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Nanophononic devices: from phonon networks to phonon CQED

Periodic Reporting for period 2 - NanoPhennec (Nanophononic devices: from phonon networks to phonon CQED)

Reporting period: 2018-08-01 to 2020-01-31

Phonons (quanta of vibration) play a major role in many of the physical properties of condensed matter. One of the most striking features of acoustic phonons is their ability to interact with virtually any other excitation in solids. Recent progress in the design, fabrication, and control of nanomechanical systems has paved the way to explore new frontiers in the classical and quantum worlds. Devices based on semiconductor quantum dots (QDs) have been recently demonstrated to perform as near-ideal single-photon sources, a very promising platform for developing a solid-state quantum network. The phonon engineering, however, remains an unexplored knob in the quantum information toolbox.

The goal of this project is to explore new horizons in nanophononics by developing novel phononic networks with full control on the phonon dynamics, and unprecedented structures capable of acoustically interact with single QDs, bridging the gap between nanophononics and semiconductor QD quantum optics.
AlGaAs based semiconductor cavities are capable of confining simultaneously photons and phonons. The building blocks of the proposed research are semiconductor pillar microcavities and single QDs deterministically positioned to maximize their interaction with the confined electromagnetic and elastic fields. To achieve our main goal we set three major objectives: 1) To develop novel one- and three-dimensional optophononic resonators and develop appropriate phononic measuring techniques; 2) To engineer nanophononic networks working in the tens-of-GHz range; and 3) To demonstrate first phonon cavity quantum electrodynamics phenomena for a single artificial atom coupled to a phononic cavity. Shaping the phononic environment opens exciting perspectives for solid-state quantum applications, by providing full control over the main source of decoherence and actually using it as a powerful resource to eventually transfer the quantum information.
One of the main challenges in nanophononics, i.e. the engineering of acoustic phonons at the nanoscale, is the absence of standard transducers to generate, control and detect acoustic phonons. The most used experimental techniques to study phonon dynamics are thus based on optical techniques. Since the beginning of the project, a considerable part of my team research time was devoted to setting up three complementary experiments to study phonon dynamics at ultra-high frequencies, essential for the achievement of the main objectives.

-High-resolution Brillouin/Raman scattering setup: this spectroscopy technique relies on a continuous-wave laser probing the phonon population in a sample by inelastic scattering. We coupled a double spectrometer and optimized the full optical setup to measure in the 15-1000 GHz range. The most important improvements in the experiment were the development of a novel stray light filtering technique based on the diffracted light from a single micropillar that allowed us to measure Brillouin scattering at 40 GHz from a micron-sized object, the coupling of an etalon filter, and the investigation of novel measuring geometries enhancing our laser-filtering efficiency.
-Coherent Phonon Generation in a Pump-Probe scheme: using ps/fs pulsed lasers we study the phonon dynamics in multilayered structures. We impemented a scanning technique that allowed us to spatially map the intensity of the generated phonon wave-package around the excitation spot.
-Micro-Photoluminescence at 4 K: a microPL experiment optimized to work with optical (mechanical) micropillars embedding quantum dots was set up. This set up is now coupled to a closed-cycle cryostat which is integrated into an optical table. Time correlation experiments were recently added, to measure the g2 of a single quantum dot coupled to an optophononic microcavity.

Likewise, we developed simulation tools to account for the measured signals. We developed a simulation code based on the transfer matrix method that allows us to numerically simulate the time and spectral domain experimental results. In addition, we developed a three-dimensional model based on finite element methods, that accounts for the three-dimensional confinement effects.
We worked on several techniques to control the propagation of acoustic phonons in multilayered structures. In particular, we engineered i) an adiabatic cavity that has the potential to be more robust under lateral etching in the fabrication of micropillars, ii) a topological cavity that is robust against a disorder that does not change the topological phases of the constituent superlattices, and iii) a general method to mimic with multilayered structures the effects of an effective acoustic potential at the nanoscale. All these concepts were based on the general idea of engineering the local acoustic band structure of a semiconductor nanometric multilayer.

Moreover, in an optical micropillar, a high-frequency acoustic resonator can replace the spacer, and both the optical cavity and acoustic cavity resonances can be independently tuned. We have developed a strategy to measure confined acoustic phonons in the 300 GHz range, using optical micropillars, and accounting for the thermal effects induced enhanced by the optical confinement.
- We study semiconductor micropillars as a new potential optomechanical platform. We showed that the mechanism governing the acoustic confinement along the pillar axis is very similar to the optical confinement; however, the Poisson ratio and boundaries conditions lead to very distinct acoustic and optical confinement in the transverse direction. The micropillar work at unprecedented high frequencies (~20 GHz), present record-high quality factor/frequency products (10^14), and high optomechanical couplings.
- We proposed a novel device based on two nested resonators. A multilayered acoustic resonator, able to confine acoustic-phonons with frequencies in the 100's GHz acts as the optical spacer in an optical micropillar cavity. This versatile structure allows to independently tune the optical and acoustic properties of the opto-phononic resonator, resulting in a tailored generator of Brillouin signals. In addition, we developed a novel technique to measure these signals.
- We were able to mimic the effect of an electric potential on a charged particle in a phononic system by locally changing the group velocity of longitudinal acoustic phonons in semiconductor multilayers. In a general way, by engineering the band structure we propose novel acoustic-cavity designs. In particular, we introduced concepts of topology to nanophononics that were at the base of the engineering of interface acoustic modes at the nanoscale.

The main results that we expect until the end of the project are:
1) To achieve full control of the propagation of high-frequency acoustic phonons in 2D landscapes. This will unveil the coherence properties of acoustic phonons and would provide a unique means to interface different classical and quantum systems.
2) To interface the confined acoustic modes in micropillars with single-photon emitters. This would allow us to use the phonons as a means to control the emission of single photons.
3) To manipulate high-frequency acoustic phonons in the quantum regime.
Artistic view of a micropillar optophononic resonator based on a scanning electron microscopy image