherent information
processing and sensing technologies; many forms of phonon dissipation pose a
barrier to this goal. We explore linear and nonlinear contributions to phonon
dissipation in silica at cryogenic temperatures using fiber-optic structures
that tightly confine both photons and phonons to the fiber-optic core. When
immersed in helium, this fiber system supports nearly perfect guidance of 9 GHz
acoustic phonons; strong electrostrictively mediated photon-phonon coupling (or
guided-wave stimulated Brillouin scattering) permits a flexible form of
laser-based phonon spectroscopy. Through linear and nonlinear phonon
spectroscopy, we isolate the effects of disorder-induced two-level tunneling
states as a source of phononic dissipation in this system. We show that an
ensemble of such two-level tunneling states can be driven into
transparency--virtually eliminating this source of phonon dissipation over a
broad range of frequencies. Experimental studies of phononic self-frequency
saturation show excellent agreement with a theoretical model accounting for the
phonon coupling to an ensemble of two-level tunneling states. Extending these
results, we demonstrate a general approach to suppress dissipation produced by
two-level tunneling states via cross-saturation, where the lifetime of a
phonons at one frequency can be extended by the presence of a high intensity
acoustic beam at another frequency. Although these studies were carried out in
silica, our findings are quite general, and can be applied to a range of
materials systems and device geometries.
[Show abstract][Hide abstract] ABSTRACT: A variety of analytical and numerical methods have been used to describe
coherent waveform generation in Kerr frequency comb systems. However, a
universal and simple analytical framework that captures the diversity of
nonlinear phenomena in such systems remains elusive. Here we identify a single
closed-form analytical solution of the driven nonlinear Schrodinger equation,
and we demonstrate that this solution reproduces a large class of the behaviors
in Kerr-comb systems. Specifically, we show that bright and dark soliton
solutions as well as periodic wavetrain solutions arise from the same solution.
In correspondence with experiments, solitons exist only at negative detuning
and dispersion whereas wavetrains can exist for any sign of dispersion and
detuning. From this closed-form solution, an area theorem is derived that
relates the pulse duration and peak powers within Kerr-comb systems; conditions
for existence of such pulses is given by a pump-detuning relation. These
results are validated using simulations, demonstrating good agreement with
established experimental results. Hence, this analytical framework provides
simple design guidelines for the prediction of new regimes of improved
[Show abstract][Hide abstract] ABSTRACT: Rapid progress in silicon photonics has fostered numerous chip-scale sensing,
computing, and signal processing technologies. However, many crucial filtering
and signal delay operations are difficult to perform with all-optical devices.
Unlike photons propagating at luminal speeds, GHz-acoustic phonons with slow
velocity allow information to be stored, filtered, and delayed over
comparatively smaller length-scales with remarkable fidelity. Hence,
controllable and efficient coupling between coherent photons and phonons
enables new signal processing technologies that greatly enhance the performance
and potential impact of silicon photonics. Here, we demonstrate a novel
mechanism for coherent information processing based on traveling-wave
photon-phonon transduction, which achieves a phonon emit-and-receive process
between distinct nanophotonic waveguides. Using this device physics-which can
support 1-20GHz frequencies-we create wavelength-insensitive radio-frequency
photonic filters with an unrivaled combination of stopband attenuation,
selectivity, linewidth, and power-handling in silicon. More generally, this
emit-receive concept is the impetus for numerous powerful new signal processing
[Show abstract][Hide abstract] ABSTRACT: Single-layer graphene exhibits exceptional mechanical properties attractive
for optomechanics: it combines low mass density, large tensile modulus, and low
bending stiffness. However, at visible wavelengths, graphene absorbs weakly and
reflects even less, thereby inadequate to generate large optical forces needed
in optomechanics. Here, we numerically show that a single-layer graphene sheet
is sufficient to produce strong optical forces under terahertz or infrared
illumination. For a system as simple as graphene suspended atop a uniform
substrate, high reflectivity from the substrate is crucial in creating a
standing-wave pattern, leading to a strong optical force on graphene. This
force is readily tunable in amplitude and direction by adjusting the suspension
height. In particular, repellent optical forces can levitate graphene to a
series of stable equilibrium heights above the substrate. One of the key
parameters to maximize the optical force is the excitation frequency: peak
forces are found near the scattering frequency of free carriers in graphene.
