Coherence of terahertz emission from photoexcited electron-hole plasma: Hydrodynamic model and Monte Carlo simulations

Abstract

Terahertz emission from the electron-hole plasma excited by the femtosecond laser pulse in semiconductor structures is analyzed. It is shown that plasma excitations in the homogeneous structures are incoherent. The incoherence of the excitations essentially reduces the intensity of terahertz radiation. The -doped heterostruc- ture for coherent plasma excitations is suggested. The Monte Carlo simulations show that the efficiency of terahertz emission from the suggested structure exceeds the efficiency of terahertz emission from semiconduc- tor surface and from p-i-n structures by 2 orders of magnitude. Our analysis of the transient dynamics of photoexcited carriers shows that the surface and contactless p-i-n terahertz emitters have serious deficiencies that significantly reduce the intensity of terahertz radiation. First, only a minor part of the excited carriers contributes to the transient current if the inverse absorption coefficient exceeds the surface depletion width or the i layer thickness of the p-i-n structure. In this case, the major part of carriers is created in the region where the built-in electric field is screened by the extrinsic carriers. Second, the plasma frequency of the excited carriers is posi- tion dependent due to the exponential decay of the created carrier density. As a result, the transient response of the cre- ated carriers is incoherent in different regions of the structure because the frequency of current oscillations is position de- pendent. Therefore, the amplitude of oscillations of the re- sulting conductivity current is significantly reduced by the interference effects. In this work, we show that terahertz en- ergy radiated from the contactless structures is significantly enhanced when such deficiencies are eliminated. The -doped p-i-n heterostructure for the efficient terahertz gen- eration is suggested. The transient dynamics of photoexcited carriers is analyzed by using the hydrodynamic model and the ensemble Monte Carlo EMC simulations. The EMC simulations of the transient dynamics of photo- excited carriers are performed following Ref. 6. The models of GaAs and AlGaAs are taken from Ref. 9. The multichan- nel intercarrier scattering, the Pauli exclusion principle, and the photoexcitation of electron-hole pairs are considered. 6 The central photon energy 0 is taken to be 1.55 eV. The full width at half maximum F of the Gaussian optical pulse in- tensity is set to 100 fs. The exponential spatial profile of the density of photoexcited carriers is considered. The carrier thermionic emission over the potential barriers of heterojunc- tions and the tunneling through the barriers are taken into account. The triangular and trapezoidal shapes of the poten- tial barriers are considered for the carrier tunneling. 9 To investigate the role of the incoherence of plasma exci- tations, we consider the contactless GaAs p-i-n structure. 5,6 We use the hydrodynamic approach for the nonlinear analy- sis of the transient dynamics of photoexcited carriers taking into account the spatially nonuniform photoexcitation. The dynamics of the excited electron-hole plasma in the contact- less p-i-n structure is analyzed under the following simplify- ing assumptions. Let us assume that carriers are instantly generated, i.e., the intensity of the optical excitation fluence It can be approximated as It=I0t. This assumption is valid if the condition Ff1 is satisfied, where f is the fre- quency of plasma oscillations. Because the net current of the conductivity and displacement currents is equal to zero in the contactless structure, the spatiotemporal variations of the electric field Ez,t, electron and hole average velocities vez,t and vhz,t, and electron and hole densities nz,t and pz,t obey the equation