Coherent Longitudinal Optical Research Articles

Decay time extension of terahertz electromagnetic waves emitted from coherent longitudinal optical phonons in GaAs epitaxial layers with the use of fast atom bombardment

We succeeded in extending the decay time of terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons in GaAs epitaxial layers with the use of fast atom bombardment, a treatment method for introducing defects and/or disorders at the surface. The decay time becomes long, up to 4.81 ± 0.15 ps, with the bombardment time of 4.0 min. This value is 2.4-times larger than the decay time of 2.04 ± 0.04 ps of the reference sample (untreated sample). We attribute the origin of the present phenomenon to the fact that the reduction of the photogenerated carrier scattering effect on the coherent LO phonon, which is caused by the presence of the surface defects and disorders formed by the fast atom bombardment, results in the extending of the decay time of the terahertz wave from the coherent LO phonon.

The effect of ion irradiation on dephasing of coherent optical phonons in GaP

The dephasing of coherent longitudinal optical (LO) phonons in ion-irradiated GaP has been investigated with a femtosecond pump-probe technique based on electro-optic sampling. The dephasing time of the coherent LO phonon is found to be dramatically prolonged by the introduction of a small amount of defects by means of Ga-ion irradiation. The maximum dephasing time observed at room temperature is 9.1 ps at a Ga+ ion dose of 1013/cm2, which is significantly longer than the value of 8.3 ps for GaP before ion irradiation. The longer dephasing time is explained in terms of the suppression of electron-LO-phonon scattering by the presence of defect-induced deep levels.

Terahertz electromagnetic waves radiated from coherent longitudinal optical (LO) phonons and LO-phonon plasmon coupled modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals

We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This finding indicates that the coupling of the LO phonon with the plasmon is not influenced by the surface orientations of the three samples.

Effects of photogenerated carrier scattering on the decay process of coherent longitudinal optical phonons in an undoped GaAs/n-type GaAs epitaxial structure investigated by terahertz time-domain spectroscopy

The authors investigated the scattering effect of photogenerated carriers on the decay of the coherent longitudinal optical (LO) phonons in an undoped GaAs/n-type GaAs epitaxial structure using a terahertz time-domain spectroscopic technique. The terahertz wave from the coherent LO phonon was observed together with those emitted both by the ultrafast photocurrent and by the coherent LO-phonon-plasmon coupled (LOPC) mode. The simultaneous observation of the coherent LO phonon and coherent LOPC mode originates from the fact that the photogenerated carrier density laterally distributes around the surface of the undoped GaAs layer owing the Gaussian profile of the pump beam. The authors found that the terahertz wave from the coherent LO phonon lives up to 5.0 ps, whereas those from the coherent LOPC mode and ultrafast photocurrent disappear within 1.0 ps. The decay time of the coherent LO phonon monotonically decreases with an increase in the pump power. This finding indicates that the dephasing of the terahertz wave from the coherent LO phonon is dominated by the photogenerated carrier scattering in the time range shorter than 1.0 ps in which photogenerated carriers remain.

Enhancement mechanism of terahertz radiation from coherent longitudinal optical phonons in undoped GaAs/n-type GaAs epitaxial structures

We have investigated the characteristics of monochromatic terahertz electromagnetic waves emitted from coherent longitudinal optical (LO) phonons in undoped GaAs/n-type GaAs epitaxial structures with various thicknesses, which range from 200 nm to 1200 nm, of the undoped GaAs layer. The terahertz waves were measured at room temperature using an optical gating method with a photoconductive dipole antenna. It was found that the intensity of the terahertz waves markedly depends on the thickness of the undoped GaAs layer. The analysis of Franz-Keldysh oscillations observed with photoreflectance spectroscopy demonstrates that the built-in electric field strength in the undoped GaAs layer, which results from the Fermi-level pinning at the surface, increases from 6.1 kV/cm to 28.4 kV/cm with a decrease in the thickness. It was revealed that the amplitude of the terahertz wave is proportional to the electric field strength in the undoped GaAs layer. This fact indicates that the efficiency of the terahertz radiation is dominated by the initial polarization of the LO phonons induced by the built-in electric field, which provides us a simple strategy for the enhancement of the terahertz radiation from the coherent LO phonons.