With a dynamically controllable Fermi level, graphene opens up new
possibilities of tunable nanoscale optomechanical devices.
[Show abstract][Hide abstract] ABSTRACT: We present the first time-domain measurement of a guided-wave nano-opto-mechanical system, resulting in the coherent excitation of multiple mechanical modes. We deconvolved the electronic and mechanical responses to observe the evolution of the coherent superposition.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that judicious selection of mirrors/substrates, operational frequency, and graphene location inside a cavity yields unprecedented optical forces on graphene, while tolerably modifies Q of the cavity .
[Show abstract][Hide abstract] ABSTRACT: We present progress towards the development of novel hybrid photonic-phononic oscillator technologies in both nanoscale silicon photonics and in fiber optic systems. These systems utilize traveling-wave photon-phonon couplings involving both stimulated Brillouin scattering processes (SBS). We explore numerous geometries that have enabled large forward-SBS processes in nanoscale silicon waveguides for the first time, and examine new approaches to achieving integrated Brillouin based signal processing.
[Show abstract][Hide abstract] ABSTRACT: We develop a general framework of evaluating the Stimulated Brillouin Scattering (SBS) gain coefficient in optical waveguides via the overlap integral between optical and elastic eigen-modes. This full-vectorial formulation of SBS coupling rigorously accounts for the effects of both radiation pressure and electrostriction within micro- and nano-scale waveguides. We show that both contributions play a critical role in SBS coupling as modal confinement approaches the sub-wavelength scale. Through analysis of each contribution to the optical force, we show that spatial symmetry of the optical force dictates the selection rules of the excitable elastic modes. By applying this method to a rectangular silicon waveguide, we demonstrate how the optical force distribution and elastic modal profiles jointly determine the magnitude and scaling of SBS gains in both forward and backward SBS processes. We further apply this method to the study of intra- and inter-modal SBS processes, and demonstrate that the coupling between distinct optical modes are necessary to excite elastic modes with all possible symmetries. For example, we show that strong inter-polarization coupling can be achieved between the fundamental TE- and TM-like modes of a suspended silicon waveguide.
[Show abstract][Hide abstract] ABSTRACT: Strong light-boundary interactions alter the nature of traveling-wave photon-phonon coupling at nanoscales. Harnessing such effects, stimulated Brillouin scattering is realized for the first time in silicon photonics via a new class of waveguides.
Integrated Photonics Research, Silicon and Nanophotonics; 07/2013
[Show abstract][Hide abstract] ABSTRACT: The dispersive properties of phononic crystals can be utilized to manipulate the phononic impedance of a material and to engineer the frequency-delay response of time domain signal processing circuits. In this paper we study, in both the frequency and time domains, the dispersive properties of phononic crystals formed in thin suspended plates of aluminum nitride.
2013 IEEE International Ultrasonics Symposium (IUS); 07/2013
[Show abstract][Hide abstract] ABSTRACT: The nonlinear of a nanoscale Brillouin-active silicon waveguide is examined through heterodyne four-wave mixing experiments. The interference between Brillouin scattering, Kerr, and dispersive free-carrier nonlinearities are analytically described to explain the characteristic line-shapes observed.
[Show abstract][Hide abstract] ABSTRACT: We develop a general framework of evaluating slow-light performance
using Stimulated Brillouin Scattering (SBS) in optical waveguides via
the overlap integral between optical and elastic eigen-modes. We show
that spatial symmetry of the optical force dictates the selection rules
of the excitable elastic modes. By applying this method to a rectangular
silicon waveguide, we demonstrate the spatial distributions of optical
force and elastic eigen-modes jointly determine the magnitude and
scaling of SBS gain coefficient in both forward and backward SBS
processes. We further apply this method to inter-modal SBS process, and
demonstrate that the coupling between distinct optical modes is
necessary to excite elastic modes with all possible symmetries.
