Terahertz Parametric Oscillator Research Articles

Highly sensitive multi-stage terahertz parametric upconversion detection based on a KTiOPO4 crystal.

In this Letter, we demonstrate a highly sensitive multi-stage terahertz (THz) wave parametric upconversion detector based on a KTiOPO4 (KTP) crystal pumped by a 1064-nm pulsed-laser (10 ns, 10 Hz). The THz wave was upconverted to near-infrared light in a trapezoidal KTP crystal based on stimulated polariton scattering. The upconversion signal was amplified in two KTP crystals based on non-collinear and collinear phase matching, respectively, to improve detection sensitivity. A rapid-response detection in the THz frequency ranges of 4.26-4.50 THz and 4.80-4.92 THz was achieved. Moreover, a dual-color THz wave generated from THz parametric oscillator using KTP crystal was detected simultaneously based on dual-wavelength upconversion. The minimum detectable energy of 2.35 fJ was realized with a dynamic range of 84 dB at 4.85 THz, which gives a noise equivalent power (NEP) of the order of 21.3 pW/Hz1/2. By changing the phase-matching angle or the wavelength of the pump laser, it is suggested that the detection of the THz frequency band of interest in a wide range from approximately 1 to 14 THz is possible.

Design and simulation of widely tunable picosecond synchronously pumped terahertz parametric oscillator based on silicon waveguide platform

AbstractA picosecond synchronously pumped terahertz optical parametric oscillator based on silicon membrane waveguide (Si‐based WTPO) is proposed and numerically simulated. Through designing the structure parameters of the silicon membrane waveguide, a broadband terahertz frequency range is obtained by satisfying the phase matching condition. The parametric oscillation process and output characteristics of Si‐based WTPO are numerically analyzed by solving the four‐wave mixing coupled‐wave equations, and the result has indicated that the parametric oscillation within the resonant cavity can apparently enhance the output terahertz wave power. Meanwhile, high bandwidth and efficiency output terahertz wave can be obtained, and too high pump power will lead to a reaction of reverse conversion. Finally, the threshold pump power is calculated. The Si‐based WTPO system we proposed is expected to make some contributions to the study of compact and efficient terahertz wave sources.

Modeling for extracavity-pumped terahertz parametric oscillators.

This paper presents a modeling method for extracavity-pumped terahertz parametric oscillators (TPO) based on stimulated polariton scattering, in which the pumping beam is from a different laser, and the Stokes beam oscillates in its cavity. After suitable approximations and assumptions, the average THz wave amplitude in the nonlinear crystal is expressed as a function of the fundamental and Stokes wave amplitudes. Then the rate equation for the Stokes wave is obtained based on the Stokes wave increment within a cavity roundtrip timescale. After solving the Stokes wave rate equation, the Stokes wave temporal evolution is considered as a known parameter, and the properties of the residual fundamental and terahertz waves are obtained by numerically solving the coupled wave equations. This modeling method is applied to an extracavity-pumped TPO based on MgO:LiNbO3 crystal. The simulation results are basically consistent with the experimental results. The main reasons causing the deviations of the simulation results from the experimental results are analyzed. To the best of our knowledge, this is the first time to perform the modeling for extracavity-pumped Q-switched TPOs.

Terahertz parametric oscillator with a rhombic ring-cavity

Terahertz parametric oscillator (TPO) with a rhombic ring-cavity is demonstrated. The oscillating cavity is composed of three mirrors and the terahertz emitting-surface. Terahertz frequency can be tuned from 0.9 THz to 2.9 THz just by shifting the position of a cavity mirror. Under a certain round-trip optical length of the Stokes beam, the pump beam size can be larger in the rhombic ring-cavity TPO than that in the conventional resonant-cavity TPO. The maximum terahertz pulse energy in the rhombic ring-cavity TPO is 3.21 μJ, which is about 3 times higher than that in the conventional resonant-cavity TPO.

