Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

Abstract

Abstract.We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers. 78.47.+p, 42.65.Re, Since the invention of the CPM ring dye laser, pump-probe techniques have widely been used to study relaxation of nonequilibria in condensed matter down to the subpicosecond time regime [1,2]. In many of these experiments periodic modulations of transmission or reflectivity changes are observed [3][4][5][6][7][8]. They are related to coherent lattice vibrations, modifying the optical dielectric functions. First theoretical explanations of this phenomenon invoked impulsive stimulated Raman scattering (ISRS) [9]. There, the coherent phonons are driven by the difference-frequency of two laser fields contained within an ultrashort laser pulse of sufficient spectral bandwidth. More recently, a displacive excitation mechanism of coherent phonons (DECP) has been discussed. In this case the phonon amplitude is driven by sudden perturbations of the electronic system [10]. Anisotropic hole distributions constitute the driving force for coherent phonons in Ge [11]. PACS:A completely different excitation mechanism is found in polar III-V-materials. Optical injection of free carriers by ultrashort laser pulses leads to an ultrafast depolarization of the crystal lattice by screening the electrical field in the surface-space-charge region [7]. This process can be so fast, that coherent longitudinal lattice vibrations are launched.In this paper, a detailed investigation of the generation mechanism for coherent LO phonons in surface-spacecharge regions of III-V-compounds will be presented. In Sect. 1, the classical equation of motion for the harmonic oscillator is presented, including specific driving force terms for coherent longitudinal optical phonons. In sect. 2 the experimental apparatus for performing time-resolved pumpprobe experiments is outlined. We use a reflective electrooptic detection geometry (time resolved REOS technique [12,13]), which is sensitive to longitudinal electrical field changes. Detailed studies of the generation process for coherent LO phonons in surface-space-charge fields are presented in Sect. 3. These experiments confirm that coherent LO phonons in surface-space-charge regions are launched by an ultrafast longitudinal depolarization of the crystal lattice induced by the screening of the surface field via the photoexcited free carriers. Driving Forces for Coherent LO PhononsThe most distinct feature of a coherent vibration is the fixed phase relationship among individual modes. Under this condition the a...

Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

“…We find the LO and LOPC amplitudes of n-GaAs to be independent of φ, in agreement with a previous transient reflectivity study 7 , and thereby confirm the TDFS generation 1 . For i-GaAs, by contrast, the amplitudes of the LO phonon [ Fig.…”

“…5(d)], indicating the direct relation between the generation and the photoexcited carrier density. B. Pump-polarization dependence at the E0 gap Kurz and coworkers examined the generation mechanisms of the coherent phonons in GaAs by changing the surface fields with external voltage 7,8 . The dependence of the phonon amplitude on the applied field was consistent with the TDFS excitation mechanism.…”

“…(3) represents ISRS, in which a broadband femtosecond optical pulse offers multiple combinations of two-photon difference frequencies required for the stimulated Raman process (hω 1 −hω 2 =hΩ) 3 . Nonlinear susceptibility of the second order (EO effect) can be included in the Raman term, while that of the third order is negligible for a zinc blende crystal in the present geometry 7 . The ISRS force depends on the direction of the applied electric field through the Raman tensor R jkl = (∂χ/∂Q) jkl .…”

“…(3) describes the ultrafast drift-diffusion current in the surface normal direction j in the presence of surface built-in field within the depletion or accumulation layer of a doped semiconductor. This current can launch coherent phonons polarized along the j direction via ultrafast screening of the built-in field (TDFS mechanism) [6][7][8] . The excitation via TDFS requires photoexcitation of carriers at sufficient density for screening of the field, but does not depend explicitly on the direction of the pump electric field for a zinc blende crystal 7 .…”

“…The coherent optical phonons have been observed as a periodic modulation in the THz frequency range, most conveniently by linear and non-linear optical techniques 1,2 . The generation mechanisms of the coherent phonons have been studied extensively in a variety of crystals and attributed, depending on the nature of the electron-phonon interaction, to impulsive stimulated Raman scattering (ISRS) 3,4 , displacive excitation of coherent phonons (DECP) 5 , and transient depletion field screening (TDFS) [6][7][8] . The time evolution of coherent phonons is in general determined by the phonon anharmonicity (phonon-phonon scattering) 9 and scattering by defects 10 .…”