Excitonic Emission of Monolayer Semiconductors Near-Field Coupled to High-Q Microresonators

We present quantum yield measurements of single layer WSe2 (1L-WSe2) integrated with high-Q (Q > 106) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe2 in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor (Q > 106) below the bandgap of 1L-WSe2. The cavity quantum yield is QYc ≈ 10–3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe2 is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.

Mid-infrared surface plasmon resonance sensing applied to refractive index measurements of liquids

Sylvain Herminjard

There has been a very strong development of the sensors based on surface plasmon resonance during the last thirty years, mostly for biological and biomedical applications. If the first experiments in this field were carried out at the beginning of the 20t century, it would have been necessary to wait for the end of the sixties to see the first devices performing accurate measurements of the refractive indices of gas and liquid samples. Indeed, the surface plasmon resonance phenomenon allows to measure refractive index variations with a high sensitivity that is hardly obtained with other optical measurement techniques. The basic principle consists of exciting the free electrons of a metallic layer with light, which induces the formation of a surface wave called surface plasmon. This resonant phenomenon depends on the wavelength of the excitation light and the refractive indices of the metallic layer and the material located close to the metallic layer. A variation of the refractive index of the latter induces a variation of the resonance condition, which allows to realize optical sensors designed to measure determined samples. An appropriate calibration allows to set a relationship between the variation of the concentration of the sample and the measured refractive index value. Among the various parameters that describe the performances of the sensor, one of the most important is the sensitivity. It is possible to demonstrate that the excitation wavelength has an impact on the sensitivity of the system : the latter increases if the measured sample has an absorption peak at a wavelength close to the excitation wavelength. The main goal of this thesis is to explore this sensitivity enhancement technique based on the absorption properties of the probed medium. A proof-of-principle is experimentally demonstrated with an optical setup constituted of laser sources working in the visible range of the electromagnetic spectrum. However, many materials have absorption peaks located in the mid-infrared range due to their fundamental molecular vibrations. Therefore, the realization of a plasmonic sensor working in the mid-infrared could allow to measure these materials with an increased sensitivity. To the best of our knowledge, surface plasmon resonance based sensors have never been developed above the near-infrared. This thesis presents for the first time the design and realization of a plasmonic sensor working in the mid-infrared range able to measure refractive index variations of gas and liquids samples. Even if the optical sources, components and detectors designed for the mid-infrared are clearly different from the ones designed for the visible range, it has been possible to successfully realize two systems working in the mid-infrared range during this project. The first is able to perform gas measurements. As the excitation wavelength depends on the measured sample, it was necessary to select a laser source that had a wavelength located close to the absorption peak of the gas under measurement. For this purpose, we designed a plasmonic sensor working at a 4.4 µm wavelength able to measure carbon dioxide (CO2) absorbing at 4.3 µm. It has been demonstrated that the experimental sensitivity of this system is five times higher than the theoretical sensitivity obtained with a system working in the visible range at a 633 nm wavelength. The second has been conceived to measure liquid samples in order to measure variations of concentration of CO2 dissolved in water. If measurements of refractive index variations (without using the sensitivity enhancement technique) could be performed successfully with variations of concentration of alcohol in water, further work is necessary to get decisive results for CO2 dissolved in water. This project was carried out in collaboration with an industrial partner in order to take into account some industrial constraints during the development of the sensor. Therefore, particular care has been taken in order to design a system that is compact, mechanically robust, without any mechanical moving parts, cheap and easily reproducible. The technological development presented in this thesis finds many applications mainly in the chemical, biological and biomedical fields. Indeed, many proteins show absorption peaks in the mid-infrared. It could be possible to measure their concentration or the modification of their properties – like their composition or structure – with an increased sensitivity. The interest of surface plasmons is that the measurement is not a transmission-based technique, but rather a "reflection-based technique", as the light does not go through the gas or liquid sample. This measurement technique is also adapted for high volume samples that are present in industrial production processes like water or soft drink quality control.