Terahertz radiation from the coherent longitudinal optical phonon-plasmon coupled mode in an i-GaAs/n-GaAs epitaxial structure

We have investigated the time-domain terahertz radiation from an undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structure using an optical gating method with a femtosecond pulse laser. We have found from the Fourier power spectra of the terahertz waveforms that two terahertz bands, the frequencies of which obviously depend on the pump power, appear in a high density excitation regime in addition to a broad band due to a surge current and a sharp band due to the coherent longitudinal optical (LO) phonon. Based on a model for the LO phonon-plasmon coupled (LOPC) mode, we reasonably explain the pump-power (photogenerated-carrier-density) dependence of the frequencies of the two bands peculiar to the high density excitation. This fact demonstrates that the two terahertz bands are assigned to the lower and upper branches of the LOPC mode. The frequencies of the LOPC mode are determined only by the pump power. Thus, the LOPC mode originates from the i-GaAs layer; namely, the plasmon component of the surge current flowing through the i-GaAs layer couples with the coherent LO phonon.

Photogenerated‐carrier‐induced band bending effects on generation of a coherent longitudinal optical phonon in a GaAs buffer layer optically masked by a GaSb top epitaxial layer

AbstractWe demonstrate that, in a GaSb/GaAs epitaxial structure, the coherent longitudinal optical (LO) phonon in a GaAs buffer layer optically masked by a GaSb top layer is observed utilizing terahertz spectroscopy. It is confirmed from Raman scattering measurements that only the optical phonons in the GaSb layer is observable. In the terahertz‐wave measurement, the Fourier power spectrum of a terahertz waveform exhibits both the coherent GaAs and GaSb LO phonon bands; namely, the coherent LO phonon in the optically masked GaAs buffer layer is observed. This fact demonstrates that the instantaneous surface potential modulation, which originates from the impulsive carrier excitation by a pump beam, reaches the GaAs buffer layer. We perform a time‐partitioning Fourier transform analysis in order to investigate the decay dynamics of the coherent GaAs and GaSb LO phonons. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Terahertz spectroscopy of dynamics of coupling between the coherent longitudinal optical phonon and plasmon in the surge current of instantaneously photogenerated carriers flowing through the i-GaAs layer of an i-GaAs/n-GaAs epitaxial structure

We demonstrate the dynamics of coupling between the coherent longitudinal optical (LO) phonon and plasmon of instantaneously photogenerated electrons in an undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structure using time-domain terahertz spectroscopy. Initially, we experimentally and numerically clarify the presence of the built-in electric field in the i-GaAs layer of the i-GaAs/n-GaAs epitaxial layer. Next, we performed the terahertz-wave measurements of the i-GaAs/n-GaAs epitaxial structure at various excitation conditions from a low density excitation regime to a high excitation regime. The LO-phonon-plasmon coupled (LOPC) mode has been confirmed from the terahertz-wave measurement. It is found that the frequency of the LOPC mode is determined by the pump-beam power. This fact demonstrates that the LOPC mode is formed in the i-GaAs layer. In addition, we performed the time-partitioning Fourier transform in order to reveal the dynamical change in the LOPC mode as a function of time delay. Using this analysis, we have observed that the disappearance of the LOPC mode immediately occurs within the time delay of 0.6 ps. Following the disappearance of the LOPC mode, only the bare coherent GaAs LO phonon dominates the terahertz waves.

Emission of the terahertz electromagnetic wave from coherent longitudinal optical phonons in a GaAs buffer layer optically masked by a GaSb top epitaxial layer

We demonstrate that, in a GaSb/GaAs epitaxial structure, the coherent longitudinal optical (LO) phonon in the GaAs layer optically masked by the GaSb top layer is observed utilizing terahertz- electromagnetic-wave spectroscopy. It is confirmed from a Raman scattering measurement that only the optical phonon in the GaSb layer is optically observable, where the photon energy of the excitation laser beam was almost the same as that of the femtosecond pulse pump beam for the terahertz wave measurement. In the terahertz wave measurement, the Fourier power spectrum of the terahertz waveform exhibits both the GaAs and the GaSb LO phonons; namely, the coherent LO phonon in the optically masked GaAs buffer layer is observed in the terahertz wave measurement. This fact demonstrates that the instantaneous surface potential modulation originating from the impulsive carrier excitation by the pump pulses reaches the GaAs buffer layer. Consequently, the above-mentioned surface potential modulation generates the coherent GaAs LO phonon.

