Detection Of Phonons Research Articles

Influence of doping profiles on coherent acoustic phonon detection and generation in semiconductors

The doping profile in different n-doped GaAs homoepitaxial structures grown by molecular beam epitaxy is investigated in the time domain by employing a laser based picosecond ultrasound technique in a contactless and noninvasive way. Experiments based on asynchronous optical sampling employ two femtosecond lasers, which allow us to detect changes in the optical reflectivity over a 1 ns time delay with a signal-to-noise ratio of 107 and 100 fs time resolution in <1 min of acquisition time. We show that the doping profile with doping densities of the order of 1018 cm−3 can be detected with picosecond ultrasound, although there is no difference in the acoustic properties of the doped and undoped region. The detection mechanism is based on a different sensitivity function for a coherent strain pulse in the doped and undoped regions. These results are corroborated by experiments at room temperature and 10 K.

Excitation and detection of high-frequency coherent acoustic phonons in low-symmetry superlattices

The generation and detection of coherent acoustic phonons in GaAs/AlAs superlattices grown on the low-symmetry, (311) and (211), planes of GaAs using femtosecond time-resolved pump-probe measurements is described. Frequencies of the excited phonons are deduced from the oscillations in the surface reflectivity of the probe and are compared with theoretical calculations assuming that Raman scattering is responsible for coherent phonon generation. The measured frequencies and relative intensities of the modes agree well with the theoretically predicted folded bulk acoustic modes and are determined by the period and symmetry of the superlattices.

Subterahertz monochromatic acoustic wave propagation using semiconductor superlattices as transducers

We demonstrate both semiconductor superlattices and nanocavities as narrow-band acoustic transducers in the subterahertz range. Picosecond ultrasonics with pump and probe incident on opposite sides of the substrate allows full decoupling of the phonon generation and detection processes. Pumping in the nanotransducers, we generate spectrally narrow wave packets which propagate over macroscopic distances. Probing in the nanotransducers, we evidence them as very sensitive and spectrally selective detectors.

Coherent acoustic phonons in a thin gold film probed by femtosecond surface plasmon resonance

AbstractWe report on coherent phonon detection in a thin gold film by a new femtosecond pump‐probe surface plasmon resonance (SPR) technique. Acoustic coherent phonons are generated impulsively in the gold film on a prism, and they are detected in the time domain through the reflectivity modulation for probe pulses that satisfy the SPR condition. We observed not only the fundamental vibration of a longitudinal acoustic phonon but also the overtones up to the fourth. Copyright © 2008 John Wiley & Sons, Ltd.

Phonon background versus analog Hawking radiation in Bose-Einstein condensates

We determine the feasibility of detecting analog Hawking radiation in a Bose-Einstein condensate in the presence of atom-loss-induced heating. We find that phonons created by three-body losses overshadow those due to analog Hawking radiation. To overcome this problem, three-body losses may have to be suppressed, for example, as proposed by Search et al. [Phys. Rev. Lett. 92, 140401 (2004)]. The reduction of losses to a few percent of their normal rate is typically sufficient to suppress the creation of loss phonons on the time scale of fast analog Hawking phonon detection.

Coherent shear phonon generation and detection with picosecond laser acoustics

Measurements on a film of silica on crystalline zinc using picosecond laser acoustics are theoretically analyzed to quantitatively explain the generation and detection of picosecond shear and longitudinal-acoustic waves. The theory encompasses the scattering of obliquely incident probe light of arbitrary polarization by a depth-dependent anisotropic permittivity modulation in a multilayer, including terms arising from the photoelastic effect, interface displacements, and local rotations. Sound velocities, ultrasonic attenuation, and photoelastic constants are experimentally derived.

Coherent phonons in semiconductor superlattice under DC electrical bias

Resonant pumping of superlattices SLs by femtosecond laser pulses results in the generation of coherent phonons with frequency centred approximately on s/dSL, where s is the speed of longitudinal sound and dSL is the SL period. The phonons can be detected by measuring the changes in reflectance of time-delayed probe pulses. To date, measurements have been made on nominally undoped SLs, and phonon frequencies are typically in the range 100 GHz - 1 THz. The motivation for these studies is that such hypersound could be used for acoustic probing of nanostructures. Here we describe the generation and detection of coherent phonons in a doped and electrically-biased SL. The studied SL consisted of 50×(6 nm GaAs and 4 nm AlAs), uniformly doped with Silicon to density 2×1022 m−3. Pump-probe measurements were made at λ∼770 nm on an optical MESA at T=12K. At zero bias, we observed a similar phonon spectrum as previously observed in comparable but undoped SLs, with a mode at ̃ 450GHz. Under applied bias this mode increased in amplitude, and the decay time was also increased. We discuss these observations in terms of the effects of the bias on the coherent phonon generation and detection process and also the possibility of coherent phonon amplification occurring in the structure.

