THz-wave Parametric Generator Research Articles

Optical up-conversion-based cross-correlation for characterization of sub-nanosecond terahertz-wave pulses.

Using a nonlinear optical mixing known as a frequency up-conversion process, we demonstrate an optical cross-correlation technique for the detection and characterization of sub-nanosecond (sub-ns) terahertz (THz)-wave pulses. A monochromatic THz-wave pulse from an injection-seeded THz-wave parametric generator (is-TPG) was mixed with a near-infrared (NIR) pump pulse to generate a NIR idler pulse in a trapezoidal-prism-shaped MgO-doped lithium niobate crystal under the noncollinear phase-matching condition. By measuring pump-energy and crystal-length dependencies, we show that the frequency up-conversion of sub-ns THz-wave pulses with and without subsequent parametric amplification can be used for sensitive detection and intensity cross-correlation characterization, respectively. Using this cross-correlation technique, we reveal that the temporal profile of THz-wave pulses from the is-TPG driven by a 351-ps 1064-nm pump laser has slightly-frequency-dependent pulse width in the range of 150-190 ps at full width at half-maximum in the tunable range of 0.95-2.00 THz.

Noise-free terahertz-wave parametric generator.

We achieved noise-free terahertz (THz)-wave output from an injection-seeded THz-wave parametric generator (is-TPG) employing high-power injection seeding. A conventional is-TPG uses a weak continuous-wave (CW) seed beam. The position in which broadband noise is generated (via spontaneous parametric down-conversion) and the position of the THz signal overlap. Thus, the output features broadband TPG noise, reducing the signal-to-noise ratio. To solve this problem, we shifted the position in which the THz signal is generated to the front of the crystal; we separated the signal from broadband TPG noise using a high-powered, pulsed seed beam that was 107-fold more powerful than the CW seed beam. Thus, we extracted only the THz signal; we achieved a noise-free is-TPG. This system features a signal-to-noise ratio of 95 dB, approximately 40 dB better than the signal-to-noise ratio of the conventional system.

Cascaded effect in a high-peak-power terahertz-wave parametric generator.

We demonstrate megawatt-level terahertz (THz)-wave generation via a Stokes-seed-injected THz-wave parametric generator and study the cascaded effect. The optical-to-THz conversion efficiency was 1.72 × 10-3, and the peak power was conservatively estimated to be 1.09 MW using the pulse width of the pump. More than 80% of the THz-wave energy came from primary parametric generation, with the rest coming from high-order parametric amplification. Clear cascaded Stokes spots of second to fourth order were observed, and the factors affecting the high-order parametric process are discussed. The cascaded parametric effect is beneficial for achieving a higher optical-to-THz conversion efficiency, thereby improving the performance of high-peak-power THz-wave parametric sources.

Improvement in THz light extraction efficiencies with antireflection subwavelength gratings on a silicon prism

An antireflection (AR) structure that incorporates a subwavelength grating (SWG) is a promising candidate for suppressing the Fresnel reflection of a silicon prism used as a component of an injection-seeded THz wave parametric generator (is-TPG) to improve the efficiency in extracting THz waves. Here, a two-dimensional binary AR-SWG with a 20 μm period is designed and then numerically and experimentally realized. The measured transmittance of the AR-SWG is discovered to be greater than 80% from 0.5 to 4.0 THz and to exceed 90% from 1.25 to 2.83 THz. Moreover, the AR-SWG is tightly attached to the Si prism of the is-TPG by direct-bonding technology. Compared to the is-TPG without AR-SWGs, the output increases by 1.3–1.5 times from 1.09 to 2.52 THz. The improvement in THz light extraction efficiency is thus successfully demonstrated.

Security screening system based on terahertz-wave spectroscopic gas detection.