Proceedings of SPIE - The International Society for Optical Engineering 03/2013; · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nanoscale modal confinement is known to radically enhance the effect of intrinsic Kerr and Raman nonlinearities within nanophotonic silicon waveguides. By contrast, stimulated Brillouin-scattering nonlinearities, which involve coherent coupling between guided photon and phonon modes, are stifled in conventional nanophotonics, preventing the realization of a host of Brillouin-based signal-processing technologies in silicon. Here we demonstrate stimulated Brillouin scattering in silicon waveguides, for the first time, through a new class of hybrid photonic-phononic waveguides. Tailorable travelling-wave forward-stimulated Brillouin scattering is realized-with over 1,000 times larger nonlinearity than reported in previous systems-yielding strong Brillouin coupling to phonons from 1 to 18 GHz. Experiments show that radiation pressures, produced by subwavelength modal confinement, yield enhancement of Brillouin nonlinearity beyond those of material nonlinearity alone. In addition, such enhanced and wideband coherent phonon emission paves the way towards the hybridization of silicon photonics, microelectromechanical systems and CMOS signal-processing technologies on chip.
[Show abstract][Hide abstract] ABSTRACT: We examine the physics of traveling-wave photon-phonon coupling within
nanoscale silicon waveguides and explore a host of new Brillouin-based
signal processing technologies enabled by tailorable stimulated
Brillouin processes in silicon photonics. Theoretical analysis of
Brillouin coupling at sub-wavelength scales is presented, revealing that
strong light-boundary interactions produce large radiation pressures
mediated Brillouin nonlinearities. Experimental results demonstrating
stimulated Brillouin scattering in silicon waveguides for the first time
are also presented, revealing 1000 times larger forward stimulated
Brillouin gain coeffcients than any prior system.
Proceedings of SPIE - The International Society for Optical Engineering 01/2013; · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that the nature of stimulated Brillouin scattering (SBS) radically changes at nanoscales due to tremendous internal radiation pressures. Coherent interplay between radiation pressure and electrostriction yields giant enhancement of SBS and tailorable photon-phonon coupling.
[Show abstract][Hide abstract] ABSTRACT: We develop a general method of calculating Stimulated Brillouin Scattering
(SBS) gain coefficient in axially periodic waveguides. Applying this method to
a silicon periodic waveguide suspended in air, we demonstrate that SBS
nonlinearity can be dramatically enhanced at the brillouin zone boundary where
the decreased group velocity of light magnifies photon-phonon interaction. In
addition, we show that the symmetry plane perpendicular to the propagation axis
plays an important role in both forward and backward SBS processes. In forward
SBS, only elastic modes which are even about this plane are excitable. In
backward SBS, the SBS gain coefficients of elastic modes approach to either
infinity or constants, depending on their symmetry about this plane at $q=0$.
[Show abstract][Hide abstract] ABSTRACT: Low-contrast all-optical Zeno switching has been demonstrated in a Silicon Nitride microdisk resonator surrounded by hot Rubidium vapor. The device is based on the suppression of the cavity field buildup due to non-degenerate two-photon absorption.
[Show abstract][Hide abstract] ABSTRACT: Low-contrast all-optical Zeno switching has been demonstrated in a silicon
nitride microdisk resonator coupled to a hot atomic vapor. The device is based
on the suppression of the field build-up within a microcavity due to
non-degenerate two-photon absorption. This experiment used one beam in a
resonator and one in free-space due to limitations related to device physics.
These results suggest that a similar scheme with both beams resonant in the
cavity would correspond to input power levels near 20 nW.
Physical Review A 06/2012; 87(2). · 2.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical forces have been traditionally decomposed into gradient force and scattering force. We show the limit of this decomposition and the non-conservative nature of optical forces, even for single particle confined in a single-mode waveguide.