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arXiv:2610.31452

Nanopillar metasurfaces uniting high optical reflectivity with ultracoherent nanomechanics

Alexander Jung, Anastasiia Ciers, Hannes Pfeifer, Albert Hansson, Johanna Örgård, Daniel Stachanow, Viktor A. Lilja, Philippe Tassin, Witlef Wieczorek

physics.opticsphysics.app-phquant-phSubmitted Oct 22, 2026• Announced Oct 23, 2026
arXivHTMLPDF

Abstract

Mechanically compliant mirrors are key components in cavity optomechanics. However, the requirements to achieve the best optical and mechanical performance are usually in direct competition. A high reflectivity for tightly focused beams entails thick reflectors, whereas ultracoherent flexural nanomechanics necessitates thin membranes. We eliminate this trade-off by introducing high-refractive-index nanopillar metasurfaces on top of a thin membrane. We pattern a 50 micrometer-diameter array of nanopillars to form a photonic crystal. The flat band behavior of guided resonances in the photonic crystal enables a reflectivity of 99.82(1)% for a tightly focused beam of 7 micrometer waist. The combination of two guided resonances creates a 100 nanometer-wide high-reflectivity band around 1550 nanometer. Simultaneously, the nanopillar patterning realizes a density-modulated phononic crystal that isolates an ultracoherent localized mechanical mode with a $Q\cdot f$-product of $2\cdot 10^{14}$Hz. The demonstrated nanopillar metasurfaces enable lightweight mirrors for room-temperature quantum optomechanics and for use as levitated mirrors or lightsails.