Tunable terahertz (THz)-wave absorption spectroscopy is a promising technique to detect trace gases suspended in ambient air owing to their strong absorption fingerprints in the THz-wave spectral region. Here, we present a THz-wave spectroscopic gas detection platform based on a frequency-tunable injection-seeded THz-wave parametric generator and compact multipass gas absorption cells. Using a 1.8-m-path-length multipass cell, we detected gas-phase methanol (CH3OH) down to a trace concentration of 0.2 ppm at the 1.48-THz transparent atmospheric window. We also developed a transportable walk-through screening prototype using a 6-m-path-length multipass cell to identify suspicious subjects. Our results demonstrate the potential of the proposed system for security screening applications.

Highly sensitive multi-stage terahertz parametric detector.

In this Letter, we developed a high-sensitivity multi-stage terahertz (THz)-wave parametric detection system that operates at room temperature. This detection system has high sensitivity over a wide wavelength range through upconversion of a THz wave to near-infrared light. The broadband noise associated with parametric generation limited the detection sensitivity in the previous setup; however, in the multi-stage configuration using multiple LiNbO3 crystals, the THz parametric detection sensitivity was improved by spatially eliminating the broadband noise using an iris between the former and latter stages. With this improvement, the minimum detectable sensitivity at 1.05 THz approached 130 zJ (zJ=10-21J), which is equivalent to 90 photons or less. Furthermore, by combining this detector with an injection-seeded THz-wave parametric generator, which is a high-power, tunable THz-wave source, the THz-wave measurement system achieved a maximum dynamic range of 125 dB.

Terahertz differential absorption spectroscopy using multifurcated subnanosecond microchip laser

Using spectral multifurcated oscillations in a passively Q-switched microchip laser, we demonstrate frequency-domain differential absorption spectroscopy in the terahertz (THz) frequency region. Within a single quasi-continuous-wave (QCW) excitation, a microchip laser comprising a 7-mm-long Nd:YAG/Cr:YAG composite ceramic provides up to three subnanosecond pulses with a 1064-nm wavelength and a 50-Hz QCW repetition rate. We have observed that the longitudinal mode of double and triple pulses shows stable bifurcation and trifurcation, respectively, induced by a spatial hole burning effect within the laser cavity. These pulses are directly used to drive an injection-seeded THz-wave parametric generator based on a MgO-doped LiNbO3 crystal, thereby generating up to three monochromatic, self-frequency-switched THz-wave pulses separated from each other in frequency by a free spectral range of the laser cavity. By precisely tuning one of the THz-wave frequencies to the gas absorption line, multifurcated THz-wave pulses facilitate the measurement of differential absorption signals every 20 ms without any active frequency modulation. We also show that first- and second-order derivative spectra of gas absorption can be derived from a single frequency sweep of multifurcated pulses without a reference spectrum and computational derivation. Our approach paves the way toward realization of a THz differential absorption lidar for use in fast gas sensing applications.

The Investigation on the Beam Spatial Intensity Distributions in the Injection-Seeded Terahertz Parametric Generator

In terahertz (THz) wave parametric sources, the large angle between the pump and terahertz waves (about 65° in LiNbO3 crystal) will affect the spatial intensity distributions. To investigate how the noncollinear phase-matching affects the spatial intensity properties of the pump, Stokes, and THz waves, we take the injection-seeded THz-wave parametric generator (is-TPG) as an example to study the beam spatial intensity properties via the numerical solutions of the coupled-wave equations. The simulated results demonstrate that the pump and Stokes intensities have significant changes along the THz-wave propagation direction. It is the first time that the spatial intensity distributions of the three waves in the THz-wave parametric sources have been studied in theory. In addition, the output intensity distributions of the pump, Stokes, and THz waves are measured in the designed is-TPG. The measured spatial intensity distributions are in coincidence with the simulated results. Investigations on the beam spatial intensity properties provide a potential approach to improve the THz-wave outputs and the pump conversion efficiency for the is-TPG and other THz-wave parametric sources.

