Acoustic whispering-gallery modes generated and dynamically imaged with ultrashort optical pulses

Abstract

Broadband coherent surface phonon wave packets up to 1 GHz in a microscopic embedded disk are excited and dynamically imaged with ultrashort optical pulses. We isolate a whispering-gallery-like mode using a technique involving quadrature detection of the out-of-plane surface motion on modulating the optical excitation beam. The eigenmode distribution is obtained from spatiotemporal Fourier transforms and a radialazimuthal decomposition demonstrates the selective excitation of a mode with 26-fold rotational symmetry and allows an estimate of the cavity Q factor to be made.

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

“…Optical WGMs show similar field localization, and this has been directly verified by spatial imaging of the optical field to sub-micron resolution [2]. Later, spatial imaging was achieved for acoustic WGMs based on sub-gigahertz surface acoustic waves (SAWs) confined in microscopic copper disks, but only one mode could be clearly identified because of the limitation in frequency resolution owing to the periodic pulsed laser source used for excitation and detection [3].…”

“…1(b), is manufactured by International Sematech, and is similar to that used in previous experiments with limited frequency resolution [3]. It 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.…”

“…It 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 [3,13]; 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. show two SAW images corresponding to the real part X of the complex signal Z X iY at 1.96 and 4.40 ns after the pump pulse arrival, respectively [3], where X (Y ) is the in-phase (quadrature) component of the measured optical phase difference.…”

Active chiral control of GHz acoustic whispering-gallery modes

“…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. In contrast to the case of WGMs on a cylinder or in a cylindrical enclosure, in our sample the WGMs are quasi-confined in two-dimensions near the rim of the copper disk by the silicon oxide walls.…”

“…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.…”