Optomechanical Research Articles

Strong Cavity-Optomechanical Transduction of Nanopillar Motion.

Nanomechanical resonators can serve as ultrasensitive, miniaturized force probes. While vertical structures such as nanopillars are ideal for this purpose, transducing their motion is challenging. Pillar-based photonic crystals (PhCs) offer a potential solution by integrating optical transduction within the pillars. However, achieving high-quality PhCs is hindered by inefficient vertical light confinement. Here, we present a full-silicon photonic crystal cavity based on nanopillars as a platform for applications in force sensing and biosensing areas. Its unit cell consists of a silicon pillar with a larger diameter at its top portion than at the bottom, which allows vertical light confinement and an energy band gap in the near-infrared range for transverse-magnetic polarization. We experimentally demonstrate optical cavities with Q factors exceeding 103, constructed by inserting a defect within a periodic arrangement of this type of pillars. Each nanopillar naturally behaves as a nanomechanical cantilever, making the fabricated geometries excellent optomechanical (OM) photonic crystal cavities in which the mechanical motion of each nanopillar composing the cavity can be optically transduced. These geometries display enhanced mechanical properties, cost-effectiveness, integration possibilities, and scalability. They also present an alternative in front of the widely used suspended Si beam OM cavities made on silicon-on-insulator substrates.

Opto-mechanical self-adjustment model of the human eye.

The eye has specific optical and biomechanical properties that jointly regulate the eye's quality of vision, shape, and elasticity. These two characteristics are interdependent and correlated. Contrary to most currently available computational models of the human eye that only focus on biomechanical or optical aspects, the current study explores the inter-relationships between biomechanics, structure, and optical properties. Possible combinations of mechanical properties, boundary conditions, and biometrics were specified to ensure the opto-mechanical (OM) integrity to compensate for physiological changes in intraocular pressure (IOP) without compromising image acuity. This study evaluated the quality of the vision by analyzing the minimum spot diameters formed on the retina and drew how the self-adjustment mechanism affects the eye globe shape by adopting a finite element (FE) model of the eyeball. The model was verified by a water drinking test with biometric measurement (OCT Revo NX, Optopol) and tonometry (Corvis ST, Oculus).

Optomechanical isolation with tunable center frequency

Optomechanical (OM) isolators have the ability to stop the optical field for one direction and to pass the field in the opposite direction. Thus they work as bandstop filter in one direction and bandpass filter in the opposite direction. The maximum isolation happens when the frequency of the field becomes equal to that of the mechanical membrane inside the isolator. This frequency, called the isolation (or center) frequency, however is restricted by the fact that the fundamental frequency of the membrane depends upon its physical parameters (such as mass and length), which are fixed during fabrication of the membrane. We propose a dynamical way of tuning this center frequency, by using a mechanical drive attached to a membrane with quadratic OM coupling in a membrane-in-the-middle setup. This provides a way for future applications where tunable filtering is required.

Foundry-processed optomechanical photonic integrated circuits: publisher’s note

This publisher’s note amends the author list of [OSA Continuum 4, 1215 (2021)10.1364/OSAC.419410].

Squeezed light generation in cascaded optomechanical systems

We theoretically investigate the enhancement of the degree and bandwidth of squeezed states of light generated in cascaded optomechanical (OM) systems. With the obtained recursion relation of the generalized output quadratures, it is possible to realize cascaded OM systems operated simultaneously in a deamplification situation for the same quadrature with a frequency-dependent phase shift between cascaded systems. Due to the cumulative OM interaction, the degree of squeezing can be significantly improved and nearly independent of frequencies. Thus the squeezing frequency band is notably broadened. Moreover, the squeezing bandwidth can be further broadened through tuning the detunings between laser and cavity frequencies. Finally, the influence of optical losses on squeezing due to the inefficient transmission between cascaded systems is also taken into account, which degrades the squeezing via introducing uncorrelated vacuum noises. However, a better squeezed state is still achievable compared to that generated in single OM system.

Displacement sensing beyond the standard quantum limit with intensity-dependent optomechanical coupling

We study an optomechanical (OM) system with optical and mechanical modes interacting through a dispersive linear and quadratic optomechanical coupling (QOC) in an unresolved sideband limit. The presence of a QOC plays a crucial role in altering the effective OM interaction and the spring constant of the mechanical resonator. The choice of a positive QOC in the system increases the spring constant, inducing a `stiffer mode.' The existence of this stiffer mode leads to unconventional behavior in the total displacement noise consisting of imprecision, backaction, and thermal noises. We further show that unlike the conventional OM system wherein the optimal total displacement noise is limited by the imprecision and backaction noises, it is due to the competing imprecision and thermal noise, making it feasible to observe displacement sensing below the conventional standard quantum limit.

