Coherent Acoustic Phonon Oscillations Research Articles

Observation of photoinduced polarons in semimetal 1T-TiSe2

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron–phonon coupling in 1T-TiSe2. The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron–phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe2, and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.

Revealing the Substrate Dependent Ultrafast Phonon Dynamics in Bi2Se3 Thin Films

AbstractThe bulk carrier excitation influences the phonon dynamics, which can alter and modulate the surface charge density of topological insulators such as bismuth selenide (Bi2Se3). This work investigates the charge carrier and phonon dynamics in Bi2Se3 grown on various substrates. The orientation of the substrate, the size of the crystallites, and the misfit of the lattice affect the carrier and phonon dynamics. Bi2Se3 thin films are grown under the same growth conditions on SiO2, Si(111), and SiN. Bi2Se3 overlayers exhibit substrate‐dependent charge carrier relaxation channels and phonon dynamics. It is evident from Raman spectroscopy and ultrafast transient absorption spectroscopy that the heterointerface interactions of all three samples affect the vibration modes of Bi2Se3 and coherent acoustic phonon oscillations in the NIR range. At 13.6, 41.2, and 34.4 GHz, the vibration modes of SiO2, Si(111), and SiN are equivalent. The propagation depth of phonon waves is shown by measuring the speed of sound in Bi2Se3 overlayers on SiO2, Si(111), and SiN. This study demonstrates that the surface and bulk‐bound charge carriers of a topological insulator determine the frequency and velocity of the generated sound.

Coherent Acoustic Phonon Oscillations and Transient Critical Point Parameters of Ge from Femtosecond Pump–Probe Ellipsometry

Herein, the complex pseudodielectric function of Ge and Si from femtosecond pump–probe spectroscopic ellipsometry with 267, 400, and 800 nm pump–pulse wavelengths is analyzed by fitting analytical lineshapes to the second derivatives of the pseudodielectric function with respect to energy. This yields the critical point parameters (threshold energy, lifetime broadening, amplitude, and excitonic phase angle) of and in Ge and in Si as functions of delay time. Coherent longitudinal acoustic phonon oscillations with a period of about 11 ps are observed in the transient critical point parameters of Ge. From the amplitude of these oscillations, the laser‐induced strain is found to be on the order of 0.03% for Ge measured with the 800 nm pump pulse, which is in reasonable agreement with the strain calculated from theory.

Generation and coherent control of terahertz acoustic phonons in superlattices of perovskite oxides

We utilized transmission-type pump–probe technique to investigate coherent acoustic phonons in the superlattices of perovskite oxides such as SrIrO3/SrTiO3 and SrRuO3/SrTiO3. Because the films in the superlattices are of high-quality and their thicknesses are only several monolayers, quasi-monochromatic acoustic phonons with THz frequency have been achieved. By investigating the propagation of coherent acoustic phonons in the superlattices with different epitaxial periods, the phonon mean free paths of SrTiO3 were studied in the frequency range between 0.5 THz and 1 THz. We further demonstrated coherent control to amplify or cease the THz coherent acoustic phonon oscillations in the superlattices. By controlling the delay of two pulses for shining the superlattices, the amplitude and phase of the THz coherent acoustic phonons were manipulated. According to the measurements of time-domain Brillouin scatterings, we found the optoacoustic conversion efficiency of SrIrO3 outperforms that of SrRuO3 for generating acoustic phonons.

Thickness-dependent ultrafast charge-carrier dynamics and coherent acoustic phonon oscillations in mechanically exfoliated PdSe2 flakes.

Recently, palladium diselenide (PdSe2) has emerged as a promising material with potential applications in electronic and optoelectronic devices due to its intriguing electronic and optical properties. The performance of the device is strongly dependent on the charge-carrier dynamics and the related hot phonon behavior. Here, we investigate the photoexcited-carrier dynamics and coherent acoustic phonon (CAP) oscillations in mechanically exfoliated PdSe2 flakes with a thickness ranging from 10.6 nm to 54 nm using time-resolved non-degenerate pump-probe transient reflection (TR) spectroscopy. The results imply that the CAP frequency is thickness-dependent. Polarization-resolved transient reflection (PRTR) measurements reveal the isotropic charge-carrier relaxation dynamics and the CAP frequency in the 10.6 nm region. In addition, the deformation potential (DP) mechanism dominates the generation of the CAP. Moreover, a sound velocity of 6.78 × 103 m s-1 is extracted from the variation of the oscillation period with the flake thickness and the delay time of the acoustic echo. These results provide insight into the ultrafast optical coherent acoustic phonon and optoelectronic properties of PdSe2 and may open new possibilities for PdSe2 applications in THz-frequency mechanical resonators.

