Optical Rectification Effect Research Articles

An interplay between optical rectification and transient photocurrent effect on THz pulse generation from bulk MoS2 layered crystal

Development of novel schemes for efficient terahertz (THz) generation from transition metal dichalcogenides are useful for realizing integrated THz devices based on them and also, understanding of the related fundamental processes from such studies will guide to suitable designs. Here, we report the THz emission efficiency of bulk MoS2 layered crystal at varying femtosecond excitation wavelengths, from 550 nm to the telecommunication wavelength of 1550 nm. By using both the below bandgap excitation at longer wavelengths and the above bandgap excitation at shorter wavelengths, we resolve THz emission contributions from resonant and non-resonant optical rectification (OR) processes, and the surface field induced transient photocurrent effect (TPE). A relatively much larger contribution to THz emission from the TPE than the resonant OR is measured for the above bandgap excitation. We have measured a clear difference between the resonant and nonresonant OR processes. The pure OR part is exclusively determined from detailed experiments using excitation intensity, polarization angle, and azimuthal angle dependent measurements. For the above bandgap excitation, the THz emission gets highly saturated with the increasing excitation intensity. Also, the value of the saturation intensity increases (decreases) with the excitation photon energy (wavelength). Interestingly, we find that the linear polarization angle and the azimuthal angle dependent THz signal due to resonant OR is π/2 phase offset relative to that due to the nonresonant OR.

Terahertz wave rectification in a ferroelectric triglycine sulfate single crystal.

The effect of optical rectification (OR) in the terahertz range (THz rectification, TR) is experimentally demonstrated. The effect consists of generating a DC voltage on the faces of a ferroelectric triglycine sulfate (TGS) single crystal under the action of pulsed radiation with a frequency of 1.57 and 1.96 THz and an electric field strength per pulse of 1.3 and 1.5 MV/m, respectively. The FLARE FELIX free-electron laser system (Radboud University, The Netherlands) was used as a THz radiation source. The TR effect makes it possible to directly determine the nonlinear susceptibilities of media (including those under conditions of strong absorption) without any reference or optical channel calibration and also without the need of Fourier transform.

Investigation of Giant Nonlinearity in a Plasmonic Metasurface with Epsilon-Near-Zero Film

Plasmonic metamaterials can exhibit a variety of physical optical properties that offer extraordinary nonlinear conversion efficiency for ultra-compact nanodevice applications. Furthermore, the optical-rectification effect from the plasmonic nonlinear metasurfaces (NLMSs) can be used as a compact source of deep-subwavelength thickness to radiate broadband terahertz (THz) signals. Meanwhile, a novel dual-mode metasurface consisting of a split-ring resonator (SRR) array and an epsilon-near-zero (ENZ) layer was presented to boost the THz conversion efficiency further. In this paper, to explore the mechanism of THz generation from plasmonic NLMSs, the Maxwell-hydrodynamic multiphysics model is adopted to investigate complex linear and intrinsic nonlinear dynamics in plasmonics. We solve the multiphysics model using the finite-difference time-domain (FDTD) method, and the numerical results demonstrate the physical mechanism of the THz generation processes which cannot be observed in our previous experiments directly. The proposed method reveals a new approach for developing new types of high-conversion-efficiency nonlinear nanodevices.

THz Pulse Generation and Detection in a Single Crystal Layout

The THz pulse of a few picosecond durations have been generated and detected via optical rectification and electro-optic effect within the same ZnTe crystal. An unbalanced single-shot detection scheme was performed to characterize the signal. As a result, a multicycle signal was obtained, in which two-photon absorption and other associated nonlinear effects were reportedly negligible. The experimental set-up is compact, economical, easy to build and has the added simplicity of facilitating an independent analysis of the horizontal or vertical polarization arm of the THz-modulated chirped probe beam. This work finds a useful application in integrated THz devices, narrow-band THz phonon spectroscopy and spectroscopic investigation of fast-occurring processes.

