Generation Of Broadband Terahertz Radiation Research Articles

Generation of optical-terahertz solitons by a few-cycle laser pulse

The generation of broadband terahertz radiation using an extremely short laser pulse of high intensity is considered. Using numerical simulation of the generalized Yajima-Oikawa system, it is shown that in the generation of an optical-terahertz soliton, in contrast to the quasi-monochromatic case, Kerr nonlinearity plays an important role for a low-period pulse, considering its dispersion.

Terahertz emission characterization of silicon based ferromagnetic heterostructures

Terahertz spectroscopy and imaging have many applications, so the generation of broadband terahertz radiation is very important, but now it faces some challenges. Opto-spintronic terahertz emitters, composed of nanometer-thin magnetic multilayer, can produce high-quality broad-band terahertz pulses. Integration of opto-spintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices. In this work, Ta/CoFeB/Ir heterostructures are deposited on thermally oxidized silicon wafers by dc magnetron sputtering. Under the illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate, a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir. Consequently, the spin current can be converted into the charge current due to the strong spin-orbit coupling. The sub-picosecond transient charge current gives rise to the terahertz radiation that enters into the free space. The terahertz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the heterostructure. The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1–2.5 THz frequency range with a maximum value of about 0.64 THz. We also investigate the dependence of THz peak-to-peak value on the pump fluence. The THz emission is found to be saturated at a pump fluence of ~0.73 mJ/cm<sup>2</sup>. Our results demonstrate the existence of the strong spin-orbit coupling in the heavy metal Ir. Furthermore, we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by changing the thickness of Ir layer. According to the thickness dependence of THz emission from the heterostructure, the propagation length of the spin current at THz frequencies is extracted to be about 0.59±0.12 nm, which is shorter than the GHz experimental measurement (~1.34 nm). Our experimental observation is consistent with that in the antiferromagnet IrMn layer, which may be attributed to different transport regimes. Theoretically, the optimized thickness values for CoFeB and Ir layers are 2.4 nm and 1.1 nm, respectively.

New Soliton Regime of Generation of Broadband Terahertz Radiation by Laser Pulses with Tilted Wave Fronts

A new soliton-like regime of generation of terahertz radiation by optical pulses with tilted wave fronts is analyzed. It has been shown that the diffraction of an optical pulse is of fundamental importance for the formation of optical–terahertz soliton. A nonsoliton broadband terahertz component is generated synchronously with the soliton component of radiation. Two matching conditions called “super-Cherenkov” and “anti-Cherenkov” have been revealed under which generation is the most efficient. In the former and latter cases, the optical terahertz soliton propagates ahead and behind the nonsoliton terahertz component, respectively.

Gas-plasma-based generation of broadband terahertz radiation with 640 mW average power.

We present a high-power source of broadband terahertz (THz) radiation covering the whole THz spectral region (0.1-30 THz). The two-color gas plasma generation process is driven by a state-of-the-art ytterbium fiber chirped pulse amplification system based on coherent combination of 16 rod-type amplifiers. Prior to the THz generation, the pulses are spectrally broadened in a multipass cell and compressed to 37 fs with a pulse energy of 1.3 mJ at a repetition rate of 500 kHz. A gas-jet scheme has been employed for the THz generation, increasing the efficiency of the process to 0.1%. The air-biased coherent detection scheme is implemented to characterize the full bandwidth of the generated radiation. A THz average power of 640 mW is generated, which is the highest THz average power achieved to date. This makes this source suitable for a variety of applications, e.g., spectroscopy of strongly absorbing samples or driving nonlinear effects for the studies of material properties.

Designing chirped aperiodically poled structures for high-energy single-cycle terahertz generation

We introduce a collinear scheme for the highly efficient generation of broadband single or few-cycle high-power terahertz (THz) pulses using optical rectification. For this purpose, two concepts are introduced and thoroughly analyzed. The first concept is the generation of chirped broadband terahertz pulses using chirped aperiodically poled electro-optic crystals. The second concept involves the compression of the terahertz pulses extracted from the aperiodically poled structure using a chirped mirror. An illustrative design employing cryogenically cooled aperiodically poled lithium niobate crystals and the appropriate chirped mirrors is presented. It is shown that the presented design allows the optical pulse to be re-used in subsequent generation stages, resulting in optical-to-THz conversion efficiencies in excess of 5% for terahertz radiation centered around ∼0.3 THz. Analytic solutions and numerical calculations are presented. In order to design a crystal with the optimum conversion efficiency, we take advantage of binary optimization techniques. This work paves the way for the generation of broadband terahertz radiation with several mJ of pulse energy using large-area aperiodically poled structures.

