Enhanced Photon–Phonon Interaction in WSe2 Acoustic Nanocavities

Acoustic nanocavities (ANCs) with resonance frequencies much above 1 GHz are prospective to be exploited in sensors and quantum operating devices. Nowadays, acoustic nanocavities fabricated from van der Waals (vdW) nanolayers allow them to exhibit resonance frequencies of the breathing acoustic mode up to f ∼ 1 THz and quality factors up to Q ∼ 103. For such high acoustic frequencies, electrical methods fail, and optical techniques are used for the generation and detection of coherent phonons. Here, we study experimentally acoustic nanocavities fabricated from WSe2 layers with thicknesses from 8 up to 130 nm deposited onto silica colloidal crystals. The substrate provides a strong mechanical support for the layers while keeping their acoustic properties the same as in membranes. We concentrate on experimental and theoretical studies of the amplitude of the optically measured acoustic signal from the breathing mode, which is the most important characteristic for acousto-optical devices. We probe the acoustic signal optically with a single wavelength in the vicinity of the exciton resonance and measure the relative changes in the reflectivity induced by coherent phonons up to 3 × 10–4 for f ∼ 100 GHz. We reveal the enhancement of photon–phonon interaction for a wide range of acoustic frequencies and show high sensitivity of the signal amplitude to the photoelastic constants governed by the deformation potential and dielectric function for photon energies near the exciton resonance. We also reveal a resonance in the photoelastic response (we call it photoelastic resonance) in the nanolayers with thickness close to the Bragg condition. The estimates show the capability of acoustic nanocavities with an exciton resonance for operations with high-frequency single phonons at an elevated temperature

The IPIN 2019 Indoor Localisation Competition—Description and Results

IPIN 2019 Competition, sixth in a series of IPIN competitions, was held at the CNR Research Area of Pisa (IT), integrated into the program of the IPIN 2019 Conference. It included two on-site real-time Tracks and three off-site Tracks. The four Tracks presented in this paper were set in the same environment, made of two buildings close together for a total usable area of 1000 m 2 outdoors and and 6000 m 2 indoors over three floors, with a total path length exceeding 500 m. IPIN competitions, based on the EvAAL framework, have aimed at comparing the accuracy performance of personal positioning systems in fair and realistic conditions: past editions of the competition were carried in big conference settings, university campuses and a shopping mall. Positioning accuracy is computed while the person carrying the system under test walks at normal walking speed, uses lifts and goes up and down stairs or briefly stops at given points. Results presented here are a showcase of state-of-the-art systems tested side by side in real-world settings as part of the on-site real-time competition Tracks. Results for off-site Tracks allow a detailed and reproducible comparison of the most recent positioning and tracking algorithms in the same environment as the on-site Tracks

Hybrid coherent control of magnons in a ferromagnetic phononic resonator excited by laser pulses

We propose and demonstrate the concept of hybrid coherent control (CC) whereby a quantum or classical harmonic oscillator is excited by two excitations: one is quasi-harmonic (i.e. harmonic with a finite lifetime) and the other is a pulsed broadband excitation. Depending on the phase relation between the two excitations, controlled by the detuning of the oscillator eigenfrequencies and the waveforms of the quasi-harmonic and broadband excitations, it is possible to observe Fano-like spectra of the harmonic oscillator due to the interference of the two responses to the simultaneously acting excitations. Experimentally, as an example, the hybrid CC is implemented for magnons in a ferromagnetic grating where GHz coherent phonons act as the quasi-harmonic excitation and the broadband impact arises from pulsed optical excitation followed by spin dynamics in the ferromag-netic nanostructure. Coherent control (CC) is well established as a powerful method to manipulate the amplitude and phase of quantum states. First used for chemical reactions [1, 2], CC has been demonstrated for single electrons [3], spins [4, 5], nanoelectromechanical oscillators [6], magnons [7, 8] and other systems [9]. The basic phenomenon governing CC is the interference of the responses of a quantum system to specific excitations, which determine the phase of the wavefunction. One of the common technical solutions for realizing CC is to use two optical pulses from ultrafast lasers with adjustable time separation or more sophisticated laser pulse shaping [10]. For CC of magnons, two microwave pulses may be used [11]. Traditionally , the excitations that lead to the interfering responses have the same origin, e.g. transitions between the ground and an excited quantum state are induced by a resonant electromagnetic field. However, there are quantum systems that may be excited by a pair of exci-tations of different origins. For example, one excitation may be broad-band and the other harmonic. Exploiting a combination of various types of excitations for hybrid CC would broaden a diversity of CC applications for quantum computing and communications. The idea of hybrid CC in the spectral domain is illustrated in Figs. 1(a) and 1(b) for a linear tunable quantum or classical oscillator with eigenfrequency ω 0 and finite lifetime. Figures 1(a) and 1(b) show the amplitude spectra of the oscillator's responses to two types of excitation: (1) quasi-harmonic (i.e. harmonic with finite lifetime) excitation with central frequency ω R detuned relative to ω 0 ; and (2) broad band excitation. Two cases of detun-ing are considered: negative (ω 0 ω R) in Fig. 1(b). The top blue curves show the spectra when only quasi-harmonic excitation is present. In this case the phase ϕ of the oscillator at ω = ω 0 changes by π when the oscillator eigenfrequency is tuned through the resonance ω = ω R , say from −π/2 to π/2 as demonstrated in the comparison of the blue spectra in Figs. 1(a) and 1(b). The middle red curves are spectral responses when the oscillator is excited by a broadband excitation (2). The oscillator's phase ϕ, e.g. ϕ = π/2, at ω = ω 0 in this case does not depend on ω 0. The lower black curves are the spectra when the two ex-citations, (1) and (2), operate together. Clearly, we get destructive [ Fig. 1(a)] or constructive [Fig. 1(b)] interference of the oscillator's responses at ω = ω 0 depending on the detuning of the oscillator eigenfrequency relative to the central frequency of the quasi-harmonic excitation, ω 0 ω R respectively. For negative detuning (ω 0 ω R) the spectral amplitude at ω = ω 0 increases by a factor of two. The interference effects represent an example of hybrid CC where two excitations have different spectra and are of different nature, for example (1) could be a coherent phonon wavepacket and (2) could be a short microwave or laser pulse. By varying the detuning, amplitudes and phases of excitations (1) and (2), it is possible to model various Fano-like spectral shapes similar to Fano spectra which appear as a result of interference of broad-and narrow-band eigenstates [12]. In the present Letter we demonstrate an example where CC is realized for the case of magnons. Magnons are a typical example for which a diversity of quantum excitations exists [13]. The quasi-harmonic excitation of magnons is coming from quasi-monochromatic surface phonons. They drive the spectrally isolated magnon mode at the frequency ω R. The broadband excitation is based on ultrafast modulation of the ferromagnet mag-netization. Both excitations are triggered optically by a femtosecond laser pulse. The magnon eigenfrequency ω 0 is tuned by the external magnetic field B. Monitoring the magnon spectrum, we observe destructive or constru