Thickness-dependent terahertz emission from Bi2S3 films under excitation below and above the band gap

Two-dimensional (2D) materials demonstrate fascinating thickness-dependent optical properties due to their van der Waals interaction and quantum confinement in the linear optical regime. However, the thickness-dependent nonlinear optical response is more complicated as virtual carriers (dipoles) and real carriers can be generated by below- and above-band-gap excitation, which calls for a more comprehensive understanding of these carriers in 2D materials. Herein, a direct band gap ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ is utilized to investigate the thickness-dependent nonlinear optical response by a terahertz (THz) emission spectroscopy. The results suggest that the THz emission intensity decreases with the increase of band gap energy under an 800-nm femtosecond laser excitation, which can be described by an empirical exponential equation. Under below-band-gap excitation, the optical rectification effect induced by instantaneous polarization (virtual carriers) dominates the THz emission mechanism in thin ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. In contrast, under above-band-gap excitation, real carriers are controlled by both resonant optical rectification and surface depletion electric field effects, resulting in shift and drift currents in thick ${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ films. The contribution ratio of the shift and drift currents is $\ensuremath{\sim}1:1$ at ${45}^{\ensuremath{\circ}}$ oblique incidence. This competition between shift and drift currents results in the elliptically polarized THz wave generation, and the major axis orientation and ellipticity of the ellipse could be manipulated by changing the polarization of the pump light. In this paper, we imply the potential for designing tunable on-chip THz sources and nonlinear optoelectronic devices based on thickness-dependent 2D materials.

Nonthermal Ultrafast Optical Control of Magnetization Dynamics by Linearly Polarized Light in Metallic Ferromagnet.

Coherent optical control of the magnetization in ferromagnetic (FM) mediums using ultrafast nonthermal effect paves a promising avenue to improve the speed and repetition rate of the magnetization manipulation. Whereas previously, only heat-induced or helicity-dependent magnetization dynamics are demonstrated in metallic ferromagnets. Here, the linearly-polarized light control of magnetization is demonstrated in FM Co coupled with ferroelectric (FE) BiFeO3 by tuning the light polarization direction. It is revealed that in the Co/BiFeO3 heterostructure excited by femtosecond laser pulses, the magnetization precession amplitude follows a sinusoidal dependence on the laser polarization direction. This nonthermal control of coherent magnetization rotation is attributed to the optical rectification effect in the BiFeO3 layer, which yields a FE polarization depending on the light polarization, and the subsequent modulation of magnetic energy in Co by the electrostriction-induced strain. This work demonstrates an effective route to nonthermally manipulate the ultrafast magnetization dynamics in metallic ferromagnets.

Transient THz Emission and Effective Mass Determination in Highly Resistive GaAs Crystals Excited by Femtosecond Optical Pulses

We present comprehensive studies on the emission of broadband, free-space THz transients from several highly resistive GaAs samples excited by femtosecond optical pulses. Our test samples are characterized by different degrees of disorder, ranging from nitrogen-implanted to semi-insulating and annealed semi-insulating GaAs crystals. In our samples, we clearly observed transient THz emissions due to the optical rectification effect, as well as due to the presence of the surface depletion electrical field. Next, we arranged our experimental setup in such way that we could observe directly how the amplitude of surface-emitted THz optical pulses is affected by an applied, in-plane magnetic field. We ascribe this effect to the Lorentz force that additionally accelerates optically excited carriers. The magnetic-field factor η is a linear function of the applied magnetic field and is the largest for an annealed GaAs sample, while it is the lowest for an N-implanted GaAs annealed at the lowest (300 °C) temperature. The latter is directly related to the longest and shortest trapping times, respectively, measured using a femtosecond optical pump-probe spectroscopy technique. The linear dependence of the factor η on the trapping time enabled us to establish that, for all samples, regardless of their crystalline structure, the electron effective mass was equal to 0.059 of the electron mass m0, i.e., it was only about 6% smaller than the generally accepted 0.063m0 value for GaAs with a perfect crystalline structure.

Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission

The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd3As2, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd3As2. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.

Enhanced terahertz emission from mushroom-shaped InAs nanowire network induced by linear and nonlinear optical effects

The development of powerful terahertz (THz) emitters is the cornerstone for future THz applications, such as communication, medical biology, non-destructive inspection, and scientific research. Here, we report the THz emission properties and mechanisms of mushroom-shaped InAs nanowire (NW) network using linearly polarized laser excitation. By investigating the dependence of THz signal to the incidence pump light properties (e.g. incident angle, direction, fluence, and polarization angle), we conclude that the THz wave emission from the InAs NW network is induced by the combination of linear and nonlinear optical effects. The former is a transient photocurrent accelerated by the photo-Dember field, while the latter is related to the resonant optical rectification effect. Moreover, the p-polarized THz wave emission component is governed by the linear optical effect with a proportion of ∼85% and the nonlinear optical effect of ∼15%. In comparison, the s-polarized THz wave emission component is mainly decided by the nonlinear optical effect. The THz emission is speculated to be enhanced by the localized surface plasmon resonance absorption of the In droplets on top of the NWs. This work verifies the nonlinear optical mechanism in the THz generation of semiconductor NWs and provides an enlightening reference for the structural design of powerful and flexible THz surface and interface emitters in transmission geometry.