Investigation of parameters of terahertz pulses generated in single-domain LiNbO3 crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.

Soliton regime of generation of terahertz radiation taking into account the phase of an optical pulse

An integrable generalization of the Yajima-Oikawa system of equations has been obtained to describe the optical generation of broadband terahertz radiation in a quadratically nonlinear medium with allowance for the effect of the phase of an optical pulse on this process. The corresponding Lax pair has been found. A soliton solution has been constructed. The analysis has indicated, in particular, that the role of the phase of the optical pulse increases significantly in the vicinity of zero value of the second-order group dispersion parameter. This behavior can result in a significant increase in the efficiency of generation in the soliton regime.

Optical terahertz bullets

Optical generation of broadband terahertz radiation in a system of resonant anisotropic molecules with a permanent dipole moment has been investigated taking into account the transverse dynamics of pulses. It has been shown that, if the carrier frequency of a femtosecond optical signal is higher than the eigenfrequency of a selected molecular transition by such a value that the spectrum of the pulse, owing to its finite length, covers the resonance transition, the generation occurs in the filamentation regime followed by transformation of the filaments into stable optical terahertz bullets localized in all directions. In this case, the energy generation efficiency can be as high as ∼10−3.

Optical methods for the generation of terahertz radiation and subterahertz acoustics of solids

Various methods to increase the efficiency of the generation of broadband terahertz radiation based on optical rectification have been analyzed. The solutions of self-consistent optical terahertz equations have been studied using the inclined front technique. The effect of symmetric and asymmetric resonant impurity centers incorporated into a crystal has been examined. Transient and asymptotic stages of generation have been considered. Attention has been focused on the evolution of the spectra of both generated terahertz and input optic signals. One- and two-frequency generation modes have been discussed. Acoustic analogs that would provide the possibility of the generation of the gigahertz acoustic supercontinuum using subterahertz elastic signals in a solid have been analyzed. The main similarities and differences between terahertz optics and subterahertz acoustics have been revealed.

Generation of terahertz radiation in the regime of resonant-nonresonant optical rectification

The generation of broadband terahertz radiation in quadratically nonlinear crystals containing additional resonant impurities having a permanent dipole moment has been theoretically analyzed. It has been shown that the contributions to the intensity of the generated radiation from resonance and nonresonance mechanisms of optical rectification are nonadditive. In semiconductor nanostructures and systems of organic molecules, the energy efficiency of generation due to the resonance mechanism can reach about 0.001, which is much higher than the corresponding efficiencies of the currently widespread tilted front technique. The spectra of the input optical pulses exhibit a noticeable red shift due to the resonance-nonresonance rectification.

Efficient Cherenkov emission of broadband terahertz radiation from an ultrashort laser pulse in a sandwich structure with nonlinear core

A scheme for efficient generation of broadband terahertz radiation by a femtosecond laser pulse propagating in a planar sandwichlike structure is proposed. The structure consists of a thin nonlinear core cladded with prisms made of a material with low terahertz absorption. The focused into a line laser pulse propagates in the core as a leaky or waveguide mode and emits Cherenkov wedge of terahertz waves in the cladding. We developed a theory that describes terahertz generation in such a structure and calculated spatial distribution of the generated terahertz field, its energy spectrum and optical-to-terahertz conversion efficiency. The developed theory predicts the conversion efficiency of up to several percent in a 1 cm long and 1 cm wide Si–LiNbO3–Si sandwich structure with a 20 μm thick nonlinear layer pumped by 8.5 μJ Ti:sapphire laser with pulse duration of 100 fs.

Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna

The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.

Broadband generation of terahertz radiation in a waveguide.

We report a novel geometry that allows for the phase-matched generation of broadband terahertz radiation in a polymer-based parallel-plate metal waveguide by means of optical rectification. Both the optical pump beam and the generated terahertz radiation propagate in the fundamental mode of the waveguide. This allows for noncritical phase matching over a broad range of terahertz frequencies. We demonstrate guided-wave interaction lengths of up to 3 mm.