53 research outputs found
Role of Optical Density of States in Two-mode Optomechanical Cooling
Dynamical back-action cooling of phonons in optomechanical systems having one
optical mode is well studied. Systems with two optical modes have the potential
to reach significantly higher cooling rate through resonant enhancement of both
pump and scattered light. Here we experimentally investigate the role of dual
optical densities of states on optomechanical cooling, and the deviation from
theory caused by thermal locking to the pump laser. Using this, we demonstrate
a room temperature system operating very close to the strong coupling regime,
where saturation of cooling is anticipated
Raman Cooling of Solids through Photonic Density of States Engineering
The laser cooling of vibrational states of solids has been achieved through
photoluminescence in rare-earth elements, optical forces in optomechanics, and
the Brillouin scattering light-sound interaction. The net cooling of solids
through spontaneous Raman scattering, and laser refrigeration of indirect band
gap semiconductors, both remain unsolved challenges. Here, we analytically show
that photonic density of states (DoS) engineering can address the two
fundamental requirements for achieving spontaneous Raman cooling: suppressing
the dominance of Stokes (heating) transitions, and the enhancement of
anti-Stokes (cooling) efficiency beyond the natural optical absorption of the
material. We develop a general model for the DoS modification to spontaneous
Raman scattering probabilities, and elucidate the necessary and minimum
condition required for achieving net Raman cooling. With a suitably engineered
DoS, we establish the enticing possibility of refrigeration of intrinsic
silicon by annihilating phonons from all its Raman-active modes simultaneously,
through a single telecom wavelength pump. This result points to a highly
flexible approach for laser cooling of any transparent semiconductor, including
indirect band gap semiconductors, far away from significant optical absorption,
band-edge states, excitons, or atomic resonances.Comment: 17 pages, 4 figure
Imaging of acoustic pressure modes in opto-mechano-fluidic resonators with a single particle probe
Opto-mechano-fluidic resonators (OMFRs) are a new platform for
high-throughput sensing of the mechanical properties of freely flowing
microparticles in arbitrary media. Experimental extraction of OMFR mode shapes,
especially the acoustic pressure field within the fluidic core, is essential
for determining sensitivity and for extracting the particle parameters. Here we
demonstrate a new imaging technique for simultaneously capturing the spatially
distributed acoustic pressure fields of multiple vibrational modes in the OMFR
system. The mechanism operates using microparticles as perturbative imaging
probes, and potentially reveals the inverse path towards multimode inertial
detection of the particles themselves.Comment: 5 pages, 5 figure
Giant Gain Enhancement in Surface-Confined Resonant Stimulated Brillouin Scattering
The notion that Stimulated Brillouin Scattering (SBS) is primarily defined by
bulk material properties has been overturned by recent work on nanoscale
waveguides. It is now understood that boundary forces of radiation pressure and
electrostriction appearing in such highly confined waveguides can make a
significant contribution to the Brillouin gain. Here, this concept is extended
to show that gain enhancement does not require nanoscale or subwavelength
features, but generally appears where optical and acoustic fields are
simultaneously confined near a free surface or material interface. This
situation routinely occurs in whispering gallery resonators (WGRs), making gain
enhancements much more accessible than previously thought. To illustrate this
concept, the first full-vectorial analytic model for SBS in WGRs is developed,
including optical boundary forces, and the SBS gain in common silica WGR
geometries is computationally evaluated. These results predict that gains
times greater than the predictions of scalar theory may appear in WGRs
even in the 100 um size range. Further, trapezoidal cross-section microdisks
can exhibit very large SBS gains approaching mW. With
resonant amplification included, extreme gains on the order of
mW may be realized, which is times greater than the
highest predicted gains in linear waveguide systems.Comment: 31 pages, 12 figure
Breaking time-reversal symmetry with acoustic pumping of nanophotonic circuits
Achieving non-reciprocal light propagation via stimuli that break
time-reversal symmetry, without magneto-optics, remains a major challenge for
integrated nanophotonic devices. Recently, optomechanical microsystems in which
light and vibrational modes are coupled through ponderomotive forces, have
demonstrated strong non-reciprocal effects through a variety of techniques, but
always using optical pumping. None of these approaches have demonstrated
bandwidth exceeding that of the mechanical system, and all of them require
optical power, which are both fundamental and practical issues. Here we resolve
both of these challenges through breaking of time-reversal symmetry using an
acoustic pump in an integrated nanophotonic circuit. GHz-bandwidth
optomechanical non-reciprocity is demonstrated using the action of a
2-dimensional surface acoustic wave pump, that simultaneously provides non-zero
overlap integral for light-sound interaction and also satisfies the necessary
phase-matching. We use this technique to produce a simple frequency shifting
isolator (i.e. a non-reciprocal modulator) by means of indirect interband
scattering. We demonstrate mode conversion asymmetry up to 15 dB, efficiency as
high as 17%, over bandwidth exceeding 1 GHz
Brillouin Cooling in a Linear Waveguide
Brillouin scattering is not usually considered as a mechanism that can cause
cooling of a material due to the thermodynamic dominance of Stokes scattering
in most practical systems. However, it has been shown in experiments on
resonators that net phonon annihilation through anti-Stokes Brillouin
scattering can be enabled by means of a suitable set of optical and acoustic
states. The cooling of traveling phonons in a linear waveguide, on the other
hand, could lead to the exciting future prospect of manipulating unidirectional
phonon fluxes and even the nonreciprocal transport of quantum information via
phonons. In this work, we present an analysis of the conditions under which
Brillouin cooling of phonons of both low and high group velocities may be
achieved in a linear waveguide. We analyze the three-wave mixing interaction
between the optical and traveling acoustic modes that participate in forward
Brillouin scattering, and reveal the key regimes of operation for the process.