Spatial localization and pattern formation in discrete optomechanical cavities and arrays

We investigate theoretically the generation of nonlinear dissipative structures in optomechanical (OM) systems containing discrete arrays of mechanical resonators. We consider both hybrid models in which the optical system is a continuous multimode field, as it would happen in an OM cavity containing an array of micro-mirrors, and also fully discrete models in which each mechanical resonator interacts with a single optical mode, making contact with Ludwig and Marquardt (2013 Phys. Rev. Lett. 101, 073603). Also, we study the connections between both types of models and continuous OM models. While all three types of models merge naturally in the limit of a large number of densely distributed mechanical resonators, we show that the spatial localization and the pattern formation found in continuous OM models can still be observed for a small number of mechanical elements, even in the presence of finite-size effects, which we discuss. This opens new venues for experimental approaches to the subject.

Microwave oscillator and frequency comb in a silicon optomechanical cavity with a full phononic bandgap

Abstract Cavity optomechanics has recently emerged as a new paradigm enabling the manipulation of mechanical motion via optical fields tightly confined in deformable cavities. When driving an optomechanical (OM) crystal cavity with a laser blue-detuned with respect to the optical resonance, the mechanical motion is amplified, ultimately resulting in phonon lasing at MHz and even GHz frequencies. In this work, we show that a silicon OM crystal cavity performs as an OM microwave oscillator when pumped above the threshold for self-sustained OM oscillations. To this end, we use an OM cavity designed to have a breathing-like mechanical mode at 3.897 GHz in a full phononic bandgap. Our measurements show that the first harmonic of the detected signal displays a phase noise of ≈−100 dBc/Hz at 100 kHz. Stronger blue-detuned driving leads eventually to the formation of an OM frequency comb, whose lines are spaced by the mechanical frequency. We also measure the phase noise for higher-order harmonics and show that, unlike in Brillouin oscillators, the noise is increased as corresponding to classical harmonic mixing. Finally, we present real-time measurements of the comb waveform and show that it can be fitted to a theoretical model recently presented. Our results suggest that silicon OM cavities could be relevant processing elements in microwave photonics and optical RF processing, in particular in disciplines requiring low weight, compactness and fiber interconnection.

Room-temperature optomechanical squeezing

One of the noise sources that currently limits gravitational wave (GW) detectors comes from the quantum nature of the light causing uncertain amplitude and phase. Phase uncertainty limits the precision of an interferometric measurement. This measurement is also subject to quantum back-action, caused by the radiation pressure force fluctuations produced by the amplitude uncertainty (QRPN). In order to lower this quantum noise, GW detectors plan to use squeezed light injection. In this thesis, I focus on using radiation-pressure-mediated optomechanical (OM) interaction to generate squeezed light. Creating squeezed states by using OM interaction enables wavelength-independent squeezed light sources that may also be more compact and robust than traditionally used non-linear crystals. We analyze the system with realistic imperfections (losses & classical noise), and use the concepts to design an experiment to obtain the most possible squeezing in a broad audio-frequency band at room temperature. This involves an optimization for the optical properties of the cavity and the mechanical properties of the oscillator. We then show its experimental implementation, and subsequent observation of QRPN as well as OM squeezing. These are the first ever direct observations of a room temperature oscillator's motion being overwhelmed by vacuum fluctuations. This is shown in the low frequency band, which is relevant to GW detectors, but poses its own technical challenges, and hence has not been done before. Being in the back-action dominated regime along with optimized optical properties has also enabled us to observe OM squeezing. That is the first direct observation of quantum noise suppression in a room temperature OM system. It is also the first direct evidence of quantum correlations in the audio frequency band, in a broad band at non-resonant frequencies.

Force sensing beyond standard quantum limit with optomechanical "soft" mode induced by nonlinear interaction.

We consider an optomechanical (OM) system that consists of a mechanical and an optical mode interacting through linear and quadratic OM dispersive couplings. The system is operated in unresolved sideband limit with a high quality factor mechanical resonator. Such a system acts as a parametrically driven oscillator, giving access to an intensity-assisted tunability of the spring constant. This enables the operation of the OM system in its "soft mode," wherein the mechanical spring softens and responds with a lower resonance frequency. We show that this soft mode can be exploited to nonlinearize backaction noise, which yields higher force sensitivity beyond the conventional standard quantum limit.

Quantum state transfer via acoustic edge states in a 2D optomechanical array

We propose a novel hybrid platform where solid-state spin qubits are coupled to the acoustic modes of a two-dimensional array of optomechanical (OM) nano cavities. Previous studies of coupled OM cavities have shown that in the presence of strong optical driving fields, the interplay between the photon-phonon interaction and their respective inter-cavity hopping allows the generation of topological phases of sound and light. In particular, the mechanical modes can enter a Chern insulator phase where the time-reversal symmetry is broken. In this context, we exploit the robust acoustic edge states as a chiral phononic waveguide and describe a state transfer protocol between spin qubits located in distant cavities. We analyze the performance of this protocol as a function of the relevant system parameters and show that a high-fidelity and purely unidirectional quantum state transfer can be implemented under experimentally realistic conditions. As a specific example, we discuss the implementation of such topological quantum networks in diamond based OM crystals where point defects such as silicon-vacancy centers couple to the chiral acoustic channel via strain.