Continuously Tunable and Energy-Enhanced Injection Pulse-Seeded Terahertz Parametric Generator Based on KTP Crystal

A continuously tunable, energy-enhanced injection, pulse-seeded THz parametric generator based on a KTP crystal (KTP ips-TPG) has been demonstrated in this paper. Theoretical analysis denotes that improving the initial Stokes intensity is favorable to achieve a continuously tunable, high-energy THz generation in a KTP crystal, which is clarified to originate from the enhancement of stimulated polariton scattering (SPS) process in the photon-like region of polariton and parametric amplification process in the phonon-like region of polariton. In the experiment, the THz continuously tunable range was from 2.93 to 6.21 THz, and the THz energies were 6.27, 2.62, 5.43, 4.59, and 5.88 μJ at the output peaks of 3.33, 3.83, 4.41, 4.85, and 5.69 THz, respectively. Compared with the traditional KTP THz parametric oscillator (KTP TPO) with discontinuous frequency tuning, the KTP ips-TPG has great improvement in terms of continuous frequency tuning range and THz output energy, where the enhancement ratios of THz output energy were 6.88× at 3.36 THz, 5.25× at 4.35 THz, 18.21× at 4.79 THz, and 1.82× at 5.69 THz. This injection pulse-seeded THz parametric source based on a KTP crystal is not only suitable to the practical applications in THz high frequency range, but also provides a technical solution for the KTP-family crystals to achieve a continuously tunable and high energy THz generation in a wide THz region.

Injection-Seeded Terahertz Parametric Oscillator with a Ring-Cavity Configuration

This paper reports the characteristics of an injection-seeded terahertz parametric oscillator (TPO) with a ring-cavity configuration based on the MgO:LiNbO3 crystal. The ring cavity is constituted of three mirrors and the THz wave output surface where the pump and Stokes beams are totally reflected. The THz pulse energies and the Stokes pulse energies as functions of the pump pulse energy for different seed powers of 47.5 mW, 150.7 mW, and 312.8 mW were investigated. The experimental results showed that the injection-seeded ring cavity TPO exhibited the benefits of lower pump thresholds and higher output energies for the Stokes and THz waves. The smaller the pump pulse energy, the more obvious the effect of the seed injection. The reasons for the laser performance improvement were analyzed.

Optimized design for a terahertz parametric oscillator based on degenerate four-wave mixing in silicon nitride

We propose terahertz wave generation using a parametric oscillator based on phase-matched and non-phase-matched degenerate four-wave mixing processes within a silicon nitride Fabry–Perot microcavity. The central frequency of the generated terahertz waves would be tunable over the frequency range of 3.6–5.73 THz by varying the wavelength of the pump wave. The best phase-matching condition is achieved at a frequency of 5 THz, and an optimum peak power of 329 W is obtained at a frequency of 4.29 THz.

0.5–4.5 THz band terahertz spectroscopy of thermally activated delayed fluorescence molecules

In this work we performed an expanded THz range spectroscopic analysis of organic electroluminescence chromophores; specifically the thermally activated delayed fluorescence (TADF) compounds 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as amorphous powders. THz parametric oscillator, injection seeded THz parametric generator, THz time-domain and Fourier transform infrared spectroscopy techniques were all used to assess these compounds over the frequency ranges of 0.50–4.5 and 18–54THz. The resulting experimental spectra were compared with the results of density functional theory (DFT) calculations at the M06-2X/6-31G(d,p) level of theory. THz absorption bands in the experimental spectra were found to be very weak because of the lack of crystallinity in these specimens, even though the DFT calculations predicted absorbance in the THz range. Based on the DFT results, the absorbance lines that were observed are attributed to the wagging, rocking, rotational and out-of-plane bending motions of the carbazole moieties.

