Phonon Spectroscopy

(1) Picosecond Ultrasonics -- In recent years, a novel approach for generation of frequency tunable shear acoustic waves in the GHz frequency range has been presented by the Nelson group using Time-Domain Coherent Brillouin Scattering (TDBS) technique. The group has demonstrated a sample and optical configuration that allows shear and longitudinal acoustic parameter measurements. The TDBS technique combines different picosecond laser ultrasonic techniques for acoustic wave generation and detection. Multiple-cycle longitudinal and shear acoustic waves can be optically generated using a crystallographically canted metal film transducer, which is obliquely deposited under ultrahigh vacuum by molecular beam epitaxy onto optical quality sapphire or glass substrates. When the transducer is hit by an optical pulse train, rapid thermal expansion launches acoustic wave packets of both longitudinal and shear polarizations, when the right configuration is met. After the wave packets propagate into and through the sample substrate, both shear and longitudinal acoustic waves are optically detected after transmission into a transparent substrate using TDBS. The signal intensity that shows time-dependent oscillations at the acoustic frequency can be acquired from the coherently scattered field, whose optical phase varies depending on the acoustic wave peak and null positions, when superposed with the reflected probe field.

(2) Transient Grating Spectroscopy -- The Transient Grating (TG) technique has been used in the Nelson group for many years. We are currently using the TG technique for both thermal and acoustic measurements. The technique involves crossing two short pump pulses to create a sinusoidal interference pattern, which when absorbed by the sample induces a sinusoidal response. In most samples the light is absorbed and the sinusoidal response is manifest as a temperature profile, and upon thermal expansion an acoustic wave is generated. The temperature profile and the acoustic wave modulate the complex index of refraction of the material to create a sinusoidal time dependent grating that diffracts an incident continuous waveform (CW) probe beam. The diffracted signal is overlapped with a reference beam to allow for heterodyne detection. Detection of thermal grating decay and acoustic response are monitored in real time using a fast photo detector and oscilloscope.

(*) Current Projects: Coherent Phonon Spectroscopy of Liquids -- The techniques above were used for optical generation and detection of gigahertz frequency longitudinal and shear acoustic waves in liquid samples, mainly DC 704 and glycerol. From MHz to lower GHz range, ultrasonics, impulsive stimulated thermal or Brillouin scattering, and spontaneous Brillouin scattering can be utilized to study dynamics. The THz longitudinal acoustic frequency ranges can be accessed using x-ray Brillouin scattering, but tens to hundreds of GHz range, where fast relaxation features occur, has only had limited access through deep-UV Brillouin scattering. TDBS study of liquid forming liquid was employed to fill the gaps that traditional techniques could not explore. Picosecond ultrasonics with access to much of the GHz-frequency longitudinal wave range has been adapted for both shear and longitudinal frequency-tunable, multipule-cycle acoustic wave generation and detection.

(*) Current Projects: Coherent Phonon Spectroscopy of Solids -- The recent efforts of coherent phonon spectroscopy of solids were focused photoacoustic determination of the speed of sound in crystals, property investigation of semiconductor super lattices, lifetime of coherent acoustic phonon, and acoustic attenuation measurements. Many energetic solid materials are molecular crystals with low crystal symmetry and anisotropic mechanical properties. Determining frequency-dependent elastic constants can be a significant step towards understanding extent of interactions between acoustic modes and other degrees of freedom. As a collaborative effort, Brillouin light scattering, impulsive stimulated light scattering, and picosecond acoustic interferometry were performed on single crystalline sample to investigate the speed of sound in the energetic crystal. Also, there is a project to measure high frequency phonon lifetimes. Phonons that contribute the most to thermal transport typically have frequencies above 1THz which is somewhat challenging to generate in materials of interest. One generation mechanism is to excite acoustic waves in a superlattice (SL) which can have very thin layers leading to high frequencies. One example of this is shown below. We are now working to generate and measure the lifetime of high frequency phonons in Si and GaAs. With high frequency phonon lifetimes we should be able to compare with and direct the development of theoretical models of thermal conductivity.

(*) Current Projects: Thermal Transport in Solids -- Thermal transport in semiconductors has a wide variety of applications particularly in the fields of thermal management of microelectronic devices and thermoelectric materials. Our group is interested in studying basic materials to further understanding of fundamental phonon physics. Using the TTG technique we can measure the decay time of the thermal grating and using the period of the sinusoidal pattern we can determine the thermal diffusivity. In addition changing the grating spacing provides information on how phonons with a given mean free path contribute to the transport because phonons with mean free path longer than the grating spacing have a reduced contribution.

(*) Current Projects: Guided Acoustic Waves -- A collaborative study of a granular crystals, which consists of close-packed, ordered arrays of elastic particles, with various engineering application was conducted with Nanophotonics and 3D Nanomanufacturing Laboratory. We have studied the interaction of surface acoustic waves (SAWs) with the contact-based resonance of microspheres forming a two-dimensional granular crystal in the order of magnitude of 1 µm using TG technique by measuring phase velocity dispersion of SAWs. SAWs are acoustic modes that propagate while confined within a very shallow penetration depth, enabling a broad range of applications in nondestructive material characterization and signal processing. The experimental method can be used to study the adhesion and contact mechanics of microparticles and enables the study of granular crystals on the microscale. A rich array of dynamical phenomena observed in macroscale granular crystals, and their promise for practical applications, suggest interesting possibilities for microscale granular crystals. Finally, the nonlinearity of the Hertzian contact holds promise for an application of our approach to developing nonlinear SAW devices.