Near-Field Analysis of Terahertz Pulse Generation From Photo-Excited Charge Density Gradients

Abstract

Abstract-Excitation of photo-current transients at semiconductor surfaces by sub-picosecond optical pulses gives rise to emission of electromagnetic pulses of terahertz (THz) frequency radiation. To correlate the THz emission with the photo-excited charge density distribution and the photo-current direction, we mapped near-field and far-field distributions of the generated THz waves from GaAs and Fe-doped InGaAs surfaces. The experimental results show that the charge dynamics in the plane of the surface can radiate substantially stronger THz pulses than the charge dynamics in the direction normal to the surface, which is generally regarded as the dominant origin of the emission.

Generation of radially-polarized terahertz pulses for coupling into coaxial waveguides

“…The transient dipoles radiate pulses of THz waves with a spatial profile determined by the optical beam profile. A recent study of the field distribution of THz pulses generated at InGaAs surfaces photo-excited at normal incidence confirmed the significance of this mechanism23, which has been corroborated with Monte Carlo simulations25. For a Gaussian beam incident on the semiconductor surface, the transient dipole moments are arranged radially and the radiated THz pulse therefore is expected to be radially-polarized with the intensity profile in a doughnut shape26 similar to the TEM mode of the coaxial metallic waveguide.…”

“…Within the THz generation model23, we expect that the profile of the generated THz pulse depends on the waist of the optical excitation beam. Specifically, the ring of maximum intensity in the doughnut shaped THz beam is expected to have a diameter similar to the optical beam waist.…”

“…However the ionization process also requires high-power amplified ultra-short laser pulses and the generated THz field in both schemes exhibits a conical wavefront. Recently, it was demonstrated that transient photo-currents excited on semiconductor surfaces by short (100 fs) optical pulses can have a large radial component due to the photo-generated charge density gradients20212223 and anisotropic conductivity24. This mechanism can provide efficiency, simplicity and tunability (i.e., dynamic control) in generation of THz beams with a tailored polarization state (e.g., radially-polarized THz beam) since the polarization state is governed by the shape of the optical beam and does not require bias.…”

“…Upon photoexcitation of a direct bandgap semiconductor by a short optical pulse, the photo-generated non-equilibrium clouds of electrons and holes undergo different dynamics leading to unbalanced transient spatial distribution of charge with corresponding dipole moments oriented in the surface plane23. The transient dipoles radiate pulses of THz waves with a spatial profile determined by the optical beam profile.…”

Generation of Terahertz Radiation from Fe-doped InGaAsP Using 800 nm to 1550 nm Pulsed Laser Excitation

“…Under optical excitation of 40 μJ/cm 2 , Fe:InGaAsP was found to withstand AC bias fields up to 35 kV/cm before breakdown. It should be noted that THz emission was also realized at zero bias voltage, which we attributed due to the gradient of the photogenerated carriers at the excitation area [29]. Fe-doped InGaAsP emitter (300 μm gap) at a 50-V bias, when it was excited with only 1 mW average laser power at 1550 nm.…”