Magnetization-induced optical rectification and inverse spin Hall effect for interfacial terahertz generation in metallic heterostructures

Metallic heterostructure as promising terahertz (THz) spintronic emitters has promoted the development of both spintronics and THz science. However, the underlying physics at the metallic interface, such as the nonlinear optical effect, remains unclear. Herein, we present interface magnetization induced THz generation from metallic heterostructure consisting of Heusler alloy CoFeMnSi (CFMS) and Pd thin films. THz generation is ascribed to 35% contribution from the magnetization-induced optical rectification (MOR) and 65% contribution from inverse spin Hall effect (ISHE) based on the pump polarization and sample azimuthal angle dependent measurement. Furthermore, the contribution ratio of the MOR decreases to 12% via lowering the CFMS grown temperature, which is due to the reduced crystalline quality and possible metal to semiconductor transformation in CFMS. Our results not only clarify MOR and ISHE in metallic heterostructure for the scientific field, but they also benefit THz source optimization for the technology field.

Terahertz emission from in-plane and out-of-plane dipoles in layered SnS2 crystal

We investigated the terahertz (THz) radiation mechanism of the layered tin sulfide (SnS2) crystal via THz emission spectroscopy. Under non-resonant excitation, THz generation from SnS2 is mainly due to the optical rectification effect, which is verified by the azimuthal angle, polarization angle, and pump fluence dependence of the THz signals. Further incident angle dependent THz signals suggest that the optical progress happened at the surface of SnS2 with both in-plane and out-of-plane photon-induced dipoles. This work puts forward THz emission spectroscopy as a noninvasive characterization tool for the in-plane and out-of-plane photon-induced dipoles in layered materials.

A Modified Drude–Lorentz Model That Enables Taking into Account Topological Characteristics of a Medium

The Drude–Lorentz model, which makes it possible to describe a nonlinear response of a dielectric or conducting medium, can be suited for the description of nonlinear nonresonant responses of some exotic media: topological insulators, a Weil semimetal, or a Dirac metal. A generalized Drude–Lorentz model and its simplified version, in which topological effects are taken into account to a minimum extent, are presented. As an example of application of the simplified model, the second-order nonlinear conductivity is derived, which is responsible for the second harmonic generation and the effect of optical rectification. It is shown that the ratio of the topological conductivity to the ordinary linear conductivity contains constants that are proportional to the fine structure constant and the axion field gradient.

Terahertz emission from layered GaTe crystal due to surface lattice reorganization and in-plane noncubic mobility anisotropy

In this work, a model based on the optical rectification effect and the photocurrent surge effect is proposed to describe the terahertz emission mechanism of the layered GaTe crystal. As a centrosymmetric crystal, the optical rectification effect arises from the breaking of the inversion symmetry due to lattice reorganization of the crystal’s surface layer. In addition, the photocurrent surge originating from the unidirectional charge carrier diffusion—due to the noncubic mobility anisotropy within the layers—produces terahertz radiation. This is confirmed by both terahertz emission spectroscopy and electric property characterization. The current surge perpendicular to the layers also makes an important contribution to the terahertz radiation, which is consistent with its incident angle dependence. Based on our results, we infer that the contribution of optical rectification changes from 90% under normal incidence to 23% under a 40° incidence angle. The results not only demonstrate the terahertz radiation properties of layered GaTe bulk crystals, but also promise the potential application of terahertz emission spectroscopy for characterizing the surface properties of layered materials.

Модифицированная модель Друде-Лоренца, позволяющая учесть топологические характеристики среды

The Drude-Lorentz model, which allows to describe nonlinear response of dielectric or conduc-tor, could be used for describing nonlinear non resonant responses of some exotic media, such as the topological insulators, the Weyl semimetals and the Dirac metals. Generalized Drude-Lorentz model and its simplified version, where the topological effects are taken into account in the minimal order, are presented in paper. A nonlinear conductance of the second order corre-sponding to second harmonic generation and optical rectification effect is obtained in paper as an example of use of the simplified model. It is shown that the ratio of the topological conductance to the ordinary linear conductance contains the constant value that is proportional to the fine structure constant and axion field gradient.