Simple strategy for enhancing terahertz emission from coherent longitudinal optical phonons using undoped GaAs/n ‐type GaAs epitaxial layer structures

AbstractWe have explored the feasibility of enhancing terahertz (THz) emission from coherent longitudinal optical (LO) phonons using simple epitaxial layer structures: undoped GaAs/n ‐type GaAs (i ‐GaAs/n ‐GaAs) epitaxial layer structures. Initially, using a numerical simulation we have confirmed that a decrease in the i ‐GaAs layer thickness d increases the built‐in electric field in the i ‐GaAs layer. From the THz wave measurement, it is observed in the THz waveforms of the samples that the monocycle oscillation, the so‐called first burst, resulting from the surge current of the photogenerated carriers, accompanies with a clear oscillatory profile with a period of 113 fs corresponding to the GaAs LO phonon frequency (8.8 THz). The Fourier transform spectra show that the intensity of the LO phonon band remarkably increases with a decrease in d that causes the enhancement of the built‐in electric field in the i ‐GaAs layer. Thus, we conclude that the utilization of the i ‐GaAs/n ‐GaAs structure is a useful way to enhance the THz emission from the coherent LO phonon. We also performed the time‐partitioning Fourier transform in order to investigate the decay time of the THz wave from the coherent LO phonon. The decay time of each sample is almost the same: 2.0 ps. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced terahertz emission from coherent longitudinal optical phonons in a quantum well structure under applied bias

We report on the characteristics of terahertz waves emitted from coherent longitudinal optical (LO) phonons in a GaAs/AlAs multiple quantum well structure under an applied electric field. It is found that the intensity of the terahertz wave from the coherent LO phonon is resonantly enhanced in comparison with the intensity in a low electric-field region, under the condition that an intersubband energy is tuned to the LO-phonon energy of GaAs. The pump-energy dependence of the terahertz-wave intensity indicates that the enhancement originates from the double resonances in the Raman scattering process.

Interactions between coherent optical phonons and excitonic quantum beats in GaAs/AlAs multiple quantum wells: strategy for enhancement of terahertz radiation from coherent optical phonons

AbstractWe have investigated the dynamical properties of coherent longitudinal optical (LO) phonons and excitonic quantum beats in GaAs/AlAs multiple quantum wells with various layer thicknesses, utilizing a reflection‐type pump‐probe technique and an optical gating method with a photoconductive dipole antenna. It is found that the pump‐probe signal of the coherent GaAs‐like LO phonon is markedly enhanced under the energy‐resonance condition of the LO phonon and the quantum beat due to the fundamental heavy‐hole and light‐hole excitons. This enhancement is caused by the coupling between the coherent LO phonon and excitonic quantum beat via the longitudinal polarization; namely, the quantum beat acts as a driving force for the coherent LO phonon. On the basis of this finding, we have demonstrated that the terahertz radiation from the coherent LO phonon is considerably intensified at room temperature by the longitudinal polarization coupling described above. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intense coherent longitudinal optical phonons in CuI thin films under exciton–excitation conditions

We have investigated the dynamical properties of the coherent longitudinal optical (LO) phonon in CuI thin films grown on a NaCl substrate by vacuum deposition. The intense coherent LO phonon in the CuI thin film is observed under the exciton–excitation conditions. Moreover, the pump-energy dependence of the amplitude of the coherent LO phonon shows peaks at the heavy-hole and light-hole exciton energies. The enhancement of the coherent LO phonon under the exciton-resonance condition is much larger than that in an ordinary semiconductor quantum well system such as a GaAs/AlAs one. These facts demonstrate that the intense coherent LO phonon is generated under the exciton–excitation condition in a material with a strong exciton–phonon interaction such as CuI.

Characteristics of coupled mode of excitonic quantum beat and coherent longitudinal optical phonon in GaAs/AlAs multiple quantum wells

We have investigated the coupled mode of excitonic quantum beats and coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells with the splitting energy of heavy- and light-hole excitons close to the LO phonon energy. The time-domain signals show the coherent oscillation of the coupled mode between the quantum beat and the LO phonon with a fast dephasing time and the long-lived coherent GaAs-like LO phonon oscillation. The dependence of the coupled-mode frequency on splitting energy exhibits an anticrossing behavior. We demonstrate that the amplitude of the long-lived coherent LO phonon is manipulated by controlling the coupled mode with a double-pulse excitation technique.

Numerical Simulation of THz Radiation by Coherent LO Phonons in GaAs p–i–n Diodes under High Electric Fields

A numerical simulation of terahertz (THz) radiation by coherent longitudinal optical (LO) phonons in GaAs p–i–n diodes under high electric fields is presented, with emphasis on the important role of a velocity overshoot of free carriers in coherent phonon generation. The dynamics of non-equilibrium photoexcited carriers is simulated by adopting a self-consistent ensemble Monte Carlo method. The explicit comparison of THz signals between calculations and experiments is carried out by taking into account the electronic and ionic accelerations. It is shown that the coherent phonon oscillations emitting the THz radiation are excited resonantly by synchronization between the proper lattice oscillation and ultrafast transients of electronic polarization within the initial period of lattice vibration. The appearance of saturation in THz amplitude due to the coherent phonon above 150 kV/cm as well as a significant phase shift of the oscillatory THz signal is predicted.