Modeling of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering

In this paper, the rate equations describing the operation of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering are given for the first time. The rate equations are obtained under the plane-wave approximation, the oscillating fundamental and Stokes waves are supposed to be round uniform beam spots. Considering the fact that the terahertz wave nearly traverses the pump and Stokes beams and using the coupled wave equations, the terahertz wave intensity is expressed as the function of the fundamental and Stokes intensities. Thus, the rate equations describing the evolution processes of the fundamental and Stokes waves are obtained in the first step. The THz wave properties are then obtained. Several curves based on the rate equations are generated to illustrate the effects of the nonlinear coefficient, the THz wave absorption coefficient, and pulse repetition rate on the THz laser characteristics. Taking the intracavity-pumped Mg:LiNbO3 TPO as an example, the THz frequency tuning characteristic and the dependences of the fundamental, Stokes, and THz wave powers on the incident diode pump power are calculated. The theoretical results are in agreement with the experimental results on the whole.

Enhancement of Intracavity-Pumped Terahertz Parametric Oscillator Power by Adopting Diode-Side Pumped Configuration Based on KTiOPO4 Crystal

In this paper, a KTiOPO4 (KTP) crystal is used as the nonlinear medium in an intracavity-pumped terahertz parametric oscillator (TPO) based on stimulated polariton scattering (SPS). Almost all the reported intracavity SPS sources adopt the diode-end pumped configuration, and the THz output power is limited by the serious thermal effect and small fundamental beam size. For improving the THz output power, we propose diode-side pumping for the laser medium to get a higher fundamental power and a larger fundamental beam size. A convex–plane fundamental laser resonator is used to further offset the thermal effect and increase the fundamental beam size. The THz frequency of the intracavity-pumped KTP terahertz parametric oscillator can be discontinuously tuned from 3.19 THz to 5.94 THz with three gaps. The fundamental beam diameter in the KTP crystal is about 1.3 mm. The maximum average THz power is 166 μW at 5.74 THz under a pulse repetition frequency (PRF) of 6 kHz and a diode pump power of 98 W. By means of the diode-side pumped configuration, the maximum THz output power is more than two-fold higher compared to the diode-end pumped configuration reported using the KTP crystal.

Theoretical and experimental study on a large energy potassium titanyl phosphate terahertz parametric source

The combination of a Stokes parametric oscillator and a Stokes parametric amplifier is used to generate high-energy terahertz pulses in KTiOPO4 (KTP) crystal at 5.70 THz. The Stokes parametric oscillator generates Stokes pulses first. And then they are amplified in the Stokes parametric amplifier and at the same time, high energy terahertz pulses are generated via stimulated polariton scattering. When the incident Stokes pulse energy is 36.6 mJ and the pump energy is 580.0 mJ, the amplified Stokes energy is 192.1 mJ and the obtained maximum terahertz pulse energy is 17.0 μJ. Compared with the reported terahertz pulse energy obtained by the KTiOPO4 terahertz parametric oscillators or generators, the terahertz pulse energy is greatly enhanced. In addition, considering the large angle non-collinear phase matching, pump beam depletion, terahertz wave surface-emitted structure, Gaussian pulse distribution, we make a theoretical simulation for the Stokes parametric amplifier through the coupled-wave equations. The theoretical results are in agreement with the experimental results on the whole.

Injection pulse-seeded terahertz-wave parametric generator with gain enhancement in wide frequency range.

An injection pulse-seeded terahertz-wave parametric generator (ips-TPG) has been demonstrated with gain enhancement in wide tuning range. Theoretical analysis denotes that the compensation of initial Stokes energy is favorable to the THz gain enhancement in wide frequency range, which is attributed to the improvement on interaction of stimulated polariton scattering (SPS) and difference frequency generation (DFG) processes. In the experiment, the THz frequency tuning range from 1.04 THz to 5.15 THz was achieved based on near-stoichiometric LiNbO3 (SLN) crystal. Compared with the traditional terahertz parametric oscillator (TPO) under the same experimental conditions, a significant enhancement of THz output energy was occurred in high frequency range. As the THz frequency increased from 1.9 THz to 3.6 THz, the enhancement ratios from 1.6 times to 34.7 times were obtained. Besides, the 3dB bandwidth of ips-TPG was measured to be 2.1 THz, which was about 2.6 times that of SLN-TPO. This THz parametric source with a relative flat gain in wide frequency range is suitable to a variety of practical applications.