Anisotropic Charge Carrier and Coherent Acoustic Phonon Dynamics of Black Phosphorus Studied by Transient Absorption Microscopy

Due to its corrugated hexagonal lattice structure, Black phosphorus (BP) has unique anisotropic physical properties, which provides an additional freedom for designing devices. Many body interactions, including interactions with phonon, is crucial for heat dissipation and charge carrier mobility in device. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation and generation were not fully interrogated. In this paper, the polarization-resolved transient absorption microscopy was conducted on BP flakes to study the dynamics of photoexcited charge carriers and coherent acoustic phonon. Polarization-resolved transient absorption images and traces were recorded and showed anisotropic and thickness-dependent charge carriers decay dynamics. The damping of the coherent acoustic phonon oscillation was found to be anisotropic, which was attributed to the polarization-dependent absorption length of the probe pulse. From the analysis of initial oscillation amplitude and phase of coherent acoustic phonon oscillation, we proposed that the direct deformation potential mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we obtained the sound velocity of the coherent acoustic phonon from the oscillation frequency and the acoustic echo, respectively, which agreed well with each other. These findings provide significant insights into the rich acoustic phonon properties of BP, and promise important application for BP in polarization-sensitive optical and optoelectronic devices.

Fano resonance between coherent acoustic phonon oscillations and electronic states near the bandgap of photoexcited GaAs

Impulsive photo-excitation of solids results in a travelling strain pulse which manifests itself as coherent acoustic phonon oscillations. These oscillations have been extensively studied using time-resolved pump-probe spectroscopy. In the present work, we report the generation of extremely long-lived, coherent longitudinal acoustic phonon oscillations in intrinsic GaAs (100), with clear and unambiguous evidence of Fano interference between these oscillations and the continuum of electronic states close to the bandgap. Fano resonance is a widespread phenomenon observed in atomic systems and condensed media that arises from quantum interference between a continuum of quantum states and a discrete quantum state. Among other techniques, Fano resonance has been investigated with respect to optical phonons studied with Raman Spectroscopy. In the present work, we investigate Fano resonance in coherent phonon oscillations generated without the aid of any capping layer, dopants or substrate/interface effects. Since Fano resonance is sensitive to changes in electronic structure, doping and defects, these observations are important to the field of picosecond ultrasonics which is used for non-destructive depth profiling of solids and for carrier diffusion studies.

Coherent acoustic phonon oscillation accompanied with backward acoustic pulse below exciton resonance in a ZnO epifilm on oxide-buffered Si(1 1 1)

Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump–probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.

Theoretical study on ultrafast dynamics of coherent acoustic phonons in semiconductor nanocrystals

We present a theoretical study on the ultrafast dynamics of coherent acoustic phonons in semiconductor quantum dots using continuum model calculations. The excitonic states and the coherent acoustic vibrational modes of semiconductor quantum dots are calculated using the effective mass approximation and continuum elastic medium model, respectively. By solving the Liouville–von Neumann equation and the equation of motion, we obtain the oscillation of coherent acoustic phonon amplitude excited by a pump pulse laser. Owing to the ultrafast excitation of coherent phonons, both the amplitude and the phase of the coherent phonon oscillation are constant with time. This coherent phonon oscillation results in conservation of the coherence of the exciton state, which cannot exist in a system interacting with incoherent phonons. We further study the amplitude and the period of coherent acoustic phonon oscillation as a function of pump pulse energy detuning, quantum dot size, and material.