Bias-dependent carrier and phonon dynamics in semiconductor–metal heterointerfaces

Coherent longitudinal optical phonons in a GaAsP/Au heterointerface are impulsively generated and detected with time-resolved second-harmonic generation. It is found that the photoexcited electrons, ballistically transporting from the metal layer into the semiconductor region, can rapidly screen the near surface depletion field. This transient screening process launches coherent longitudinal optical (LO) phonons in the heterointerface. The relaxation time of photoexcited nonthermal equilibrium electrons and the dephasing times of coherent LO phonons decrease significantly as a forward bias is applied to the heterointerface. This phenomenon is attributed to the strong carrier–carrier and carrier–phonon scatterings induced by a large direct current flowing across the heterointerface.

Coupling of coherent longitudinal optical phonons to excitonic quantum beats in GaAs/AlAs multiple quantum wells

We have investigated the dynamical properties of the coherent oscillations due to the GaAs-like longitudinal optical (LO) phonon and the quantum beat of excitons in two samples of GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole and light-hole excitons. The coherent oscillations were measured with a reflection-type pump-probe technique. In the multiple quantum well with the splitting energy almost equal to the energy of the GaAs-like LO phonon, the intensity of the coherent LO phonon are markedly enhanced in comparison with that of the other sample, and the pump-energy dependence of the intensity of the coherent GaAs-like LO phonon shows similar profile to that of the excitonic quantum beat. Moreover, the intensity of the coherent GaAs-like LO phonon is linearly increased with an increase in pump power, which is contrary to a usual generation mechanism, i.e., the displacive excitation of coherent phonon leading to a square dependence. These facts indicate that the excitonic quantum beat acts as a driving force for the coherent GaAs-like LO phonon in the case that the splitting energy of the heavy-hole and light-hole excitons is almost equal to the LO phonon energy.

Coherent phonon spectroscopy of GaP Schottky diode

Coherent longitudinal optical and acoustic phonons in GaP Schottky diode are impulsively generated and detected with time-resolved second-harmonic generation. A coherent longitudinal optical (LO) phonon is launched in the near-surface depletion region of GaP. The dephasing time of this coherent LO phonon is ∼10 ps and mainly due to the anharmonic decay into two zone-edge half-energy longitudinal acoustic (LA) phonons. Simultaneously, a coherent LA wave packet is created in the metal–semiconductor interface via transient thermal absorption in the metal thin layer. This acoustic wave packet propagates into the bulk with the sound velocity ∼5.8×105 cm/s of a GaP LA phonon.

Simultaneous Observation of Coherent GaSb-like and AlSb-like Longitudinal Optical Phonons in GaSb/AlSb Superlattices

We have investigated coherent longitudinal optical (LO) phonons in GaSb/AlSb superlattices. The coherent AlSb-like LO phonon in the barrier layer and the coherent GaSb-like LO phonon in the well layer have been simultaneously observed. The GaSb-like LO phonon exhibits a cosine-type oscillation, which is the dominant signal. We have analyzed the initial phase of the AlSb-like LO phonon by plotting the imaginary part of the Fourier-transform components as a function of the real part, the so-called Cole–Cole's plot. We have found from this plot that the difference in the initial phase between the GaSb-like and AlSb-like LO phonons is about 90 degrees: The AlSb-like LO phonon has a sine-type oscillation. This difference in the initial phase indicates the difference in the generation mechanism between the coherent GaSb-like and AlSb-like LO phonons. We discuss the generation mechanism based on some proposed models.

Dynamics of coherent optical phonon generation and decay in GaP.

Coherent longitudinal optical (LO) phonons are generated through nonlinear excitation by laser light. It is observed that the vibronic sideband structure associated with coherent LO phonons, and that of the acoustic phonons, generated during the dephasing of the LO phonons have an unexpected dependence on the power of the excitation lasers. The effect is traced to the presence of an electron-hole plasma which is shown to influence the phonon generation process. Moreover, it is directly verified that the dephasing of coherent near-zone-center longitudinal optical phonons in GaP is dominated by anharmonic decay into two half-energy longitudinal acoustic phonons.