Our calculations indicate that measurable cooling may occur in a system having
phonons with spatial loss rate that is of the same order as the spatial optical
loss rate. If the Brillouin gain in such a waveguide reaches the order of
10 mW, appreciable cooling of phonon modes may be observed
with modest pump power of a few mW
Dynamic suppression of Rayleigh light scattering in dielectric resonators
The ultimate limits of performance for any classical optical system are set
by sub-wavelength fluctuations within the host material, that may be frozen-in
or even dynamically induced. The most common manifestation of such
sub-wavelength disorder is Rayleigh light scattering, which is observed in
nearly all wave-guiding technologies today and can lead to both irreversible
radiative losses as well as undesirable intermodal coupling. While it has been
shown that backscattering from disorder can be suppressed by breaking
time-reversal symmetry in magneto-optic and topological insulator materials,
common optical dielectrics possess neither of these properties. Here we
demonstrate an optomechanical approach for dynamically suppressing Rayleigh
backscattering within dielectric resonators. We achieve this by locally
breaking time-reversal symmetry in a silica resonator through a Brillouin
scattering interaction that is available in all materials. Near-complete
suppression of Rayleigh backscattering is experimentally confirmed through
three independent measurements -- the reduction of the back-reflections caused
by scatterers, the elimination of a commonly seen normal-mode splitting effect,
and by measurement of the reduction in intrinsic optical loss. More broadly,
our results provide new evidence that it is possible to dynamically suppress
Rayleigh backscattering within any optical dielectric medium, for achieving
robust light propagation in nanophotonic devices in spite of the presence of
scatterers or defects.Comment: 14 pages, 3 figures with supplementary informatio
Direction reconfigurable non-reciprocal acousto-optic modulator on chip
Non-reciprocal components are essential in photonic systems for protecting
light sources and for signal routing functions. Acousto-optic methods to
produce non-reciprocal devices offer a foundry-compatible alternative to
magneto-optic solutions and are especially important for photonic integration.
In this paper, we experimentally demonstrate a dynamically reconfigurable
non-reciprocal acousto-optic modulator at telecom wavelength. The modulator can
be arranged in a multitude of reciprocal and non-reciprocal configurations by
means of an external RF input. The dynamic reconfigurability of the device is
enabled by a new cross-finger interdigitated piezoelectric transducer that can
change the directionality of the reciprocity-breaking acoustic excitation based
on the phase of the RF input. The methodology we demonstrate here may enable
new avenues for direction dependent signal processing and optical isolation
Non-Reciprocal Brillouin Scattering Induced Transparency
Electromagnetically induced transparency (EIT) provides a powerful mechanism
for controlling light propagation in a dielectric medium, and for producing
slow and fast light. EIT traditionally arises from destructive interference
induced by a nonradiative coherence in an atomic system. Stimulated Brillouin
scattering (SBS) of light from propagating hypersonic acoustic waves has also
been used successfully for the generation of slow and fast light. However,
EIT-type processes based on SBS were considered infeasible because of the short
coherence lifetime of hypersonic phonons. Here, we report a new Brillouin
scattering induced transparency (BSIT) phenomenon generated by acousto-optic
interaction of light with long-lived propagating phonons. We demonstrate that
BSIT is uniquely non-reciprocal due to the propagating acoustic phonon wave and
accompanying momentum conservation requirement. Using a silica microresonator
having naturally occurring forward-SBS phase-matched modal configuration, we
show that BSIT enables compact and ultralow-power slow-light generation with
delay-bandwidth product comparable to state-of-the-art SBS systems.Comment: 26 pages, 12 figure
Demonstration of a quantized microwave quadrupole insulator with topologically protected corner states
The modern theory of electric polarization in crystals associates the dipole
moment of an insulator with a Berry phase of its electronic ground state [1,
2]. This concept constituted a breakthrough that not only solved the
long-standing puzzle of how to calculate dipole moments in crystals, but also
lies at the core of the theory of topological band structures in insulators and
superconductors, including the quantum anomalous Hall insulator [3, 4] and the
quantum spin Hall insulator [5-7], as well as quantized adiabatic pumping
processes [8-10]. A recent theoretical proposal extended the Berry phase
framework to account for higher electric multipole moments [11], revealing the
existence of topological phases that have not previously been observed. Here we
demonstrate the first member of this predicted class -a quantized quadrupole
topological insulator- experimentally produced using a GHz-frequency
reconfigurable microwave circuit. We confirm the non-trivial topological phase
through both spectroscopic measurements, as well as with the identification of
corner states that are manifested as a result of the bulk topology. We
additionally test a critical prediction that these corner states are protected
by the topology of the bulk, and not due to surface artifacts, by deforming the
edge between the topological and trivial regimes. Our results provide
conclusive evidence of a unique form of robustness which has never previously
been observed.Comment: Main Text: 14 pages, 4 figures. Supplementary Information: 6 pages, 3
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