Terahertz-wave differential detection based on simultaneous dual-wavelength up-conversion

We report a terahertz (THz)-wave differential detection based on simultaneous dual-wavelength up-conversion in a nonlinear optical MgO:LiNbO3 crystal with optical and electronic THz-wave sources. The broadband parametric gain and noncollinear phase-matching of MgO:LiNbO3 provide efficient conversion from superposed THz waves to spatially distributed near-infrared (NIR) beams to function as a dispersive THz-wave spectrometer without any additional dispersive element. We show that the μW-level THz waves from two independent sources, a 0.78-THz injection-seeded THz-wave parametric generator (is-TPG) and a 1.14-THz resonant tunneling diode (RTD), are simultaneously up-converted to two NIR waves and then detected with two NIR photodetectors. By applying a balanced detection scheme to this dual-frequency detection, we demonstrate THz-wave differential imaging of maltose and polyethylene pellets in the transmission geometry. This dual-wavelength detection is applicable to more than three frequencies and broadband THz-wave radiation for real-time THz-wave spectroscopic detection and imaging.

Kilowatt-peak Terahertz-wave Generation and Sub-femtojoule Terahertz-wave Pulse Detection Based on Nonlinear Optical Wavelength-conversion at Room Temperature

Intense Terahertz (THz)-wave generation and highly sensitive THz-wave detection were obtained by wavelength conversion with nonlinear optical susceptibility χ(2) of LiNbO3 crystals. Maximum peak output of about 50 kW (5 μJ/pulse) was demonstrated in an injection-seeded THz-wave parametric generator pumped by post-amplified emission from a microchip Nd:YAG laser. Using the sub-nanosecond pulse duration of the laser proposed herein provides effective mitigation of stimulated Brillouin scattering in LiNbO3, producing higher gain for wavelength conversion between near-infrared (near-IR) pump light and THz waves. Monochromatic THz radiation was obtained in the continuous tuning range of 0.7–2.9 THz. Additionally, highly sensitive THz-wave detection was demonstrated based on up-conversion from THz waves to near-IR light as well as efficient THz-wave generation. The signal generated with non-collinear phase-matching condition showed spectroscopic detection on the screen apart from the LiNbO3 crystal. Highly sensitive detection with minimum energy of about 80 aJ/pulse (0.8 μW at peak) and a large dynamic range of more than 100 dB were achieved in this experiment.

Investigation of Electron Beam Parameter in Seeded THz-FEL Amplifier using Photocathode RF Gun

Simple, compact, and powerful seeded terahertz-free electron laser (THz-FEL) which consists of a photocathode radio-frequency gun, a focusing solenoid, an undulator and a THz wave parametric generator has been designed. A start-to-end calculation has been performed by using the particle tracking code PARMELA and the FEL gain simulation code GENESIS1.3. To obtain a higher FEL gain, we searched a better condition of the electron beam parameter and the seed light. In the present design scheme, the FEL peak power was calculated to be 3kW at the FEL resonant wavelength of 179 m. Since the power of the seed light was 0.20W, the light power was amplified almost 104 times.

Output power enhancement of a palmtop terahertz-wave parametric generator

Broadband terahertz (THz) waves were generated by optical parametric processes based on laser light scattering from the polariton mode of a nonlinear crystal. By using the parametric oscillation of a MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized a broadband, high-energy and compact THz-wave source. We report the development of a THz-wave parametric generator (TPG) using a small pump source with a short pulse width and a top-hat beam profile. These characteristics of the pump beam permit high-intensity pumping especially close to the output surface of the THz wave without thermal damage to the crystal surface. We also calculated the outcoupled THz wave for beams with two different intensity profiles: a top-hat beam (in this experiment) and a Gaussian beam (previously reported). The result shows the mechanism of the output energy and/or power enhancement.

Terahertz-wave spectrometer based on a coherent terahertz-wave parametric generator

We developed a fast data-acquisition rate terahertz (THz)–wave spectrometer based on an all-solid-state achromatically injection-seeded THz-wave parametric generator (is-TPG). It takes only seconds to smoothly sweep the frequency range from 0.9 THz to 2.4 THz. This system has been used successfully in THz-wave absorption spectrum measurement with high brightness, a fast data-acquisition rate, and ease of handling.

