Phonon Pulses Research Articles

Erratum: Picosecond acoustic phonon pulse generation in nickel and chromium [Phys. Rev. B67, 205421 (2003)

Erratum: Picosecond acoustic phonon pulse generation in nickel and chromium [Phys. Rev. B<b>67</b>, 205421 (2003)

Evolution of a pulse of noninteracting quasiparticles with dispersion and initial angular width

The evolution of a pulse of noninteracting quasiparticles, caused by their different velocities and angular distribution of momenta, is studied theoretically. Equations are found that describe the shape of the pulse surface at any time. The times of the beginning and end and the duration of the quasiparticle energy flux density is determined at a general spatial point. The quasiparticle energy density is considered at all times and positions, and it is shown that in the region of high energy density, in the middle of the pulse, it is equal to the initial energy density under certain conditions. These theoretical results are discussed in relation to experimental data on the evolution of a pulse of noninteracting phonons in superfluid helium.

High-energy phonon pulses in liquid4He

We have discovered a method for selectively detecting high-energy phonons which has enabled us to make detailed measurements of the shapes of high-energy phonon pulses in liquid 4 He at 50 mK. These phonons are created from pulses of low-energy phonons, injected into the helium by a heater. We suppress the detection of the low-energy phonons by orienting a thin film superconducting Zn bolometer at an angle to the phonon beam. We find that the high-energy phonons are created continuously along the propagation path. As there is also velocity dispersion, the signal at any time is due to a range of phonon energies. Computer simulations of the signals from short pulses reproduce the main features of the measured results. The behavior of the high-energy phonon signals as functions of heater power and pulse length are analyzed and this leads us to suggest how the low-energy phonon pulse develops. In the following paper [I. N. Adamenko et al., Phys. Rev. B 69. 144525 (2004)] the pulse shape for short pulses is derived analytically.

Signal of high-energy phonons created by low-energy phonons in superfluid helium

Starting from the solution of the kinetic equation, we have calculated the distribution function for the high-energy phonons, which are created by a short pulse of low-energy phonons moving in superfluid helium. This enables an explicit expression for the energy density flux to be derived. Hence we find the amplitude of the high-energy phonon signal as a function of time on a bolometer. We divide this signal into two halves: the ``head'' and ``tail'' which arrive before and after the peak signal, respectively. We analyze which high-energy phonons form the head and tail of the signal. The half-widths of head and tail are calculated and approximate formulas which describe the shapes of them are obtained. The partial contribution of high-energy phonons, with different momenta, to the total signal at different times is determined. These results are compared with the experimental results given in the preceding paper [R. V. Vovk, C. D. H. Williams, and A. F. G. Wyatt, Phys. Rev. B 69, 144524 (2004)].

Phonon excitation of quasiparticles in niobium and tantalum superconducting tunnel junction photon detectors

We report phonon pulse measurements of quasiparticle (qp) self-recombination and trapping by Abrikosov vortices in Nb and Ta superconducting tunnel junctions. The quasiparticle loss times, determined from the shape of the tunnel junction output pulse, were found to be strongly correlated with flux trapping. The measured results were compared with the model of Golubov and Houwman. It was found that qp self-recombination, as measured from the dependence of responsivity on absorbed phonon energy, was also modified by the presence of traps. A process of phonon-mediated de-trapping is proposed to explain the observed effects.

A study of the bolometric capabilities of p‐type GaSb and InSb:Ge for phonon pulse experiments

The potential of p-type GaSb and InSb:Ge for use as low-temperature hopping bolometric detectors of particles and photons has been investigated. In the range 1.4 K ≤ T ≤ 4.2 K, GaSb conductivity followed ln ρ(T) ∝ exp (T0/T)1/4 law with sensitivity, α(T) = (1/R)dR/dT, varying from 0.6 K−1 to 0.2 K−1. As a detector at 1.4 K, phonon pulses generated in a silicon absorber bonded to GaSb were easily distinguished with a response time of 1 μs. Conductivity in InSb detectors exhibited thermally activated-hopping conduction, ρ(T) ∝ exp (ε3/kBT), with an activation energy of ε3 ∼ 0.12 meV. In the range 1.4 K ≤ T < 4.2 K, α(T) of InSb varied from 0.7 K−1 to 0.2 K−1 with an abrupt increase above 4.2 K rising to a maximum value of 8.1 K−1 at 4.5K. For the range 5 K ≤ T ≤ 10 K, α(T) had an exponential decay. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of a cool phonon pulse propagating in superfluid helium

We derive the system of equations which describes the evolution of a cool phonon pulse during its propagation in superfluid helium. Solutions are found both for longitudinal deformation of the pulse along the pulse axis and for the transverse evolution perpendicular to the pulse axis. It is shown that the longitudinal motion of the pulse is described by a simple running wave, and the transverse evolution is similar to the expansion of a gas in a vacuum. From these solutions, the angular distribution of phonon-energy density is calculated at various distances from the source. We discuss the very unusual evolution of the phonon pulse and compare it to experimentally observed phenomena.

