Efficient THz Generation Research Articles

Optimal Te-doping in GaSe for non-linear applications

Centimeter-sized Te-doped GaSe ingots were grown from the charge compositions of GaSe with nominals 0.05, 0.1, 0.5, 1, and 3 mass% Te, which were identified as ε-GaSe:Te (0.01, 0.07, 0.38, 0.67, and 2.07 mass%) single crystals. The evolution of the absorption peaks of the phonon modes E'(2) (≈ 0.584 THz) and E"(2) (1.77 THz) on Te-doping in GaSe:Te crystals was studied by THz time-domain spectroscopy. This study proposes that the evolution of both E'(2) and E''(2) absorption peaks correlates well with the optical quality of Te-doped GaSe crystals, which was confirmed by experimental results on the efficiency of THz generation by optical rectification. Maximal intensity of the absorption peak of the rigid layer mode E'(2) is proposed as a criterion for identification of optimal Te-doping in GaSe crystals.

Highly Efficient Organic THz Generator Pumped at Near‐Infrared: Quinolinium Single Crystals

AbstractA novel highly efficient ionic electro‐optic quinolinium single crystals for THz wave applications is reported. Acentric quinolinium derivatives, HMQ‐T (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methylbenzenesulfonate) and HMQ‐MBS (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methoxybenzenesulfonate) exhibit high order parameters cos3θp = 0.92 and cos3θp = 1.0, respectively, as well as a large macroscopic optical nonlinearity, which is in the range of the benchmark stilbazolium DAST (N,N‐dimethylamino‐N’‐methylstilbazolium 4‐methylbenzenesulfonate) and phenolic polyene OH1 (2‐(3‐(4‐hydroxystyryl)‐5,5‐dimethylcyclohex‐2‐enylidene)malononitrile) crystals. As‐grown unpolished bulk HMQ‐T crystals with a side length of about 6 mm and thickness of 0.56 mm exhibit 3.1 times higher THz generation efficiency than 0.37 mm thick OH1 crystals and about 8.4 times higher than 1 mm thick inorganic standard ZnTe crystals at the near‐infrared fundamental wavelength of 836 nm. Therefore, HMQ crystals with high order parameter obviously have a very high potential for high power THz‐wave generation and its applications.

Terahertz radiation generation by beating of two spatial-Gaussian lasers

Terahertz radiation generation is proposed based on beating of two spatial-Gaussian lasers in a periodic density plasma, where a transverse component of current is realized that resonantly excites the radiation with efficiency ∼10 −3. Importance of laser-beam-width, amplitude and periodicity of density structure is discussed for the efficient THz generation.

Temperature dependent narrow-band terahertz pulse generation in periodically poled crystals via difference frequency generation

Femtosecond optical pulse is used to generate narrow-band terahertz pulses depending on a quasi-phase-matched condition in periodically poled lithium niobate (PPLN) and stoichiometric lithium tantalate (PPSLT) crystals by difference frequency generation. The origin of narrow-band THz generation proved that the two frequency components of the fs pulse contribute to the frequency mixing. By cryogenic cooling, the absorption of THz waves in the crystal is significantly reduced which results in efficient THz generation. Simultaneously generated forward and backward THz pulses were 1.38 and 0.65THz with as narrow as the bandwidth of 32GHz in the PPSLT sample. Temperature dependence of the generated THz waveforms had good agreement with the simulation result using one dimensional plane-wave propagation model.

Large-Size Pyrrolidine-Based Polyene Single Crystals Suitable for Terahertz Wave Generation

We report on the growth of large pyrrolidine-based polyene single crystals from methyl ethyl ketone (MEK) solutions for nonlinear optical applications. The polyene crystals grown exhibit a suitable morphology and surface roughness and good optical quality. We successfully demonstrated THz wave generation with as-grown polyene crystals. Although the crystal cut was not optimized and phase-matching conditions were not fulfilled, we achieved a similar or higher THz generation efficiency compared to the well-known inorganic ZnTe emitter crystals.

Design of an efficient terahertz source using triply resonant nonlinear photonic crystal cavities

We propose a scheme for efficient cavity-enhanced nonlinear THz generation via difference-frequency generation (DFG) processes using a triply resonant system based on photonic crystal cavities. We show that high nonlinear overlap can be achieved by coupling a THz cavity to a doubly-resonant, dual-polarization near-infrared (e.g. telecom band) photonic-crystal nanobeam cavity, allowing the mixing of three mutually orthogonal fundamental cavity modes through a chi((2)) nonlinearity. We demonstrate through coupled-mode theory that complete depletion of the pump frequency - i.e., quantum-limited conversion - is possible. We show that the output power at the point of optimal total conversion efficiency is adjustable by varying the mode quality (Q) factors.

