Cavity optomechanics

Résumé

Cavity optomechanics is a branch of physics which focuses on the interaction between light and mechanical objects on low-energy scales. It is a cross field of optics, quantum optics, solid-state physics and materials science. The motivation for research on cavity optomechanics comes from fundamental effects of quantum theory and gravity, as well as technological applications. The name of the field relates to the main effect of interest: the enhancement of radiation pressure interaction between light (photons) and matter using optical resonators (cavities). It first became relevant in the context of gravitational wave detection, since optomechanical effects must be taken into account in interferometric gravitational wave detectors. Furthermore, one may envision optomechanical structures to allow the realization of Schrödinger's cat. Macroscopic objects consisting of billions of atoms share collective degrees of freedom which may behave quantum mechanically (e.g. a sphere of micromete

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https://en.wikipedia.org/wiki/Cavity_optomechanics

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Integrated Gallium Phosphide Photonics

Katharina Schmeing

The integration of new materials mediating light-matter interaction in nanoscale devices is a persistent goal in nanophotonics. One of these materials is Gallium phosphide, which offers an attractive combination of a high refractive index (n=3.05 at a wavelength of 1550 nm) and a large bandgap (Eg =2.26 eV), enabling photonic devices with strongly confined light fields, not suffering from heating due to two-photon absorption at telecommunication wavelengths. Furthermore, due to its non-centrosymmetric crystal structure, it has a non-vanishing second-order susceptibility and is piezoelectric. Related to its large refractive index is a high third-order susceptibility. Prior to this work the use of GaP for photonic devices was limited to individual non-integrated components, as GaP was not available on a substrate with substantially lower refractive index equivalent to SOI-wafers for silicon. In this work a process was developed that allows the integration of GaP devices onto SiO2. It exploits direct wafer bonding of a GaP/AlxGa1-xP/GaP heterostructure onto a SiO2-on-Si wafer. After substrate removal, photonic devices are patterned by dry-etching in the top GaP device layer. The GaP devices investigated here are used to explore nonlinear optics and optomechanics. In the area of nonlinear optics, second- and third-harmonic generation are observed. The Kerr coefficient is experimentally estimated as n2[1550nm] = 1.2(5)x10^17m^2/W, for the first time in a precision measurement at telecommunication wavelengths. Four-wave mixing is used for broadband frequency comb generation, where a power threshold as low as 3 mW is obtained. The combination of four-wave mixing and second-harmonic generation leads to frequency-doubled combs. The optomechanical properties of GaP one-dimensional photonic crystal cavities are optimized by simulations and fabricated devices are characterized. Optical quality factors of Qo>10^5 and optomechanical coupling strengths of g0/2pi=400 kHz are measured. Dynamical backaction in the form of the spring effect and the parametric amplification are observed, as well as optomechanically induced transparency and absorption. A device design for a microwave-to-optical transducer is developed, relying on the piezoelectricity of GaP. It combines electromechanical and optomechanical transduction. The predicted electromechanical coupling strength is in the MHz range. Furthermore, photonic crystal cavity designs containing a slot at the center of the cavity are studied. According to simulations for slot widths below 30 nm, optomechanical coupling strengths g0/2pi>1 MHz could be achieved. Fabricated silicon photonic crystal cavities show high quality factors of Qo=8x10^4 while hosting a mechanical eigenmode with a frequency of 2.7 GHz. Because of process technology limitations, only slot widths as narrow as 40 nm can be fabricated, the achieved g0/2pi is limited to 300 kHz. The new GaP-on-insulator material platform opens the door to integrated GaP devices. Frequency combs are of interest for soliton comb formation, mid-IR frequency combs, and ultra-broadband supercontinuum generation. Microwave-to-optical transducers are on the one hand desired for quantum information processing, on the other hand they are applicable as efficient modulators or detectors for classical signals.

EPFL2019

Chargement

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017

Chargement

Thermal motion of a room-temperature mechanical resonator typically dominates the quantum backaction of its position measurement. This is a longstanding barrier for exploring cavity optomechanics at room temperature. In order to enter the quantum regime of the optomechanical interaction, we need to be in the "backaction-dominated" regime, where we can study the limits of quantum measurement and how to circumvent them. To create a system which can enter the quantum regime, the optomechanical transducer has to satisfy certain criteria. A quantum-enabled optomechanical system consists of an optical cavity and a mechanical resonator with ultra low quantum decoherence, which are strongly coupled to each other via the optomechanical interaction.High stress silicon nitride has enabled nanomechanical resonators with exceptionally low dissipation at room temperature via several soft-clamping techniques. Achieving high optomechanical coupling to these coherent nanobeams results in high optomechanical cooperativities, thus alleviating the thermal motion barrier for room temperature quantum optomechanics. Monolithic integration of high-Q nanobeam resonators with optical cavities has been limited to doubly-clamped nanobeams due to the complexity of the device fabrication and long device sizes required for conventional soft-clamping using phononic crystals. In addition, the previous demonstrations of such systems showed limited optomechanical couplings thus a limited single photon cooperativity.I present the design, fabrication and characterization of three different classes of nanomechanical resonators clamp-tapered, fractal-like, and polygon resonators, which support perimeter modes, with Q factors exceeding 3 billion at room temperature and their optical readout using an integrated nearfield nano-optomechanical transducer using high stress silicon nitride. Our transducer features a one-dimensional Fabry-Perot optical cavity integrated with a high-Q nanomechanical resonator. The Fabry-Perot optical cavity is formed by patterning two photonic crystal reflectors on a silicon nitride waveguide designed for high-Q optical modes. Our approach allows individual optimization of optical and mechanical resonators, while maintaining a high optomechanical coupling rate due to large optomechanical mode overlap. Our best performing devices show on-chip optomechanical transducers with single photon cooperativities as high as 123 with mechanical quality factor of 120 million at room temperature. The developed system is of great interest to the optomechanics and sensing community. In quantum optomechanics, it will serve as a platform for quantum feedback control of the nanomechanical resonators to achieve motional ground state of a macroscopic resonator and generation of squeezed light at room temperature. Owing to their record value mechanical quality factors, the room temperature force sensitivity of our highest Q perimeter modes is on par with atomic force microscopy cantilevers at millikelvin temperature.Complete documentation of a nanofabrication process is the key to its reproducibility. Process-specific details of the fabrication techniques are usually missing in journal publications. To bridge this gap, I developed NanoFab-net.org. An online open-access tool that allows sharing process-specific fabrication reports, extracting the metadata and obtaining a unique digital object identifier (DOI) for each report.

EPFL2022

Chargement

\sim\!10^4

EPFL2017

EPFL2022

Integrated Gallium Phosphide Photonics

Katharina Schmeing

EPFL2019