Monochromatic high frequency coherent phonons propagation with superlattice transducers

The availability of efficient and compact phonons transducers in the THz range would be very interesting for phonons spectroscopy, acoustic microscopy and study of vibrational and electronic properties of nanostructures. Thanks to epitaxial growth of semiconductors multilayers, high quality phononic nanostructures with standard semiconductors, such as superlattices (SL) and nanocavities can be obtained for the GHz and THz transduction. Picosecond ultrasonics experiments have been performed in transmission geometry with pump and probe incident on opposite sides of the substrate, allowing discoupling acoustic generation and detection processes. By these means, we have shown independently that SL are very efficient high frequency monochromatic phonon generators and detectors. We report on experiments where two superlattices have been grown on the opposite sides of a substrate: a first SL with uniform layer thickness over the whole surface sample is used as a generator; the other one, used as the detector, presents a thickness gradient and the location of the detection is chosen in order to have the best matching with the emitted frequency. This setup is used to study the propagation of monochromatic high frequency coherent phonons as a function of temperature.

Hopping photoconductivity and the effectiveness of phonon detection in GaAs:Zn bolometers

We have studied the effect of white light on hopping conduction and the effectiveness of non-equilibrium phonon detection in Zn-doped GaAs bolometers over the temperature range 1.35 K ⩽ T ⩽ 2.15 K. The temperature dependence of the low electric field resistance indicates that the mechanism of conduction is due to variable range hopping. Using a heat-pulse technique we show that the sensitivity of a GaAs:Zn bolometer with an acceptor concentration, n a = 4.2 × 10 17 cm −3 is significantly enhanced in the presence of suitably applied irradiation, despite the value of its temperature coefficient of resistance, α ( T ) = ( 1 / R ) ( ∂ R / ∂ T ) , actually being decreased by light. It is proposed that the effect of light is to populate excited states of the acceptors, which have larger wave functions and hence show enhanced hopping.

Properties of Tungsten Thin Films Produced with the RF-Sputtering Technique

For the purpose of building very sensitive light and phonon detectors, as e.g. applied in the Dark Matter (DM) experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers), transition edge sensors (TESs) in combination with a massive absorber crystal are used. To ensure high sensitivity of the detectors, low heat capacities, i.e. low working temperatures of about 10 mK are aimed at. Therefore, TESs made of tungsten thin films exhibiting the alpha-tungsten (α-W) phase with transition temperatures of T c =10–15 mK are required. We have produced tungsten thin films with T c in the range of 25–55 mK by rf-sputtering. To decouple the thermometer production from the choice of the target material and to avoid heating cycles of the absorber crystal, a composite design for detector production is applied. The composite design includes fabrication of the TES on a separate substrate and then attaching of this separate TES to a massive absorber crystal by gluing. For this purpose small sapphire substrates are used for the deposition of the TES. Properties of tungsten thin films grown with the rf-sputtering technique as well as first results of composite detectors built with these films acting as TESs will be presented.

Fabrication of Robust Superconducting Granular Aluminium/Palladium Bilayer Microbolometers with Sub-Nanosecond Response

We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminium bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminium film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al. (Acta Phys. Pol. A 103(4):325, [2003]) and direct nanosecond-pulsed photoexcitation to obtain the microbolometer’s characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al. (J. Phys. E Sci. Instrum. 22:582, [1989]) to obtain the phonon flux in a heat pulse experiment with nanosecond resolution.

Decoupling of optical generation and detection of acoustic phonons in semiconductor superlattices

We present a study of the generation and detection of terahertz acoustic phonons in GaAs/AlAs superlattices through femtosecond pump and probe optical pulses, where the spectral contributions of the generation and detection processes are decoupled and identified. The processes are spatially separated in different superlattices grown with an intermediate 1-µm-thick GaAs layer. Pumping and probing occur on opposite superlattices. A thickness gradient present on one of the superlattices allows us to identify the features in the spectra resulting either from the generation or from the detection. A Raman scattering characterization of the sample is also presented.

High-speed asynchronous optical sampling for high-sensitivity detection of coherent phonons

A new optical pump-probe technique is implemented for the investigation of coherent acoustic phonon dynamics in the GHz to THz frequency range which is based on two asynchronously linked femtosecond lasers. Asynchronous optical sampling (ASOPS) provides the performance of on all-optical oscilloscope and allows us to record optically induced lattice dynamics over nanosecond times with femtosecond resolution at scan rates of 10 kHz without any moving part in the set-up. Within 1 minute of data acquisition time signal-to-noise ratios better than 107 are achieved. We present examples of the high-sensitivity detection of coherent phonons in superlattices and of the coherent acoustic vibration of metallic nanoparticles.

