Investigation On Ultrafast Dynamics Of Coherent Acoustic Phonons In GaAs Crystals

With the development in femtosecond lasers and ultrafast spectroscopy,it's much easier to detect the oscillation modes of coherent phonon directly.In particular,coherent acoustic phonons(CAPs)excited by ultrashort optical pulses have been vastly investigated in semiconductors,metal films,multiple quantum wells and heterostructures by using pump-probe techniques.Femtosecond pump pulses are absorbed at sample surfaces,which initiates the generation of coherent phonons.The interaction between the coherent phonons and the electronic or excitonic states leads to the modulation of refractive index and the reflectivity of the materials.Thus,the transient signals of coherent phonons can be detected by monitoring a time-delayed probe light.This technique improves the application of nondestructive testing of CAPs.At the same time,the oscillation signals of CAPs are of great significance in detecting the thickness of interface layers of different materials,measuring the speed of sound,and studying the optical and acoustic properties of some materials.Our results can facilitate the understanding of ultrafast phonon dynamics in GaAs crystals.The main contents of this paper are as follows:(1)The transient reflection pump-probe optical system has been designed and set up,and a microscopic test system(K?hler illumination)was introduced into the system.Then,the CdSSe nanobelt sample was characterized and the position to be tested was marked by the microscopic system.Finally,this sample is used as a standard to determine the zero point of the pump-detection optical system,laying the foundation for later research.(2)We investigate the ultrafast dynamics of GaAs crystals with[100]orientation by using a two-color femtosecond pump-detection optical system.Under an above-bandgap pumping at 400 nm,the CAP_S wave is generated on the surface of the sample and then it propagates into the crystals and is detected by time-resolved relative variation of reflectivity,?R/R,of the probe beam at 800 nm.Experimental results show that as the pumping intensity is increased,the amplitude and lifetime of generated CAPs in GaAs crystals are increased and decreased respectively,while the phonons oscillation frequency remains a constant of~44.3 GHz.At the same time,we also explain the physical mechanism of the generation of CAP_S in GaAs crystals and the physical origin of the chirp.(3)We investigate the anisotropy of GaAs single crystals with different orientations of[100],[111],and[110].It was found that the CAPs frequency varies in GaAs single crystals with different orientations,which is essentially determined by the elastic anisotropy of GaAs crystals.Since the lattice stresses in GaAs is either tensile or compressive,the oscillation phase of[100]opposite to[111]and[110].The experimental results of the CAP_S oscillation frequency in GaAs crystals with different orientations tested by pump-probe technique are in good agreement with the theoretical calculation results.In addition,we also analyzed the CAP_S oscillation signal of Si-doped GaAs and found that the doped GaAs Brillouin oscillation frequency is slightly lower than that of undoped GaAs.