Terahertz phonon optics in GaAs/AlAs superlattice structures

We have generated pulsed beams of $\ensuremath{\sim}\mathrm{THz}$ monochromatic longitudinal acoustic phonons by ultrafast laser excitation of GaAs/AlAs superlattices. The phonons propagated ballistically across the GaAs substrates at low temperatures and were detected using superconducting aluminum bolometers. Between the generator superlattice and the bolometer was a second superlattice structure which acted as a reflective band-stop filter. We have made measurements of the phonon pulses in various samples for which the filter stop band was either in tune or out of tune with the generator frequency. These have enabled us to place an upper bound on the bandwidth of the monochromatic phonon beam. We have also determined the relative proportions of the detected longitudinal acoustic phonon signal that are due to monochromatic phonons and broadband phonons from carrier relaxation. We present this experiment as an example of ``phonon optics'' using propagating monochromatic phonons in the THz frequency range.

The fabrication and characterization of polycrystalline CuSn bolometers

Polycrystalline CuSn thin films can be used as highly reproducible andeasily fabricated bolometers for the temperature region of 1.3–3 K, animportant range for both phonon pulse experiments and photon and particledetection. We report their fabrication and the dependence of theircharacteristic parameters on the evaporation melt and deposition conditions.The superconducting transition temperature of the films was determinedpredominantly by the alloy composition, the normal state resistance bythe film thickness, and the bolometer sensitivity by the deposition rate.X-ray diffraction showed that the films were composites of Cu6Sn5complexes and pure tin regions.

Picosecond acoustic phonon pulse generation in nickel and chromium

The generation mechanisms for picosecond acoustic phonon pulses excited by ultrashort optical pulses in metals with strong electron-phonon coupling are elucidated. By means of an interferometric probing technique to extract changes in optical phase and amplitude, the acoustic pulse shapes in the transition metals Ni and Cr are shown to be governed by both thermal and nonequilibrium electron diffusion. Quantitative comparison of the results with a model that takes into account the nonequilibrium electron dynamics and ultrasonic attenuation gives excellent agreement.

Phonon absorption by superconducting tunnel junction x-ray detectors

Pulses of nonequilibrium phonons (heat pulses) have been used to mimic the absorption of x-ray photons in superconducting niobium tunnel junctions. For device characterization, the technique provides a valuable alternative to photoabsorption with good time resolution and continuous variability of absorbed energy. In addition, excitation is uniform across the tunnel junction, so that the effects of quasiparticle diffusion can be neglected in the analysis, and hence, the Rothwarf–Taylor equations solved exactly. Consistency is obtained between device parameters obtained from phonon measurements and those inferred from modeling of photoabsorption. In addition, the quasiparticle recombination rate can be determined directly from the nonlinearity of the energy response.

Acoustic phonon-assisted tunneling in GaAs/AlAs superlattices

Measurements have been made of the transient changes in tunnel current $\ensuremath{\Delta}I$ through a biassed 50 period Si doped GaAs/AlAs superlattice produced by pulses of nonequilibrium acoustic phonons. The bias ${V}_{\mathrm{max}}$ at which $\ensuremath{\Delta}I$ is greatest varies linearly with the heater temperature ${T}_{h}$ and hence with the dominant phonon frequency, the SL is acting as a phonon spectrometer with a linewidth (HWHM) $\ensuremath{\lesssim}{k}_{B}{T}_{h}/h\ensuremath{\sim}200\mathrm{GHz}$ (0.8 meV) for ${T}_{h}=10\mathrm{K}.$ The spectral response was confirmed using the phonons that are scattered and hence delayed in their passage across the substrate as a source of higher average frequency than those crossing ballistically. The measured values of $\ensuremath{\Delta}I(V)$ are compared with values calculated using a similar model to that of Glavin et al. [JETP Lett. 51, 191 (2000)].

Wavelength selective photoexcitation of picosecond acoustic-phonon pulses in a triple GaAs/Al 0.3Ga 0.7As quantum well structure

Ultrashort coherent phonon-pulse generation and detection is investigated in a GaAs–Al 0.3Ga 0.7As quantum well structure containing three wells of different widths using an optical pump and probe method. The pump photon energy is tuned to the region of the hh1– e1 transition energies of the wells and the probe photon energy is chosen for detection of the phonon pulses that reach the sample surface. By studying the dependence of the probe reflectance change on the pump photon energy, we demonstrate the possibility of wavelength-selective excitation of picosecond acoustic-phonon pulses in quantum wells.

