Noncollinear Phase-matching Condition Research Articles

Collinear injection-seeded terahertz parametric generator

In a conventional injection-seeded terahertz (THz) parametric generator (is-TPG), a complicated achromatic optical system controlling the angle of incidence of the seed beam is used to ensure tunability because both the pump and seed beams must satisfy non-collinear phase-matching conditions. In this study, we found that a THz output and tunability similar to those characteristics of a conventional is-TPG can be obtained even when the pump and seed beams are coaxially injected into the LiNbO3 crystal. In this new generation mechanism, a weak THz-wave is generated by the difference frequency of optical mixing between the pump and seed beams at the Cherenkov phase-matching angle and then strongly parametrically amplified. Thus, we describe our device as a collinear is-TPG. Simplicity is improved by eliminating any need for achromatic optics.

Widely tunable NCPM-KTA OPO based on non-collinear phase-matching in a four-mirror ring cavity.

Based on non-collinear phase-matching (PM), a new method for widely tunable wavelength was proposed and demonstrated in a four-mirror ring cavity of non-critical phase-matching (NCPM) KTiOAsO4 (KTA) optical parametric oscillator (OPO). Wavelength tuning range of 141 nm from 1535.56 nm to 1676.73 nm was achieved by moving one mirror of ring cavity back and forth. The tuning theory and tuning method of non-collinear PM were analyzed in detail. The output energy and pulse width of signal were measured and compared in collinear and non-collinear PM condition. This method is also applicable to OPOs of other nonlinear crystals based on four-mirror ring cavity.

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.

Full 3D + 1 modeling of tilted-pulse-front setups for single-cycle terahertz generation

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase-matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3 D + 1 ( x , y , z , t ) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences among 1 D + 1 , 2 D + 1 , and 3 D + 1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatiotemporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is generated only near the apex of the prism. Even though the part of the beam farther from the apex can contain a large fraction of the energy, the terahertz waveform shows less few-cycle character. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves, and terahertz high-harmonic generation.

Adaptive spatiotemporal optical pulse front tilt using a digital micromirror device and its terahertz application.

We report a new method to temporally and spatially manipulate the pulse front tilt (PFT) intensity profile of an ultrashort optical pulse using a commercial microelectromechanical system, also known as a digital micromirror device (DMD). For our demonstration, we show terahertz generation in a lithium niobate crystal using the PFT pumping scheme derived from a DMD chip. The adaptive functionality of the DMD could be a convenient alternative to the more conventional grating required to generate a laser beam with a PFT intensity profile that is typically used for efficient optical rectification in noncollinear phase-matching conditions. In contrast to a grating, PFT using DMD does not suffer from wavelength dispersion, and exhibits overlap properties between grating and a stair-step echelon mirror.

Theoretical analysis of stimulated polariton scattering from the A1-symmetry modes of KNbO3 crystal

Stimulated polariton scattering (SPS) based on noncollinear phase matching scheme from the A1-symmetry modes of KNbO3 crystal is investigated for generating terahertz (THz) wave. Frequency tuning characteristics of THz wave by varying the phase matching angle and pump wavelength are analyzed. The expression of the effective parametric gain length under the noncollinear phase matching condition is deduced. Parametric gain and absorption characteristics of THz wave in KNbO3 are theoretically simulated. The characteristics of KNbO3 for parametric oscillator (TPO) are compared with those of MgO:LiNbO3. The analysis results indicate that KNbO3 is an excellent optical crystal for TPO to enhance the output of THz wave.

Gain and bandwidth investigation in dual-pumped non-collinear optical parametric chirped pulse amplification

Dual-pumped non-collinear parametric amplification of a chirped laser pulse is numerically studied by employing the pump beams of different wavelengths. Our results reveal that the output bandwidth and the efficiency-bandwidth product are improved, when non-collinear angles and phase matching conditions are separately optimized for each individual pump beam. Moreover, by studying the dependence of the bandwidth of the amplified signal on the pump intensity, gain stability and its fluctuations are compared in small signal gain and saturation regimes. It is verified that the effective interaction length in this dual-pumped configuration is reduced as a consequent of increased bandwidth, compared to that in a single pump scheme.

Nonlinear optical detection of terahertz-wave radiation from resonant tunneling diodes.