Optical Rectification and Generation of Harmonics Under Condition of Propagation of Few-Cycle Pulses in the Birefringent Medium with Asymmetric Molecules

Without using the approximation of slowly varying envelopes, we investigate spectral transformations of vector few-cycle and quasi-monochromatic solitons under propagation in the birefringent medium of asymmetric molecules possessing permanent dipole moments. We show that when an ordinary pulse enters the medium, the self-modulation effect arises. As a result, the spectrum of the ordinary component undergoes significant changes from the violet shift and the generation of odd harmonics with close frequency satellites to the supercontinuum including zero frequency. The spectrum of the extra-ordinary few-cycle pulse also has the properties of a supercontinuum. When a quasi-monochromatic ordinary pulse enters the medium in the spectrum of an extra-ordinary component, the effect of optical rectification with simultaneous generation of the second harmonics is clearly manifested.

Enhanced terahertz response of diluted magnetic semiconductor Zn_1-xMnxTe crystals

Single crystalline ZnTe is undoubtedly the leading nonlinear optical material for the promising application of terahertz (THz) generation and detection through optical rectification and electro-optic effect, respectively. In this work, we report the capability of the diluted magnetic semiconductor Zn1-xMnxTe as potential THz emitters and sensors. In Zn1-xMnxTe crystals, a significantly enhanced THz response as high as 10.4%-18.9% (emitter) and 16.9-28.0% (sensor) is observed over intrinsic ZnTe. Both the reduced carrier concentration and the better phase-match condition are proven to be responsible for the enhanced THz emission and detection. The resulting terahertz waves from Zn1-xMnxTe reveal an optimum x value of ~0.028. The magnetic, optical, and electrical properties of as-grown Zn1-xMnxTe crystals have also been evaluated.

Optical rectification in surface layers of germanium

In this Letter, we have demonstrated significant electric field induced (EFI) optical rectification (OR) effects existing in the surface layers of germanium (Ge) and measured the distributions of EFI OR signals along the normal directions of surface layers of Ge samples. Based on the experimental results, the ratios of the two effective second-order susceptibility components χzzz(2eff)/χzxx(2eff) for Ge(001), Ge(110), and Ge(111) surface layers can be estimated to be about 0.92, 0.91, and 1.07, respectively. The results indicate that the EFI OR can be used for analyzing the properties on surface layers of Ge, which has potential applications in Ge photonics and optoelectronics.

Theory of the optical-rectification effect in metallic thin films with periodic modulation

We conducted theoretical and numerical investigations of the optical rectification (OR) effect in metallic structures with periodic modulation. A new formulation of the OR effect is presented, and the mechanism by which the OR effect is generated, which has been a controversial issue in previous studies, is clarified. We reveal that the OR effect is strongly enhanced by a combination of spatial variation of the metallic structure and local electric field enhancement. Our theory was numerically evaluated and agreed fairly well with experiment.

Optical rectification and Pockels effect as a method to detect the properties of Si surfaces

The depth profile of electric-field-induced (EFI) optical rectification (OR) and EFI Pockels effect (PE) in a Si(110) crystal are investigated. The results show that EFI OR and PE signals are very sensitive to the electric field strength in the surface layers of the Si crystal. Theoretical formulas that include the electric field parameters and the widths of the space-charge region are presented and agreed very well with the experimental results. The experiments and simulations indicate that EFI OR and PE are potential methods for researching the surface/interface properties along the depth direction in centrosymmetric crystals such as Si.

Physical mechanisms of terahertz pulse emission from photoexcited surfaces of tellurium crystals

We report on terahertz (THz) emission from tellurium crystal surfaces excited by femtosecond optical pulses. Measurements were performed on three differently cut Te samples and with different wavelength optical excitation pulses. THz pulse amplitude dependences on the azimuthal angle measured at various excitation wavelengths have evidenced that three different mechanisms are responsible for THz generation in tellurium: second order nonlinear optical rectification effect, dominating at lower excitation photon energies, as well as transverse and ordinary photo-Dember effects, which emerge at energies larger than 0.9 eV. The shapes of the azimuthal angle dependences were also explained by theoretical model.