Terahertz Surface Emission Research Articles

Transient Anisotropic Photocurrent Induced Terahertz Emission from the Surface of Black Phosphorus

Anisotropic second‐order nonlinear optical effects have not been experimentally observed in bulk black phosphorus (BP) because of the presence of inversion symmetry. Herein, it is experimentally shown that the anisotropic second‐order nonlinear optical effect can be observed at the surface of BP with inversion symmetry breaking by terahertz emission spectroscopy. The dependence of the terahertz emission on the azimuthal angle, pump power, and polarization states is investigated in a reflection configuration. The results can be well fitted by photogalvanic theory, which proves that the terahertz emission process is dominated by a second‐order nonlinear optical effect. In addition, it is demonstrated that the oxidization process at the surface can protect the terahertz surface emission from the bulk BP. These results prove the BP crystal can be used as an alternative terahertz emitter and the radiation mechanism shows that the second‐order nonlinear optical effect can occur at the surface of the bulk BP crystal.

Terahertz Surface Emission from MoSe2 at the Monolayer Limit.

The surface-charge region of bulk and monolayer MoSe2 is analyzed directly by terahertz (THz) surface emission spectroscopy in a nondestructive way. Both surface nonlinear optical polarization and surface field-induced photocurrent contribute to the THz radiation in both bulk and monolayer MoSe2. The first THz emission mechanism is due to the surface optical rectification and the second one is due to the photogenerated carriers accelerated by the surface depletion field. The THz radiation contribution from the surface optical rectification is basically the same for both bulk and monolayer MoSe2 because of the same symmetry at the surface. However, the contribution from the surface field-induced photocurrent is ∼94.2% in bulk MoSe2 and it goes down to 74.5% in monolayer MoSe2. This is due to the larger surface depletion field in bulk MoSe2 (∼2.54 × 107 V/m) compared with that in monolayer MoSe2 (∼5.42 × 105 V/m), as such THz emission from the bulk is approximately four times larger than that from monolayer MoSe2. This work not only proves the clear THz radiation mechanism from MoSe2 crystals but also affords a THz technology for the surface characterization of two-dimensional materials.

Terahertz surface emission from layered semiconductor WSe2

Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggests that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2 confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this semiconductor for THz devices.

Terahertz Surface Emission from Layered MoS2 Crystal: Competition between Surface Optical Rectification and Surface Photocurrent Surge

Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable-angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make an important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump-dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary conditions. The maximum of the surface depletion field is estimated to be 1.45 × 104 V/cm with 130 nm in depth under −40° incidence. Interestingly, when the incident angle is tuned from −40° to 0°,...

Transversely superimposed gratings for narrow far-field terahertz surface emission of nonlinear quantum cascade lasers

We present terahertz (THz) surface emission by difference frequency generation in nonlinear quantum cascade lasers operating at room temperature. The device comprises two separate, transversely superimposed gratings, one for selective feedback for the midinfrared (MIR) modes and one for normal surface emission of the generated THz radiation. This allows for narrow far-field THz emission with an enhanced extraction over the entire device length with improved electrical and thermal properties compared to previous devices relying on Cherenkov phase matching. The MIR grating is realized as a higher order loss-coupled distributed feedback grating for single-mode emission at two distinct wavelengths. Its position determines the position of the standing wave pattern of the MIR modes and of the nonlinear polarization wave. This allows for a precise placement of the second-order top grating, which leads to perpendicular surface emission. The device emits in a single-lobed far-field with a full width half maximum of 3.5 deg in single-mode operation at room temperature.

Terahertz surface emission from Cu2ZnSnSe4 thin film photovoltaic material excited by femtosecond laser pulses

We observed efficient terahertz (THz) emission from sol-gel grown Cu2ZnSnSe4 (CZTSe) thin films using THz time domain spectroscopy technique. The THz emission bandwidth exceeds 2 THz with a dynamic range of 20 dB in the amplitude spectrum. The THz emission amplitude from CZTSe is found to be independent of external magnetic fields. Comparing the polarity of THz emission waveforms of CZTSe and GaAs, we suggest that the acceleration of photo-carriers in the surface accumulation layer of CZTSe is the dominant mechanism of radiation emission. Optical excitation fluence dependence measurements show that the saturation fluence of the CZTSe thin film reaches 1.48 μJ/cm2.

A review of terahertz sources

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

Terahertz emission from semi-insulating GaAs with octadecanthiol-passivated surface

Terahertz (THz) emission from octadecanthiol (ODT) passivated (100) surface of the semi-insulating GaAs was measured, and compared with those from the native oxidized and the fresh surfaces. It was shown that the self-assembled ODT monolayer can stabilize the GaAs (100) surface, and maintain a THz surface emission 1.4 times as efficient as the native oxidized surface under equal conditions. Surface passivation can reduce the built-in electric field in the depletion region of the GaAs (100), resulting in the suppression of the THz radiation to a different extent. Oxidation of GaAs surface reduces the THz amplitude mainly in the low-frequency region. These results indicate that GaAs can be made a more effective THz source by choosing molecular passivation technique. Conversely, the THz emission features such as polarity, amplitude, and phase from molecule-passivated surfaces may be used to characterize the attached molecules.

Impact of n‐type doping on the terahertz surface emission from c‐plane InN

AbstractWe theoretically investigate the influence of n‐type doping on the terahertz (THz) electric field emission from unbiased c‐plane InN surfaces using the ensemble Monte Carlo (MC) method. It is shown that the increase of n‐type doping has twofold effect on the THz surface emission. The detrimental effect of electron drift mobility decrease is compensated by doping‐derived electrons constructively contributing to the total dynamic dipole responsible for a generation of the THz electric field pulse emission from InN surface.

Influence of band structure and intrinsic carrier concentration on the THz surface emission from InN and InAs

We investigate the band structure effects on the terahertz (THz) electric field emission from unbiased InN and InAs surfaces using the ensemble Monte Carlo (MC) simulation method. It is shown that the nonparabolicity of the central conduction band valley and the energy separations between the central and satellite valleys are crucial parameters to provide a high efficiency of THz surface emitters. The THz emission from InN and InAs surfaces exposed to femtosecond laser pulses are compared over a wide range of photon energies from 0.85 eV (λ = 1550 nm) up to 1.55 eV (λ = 800 nm). We also demonstrate that the relatively high intrinsic carrier concentration of InAs (∼8 × 1014 cm−3) has a negligibly small effect on the THz surface emission.