The sensitive detection of terahertz (THz)-wave radiation from compact sources at room temperature is crucial for real-world THz-wave applications. Here, we demonstrate the nonlinear optical detection of THz-wave radiation from continuous-wave (CW) resonant tunneling diodes (RTDs) at 0.58, 0.78, and 1.14 THz. The up-conversion process in a MgO:LiNbO3 crystal under the noncollinear phase-matching condition offers efficient wavelength conversion from a THz wave to a near-infrared (NIR) wave that is detected using a commercial NIR photodetector. The minimum detection limit of CW THz-wave power is as low as 5 nW at 1.14 THz, corresponding to 2-aJ energy and 2.7 × 103 photons within the time window of a 0.31-ns pump pulse. Our results show that the input frequency and power of RTD devices can be calibrated by measuring the output wavelength and energy of up-converted waves, respectively. This optical detection technique for compact electronic THz-wave sources will open up a new opportunity for the realization of real-world THz-wave applications.

Tunable terahertz waves from 4-dimethylamino-N′-methyl-4′-stibazolium tosylate pumped with dual-wavelength injection-seeded optical parametric generation

We developed a widely tunable terahertz (THz)-wave source covering the sub-THz frequency by difference frequency generation using a 4-dimethylamino-N′-methyl-4′-stibazolium tosylate (DAST) crystal. Near-infrared waves generated by dual-wavelength injection-seeded β-BaB2O4 optical parametric generation (is-BBO-OPG) were used for pumping the DAST crystal, which had separated wavelengths in the spectrum with a difference frequency of sub-THz. Furthermore, the non-collinear phase-matching condition was designed to compensate the walk-off effect of the BBO crystal. Consequently, tunable THz-waves from 0.3 to 4 THz were generated by tuning the wavelength of one of the seeding beams. The generated sub-THz-waves were monochromatic (dν < 33 GHz) with a maximum energy of 80 pJ at 0.65 THz.

High-Brightness Continuously Tunable Narrowband Subterahertz Wave Generation

We report on the generation of high-brightness (high peak power and narrow linewidth) continuously tunable subterahertz waves by optical parametric wavelength conversion using lithium niobate as a nonlinear optical crystal. The high-brightness subterahertz waves were achieved by expanding the interaction volume of the pumping beam and the idler wave, as well as the subterahertz wave, during wavelength conversion and by injecting the seeding beam into the idler wave. We could continuously tune the wavelengths of the generated subterahertz waves by controlling the angle and wavelength difference between the pumping and injection seeding beams to satisfy the noncollinear phase-matching conditions. The high-brightness (maximum peak output power, linewidth) and tunable range were about 10 MW/sr· cm 2 (>7 kW, <;5 GHz), and 760-150 μm (0.39-2 THz), respectively.

Theoretical analysis of terahertz parametric oscillator using KTiOPO4 crystal

Terahertz parametric oscillator (TPO) using KTiOPO4 (KTP) crystal with a noncollinear phase-matching scheme is investigated. Frequency tuning characteristics of terahertz wave (THz-wave) by varying the phase-matching angle and pump wavelength are analyzed. The expression of the effective parametric gain length under the noncollinear phase matching condition is deduced. Parametric gain and absorption characteristics of THz-wave in KTP are theoretically simulated for the first time. The characteristics of KTP for TPO are compared with MgO:LiNbO3. The analyses indicate that KTP is more suitable than MgO:LiNbO3 for TPO.

Terahertz wave parametric oscillations at polariton resonance using a MgO:LiNbO3 crystal.

Terahertz wave (THz-wave) parametric oscillations with a noncollinear phase-matching scheme at polariton resonance using a MgO:LiNbO3 crystal with a surface-emitted configuration are investigated. We investigate frequency tuning characteristics of a THz-wave via varying the wavelength of the pump wave and phase-matching angle. The effective parametric gain length under the noncollinear phase-matching condition is calculated. Parametric gain and absorption characteristics of a THz-wave in the vicinity of polariton resonances are analyzed.

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.

Broadband THz-wave generation by satisfying the noncollinear phase-matching condition with a reflected signal beam

We demonstrated broadband terahertz (THz) wave generation by satisfying the noncollinear phase-matching condition with a reflected signal beam. We constructed a dual-wavelength optical parametric oscillator with two potassium titanium oxide phosphate crystals pumped by a frequency-doubled Nd:YAG laser. The collinear pump and signal waves were irradiated into a lithium niobate crystal. The pump and the signal waves were reflected at the crystal surface. Because the pump and the signal waves have a finite beam diameter, when the reflected signal wave and unreflected pump wave were irradiated at the correct angle, the noncollinear phase-matching condition was satisfied. By changing the incident angle to the crystal, broadband THz-wave generation with a range of over 0.2-7.2THz was achieved.