SIMULATION AND EXPERIMENTAL RESULTS ON GaN BASED ULTRA-SHORT PLANAR NEGATIVE DIFFERENTIAL CONDUCTIVITY DIODES FOR THz POWER GENERATION

A GaN based negative differential conductivity diode utilizing transient ballistic transport effects is proposed and large-signal circuit simulations along with preliminary experimental results are presented. The diode is an n +- n - n + structure and transport is described by an empirical velocity-field relation which is derived directly from femtosecond pulse-probe measurements available in literature and incorporated into the simulations through curve fitting. Efficient THz generation is predicted as a result of ~2.8 peak-to-valley ratio. Pulsed current-voltage characteristics were measured and N -type dependence was observed.

Excitation spectra of terahertz emission from semiconductor surfaces

Spectral dependences of THz radiation from laser-illuminated surfaces of InAs and InSb have been investigated experimentally at high optical fluences for laser wavelengths ranging from 0.6μm to 2μm. Efficient THz generation was discovered in the excitation range around 1.6μm. The influence of inter-valley scattering was evident. The energy position of subsidiary conduction band valleys was evaluated to be equal to 1.08eV and 0.53eV for InAs and InSb, respectively.

High-efficiency THz generation in optically contacted GaAs with near-Brewster angle pumping

High-efficiency THz generation in quasi-phase matched (QPM) optically contacted GaAs (OC-GaAs) with near-Brewster angle pumping is studied numerically. The effective nonlinear coefficients d(eff) for different incident angles and polarization directions are investigated. Compared with the normal incidence case, reflection loss of the pump energy at OC-GaAs interfaces can be reduced by propagating the pump at a near-Brewster angle (66.92?). The effect of the air-gap spacings between adjacent OC-GaAs layers to the number of optimal QPM periods and the efficiency of THz generation are calculated. In our study, the number of optimal QPM periods of OC-GaAs is increased significantly from 12 in the normal incidence configuration to 25 in the near-Brewster angle pumped configuration, while the efficiency of THz generation is enhanced by more than 16 times.

Efficient terahertz generation by optical rectification at 1035 nm

We demonstrate efficient generation of THz pulses by optical rectification of 1.03 um wavelength laser pulses in LiNbO3 using tilted pulse front excitation for velocity matching between the optical and THz fields. Pulse energies of 100 nJ with a spectral bandwidth of up to 2.5 THz were obtained at a pump energy of 400 uJ and 300 fs pulse duration. This conversion efficiency of 2.5x10(-4) was an order of magnitude higher than that obtained with collinear optical recitification in GaP, and far higher still than that measured using ZnTe in an optimized geometry. Using a simple model we demonstrate that two- and three-photon absorption strongly limit the THz generation efficiency at high pump fluences in ZnTe and GaP.

Efficient THz generation and frequency upconversion in GaP crystals

We have developed a widely-tunable monochromatic THz source based on phase-matched difference-frequency generation (DFG) in a GaP crystal. By using a 20-mm-long GaP crystal the output wavelength was tuned in the range of 71.1–2830 μm whereas the highest peak power was 15.6 W. The advantage of using GaP over birefringent non-linear-optical crystals is obvious: crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within a bandwidth of as narrow as 15.3 nm. Most recently, we observed THz frequency upconversion by mixing a THz wave with an infrared laser beam in another GaP crystal. The lowest energy per pulse measured by us was 780 pJ whereas the wavelength range of the detectable THz waves was 113.6–128.3 μm. This scheme allows us to measure the pulse energy density, wavelength, linewidth, and pulse width of a THz beam at non-cryogenic temperatures. We have also discussed a few mechanisms for further enhancing the conversion efficiencies of the two parametric processes.

Spectral dependencies of terahertz emission from InAs and InSb

Spectral dependences of the THz radiation from the laser-illuminated surfaces of InAs and InSb have been investigated experimentally at high optical fluences for the laser wavelengths ranging from 0.6 to 2μm. Efficient THz generation was discovered in the excitation range around 1.6μm. The influence of the intervalley scattering was clearly evidenced. The energy position of the subsidiary conduction band valleys was evaluated from this study to be equal 1.08 and 0.53 eV for InAs and InSb, respectively. It has been concluded that THz emission at high excitation fluencies is dominated by the shift current effect.

THz instability by streaming carriers in high mobility solid-state plasmas

It is described how the interaction of streaming electron plasma waves with lattice waves may lead to a highly unstable coupled plasmon-phonon excitation in the THz range, in high mobility semiconductor materials. The underlying physical mechanism is related to the ubiquitous beam-plasma instability in electrostatically coupled fluids. Continuous amplification in transit, rather than localized amplification at the system boundary reflections, yields high growth rates and may lead to efficient THz generation.

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO3 crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.

Optimized generation of THz pulses via optical rectification in the organic salt DAST

Short THz pulses can be generated via optical rectification in the organic salt DAST (4- N, N-dimethylamino-4 ′- N ′-methyl stilbazolium tosylate) due to its large second-order nonlinear susceptibility d=1010 pm/V (at λ=1318 nm). The efficiency of THz generation is affected by absorption and dispersion caused by several resonances of the crystals in the THz range. We found optimum generation conditions for sub-picosecond pump pulses around λ=1050 nm and a center frequency of 0.65 THz. This is in agreement with theory taking into account absorption and velocity-matching.