Imaging arbitrary acoustic whispering-gallery modes in the gigahertz range with ultrashort light pulses

Abstract

Using an ultrafast optical technique with enhanced frequency control, we image surface-acoustic whispering-gallery-like modes in a microscopic disk at various frequencies up to 1 gigahertz (GHz), allowing experimental determination of their dispersion. This is made possible by intensity-modulated optical pumping and probing with a periodic femtosecond light source. Spatiotemporal Fourier transforms of the two-dimensional acoustic fields measured to micron resolution allow us to isolate individual whispering-gallery modes of first and second radial order as well as their mode patterns and Q factors to unprecedented frequency resolution. We thereby demonstrate arbitrary-frequency ultrafast control and imaging of a micro-acoustic system with an optical time-resolved technique. Applications include quality control of surface acoustic wave filters in telecommunications.

Active chiral control of GHz acoustic whispering-gallery modes

“…2(e), whereas for the arc sources different Fourier amplitudes for counter-propagating waves are evident: WGMs with positive (negative) azimuthal order have a larger amplitude when the lower (upper) arc source is used. The mode with the highest Fourier amplitude at this frequency corresponds to = ±15, and fits well on the previously measured dispersion curves 5 . Crosssectional views of the amplitude A n m (r) at = ±15 are plotted in Fig.…”

“…3 (a) and (c), respectively (Multimedia view). WGMs for = 9, 12, 22 are also observed at frequencies in agreement with the frequency vs dispersion curve 5 . In all these cases, we find that the lower (upper) arc enhances CCW (CW) waves compared to the reference (point source) case, as observed for = 15, although accurate values for the ratios R could not be obtained for these other values.…”

“…This was first achieved in optics in a cylindrical silicon-nitride microcavity by use of visible light 3 , and later in acoustics in a microscopic copper disk using ∼1 GHz surface acoustic waves (SAWs) 4,5 . In the case of acoustics, WGM-like modes are used in different applications such as in the non-destructive testing of pipes, and various cylindrical geometries have been investigated numerically or experimentally, including by means of laser acoustics [6][7][8][9] and picosecond surface-wave acoustics 4,5 . In spite of these investigations, few studies 2 have been specifically designed to break the symmetry in the rotation direction of acoustic WGMs, in spite of this being an interesting and topical subject for optical WGMs 10,11 , and, in particular, none have involved acoustic active control.…”

“…It is chopped by an electrooptic modulator at frequency f p = 1 MHz. We implement an arbitrary frequency technique 5,17,18 to access the lower and upper sidebands of the acoustic signal, i.e., at f gen = nf rep + mf p , where n 0 is a positive integer or zero, and m = ±1 correspond to the lower sideband (LSB, m = −1) or upper sideband (USB, m = 1). The pump beam is reflected by a SLM at an angle close to normal incidence to illuminate all SLM phaseretardation-controlled reflectors simultaneously.…”

“…1(b). It has the same geometry as that used in previous experiments 4,5 , and consists of a polycrystalline copper disk of radius r 0 =18.75 µm embedded in a silicon oxide layer (thickness 370 nm) lying on a silicon nitride layer (thickness 100 nm), another silicon oxide layer (thickness 550 nm), and a Si (100) substrate. Polishing the sample to remove the excess deposited copper results in a concave disk surface 4,20 ; atomic-force microscopy measurements indicate that the center of the disk is recessed with respect to the surface of the silicon oxide layer by ∼150 nm.…”