Generation of ∼100 kW narrow-line far-infrared radiation from a KTP off-axis THz parametric oscillator

In the far-infrared spectrum between 20 and 60 μm, the free-electron laser (FEL) is the only wavelength-tunable coherent radiation source capable of generating kilowatt to megawatt peak powers with a linewidth of the order of 1%. Here, we report the detection of >70 kW radiation power at about 52 μm in a 45 kW radiation power for the far-infrared radiation. With 63% coupling efficiency of the silicon prism atop the KTP crystal, the measured >70 and >45 kW far-infrared radiation correspond to >111 and >71 kW powers extracted from the KTP crystal of the seeded off-axis THz parametric oscillator. The radiation source accomplished in this work has great potential to become a tabletop and economical alternative for the bulky and expensive far-infrared FELs in national facilities.

Efficient Ring-Cavity Terahertz Parametric Oscillator With Pump Recycling Technique

The energy scaling of THz wave in wide range based on surface-emitted (SE) ring-cavity terahertz parametric oscillator (TPO) with pump recycling technique is demonstrated in this paper. The continuously tunable range from 1.24 to 3.77 THz was achieved with full pump beam recycling, whereas the tunable range was 1.24-2.72 THz without pump beam recycling. The terahertz output energy and conversion efficiency increased more than seven times throughout the whole tuning range. The maximum enhancement ratio of the terahertz output energy was 12.8 times at 2.72 THz. Additionally, under the same experimental conditions, THz conversion efficiency was increased from 7.77 × 10 -6 to 7.85 × 10 -5 and the threshold energy was significantly reduced by 41% at 1.63 THz. Moreover, through the theoretical analysis, the enhancement mechanism of TPO by pump recycling was verified to originate from the threshold reduction of the stimulated polariton scattering (SPS) process and the interaction improvement between the pump and Stokes waves in the difference frequency generation (DFG) process.

Off-axis terahertz parametric oscillator

We study an off-axis terahertz (THz) parametric oscillator and show its superior performance over a conventional THz parametric oscillator. Furthermore, by pumping an off-axis lithium-niobate oscillator with a pulsed Nd:YAG laser having an axial-mode spacing matched to the THz oscillator’s mode spacing, we generated an ultra-broad red-shifted frequency comb between 1065 and 1085 nm at the output, which corresponds to a four-octave-spanning THz-frequency comb in the idler spectrum. We also demonstrated a signal-seeded off-axis THz parametric oscillator, which generates 8 nJ narrow-line radiation at 2.1 THz and achieved 46% pump depletion in a 6.5 cm long crystal with only 4 mJ pump energy in a 460 ps pump pulse width. Furthermore, tuning between 3.6 and 4.9 THz for the generated radiation is also demonstrated in this work.

Energy scaling and extended tunability of a ring cavity terahertz parametric oscillator based on KTiOPO4 crystal**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB755403 and 2014CB339802), the National Key Research and Development Program of China (Grant No. 2016YFC0101001), the National Natural Science Foundation of China (Grant Nos. 61775160, 61771332, and 61471257), China Postdoctoral Science Foundation (Grant

A wide terahertz tuning range from 0.96 THz to 7.01 THz has been demonstrated based on ring-cavity THz wave parametric oscillator with a KTiOPO4 (KTP) crystal. The tuning range was observed intermittently from 0.96 THz to 1.87 THz, from 3.04 THz to 3.33 THz, from 4.17 THz to 4.48 THz, from 4.78 THz to 4.97 THz, from 5.125 THz to 5.168 THz, from 5.44 THz to 5.97 THz, and from 6.74 THz to 7.01 THz. The dual-Stokes wavelengths resonance phenomena were observed in some certain tuning angle ranges. Through the theoretical analysis of the dispersion curve of the KTP crystal, the intermittent THz wave tuning range and dual-wavelength Stokes waves operation during angle tuning process were explained. The theoretical analysis was in good agreement with the experiment results. The maximum THz output voltage detected by Golay cell was 1.7 V at 5.7 THz under the pump energy of 210 mJ, corresponding to the THz wave output energy of 5.47 μJ and conversion efficiency of 2.6 × 10−5.

