Cavity optomechanics with photonic bound states in the continuum

We propose a versatile, free-space cavity optomechanics platform built from two photonic crystal membranes, one of which is freely suspended, and designed to form a microcavity less than one wavelength long. This cavity features a series of photonic bound states in the continuum that, in principle, trap light forever and can be favorably used together with evanescent coupling for realizing various types of optomechanical couplings, such as linear or quadratic coupling of either dispersive or dissipative type, by tuning the photonic crystal patterning and cavity length. Crucially, this platform allows for a quantum cooperativity exceeding unity in the ultrastrong single-photon coupling regime, surpassing the performance of conventional Fabry-Perot-based cavity optomechanical devices in the nonresolved sideband regime. This platform allows for exploring new regimes of the optomechanical interaction, in particular in the framework of pulsed and single-photon optomechanics

Micromechanical high-Q trampoline resonators from strained crystalline InGaP for integrated free-space optomechanics

Tensile-strained materials have been used to fabricate nano- and micromechanical resonators with ultra-low mechanical dissipation in the kHz to MHz frequency range. These mechanical resonators are of particular interest for force sensing applications and quantum optomechanics at room temperature. Tensile-strained crystalline materials that are compatible with epitaxial growth of heterostructures would thereby allow realizing monolithic free-space optomechanical devices, which benefit from stability, ultra-small mode volumes and scalability. In our work, we demonstrate micromechanical resonators made from tensile-strained InGaP, which is a crystalline material that can be epitaxially grown on III-V heterostructures. The strain of the InGaP layer is defined via its Ga content when grown on (Al,Ga)As. In our case we realize devices with a stress of up to 470\,MPa along the $[1\,1\,0]$ crystal direction. We characterize the mechanical properties of the suspended InGaP devices, such as anisotropic stress, anisotropic yield strength, and intrinsic quality factor. We find that the latter degrades over time. We reach mechanical quality factors surpassing $10^7$ at room temperature with a $Q\cdot f$-product as high as $7\cdot10^{11}$ with trampoline-shaped micromechanical resonators, which exploit strain engineering to dilute mechanical dissipation. The large area of the suspended trampoline resonator allows us to pattern a photonic crystal to engineer its out-of-plane reflectivity in the telecom band, which is desired for efficient signal transduction of mechanical motion to light. Stabilization of the intrinsic quality factor together with a further reduction of mechanical dissipation through hierarchical clamping or machine learning-based optimization methods paves the way for integrated free-space quantum optomechanics at room temperature in a crystalline material platform.Comment: fixed typos, added more material on crystal anisotropy and on fabrication; 15 pages incl. appendix, 13 figure

Optomechanical cooling with coherent and squeezed light: the thermodynamic cost of opening the heat valve

Ground-state cooling of mechanical motion by coupling to a driven optical cavity has been demonstrated in various optomechanical systems. In our work, we provide a so far missing thermodynamic performance analysis of optomechanical sideband cooling in terms of a heat valve. As performance quantifiers, we examine not only the lowest reachable effective temperature (phonon number) but also the evacuated-heat flow as an equivalent to the cooling power of a standard refrigerator, as well as appropriate thermodynamic efficiencies, which all can be experimentally inferred from measurements of the cavity output light field. Importantly, in addition to the standard optomechanical setup fed by coherent light, we investigate two recent alternative setups for achieving ground-state cooling: replacing the coherent laser drive by squeezed light or using a cavity with a frequency-dependent (Fano) mirror. We study the dynamics of these setups within and beyond the weak-coupling limit and give concrete examples based on parameters of existing experimental systems. By applying our thermodynamic framework, we gain detailed insights into these three different optomechanical cooling setups, allowing a comprehensive understanding of the thermodynamic mechanisms at play.Comment: 28 pages, 14 figures, 2 tables Small revision of the main text, corrected typos in the appendices, added study of the stability of the systems and comparison with absorption refrigerators in appendi

Integrated microcavity optomechanics with a suspended photonic crystal mirror above a distributed Bragg reflector

Increasing the interaction between light and mechanical resonators is an ongoing endeavor in the field of cavity optomechanics. Optical microcavities allow for boosting the interaction strength through their strong spatial confinement of the optical field. In this work, we follow this approach by realizing a sub-wavelength-long, free-space optomechanical microcavity on-chip fabricated from an (Al,Ga)As heterostructure. A suspended GaAs photonic crystal mirror is acting as a highly reflective mechanical resonator, which together with a distributed Bragg reflector forms an optomechanical microcavity. We demonstrate precise control over the microcavity resonance by change of the photonic crystal parameters. The interplay between the microcavity mode and a guided resonance of the photonic crystal modifies the cavity response and results in a stronger dynamical backaction on the mechanical resonator compared to conventional optomechanical dynamics.Comment: 11 pages, 6 figures + Supplementary Material with 10 pages, 12 figures, 2 table