The influence of charge and magnetic order on polaron and acoustic phonon dynamics in LuFe2O4

Femtosecond optical pump-probe spectroscopy is used to reveal the influence of charge and magnetic order on polaron dynamics and coherent acoustic phonon oscillations in single crystals of charge-ordered, ferrimagnetic LuFe2O4. We experimentally observed the influence of magnetic order on polaron dynamics. We also observed a correlation between charge order and the amplitude of the acoustic phonon oscillations, due to photoinduced changes in the lattice constant that originate from the photoexcited electrons. This provides insight into the general behavior of coherent acoustic phonon oscillations in charge-ordered materials.

Coherent acoustic phonon generation and detection by femtosecond laser pulses in ZnTe single crystals

We report experiments of coherent acoustic phonon generations and detections using a two-color femtosecond pump-probe technique in [001] and [110] oriented ZnTe single crystals. The above-bandgap pump excitation initiates strain waves to yield coherent acoustic phonons. The generation and propagation of coherent acoustic waves not only modulate the intensity of the reflected probe light but also change its polarization periodically due to strain-induced birefringence. The nonuniform excitation due to Gaussian spatial distribution of pump pulses takes an important role in the polarimetric detection of coherent acoustic phonon oscillations.

Dynamics of coherent acoustic and optical phonon oscillations in nanostructures studied by a femtosecond pump-probe technique

Coherent phononic oscillations in Bi and Ag nanowires were studied with a femtosecond pump-probe technique. In Bi nanowires laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the THz range. The transmission of nanowires on a glass substrate and the light scattered from free standing nanowires were measured. In Ag nanowires laser-induced acoustic oscillations at different excitation levels were studied. The observed reduction of the oscillation frequency at higher pump energy was related to a transient softening of the material. This was directly confirmed for optical phonons by experiments with a pre-pump pulse of variable energy, producing different excitation densities.

Observation of coherent acoustic and optical phonons in bismuth nanowires by a femtosecond pump-probe technique

Coherent acoustic and optical phonon oscillations in Bi nanowire samples were studied with a femtosecond pump-probe technique. Laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the terahertz range. The transmission signal of nanowires on a glass substrate and the signal of light scattered from freestanding nanowires were measured. The acoustic velocity in nanowires was found to be close to that of bulk polycrystalline material. The changes in the optical phonon frequency at different laser fluences were simulated taking into account excitation inhomogeneity, lattice anharmonicity, diffusion, and recombination of the carriers and gave good agreement with experimental results.

Time-resolved dynamics of coherent acoustic phonons in CdMnTe diluted-magnetic single crystals

The authors report on observation of pronounced coherent acoustic phonon (CAP) oscillations in diluted-magnetic Cd1−xMnxTe (x=0.09) single crystals by means of femtosecond, optical single-color, and two-color pump-probe spectroscopy. The observed properties of CAP oscillations are consistent with the propagating-strain-pulse model, with electronic stress as the main CAP generation mechanism. The experimental dispersion relation between the CAP oscillation frequency and the probe-beam wave vector allowed them to extract the Cd0.91Mn0.09Te refractive index dependence on wavelength, while the dependence of oscillation amplitudes on the photon energy near the Cd0.91Mn0.09Te band gap edge indicated that the material energy gap is 1.63eV. Two-color pump-probe studies showed that the speed of sound in our material is 3590m∕s and that the absorption depth of probe light, rather than the decay time of CAPs, is the dominant source of the experimentally observed oscillation dephasing time. The intrinsic lifetime of CAPs in CdMnTe crystals has been found to be at least in the nanosecond range.

Femtosecond time-resolved study in InxGa1−xN (0001) ultrathin epilayers: Effects of high indium mole fraction and thickness of the films

In view of promising full-solar-spectrum photovoltaic systems based on InxGa1−xN ternary alloys, femtosecond time-resolved study in ultrathin epilayers was employed in order to extract the fundamental properties of material. Two different thicknesses of epilayers were employed with relative high indium mole fractions. State filling effect at various probing energy states has been observed for both epilayers. Saturation of state filling as well as enhanced photoinduced absorption occurred at higher probing wavelengths. Furthermore, coherent acoustic phonon oscillations were also observed for both ultrathin epilayers with a thickness dependent oscillation frequency. Finally, absorption band edge of these alloys has been determined.