Perspective: Terahertz wave parametric generator and its applications

The injection-seeded terahertz (THz) parametric generator (is-TPG) is one of the most high-power single-longitudinal mode THz-wave sources. Our system is less influenced by scattering, refraction, and multiple reflections by samples because it is a narrow-linewidth source, and the detection area of the THz parametric detector is large. Thus, it is suitable for nondestructive inspection of practical samples in the real world. In 2003, we reported on the development of a mail inspection system that employed a THz parametric oscillator. However, with a dynamic range of less than four orders of magnitude, this system could only identify reagents through thin envelopes. Recently, we succeeded in developing a high-power, highly sensitive THz-wave spectroscopic imaging system with a dynamic range of 100 dB using the is-TPG and a THz parametric detector. Nondestructive inspection of reagents inside thick envelopes and three-dimensional computed tomography of plastics, which attenuate THz-waves by more than 60 dB, were conducted using this system. More recently, we have focused our efforts on a real-time measurement system using a multiwavelength is-TPG, which gives rise to numerous potential applications, given the significantly shorter measurement times. Thus, this system will facilitate the implementation of THz-wave measurements in real-world applications. In this paper, we report on our recent results and provide a perspective on the is-TPG.

Terahertz parametric oscillator with the surface-emitted configuration in RbTiOPO4 crystal

Terahertz parametric oscillator based on RbTiOPO4 (RTP) crystal utilizing the surface-emitted configuration under high pumping pulse energy and low pulse repetition rate is investigated. The obtained THz wave was discontinuously tuned from 3.01 to 3.09 THz, from 3.53 to 4.17 THz, from 4.54 to 4.66 THz, and from 5.35 to 5.89 THz, respectively. The maximum output THz wave pulse energy was 1.77 μJ at 3.85 THz. It was obtained when the pumping pulse energy was 120 mJ and the pumping beam diameter was 3.0 mm. The polarization of THz wave was along the z axis of the RTP crystal. The THz beam on its output surface was approximately elliptical with a major axis of 6.5 mm and a minor axis of 3.5 mm. The divergence angles were 2.45° in the horizontal direction, and 3.88° in the vertical direction.

Single pixel imaging with tunable terahertz parametric oscillator.

A method of active terahertz imaging based on compressive sampling is demonstrated. A metal mask structure is designed with all modulation matrices engraved on. The imaging approach based on the mask eliminates the need for imaging object movement in point-wise scanning and shows high sensitivity. A terahertz parametric oscillator with tunability from 0.5 to 2.7 THz was used as the light source. Holes with circular, rectangular, and letter "H" shapes were imaged at 1.75 THz at 20% sampling rate. The influence of sampling rates and averaging times on the image was analyzed. Imaging of the letter "H" at different frequencies from 1.0 to 2.2 THz was tested and evaluated, and recognizable results were obtained in the range of 1.4-2.0 THz.

Tunable terahertz generation via a cascaded optical parametric device

A compact cascaded optical parametric device generating a coherent pulse terahertz (THz) wave is demonstrated. The terahertz parametric oscillator (TPO) and the difference frequency generation (DFG) are designed for cascaded operation use with two outputs producing tunable THz wavelengths. From the first optical parametric device, a TPO with a MgO: LiNbO3 crystal pumped by a Q switch laser of 1.064 μm, 1.8 mJ idler pulse and 5.5 mJ residual pump pulse is obtained. Both of the two beams are employed as the pump and signal beams in the second optical parametric device DFG with a GaSe crystal. More than 0.6 μJ and about 2.1 ns THz pulse at 183 μm is achieved from the DFG. A tunable THz source in the range 104–226 μm via tuning the external phase matching (PM) angles of the TPO and the DFG flexibly under room temperature is obtained. The observed tunable THz wavelengths from the DFG are the same as those from the TPO.