Coherent generation of acoustic phonons in an optical microcavity

Coherent acoustic phonons confined in a nanocavity are generated in an optical microcavity. The confinement of the femtosecond light pulse in the optical resonator amplifies both processes, generation and detection of the acoustic phonons. In addition, due to the standing wave character of the photon field, phonons of wavevector q = 0 and q = 2k (k is the light wavenumber) contribute to both the generation and detection in time resolved reflectivity measurements, further optimizing the pump and probe experiments. Time resolved differential reflectivity experiments are reported as a function of laser energy. The optical cavity resonance is apparent in the amplitude and spectral features of the Fourier transformed signals.

Ultra-long-lived coherent acoustic phonons in GaN single crystals

We report our experimental and theoretical studies on generation and time-resolved detection of coherent acoustic phonons (CAPs) in bulk GaN single crystals, using the femtosecond optical pump-probe spectroscopy. A train of 100-fs-wide, ultraviolet laser pump pulses with the photon energy far above of the GaN bandgap induced electronic stress at the crystal surface and triggered propagation of CAP oscillations. Subsequently, time-delayed, low-intensity, 100-fs-wide, ultraviolet/infrared (one/two-colour setup) probe pulses monitored CAP propagation either near the surface or deeply inside the GaN crystal. The amplitude of CAP oscillations was on the order of 10-5-10-6 of the probe-beam transient differential reflectivity signal, and was only dependent on the pump-beam absorption coefficient. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to 8000 m/s-the speed of sound in GaN. In our two-colour arrangement, we could detect CAP oscillations as deeply as ~0.1 mm inside the crystal (the pump-probe delay ~13.2 ns), since the CAP signal attenuation was only limited by the absorption of the infrared probe light in GaN. The latter indicated that the intrinsic CAP lifetime in our crystals was actually ultra long and was estimated to be of the order of at least 100 ns.

Coherent phonons in a doped GaAs/AlAs superlattice

In this paper we report the excitation and detection of coherent zone-folded longitudinal phonons in a n-doped AlAs/GaAs superlattice using femtosecond pump-probe measurements. Oscillations in the probe reflectance were observed at a frequency of about 440 GHz, which is the frequency of the first mini-zone centre phonon mode. By applying an electrical bias across the superlattice, the intensity of the oscillations was increased when the energy drop per period of the superlattice was approximately equal to the energy of the first mini-zone centre mode. We attribute this observation to phonon amplification occurring in the superlattice.

Fabrication of robust superconducting granular aluminium/palladium bilayer microbolometers with sub-nanosecond response

We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminium bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminium film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al. [1], and direct nanosecond-pulsed photoexcitation to obtain the microbolometer's characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al. [2] to obtain the phonon flux in a heat pulse experiment with nanosecond resolution.

Coherent Generation of Acoustic Phonons in an Optical Microcavity

Ultrafast coherent generation of acoustic phonons is studied in a semiconductor optical microcavity. The confinement of the light pulse amplifies both the generation and the detection of phonons. In addition, the standing wave character of the photon field modifies the generation and detection phonon bandwidth. Coherent generation experiments in an acoustic nanocavity embedded in an optical microcavity are reported as a function of laser energy and incidence angle to evidence the separate role of the optical and exciton resonances. Amplified signals and phonon spectra modified by the optical confinement are demonstrated.

Femtosecond optical generation and detection of coherent acoustic phonons in GaN single crystals

We report our experimental and theoretical studies on the time-resolved generation and detection of coherent acoustic phonons (CAPs) in very high quality bulk GaN single crystals, performed using a femtosecond, two-color, all-optical pump-probe technique. A train of ultraviolet laser pulses with energy above the GaN energy gap induced the transient electronic stress at the GaN surface, responsible for the CAP generation. Subsequent CAP oscillations, propagating without any measurable intrinsic attenuation, were observed by scanning the transient differential reflectivity signal $(\ensuremath{\Delta}R∕R)$ of the near-infrared, far-below-band-gap probe beam. The $\ensuremath{\Delta}R∕R$ CAP oscillation amplitude was of the order of ${10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}6}$ and was dependent only on the pump-photon absorption coefficient spectral characteristics. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector) with the slope corresponding to $8002\ifmmode\pm\else\textpm\fi{}22\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$---the speed of sound in GaN---while the CAP signal phase was constant within the entire range of our experiments. The above experimental results are in excellent agreement with our theoretical modeling and the published literature data.

Generation and detection of coherent longitudinal acoustic phonons in the La0.67Sr0.33MnO3 thin films by femtosecond light pulses

The authors report on an anomalous first-to-zero sound crossover in the colossal magnetoresistance compound La0.67Sr0.33MnO3 (LSMO) using ultrafast time-resolved optical techniques. Two-color pump-probe setup was employed to record the photoinduced reflectance oscillations due to excitations of longitudinal acoustic phonons. By changing the sample thickness and probe wavelength, they determine the longitudinal sound velocities of both the thin film and substrate . The values of the sound velocity in LSMO at gigahertz frequencies are more than 20% larger than those from ultrasound experiments in the megahertz regime near TC. The results show the possible existence of dynamic polaron correlations in LSMO.