Abstract
Coherent excitation and control of lattice motion by electromagnetic radiation in optical frequency range has been reported through variety of indirect interaction mechanisms with phonon modes. However, coherent phonon excitation by direct interaction of electromagnetic radiation and nuclei has not been demonstrated experimentally in terahertz (THz) frequency range mainly due to the lack of THz emitters with broad bandwidth suitable for the purpose. We report the experimental observation of coherent phonon excitation and detection in GaAs using ultrafast THz-pump/optical-probe scheme. From the results of THz pump field dependence, pump/probe polarization dependence, and crystal orientation dependence, we attributed THz wave absorption and linear electro-optic effect to the excitation and detection mechanisms of coherent polar TO phonons. Furthermore, the carrier density dependence of the interaction of coherent phonons and free carriers is reported.
Highlights
THz wave technologies have been providing a powerful tool to access and manipulate variety of material properties including molecular vibration[1,2], lattice and electronic dynamics[3,4,5], coherent antiferromagnetic spin wave[6], and so on[7,8,9]
We show that the transverse optical (TO) phonon excitation mechanism by THz pulses is the direct coupling of electromagnetic wave to infrared active mode unlike the case of optical excitation of coherent phonon in GaAs where plasmon-longitudinal optical (LO)-phonon coupling mediates the optical excitation of phonons
The linear dependence of the phonon amplitude on the pump electric field indicates the direct coupling between the THz pump electric field and dipole moment carried by polar phonons as the generation mechanism for TO phonons
Summary
THz wave technologies have been providing a powerful tool to access and manipulate variety of material properties including molecular vibration[1,2], lattice and electronic dynamics[3,4,5], coherent antiferromagnetic spin wave[6], and so on[7,8,9]. Ionic Raman scattering (IRS)[20] using mid-infrared pulses has been demonstrated where a rectified phonon field can exert a force onto the crystal lattice and initiate coherent vibrations of atoms In this phonon generation mechanism, photons interact with nuclei and higher energy mid-infrared phonon state plays a role of intermediate energy state of Raman process instead of electronic energy state. The excitation of coherent optical phonons by the direct coupling of photon and phonon provides a new tool of coherent control of polar lattice motions while electronic state remains unperturbed. The main reason of the lack of the study on this mechanism is due to the limited bandwidths of commonly used solid state THz radiation sources such as LiNbO3, ZnTe, GaP27–29 In many of these THz emitter crystals, the narrower bandwidth originates from the phonon absorption within the emitter crystals.
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