EPFL2010

Quantum limits on measurement and control of a mechanical oscillator

Vivishek Sudhir

The precision measurement of position has a long-standing tradition in physics. Cavendish's verification of the universal law of gravitation using a torsion pendulum, Perrin's confirmation of the atomic hypothesis via the precise measurement of the Brownian motion, and, the verification of the mechanical effect of electromagnetic radiation, all belong to this classical heritage. Quantum mechanics posits that the measurement of position results in an uncertain momentum; an idea developed to full maturity within the context of interferometric searches for gravity waves. Over the past decade, standing at the confluence of quantum optics and nanomechanics, cavity optomechanics has emerged as a powerful platform to study the quantum limits of position measurements. The subject of this thesis is the precision measurement of the position of a nano-mechanical oscillator, the fundamental limits of such measurements, and its relevance to measurement-based feedback control. The nano-mechanical oscillator is coupled to light confined in an optical micro-cavity via radiation pressure. The fluctuations in the position of the oscillator are transduced onto the phase of the light, while quantum fluctuations in the amplitude of the light leads to a disturbance in the momentum of the oscillator. We perform an interferometric position measurement with a sensitivity that is 10^5 times below what is required to resolve the zero-point motion of the oscillator, constituting the most precise measurement of thermal motion yet. The resulting disturbance -- measurement back-action -- is observed to be commensurate with the uncertainty principle, leading to a 10% contribution to the total motion of the oscillator. The continuous record of the measurement (performed in a 4 K cryogenic environment) furnishes the ability to resolve the zero-point motion of the oscillator within its decoherence rate - the necessary condition for measurement-based feedback control of the state of the oscillator. Using the measurement record as error signal, the oscillator is cooled towards its ground state, resulting in a factor 10^4 suppression of its total (thermal and back-action) motion, to a final occupation of 5 phonons on average. Measurements generally proceed by establishing correlations between the system being measured and the measuring device. For the class of quantum measurements employed here - continuous linear measurements - these correlations arise due to measurement back-action. These back-action-induced correlations appear as correlations between the degrees of freedom of the measuring device. For interferometric position measurements, quantum correlations are established between the phase and amplitude of the light. In a homodyne measurement, they lead to optical squeezing, while in a heterodyne measurement, they appear as an asymmetry in the sidebands carrying information about the oscillator position. Feedback is used to enhance sideband asymmetry, a first proof-of-principle demonstration of the ability to control quantum correlations using feedback. In the regime where amplified vacuum noise dominates the feedback signal, the disappearance of sideband asymmetry visualises a fundamental limit of linear feedback control. Using a homodyne detector, we also characterise these quantum correlations manifested as optical squeezing at the 1% level.

EPFL2016

Quantum Measurement of Mechanical Motion close to the Standard Quantum Limit

Liu Qiu

In quantum mechanics, the Heisenberg uncertainty principle places a fundamental limit in the measurement precision for certain pairs of physical quantities, such as position and momentum, time and energy or amplitude and phase. Due to the Heisenberg uncertainty principle, any attempt to extract certain information from a quantum object would inevitably perturbitinanunpredictableway. This raises one question,"What is the precision limit in such quantum measurements?" The answer, standard quantum limit (SQL), has been obtained by Braginsky to figure out the fundamental quantum limits of displacement measurement in the context of gravitational wave detection. To circumvent the unavoidable quantum back-action from the priori measurement, quantum non-demolition measurement (QND) methods were introduced by Braginsky and Thorne. To surpass the SQL of the displacement measurement in an interferometer, one can measure only one quadrature of the mechanical motion while give up the information about the other canonically conjugated quadrature. Such measurements can be performed by periodic driving the mechanical oscillator, i.e. the back-action evading (BAE) measurement. Cavity optomechanics provides an ideal table-top platform for the testing of the quantum measurement theory. Mechanical oscillator is coupled to electromagnetic field via radiation pressure, which is enhanced by an optical micro-cavity. Over the last decade, laser cooling has enabled the preparation of mechanical oscillator in the ground state in both optical and microwave systems. BAE measurements of mechanical motion have been allowed in the microwave electromechanical systems, which led to the observations of mechanical squeezing and entanglement. However, despite the theoretical proposal almost 40 years ago, the sub-SQL measurements still remain elusive. This thesis reports our efforts approach the sub-SQL with a highly sideband-resolved silicon optomechanical crystal (OMC) in a 3He buffer gas environment at 2K. The OMC couples an optical mode at telecommunication wavelengths and a colocalized mechanical mode at GHz frequencies. The Helium3 buffer gas environment allows sufficient thermalization of the OMC despite the drastically decreased silicon thermal conductivity. We observe Floquet dynamics in motional sideband asymmetry measurement when employing multiple probe tones. The Floquet dynamics arises due to presence of Kerr-type nonlinearities and gives rise to an artificially modified motional sideband asymmetry, resulting from a synthetic gauge field among the Fourier modes. We demonstrate the first optical continuous two-tone backaction-evading measurement of a localized GHz frequency mechanical mode of silicon OMC close to the ground state by showing the transition from conventional sideband asymmetry to backaction-evading measurement. We discover a fundamental two-tone optomechanical instability and demonstrate its implications on the back-action evading measurement. Such instability imposes a fundamental limitation on other two-tone schemes, such as dissipative quantum mechanical squeezing. We demonstrate state-of-art laser sideband cooling of the mechanical motion to a mean thermal occupancy of 0.09 quantum, which is 7.4dB of the oscillator's zero-point energy and corresponds to 92% ground state probability. This also enables us to observe the dissipative mechanical squeezing below the zero-point motion for the first time with laser light.

EPFL2020