Counting the cycles of light using a self-referenced optical microresonator

Phase coherently linking optical-to-radio frequencies with femtosecond mode-locked laser frequency combs has enabled counting the cycles of light and is the basis of optical clocks, absolute frequency synthesis, tests of fundamental physics, and improved spectroscopy. Using an optical microresonator frequency comb to establish a coherent link between optical and microwave frequencies will extend optical frequency synthesis and measurements to areas requiring compact form factor, on-chip integration, and comb line spacing in the microwave regime, including coherent telecommunications, astrophysical spectrometer calibration, or microwave photonics. Here we demonstrate a microwave-to-optical link with a microresonator. By using a temporal dissipative single soliton state in an ultrahigh-Q crystalline microresonator that is broadened in highly nonlinear fiber, an optical frequency comb is generated that is self-referenced, allowing us to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates that precision optical frequency measurements can be realized with compact high-Q microresonators. (C) 2015 Optical Society of America

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

Functional properties of III-V nanowires addressed by Raman spectroscopy

Francesca Amaduzzi

In recent years, semiconductor nanowires have attracted considerable attention as a result of their unique properties and potential applications in many fields. In particular, they can be very attractive materials for certain optoelectronic and electronic devices, such as lasers, detectors and solar cells, which benefit from the photonic properties of nanowires. In order for these future technologies to become a reality, a good understanding of the functional properties of nanowires is fundamental. In this thesis we have investigated the optical and the electrical properties of III-V semiconductors nanowires by the means of Raman spectroscopy. Thanks to its non-destructive and spatial resolution, Raman spectroscopy is a powerful contact-less tool for the characterization of semiconductor nanowires. Raman spectroscopy can provide information about crystallinity, orientation, size and chemical composition. In polar semiconductors it is also possible to characterize the free carriers, through the coupling of plasmons with longitudinal optical modes. Due to the small size and particular morphology of nanowires, the interaction of light can be more complex than in thin films. In particular, the existence of photonic modes alters significantly the corresponding light-matter interaction. In this thesis we exploit the use of photonic modes for the compositional mapping of nanowire core-shell heterostructures and also to circumvent the macroscopic selection rules. In the first part of this thesis, we have performed Raman scattering measurements on GaAs/AlGaAs core/shell nanowires. We have shown that it is possible to select and characterize regions of the structure with different Aluminum content, by performing the measurements at different laser wavelengths. Then, we have shown that the photonic modes can be modified by suspending the nanowires on a trench. We have shown that in this case it is possible to enhance the response of the longitudinal optical phonon mode. We have then applied this configuration for the characterization of the hole concentration on p-type GaAs nanowires in back-scattering geometry. The second part of the thesis focused on the assessment of free carriers by Raman spectroscopy in systems with an expected high electron mobility: GaAs nanowires with a modulation doped structure and InAs(Sb) nanowires. Raman measurements were performed as a function of the temperature on modulation doped GaAs/AlGaAs nanowire. By characterizing the coupling between free carrier and the LO phonons, we have extracted the concentration and the mobility of carriers. We have found that Si donors are almost ionized for a temperature above 50 K. We have shown that the mobility is limited by interface scattering, with values of 400 cm^2/Vs at room temperature and 2700 cm^2/Vs at low temperature. Finally, we investigated the electronic properties of InAs(Sb) nanowires as a function of the temperature. The effect of a dielectric coating on the electronic properties was also studied. We have found an increase of mobility and electron concentration with the antimony content , moving from 5100 cm^2/Vs for InAs nanowires, to 17500 cm^2/Vs for InAsSb with 35% of antimony at 14K. Moreover, we have shown that in the case of InAs electrons are located in the accumulation layer at the surface, while for InAsSb our measurements are consistent with the carriers located in the nanowire core.

EPFL2017

Soliton Kerr Frequency Combs Generated in Integrated Photonic Chips

Victor Brasch

In 2007 a new method to generate optical frequency combs was discovered. In contrast to conventional frequency combs which are generated from pulsed laser sources, these Kerr frequency combs (also known as microresonator frequency combs) are generated entirely by nonlinear effects from a single, strong continuous wave pump laser that is coupled to a microresonator. Within the first few years the field of Kerr frequency combs was able to achieve many milestone results such as fully stabilized spectra, demonstrations of several applications and octave spanning spectra. However, in particular broad Kerr frequency comb spectra showed low coherence. The reason for this was the emergence of noise intrinsic to the process behind the generation of these early Kerr frequency combs. Within the first year of my PhD this situation changed radically with the discovery of temporal dissipative Kerr solitons in crystalline microresonators in our group. These states feature a pulsed output that originates from the nonlinear dynamics inside the microresonator which enables broadband and coherent Kerr frequency combs. The dissipative Kerr solitons inside the microresonator represent a pulse shape that maintains itself indefinitely while propagating around the resonator. This is possible because all the losses that occur are compensated by the continuous wave pump laser. The deformation of the waveform that is usually caused by the dispersion of the cavity on the other hand is compensated by the nonlinearity of the medium. My work shows for the first time that these dissipative Kerr soliton states do also exist in integrated microresonator platforms. The microresonator platform that was used here are planar silicon nitride waveguide resonators with silicon dioxide cladding. In order to allow for soliton generation the fabrication of the integrated microresonators had to be optimized. Once the desired soliton states were demonstrated in the integrated microresonators an experimental protocol to stabilize these otherwise instable soliton states was developed. Thanks to this protocol it was possible to investigate key properties of the soliton states. In particular it was possible to observe a phenomenon known as dispersive wave or soliton Cherenkov radiation in microresonators for the first time and to show that the dissipative Kerr soliton states remain stable and coherent in the presence of the dispersive wave. In addition the dispersive wave broadens the optical spectrum substantially. The resulting gain in optical bandwidth allowed for the self-referencing of the Kerr frequency comb without using conventional broadening techniques. In an independent experiment the full phase stabilization with respect to an optical reference was also demonstrated and verified. Several additional effects were discovered which had substantial influence on the soliton dynamics inside the microresonator and which had not been observed before in microresonators such as the Raman frequency shift, a shift of the dispersive wave and the interaction of multiple solitons inside the cavity. In summary my work opens the door towards coherent Kerr frequency combs with large bandwidth from integrated photonic chips. Such frequency combs can find use in different applications such as optical data transmission, which we also demonstrated in a collaboration with a research group from the Karlsruhe Institute of Technology.

EPFL2016