Suspended photonic crystal membranes in AlGaAs heterostructures for integrated multi-element optomechanics

We present high-reflectivity mechanical resonators fabricated from AlGaAs heterostructures for use in free-space optical cavities operating in the telecom wavelength regime. The mechanical resonators are fabricated in slabs of GaAs and patterned with a photonic crystal to increase their out-of-plane reflectivity. Characterization of the mechanical modes reveals residual tensile stress in the GaAs device layer. This stress results in higher mechanical frequencies than in unstressed GaAs and can be used for strain engineering of mechanical dissipation. Simultaneously, we find that the finite waist of the incident optical beam leads to a dip in the reflectance spectrum. This feature originates from coupling to a guided resonance of the photonic crystal, an effect that must be taken into account when designing slabs of finite size. The single- and sub-\upmum-spaced double-layer slabs demonstrated here can be directly fabricated on top of a distributed Bragg reflector mirror in the same material platform. Such a platform opens a route for realizing integrated multi-element cavity optomechanical devices and optomechanical microcavities on chip.Comment: close to published version, 4+9 pages, 6+11 figure

Free-space cavity optomechanical systems on a chip with III-V heterostructures

Cavity optomechanics examines the mutual interaction between light and mechanical motion for controlling mechanical resonators down to the quantum regime. A major challenge in the field of cavity optomechanics remains accessing a strong interaction between the light field and mechanics on the level of single quanta. Optomechanical systems with small mode volumes show considerable enhancement in the interaction strength. However, in a majority of these systems the increase in the interaction strength comes at the cost of additional optical losses. Therefore, one cannot exploit the novel capabilities of such systems often.This thesis is about the development of a monolithic cavity optomechanical platform using III-V materials which demonstrates a pathway to combine a free-space optical cavity with an integrated mechanical system. To this end, we showcase the design, fabrication and characterization of optomechanical microresonators in AlGaAs/InGaP heterostructures. We demonstrate the enhancement of the out-of-plane reflectivity by reflectance engineering using photonic crystals. We utilize the features of III-V heterostructures by realizing monolithic fully-suspended micromechanical resonator arrays with sub-\ub5m gap in GaAs. This would enable the possibility of enhancing the optomechanical interaction using the concept of multi-element optomechanics. We explore integrated cavity optomechanical systems formed by two photonic crystals reflectors and by a photonic crystal reflector with an integrated distributed Bragg reflector mirror. Furthermore, we propose the use of highly-frequency dependent photonic crystal reflectors in the optomechanical system for realizing photonic bound states in a continuum, which decouple the otherwise coupled cavity loss rates and coupling strength.The quality factor of the mechanical resonator can be increased by using tensile-strained InGaP which is compatible with AlGaAs heterostructures growth. We determine the material properties of InGaP relevant for mechanical resonators. We demonstrate quality factors of 10^7 in trampoline resonators in InGaP at room temperature. The quality factor is pressure limited and can be enhanced using strain engineering. Free-space integrated multi-element cavity optomechanical systems in III-V heterostructures have the potential to enter the quantum optomechanics regime at room temperature

Nanophotonic Structures for Cavity Optomechanics

In this contribution, we will show how nanophotonic structures can be used to gain access to previously inaccessible regimes in cavity optomechanics [1]. We introduce a novel optomechanics platform, built from two photonic crystal membranes [2] , one of which is freely suspended (see Fig. 1 ). This cavity supports a series of photonic bound states in the continuum (BICs) that, in principle, trap light forever [3] and can be favourably used together with evanescent coupling for realizing various types of optomechanical couplings, such as linear or quadratic coupling of either dispersive or dissipative type, by tuning the photonic crystal patterning and cavity length. By combining light propagation in both free-space (between the photonic-crystal membranes) and guided-mode (over the photonic-crystal membranes) form, our platform merges the strengths offered by in-plane and out-of-plane optomechanical systems

Double layer photonic crystal membranes in AlGaAs heterostructures for integrated cavity optomechanics

We characterize the opto-mechanical properties of double-layer mechanical devices. These closely spaced photonic crystal membranes can exhibit photonic bound states in the continuum, which could enable the realization of a strongly coupled, integrated optomechanical system

Integrated free-space optomechanics with AlGaAs heterostructures

We fabricated and characterized suspended bi-layered photonic crystal slabs in AlGaAs heterostructures. Our approach allows to create integrated, closely spaced membranes, which can exhibit photonic bound states in the continuum to increase light-matter interaction