High-powered tunable terahertz source based on a surface-emitted terahertz-wave parametric oscillator

A high-powered pulsed terahertz (THz)-wave has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO). The effective parametric gain length under the noncollinear phase matching condition was calculated for optimization of the parameters of the TPO. A large volume crystal of MgO:LiNbO3 was used as the gain medium. THz-wave radiation covering a frequency range from 0.87 to 2.73 THz was obtained. The average power of the THz-wave was 9.12 μW at 1.75 THz when the pump energy was 94 mJ, corresponding to an energy conversion efficiency of about 9.7×10−6 and a photon conversion efficiency of about 0.156%. The THz-wave power in our experiments is high enough for practical applications to spectrum analysis and imaging.

Wavelength Tuning Characteristics of Idler Waves in Terahertz-Wave Parametric Oscillator Using Optical Double Resonance

We experimentally investigated the wavelength tuning of oscillating idler (Stokes) waves by slightly translating the position of a mirror constituting an enhancement cavity in a terahertz (THz)-wave parametric oscillator (TPO) with optical double resonance. The wide tuning range of the idler wavelength was from 781.5 to 787.3 nm, corresponding to the frequency range of THz (signal) waves from 0.7 to 3.5 THz. The measured intersecting angle between pump and idler waves was in good agreement with the theoretical calculation of the noncollinear phase-matching condition in all the above tuning range.

Frequency-agile terahertz-wave parametric oscillator in a ring-cavity configuration

We demonstrate a frequency-agile terahertz wave parametric oscillator (TPO) in a ring-cavity configuration (ring-TPO). The TPO consists of three mirrors and a MgO:LiNbO(3) crystal under noncollinear phase-matching conditions. A novel, fast frequency-tuning method was realized by controlling a mirror of the three-mirror ring cavity. The wide tuning range between 0.93 and 2.7 THz was accomplished. For first demonstration using the ring-TPO, terahertz spectroscopy was performed as the verification of the frequency-agile performance, measuring the transmission spectrum of the monosaccharide glucose. The spectrum was obtained within about 8 s in good comparison to those of Fourier transform infrared spectrometer.

Generation of multicycle terahertz phonon-polariton waves in a planar waveguide by tilted optical pulse fronts

We demonstrate generation of frequency-tunable, multicycle terahertz phonon-polariton waves in a LiNbO3 slab waveguide. Because the waveguide modes show considerable phase-velocity dispersion, we are able to enhance frequency-selected narrowband terahertz waves by using femtosecond optical pulses whose intensity fronts are tilted at angles that meet the appropriate noncollinear phase-matching conditions. The pump light is spread across a large area of the crystalline waveguide within which coherent terahertz wave generation occurs, averting material damage, while yielding peak-to-peak terahertz field amplitudes in the waveguide of 50 kV/cm.

Investigation of a novel frequency-tuning method for terahertz-wave parametric oscillators

A novel frequency tuning scheme for terahertz-wave parametric oscillators (TPOs) is proposed. We demonstrate that the generation of the tunable terahertz wave can be realized by continuously varying the incident pump wavelength at a fixed angle of incidence of pump relative to the TPO resonator axis, based on the variation of noncollinear phase-matching conditions between the pump and Stokes/THz waves in the process of simulated polariton scattering. Combined with the angle-tuning method, this potential pump-wavelength tuning technique can further extend the TPO tuning range, especially in low-frequency THz region. However, the stability of the output direction of THz waves from the Si prism under this tuning method has yet to be further improved. The characteristics of THz-wave parametric gain involved in this frequency tuning scheme are also studied.

Experimental Performance of a Two-Stage Periodically Poled Lithium Niobate Parametric Amplifier

A two-stage periodically poled lithium niobate parametric amplifier is used to boost the idler wave energy by almost a factor of three near the degenerate wavelength of the second stage. For nondegenerate values, the idler wave energy is boosted by a factor of two. Our experimental results validate a numerical model that includes arbitrary beam profiles, beam walkoff, pump depletion, and optical elements between crystals. Experimental performance of the whole system was found to be much more stable over a wide range of temperatures due to noncollinear phase-matching conditions in the second stage.