Strong optomechanical coupling of light and highly confined acoustic phonons in slot dual-beam phoxonic crystal cavities

In this study, strong optomechanical (OM) coupling is proposed through the introduction of a slot in a dual-beam phoxonic crystal cavity structure. The structure can support a confined optical slot mode and localized phononic cavity modes to interact effectively through the slot surfaces. In addition to using conventional OM coupling rates for evaluating the coupling strength of photonic and phononic modes, we consider the optical forces induced by the optical slot mode to discuss the coupling between different mode pairs and the underlying mechanisms of the strong OM coupling. The optical field can induce radiation pressure and electrostrictive forces in the structure. We demonstrate that the radiation pressure dominates the coupling enhancement of photonic and phononic modes because of the slot, whereas the electrostrictive surface pressure plays a minor role and the electrostrictive body force has a negligible contribution. On the basis of the optical forces, we can then calculate the acoustic phonon spectrum through optical excitation. The spectrum indicates the appearance of strong OM coupling in the additional phononic cavity modes. The results suggest that the slot dual-beam cavity structure can be a promising choice for tailoring effective optical forces in micro- and nano-optomechanical systems for enhancing OM coupling.

Programmable Thermal Metamaterials Based on Optomechanical Systems

Programmable Thermal Metamaterials Based on Optomechanical Systems

Mimicking a hybrid-optomechanical system using an intrinsic quadratic coupling in conventional optomechanical system

ABSTRACTWe consider an optical and mechanical mode interacting through both linear and quadratic dispersive couplings in a general cavity-optomechanical set-up. The parity and strength of an intrinsic quadratic optomechanical coupling (QOC) provides an opportunity to control the optomechanical (OM) interaction. We quantify this interaction by studying normal-mode splitting (NMS) as a function of the QOC's strength. The proposed scheme exhibits NMS features equivalent to a hybrid-OM system containing either an optical parametric amplifier or a Kerr medium. Such a system in reality could offer an alternative platform for devising state-of-art quantum devices with requiring no extra degrees-of-freedom as in hybrid-OM systems.

Radiation-pressure-mediated control of an optomechanical cavity

We describe and demonstrate a method to control a detuned movable-mirror Fabry-Perot cavity using radiation pressure in the presence of a strong optical spring. At frequencies below the optical spring resonance, self-locking of the cavity is achieved intrinsically by the optomechanical (OM) interaction between the cavity field and the movable end mirror. The OM interaction results in a high rigidity and reduced susceptibility of the mirror to external forces. However, due to a finite delay time in the cavity, this enhanced rigidity is accompanied by an anti-damping force, which destabilizes the cavity. The cavity is stabilized by applying external feedback in a frequency band around the optical spring resonance. The error signal is sensed in the amplitude quadrature of the transmitted beam with a photodetector. An amplitude modulator in the input path to the cavity modulates the light intensity to provide the stabilizing radiation pressure force.

Higher-order quantum dynamics of a generic quadratically-coupled optomechanical system: entanglement, antibunching and spin squeezing

Higher-order quantum dynamics of a generic quadratically-coupled optomechanical system: entanglement, antibunching and spin squeezing

Anderson localization of composite excitations in disordered optomechanical arrays

Optomechanical (OMA) arrays are a promising future platform for studies of transport, many-body dynamics, quantum control and topological effects in systems of coupled photon and phonon modes. We introduce disordered OMA arrays, focusing on features of Anderson localization of hybrid photon–phonon excitations. It turns out that these represent a unique disordered system, where basic parameters can be easily controlled by varying the frequency and the amplitude of an external laser field. We show that the two-species setting leads to a non-trivial frequency dependence of the localization length for intermediate laser intensities. This could serve as a convincing evidence of localization in a non-equilibrium dissipative situation.

Optomechanical coupling in phoxonic–plasmonic slab cavities with periodic metal strips

We theoretically investigate the optomechanical (OM) coupling of submicron cavities formed in one-dimensional phoxonic–plasmonic slabs. The phoxonic–plasmonic slabs are structured by depositing periodic Ag strips onto the top surfaces of dielectric GaAs slabs to produce dual band gaps for both electromagnetic and acoustic waves, thereby inducing the coupling of surface plasmons with photons for tailoring the OM coupling. We quantify the OM coupling by calculating the temporal modulation of the optical resonance wavelength with the acoustic phonon-induced photoelastic (PE) and moving-boundary (MB) effects. We also consider the appearance of a uniform Ag layer on the bottom surface of the slabs to modulate the photonic–plasmonic coupling. The results show that the PE and MB effects can be constructive or destructive in the overall OM coupling, and their magnitudes depend not only on the quality factors of the resonant modes but also on the mode area, mode overlap, and individual symmetries of the photonic–phononic mode pairs. Lowering the mode area could be effective for enhancing the OM coupling of subwavelength photons and phonons. This study introduces possible engineering applications to achieve enhanced interaction between photons and phonons in nanoscale OM devices.

Focus on optomechanics

We provide a brief overview of the various topics addressed in this ‘focus on’ collection on optomechanics.

A one-dimensional optomechanical crystal with a complete phononic band gap.

Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections.