Optical emission of graphene and electron-hole pair production induced by a strong terahertz field

Abstract

We report on the first experimental observation of graphene optical emission induced by the intense THz pulse. P-doped CVD graphene with the initial Fermi energy of about 200 meV was used, optical photons was detected in the wavelength range of 340-600 nm. Emission started when THz field amplitude exceeded 100 kV/cm. For THz fields from 200 to 300 kV/cm the temperature of optical radiation was constant, while the number of emitted photons increased several dozen times. This fact clearly indicates multiplication of electron-hole pairs induced by an external field itself and not due to electron heating. The experimental data are in a good agreement with the theory of Landau-Zener interband transitions. It is shown theoretically that Landau-Zener transitions are possible even in the case of heavily doped graphene because the strong THz field removes quasiparticles from the region of interband transitions during several femtoseconds, which cancels the Pauli blocking effect. *oladyshkin@gmail.comThe nonlinear optical properties of graphene are currently actively investigated in view of their prospective use in plasmonics, optoelectronics, and photonics [1]. The specific features of gapless dispersion of Dirac fermions, including the so-called Dirac cones, make graphene a unique material. In the neighborhood of the Dirac point, fermions have a massless dispersion law up to energies of order 1.5 eV and high Fermi velocity 8 F 10 cm/s  , thus providing ultrahigh nonlinear susceptibility (both quadratic and cubic) of graphene in the infrared and terahertz ranges [2][3][4][5][6][7][8][9][10]. The interaction of graphene with terahertz radiation is presently arousing particular interest from the viewpoint of various applications [11]. Terahertz video pulses are also used for studying relaxation processes in graphene [12,13].An interesting effectthe THz-pulse-initiated carrier multiplication (CM) was studied experimentally in [12]. This effect was attributed in [12] to impact ionization (II), i.e., to interband reverse Auger recombination. At the same time, according to the theoretical results obtained in [14] within the framework of the 2D Fermi-liquid model, the Auger processes have low efficiency in the region of the linear dispersion law 1 ; consequently, in the simulation made in [12] the Auger resonances were broadened "artificially" within the framework of the numerical scheme 2 . The CM effect was also observed in the case of optical pumping [13,[16][17][18][19][20][21][22], when the contribution of the reverse Auger 1 The situation is different for graphene in magnetic field (see, e.g., [15]). 2 The authors of [12] assumed that dislocations could be a possible physical mechanism of such broadening.relaxation to the formation of electron population in the conducting band did not exceed 5-10% [17].It is worth noting that the ballistic (directly initiated by the field) interband transitions were neglected in [12] in view of the small photon energy as compared to the typical kinetic energy of fermions in doped graphe...

Nonequilibrium excitations and transport of Dirac electrons in electric-field-driven graphene

“…Figure 7(c) shares the characteristics proposed in Eq. (18). However, coupling to the inelastic phonons results in important differences.…”

Second harmonic generation in graphene dressed by a strong terahertz field

“…Indeed, in recent experiments including our experiment the THz pulses of several hundreds fs duration and field amplitudes E 0 ≈ 100 − 300 kV/cm were used; see e.g. [39,40]. Such fields distort the original Fermi distribution far beyond small perturbations: an electron acceleration to energies of the order of 0.2 eV (which is a typical Fermi energy for CVD graphene on a glass substrate) happens over a timescale of only a few fs [40].…”

“…[39,40]. Such fields distort the original Fermi distribution far beyond small perturbations: an electron acceleration to energies of the order of 0.2 eV (which is a typical Fermi energy for CVD graphene on a glass substrate) happens over a timescale of only a few fs [40]. In such strong fields, a standard expression for the current J Dr using the Drude relaxation time of ∼ 1 ps adopted in [11] would yield a current amplitude much higher than the maximum possible value ev F N , where N is the surface density of carriers!…”

“…In such strong fields, a standard expression for the current J Dr using the Drude relaxation time of ∼ 1 ps adopted in [11] would yield a current amplitude much higher than the maximum possible value ev F N , where N is the surface density of carriers! Furthermore, in ultra-strong fields the carrier density gets multiplied by a large factor during the THz pulse due to the electron-hole pair creation [39,40].…”

Simulating dynamically assisted production of Dirac pairs in gapped graphene monolayers

“…Indeed, recent measurements of optical radiation emitted from regular graphene layers [∆ε = 0] irradiated by ultrashort terahertz pulses, provide strong evidence about the interband transition of electrons in the field of the THz pulse. The radiation is presumably caused by the recombinations of electronhole pairs created via the Schwinger mechanism [25]. See also Refs.…”