All-solid-state, narrow linewidth, wavelength-agile terahertz-wave generator

An all-solid-state, narrow linewidth, wavelength-agile THz-wave parametric generator was developed. The THz-wave frequency can be set randomly or tuned rapidly and smoothly over the range from 0.6to2.4THz with a narrow linewidth of 50MHz. Using this THz-wave parametric generator, accurate and rapid measurements were made of the absorption spectrum of water vapor, with a wide frequency range from 0.8to2.1THz, from a single scan of the THz-wave frequencies. These results demonstrate that this THz-wave parametric generator is a powerful tool with practical applications in THz-wave spectroscopy.

Narrow-linewidth operation of a compact THz-wave parametric generator system

We developed a compact THz-wave parametric generator (TPG) system with a diode-pumped Nd:YAG laser that was specially designed for pumping the TPG. The pump laser produced an output energy of 11 mJ in a single Q-switched pulse with a pulse width of 6 ns. Using MgO:LiNbO 3 crystals, a THz-wave energy of 1.7 pJ was obtained in unseeded operation, and a broad THz-wave spectrum (1.4–2.2 THz) was then observed. Furthermore, a narrow THz-wave linewidth of less than 33 GHz was achieved by injection seeding of the idler wave. The THz-wave energy was increased by a factor of 3 compared to the unseeded output.

Injection-seeded terahertz-wave parametric generator with wide tunability

We report on the development of a widely tunable (frequency: 0.7–2.4 THz and wavelength: 125–430 μm), injection-seeded THz-wave parametric generator (IS-TPG), which operates at room temperature. The spectral resolution (<100 MHz and 0.003 cm−1) is the Fourier transform limit of the nanosecond THz-wave pulse. The continuous scanning and narrow spectral bandwidth of the IS-TPG were verified in the absorption spectrum of low-pressure water vapor. The high peak power (>200 mW) of the output wave and the small beam divergence are suited to a variety of applications.

Fourier-Transform Spectrometer with a Terahertz-Wave Parametric Generator

A novel terahertz (THz)-wave spectrometer that combines a bright broad-band source of a laser-pumped THz-wave parametric generator (TPG) and a Martin-Puplett interferometer was developed. The TPG was based on stimulated polariton scattering via the lowest A1-symmetry mode in lithium niobate (LN). Broad-band THz-wave generation with nanosecond-pulse duration was achieved by single-pass pumping of the crystals with a Q-switched Nd:YAG laser. An array of silicon prisms was used to couple out THz waves efficiently. To optimize the output, various configurations of the LN crystals were examined. A peak power of 280 microwatts was achieved, and the spectral range extended from 30 to 120 cm-1 (0.9–3.6 THz), due to the first- and second-order Stokes scattering processes. The THz-wave spectrometer system with a nitrogen-purge unit was of tabletop size and had a maximum resolution of 0.025 cm-1 (750 MHz). THz spectroscopic data measured with the spectrometer were also presented.

Transform-limited, narrow-linewidth, terahertz-wave parametric generator

An injection-seeded nanosecond terahertz (THz) wave parametric generator was demonstrated using nonlinear crystals that were pumped by a single frequency Nd:yttrium–aluminum–garnet laser. Spectrum narrowing to the Fourier transform limit (ν=1.58 THz, Δν<200 MHz) was achieved by injection seeding the idler wave (near-infrared Stokes). This resulted in a THz-wave output power (900 pJ/pulse, >100 mW peak) approximately 300 times higher than that of a conventional THz-wave parametric generator, which has no injection seeder. This compact system operates at room temperature and promises to be a widely tunable THz-wave source that will compete with free-electron lasers and p-Ge lasers.

THz-Wave Parametric Generator with Injection Seeding.

A novel THz-wave parametric generator (TPG) was demonstrated by introducing an injection seeding technique.The linewidth was narrowed to the Fourier transform limit of the pulsewidth, and the purity of thefrequency was dramatically improved to Δv/v′10-4. The resolution of less than 100 MHz (0.003 cm-1) wasclearly shown by the absorption spectrum measurement of low pressure water vapor. The linewidth couldbecome much narrower by using longer pump pulse. Simultaneously, the obtained THz-wave output wasmore than 300 times higher than that of conventional TPG which has no injection seeder, and about 5 timeshigher than that of TPO.