Ultrafast acoustic phonon pulse generation in chromium

The generation mechanisms for longitudinal acoustic-phonon pulses excited in the transition metal chromium with sub-picosecond optical pulses are studied. The temporal and spatial evolution of the electron and lattice temperatures up to several picoseconds after laser photon absorption have a major impact on the final profile of the acoustic-phonon pulses. These phonon pulse shapes can be explained by considering both thermal and electron diffusion, and taking into account the effects of frequency-dependent ultrasonic attenuation.

The density of high energy phonons created from a low energy phonon pulse in liquid 4He

We calculate the energy density of high energy phonons created from a pulse of low energy phonons spatially confined to a narrow cone. We include the tangential expansion of the propagating pulse and the interactions between high energy phonons within the pulse as well with low energy phonons. The expansion of the pulse causes it to cool so much that the creation of high energy phonons stops within ≈3 mm of the heater. The decay rate of high energy phonons governs the dynamic equilibrium inside the pulse. The magnitude of the high energy phonons, emitted by the pulse into the cold liquid helium, is calculated for different length pulses. It is found that small heaters can produce short pulses of high energy phonons from relatively long injected pulses.

Narrow-Angle Beams of Strongly Interacting Phonons

We demonstrate that narrow-angle phonon pulses of low energy phonons in liquid 4He are strongly interacting and rapidly come into equilibrium within a narrow cone in momentum space. The effect of collimation on such a system is to strongly reduce the axial phonon flux. This gives a method of separating high and low energy phonons.

The Relaxation of High Energy Phonons in Anisotropic Distributions in Superfluid 4He

Anisotropic distributions of low energy phonons in momentum space, in superfluid helium, create high energy phonons with energy e/k B>10K. It has been shown that phonons with e/k B>11K cannot decay by the inverse of the process that creates them, if the low energy phonons are in such a momentum cone of 11°. Here we investigate other possible phonon scattering processes that could annihilate high energy phonons. We find that interactions between two high energy phonons, which can create one low and one high energy phonon, is effective. We calculate the scattering rates for all the four phonon processes as functions of momentum of the high energy phonon, cone angle and the temperature of the low energy phonons. We obtain analytic results from which we can understand the physical reasons for all the functional dependencies. The analytic results are in good agreement with our computer evaluations. We show that a dynamic equilibrium number of high energy phonons can be established in a propagating pulse of phonons and the distribution will be suprathermal.

Ultrafast carrier diffusion in gallium arsenide probed with picosecond acoustic pulses

We describe an experimental investigation of the generation and detection of picosecond acoustic-phonon pulses in a thin slab of GaAs using ultrashort optical pulses. Comparison of the optical phase variation with a simple theory for ambipolar diffusion indicates that carrier diffusion has a significant effect on the shape of the phonon pulses generated. The phonon pulse duration is measured to be \ensuremath{\sim}25 ps, four times longer than that expected from optical-absorption considerations alone, indicating that hot carriers penetrate more than 100 nm into the sample during the phonon pulse generation process.

The analysis of structural defects in piezo and ferroelectric materials by the “heat pulse” method

We consider the “heat pulse” technique for the analysis of structural imperfections. This method is based on the propagation of a pulse or packet of pulses of non-equilibrium acoustic phonons with 1011–1012 Hz frequencies. Analyse des défauts structuraux dans les matériaux piézo-ferroélectriques par la méthode “heat pulse”. Cette méthode, basée sur la propagation pulsée de phonons acoustiques hors équilibre vers 1011–1012 Hz, permet l'étude des défauts structuraux et autres imperfections présents dans les cristaux piézo-ferroélectriques.

Phonon Interactions in an Anisotropic Phonon System Leading to a Suprathermal Distribution in Liquid 4He

The relaxation rates of high energy phonons are calculated for a highly anisotropic phonon system which can be realised by phonon pulses in liquid 4He. We find that the creation rate for these phonons is very much greater than the decay rate over almost all of the momentum range. As a consequence, we have the unusual behaviour of the distribution function, for the high energy phonons, not being the Bose–Einstein distribution function. From our results we expect that the distribution function will be very much greater than the Bose–Einstein one and also to have a quite different momentum dependence. The analytic expressions which are derived, enable us to explain the physical reasons for a suprathermal distribution of high energy phonons which are predicted to occur in highly anisotropic phonon systems.