Comments
9 pages, 4 figures; incl supporting information of 29 pages

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•Aug 29, 2019•0 citations
Mechanically compliant mirrors are key components in cavity optomechanics. However, the requirements to achieve the best optical and mechanical performance are usually in direct competition. A high reflectivity for tightly focused beams entails thick reflectors, whereas ultracoherent flexural nanomechanics necessitates thin membranes. We eliminate this trade-off by introducing high-refractive-index nanopillar metasurfaces on top of a thin membrane. We pattern a 50 micrometer-diameter array of nanopillars to form a photonic crystal. The flat band behavior of guided resonances in the photonic crystal enables a reflectivity of 99.82(1)% for a tightly focused beam of 7 micrometer waist. The combination of two guided resonances creates a 100 nanometer-wide high-reflectivity band around 1550 nanometer. Simultaneously, the nanopillar patterning realizes a density-modulated phononic crystal that isolates an ultracoherent localized mechanical mode with a Q⋅fQ\cdot fQ⋅f-product of 2⋅10142\cdot 10^{14}2⋅1014Hz. The demonstrated nanopillar metasurfaces enable lightweight mirrors for room-temperature quantum optomechanics and for use as levitated mirrors or lightsails.
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The motion of a micromechanical resonator coupled simultaneously to a high-finesse optical cavity and a microwave resonator can be used to transduce the fragile quantum states generated in the realm of microwave superconducting circuits into optical photons that can transport quantum information over large distances in telecom optical fibers at room temperature. We have engineered the optical and mechanical properties of thin membranes patterned in a suspended high-stress Si3N4 film to realize such a hybrid opto-electro-mechanical system. An optical Fabry-Perot cavity with a finesse exceeding 12 000 was realized between a highly reflective photonic crystal patch realized on the suspended membrane and a conventional high-reflectivity mirror. The electromechanical coupling with a nearby planar superconducting circuit is achieved by approaching the membrane at a sub-micron distance from an on-chip interdigitated capacitor. By operating the hybrid opto-electro-mechanical system in a cryogenic environment, the photon-phonon swap rates between microwave, mechanical, and optical degrees of freedom is expected to overcome the mechanical decoherence rate.
Optomechanical systems provide a versatile platform for precision measurements and investigations of fundamental physics, where bringing macroscopic resonators into the quantum regime is a widely pursued goal. Achieving such quantum behavior of solid-state mechanical resonators at room temperature would greatly broaden their applications by removing the need for cryogenic environments. Reaching this goal requires efficient cooling of mechanical motion, among various laser cooling methods, dynamical backaction cooling (DBC) is widely utilized in experiments but fundamentally limited when operating in the sideband-unresolved regime. Coherent feedback cooling (CFC) can overcome this limitation, while avoiding state collapse and the electronic restrictions inherent to measurement-based feedback. Here, we experimentally demonstrate CFC using an ultracoherent density phononic crystal membrane. By combining CFC with strong DBC in a relatively narrow cavity, we achieve a phonon occupation reduction from 5.5×1065.5\times10^{6}5.5×106 to 166±7166\pm7166±7, corresponding to a cooling factor of 3.3×1043.3\times10^{4}3.3×104 at room temperature, even with current experimental limitations. Our results show the potential of CFC for approaching the ground state of high-QQQ membranes at room temperature.
Summary form only given. The motion of a micromechanical resonator coupled simultaneously to a high-finesse optical cavity and a microwave resonator can be used to transduce the fragile quantum states generated in the realm of microwave superconducting circuits into optical photons that can transport quantum information over large distances in telecom optical fibers at room temperature. We have engineered the optical and mechanical properties of thin membranes patterned in a suspended high-stress Si3N4 film to realize such a hybrid opto-electro-mechanical system. An optical Fabry-Perot cavity with a finesse exceeding 12 000 was realized between a highly reflective photonic crystal patch realized on the suspended membrane and a conventional high-reflectivity mirror. The electromechanical coupling with a nearby planar superconducting circuit is achieved by approaching the membrane at a sub-micron distance from an on-chip interdigitated capacitor. By operating the hybrid opto-electro-mechanical system in a cryogenic environment, the photon-phonon swap rates between microwave, mechanical, and optical degrees of freedom is expected to overcome the mechanical decoherence rate. An artist view of the device envisioned is depicted on Figure 1.
Interactions between light and mechanics provide a powerful interface between optical and microwave-frequency signals, with applications spanning classical signal processing and quantum technologies. High-performance optomechanical devices require both strong photon-phonon coupling and tolerance to parasitic laser heating. Release-free optomechanical crystals provide improved thermal anchoring compared to suspended nanobeams, but have so far exhibited weaker vacuum optomechanical coupling rates, leaving a trade-off between coupling strength and thermal robustness. Here, we largely close this gap: we design and experimentally demonstrate a release-free silicon optomechanical crystal with a record vacuum optomechanical coupling rate of about gOM/(2π)=800g_\text{OM} / (2 π) = 800gOM​/(2π)=800 kHz, comparable to suspended state-of-the-art devices. The resulting optomechanical scattering rate ΓOM/(2π)=1.1Γ_\text{OM}/(2 π)= 1.1ΓOM​/(2π)=1.1 kHz is nearly twice that of previous release-free implementations. This performance is achieved by combining physics-guided human intuition with a multiphysics inverse-design algorithm introduced here for resonant optomechanical structures. Beyond the specific device demonstrated, the inverse-design framework is applicable to co-optimizing optical and mechanical resonances and eigenmodes more broadly. These results strengthen release-free optomechanical crystals as a platform for fast, low-noise classical and quantum optomechanics.
The focus of this thesis is to investigate and fabricate a platform that can facilitate the enhancement of light-matter interactions. By tightly confining the photons and atoms to the same region of space, the probability of interaction is drastically increased as compared to free space interactions. Specifically, we focus on forming an optical cavity (or resonator) incorporated into a hollow-core photonic crystal fiber (HCPCF) using two distinct types of reflective mirrors along the axis of the fiber while still permitting atoms to be loaded into the region of high field confinement. \n \nOne means of pursuing this goal that we explored was to propose and numerically simulate two methods for implementing Bragg gratings in a HCPCF. These two methods leave the hollow-core unobstructed and are both based on controlled selective injection of photosensitive polymers into the photonic-crystal region of the hollow-core fiber, followed by interference photolithography. We report the results of numerical simulations for the hollow core fiber with Bragg gratings formed by the two methods. We find that a reflectivity of > 99.99% should be achievable from such fiber-integrated mirrors. Such a device could support high cooperativity and strong coupling regimes to be achieved. \n \nWe also demonstrate a fiber-integrated Fabry-Pérot cavity formed by attaching a pair of dielectric metasurfaces to the ends of a hollow-core photonic-crystal fiber segment. The metasurfaces consist of perforated membranes designed as photonic-crystal slabs that act as planar mirrors but can potentially allow injection of gases through their holes into the hollow core of the fiber. We have so far observed cavities with finesse of 11 and Q-factors of ~ 4.5 × 10^5, but much higher values should be achievable with improved fabrication procedures. We expect this device to enable the advancement of new fiber lasers, enhanced gas spectroscopy, and studies of fundamental light-matter interactions and nonlinear optics. These mirrors can be designed to be polarization dichroic — transparent for one polarization and reflective for another. This unique property can be exploited to allow for all signals to be directed along the high optical depth axis of the cavity and may provide a excellent platform for applications such as optical switching. \n \nFinally, we develop a novel protocol for a single photon all-optical transistor and how it may be implemented in the above mentioned fiber cavity systems. This unique scheme utilizes a far off-resonant vacuum cavity mode to stimulate a Raman absorption process of a source photon which may be switched off by the insertion of a single gate photon into the cavity mode. Relatively high switching contrasts and ratios for the source photon transmission can be obtained in our system.