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers: Effects of high fluence excitation

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x=0.07, 0.15, and 0.33, over a fluence range of 1–12mJ∕cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation.

Long-lived, coherent acoustic phonon oscillations in GaN single crystals

We report on coherent acoustic phonon (CAP) oscillations studied in high-quality bulk GaN single crystals with a two-color femtosecond optical pump-probe technique. Using a far-above-the-band gap ultraviolet excitation (∼270nm wavelength) and a near-infrared probe beam (∼810nm wavelength), the long-lived, CAP transients were observed within a 10 ns time-delay window between the pump and probe pulses, with a dispersionless (proportional to the probe-beam wave vector) frequency of ∼45GHz. The measured CAP attenuation corresponded directly to the absorption of the probe light in bulk GaN, indicating that the actual (intrinsic) phonon-wave attenuation in our crystals was significantly smaller than the measured 65.8cm−1 value. The velocity of the phonon propagation was equal to the velocity of sound in GaN.

Spectral analysis of high-harmonic coherent acoustic phonons in piezoelectric semiconductor multiple quantum wells

Multiple harmonic components have been observed in the Fourier spectra of the coherent acoustic phonon oscillations in InGaN/GaN multiple quantum wells (MQW's). The dependence of the relative magnitudes of the first two Fourier components on the sample geometry was analyzed by conducting time-resolved pump-probe measurements on MQW samples with different well-to-barrier ratios. The results were also compared with the recently proposed loaded-string model [Phys. Rev. B 64, 235316 (2001)], and an overall agreement is obtained which validates the picture of string oscillation with inhomogeneous loadings. We also demonstrated manipulation of the spectral components using the coherent optical control technique. Our demonstration can be regarded as a first step to the realization of wave-form synthesis of nanoacoustic waves.

Excitation of evanescent acoustic waves by ultrafast laser action on crystal surface (abstract)

Following the advances in femtosecond laser technology, significant progress was achieved during the last 10 years in excitation of terahertz coherent lattice vibrations [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Coherent terahertz phonons were excited in various type of crystals (in particular, in semiconductors and superconductors). However, until recently all-optical excitation and detection were reported on optical phonons only [W. A. Kütt, W. Albrecht, and H. Kurz, IEEE J. Quantum Electron. 28, 2434 (1992)]; [R. Merlin, Solid State Communications, 102, 207 (1997)]. Last year, ultrafast impulsive excitation of coherent acoustic phonon oscillations at frequencies around 4 THz was achieved [M. D. Cummings and A. Y. Elezzabi, Appl. Phys. Lett. 79, 770 (2001)]. We present here the results of theoretical analysis of acoustic signals excited by laser-induced mechanical stresses, which have spectral components around and above the cutoff frequency for propagating acoustic waves. For the analysis of both excitation process and of photo-excited wave propagation, discrete periodic structure of material should be taken into account. We are using for this purpose the simplest chain model of crystal. Analytical solutions, which reveal the role of strong acoustic velocity dispersion and the role of forbidden frequency band existence (where only evanescent modes are excited), are derived. Frequency spectra and temporal profiles of mechanical displacement are determined both at front (laser-irradiated) surface and at rear surface of photo-excited thin film. The contribution of the excited evanescent modes to the motion of the front surface is found. It is expected that evanescent modes might be very sensitive to possible surface and subsurface defects, and because of this they might be useful in nondestructive photoacoustic testing of surfaces and interfaces.

Generation of coherent acoustic phonons in strained GaN thin films

Coherent acoustic phonon oscillations were generated and studied in strained GaN thin films. Inside the bulk GaN film, the longitudinal interference of an ultraviolet femtosecond pump pulse created periodic carrier distribution that screened out the strain-induced piezoelectric field and initiated the coherent longitudinal acoustic phonon oscillations corresponding to the carrier periods. The created coherent phonon oscillation modulated the piezoelectric field thus modified the absorption property of the GaN thin film through Franz–Keldysh effect. This time-dependent absorption modulation was reflected in the transmission variation of the followed probe pulses, resulting a long decay time ∼300 ps for the initiated coherent phonon oscillations.