New method to determine the refractive index and the absorption coefficient of organic nonlinear crystals in the ultra-wideband THz region

Coupling effects of the sum-frequency process and difference-frequency process on upconversion terahertz-wave detection via a DAST crystal

Frequency upconversion technology with good performance including high sensitivity, fast response, and room-temperature operation is a promising method for terahertz-wave detection. The sum-frequency conversion and difference-frequency conversion jointly affect the detection ability for upconversion detection using organic crystals as nonlinear media. The concurrence of both processes has been ignored in past studies, which results in discrepancies between theoretical simulations and experimental results. In this paper, four-wave interaction equations involving two nonlinear conversion processes are proposed, and the effect of the sum-frequency process is analyzed in upconversion terahertz-wave detection via a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. The ratio of the sum-frequency signal to the difference-frequency signal varies for different terahertz frequencies and crystal thicknesses. Experiments suggest that theoretical simulations are good at predicting physical processes. Under certain conditions, the detection efficiency can be improved by simultaneously utilizing the two signals. The total signal photon number is not sensitive to the crystal thickness. Furthermore, the theoretical exploration of terahertz single-photon detection provides a noteworthy reference for future experiments.

Cascaded difference frequency generation in organic crystal 4'-dimethylamino-N-methyl-4-stilbazolium tosylate

In this Letter, we verified, for the first time to the best of our knowledge, an efficient cascaded difference frequency generation (DFG) with multi-order red shift. The experiment was performed with homemade organic crystals pumped by an optical parametric oscillator. The characteristics of energy conversion during DFG were investigated comprehensively at different frequencies and crystal thicknesses. Theoretical explanations were provided, based on the coupled-wave equations. The calculations predicted that the cascading effect contributed to a sustained growth of the DFG output (free from the saturation and shrinkage), especially in thick crystals. By measuring the optical spectrum, remarkable cascading processes of at least seven orders were observed. This effect shows the potential to overcome the quantum defect of the terahertz-wave DFG and fundamentally improve the conversion efficiency.

Organic Crystals for THz Photonics

Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.

Tunable narrow band difference frequency THz wave generation in DAST via dual seed PPLN OPG

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST.

10 aJ-level sensing of nanosecond pulse below 10 THz by frequency upconversion detection via DAST crystal: more than a 4 K bolometer

By using frequency upconversion detection of terahertz (THz) waves via 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal with an optimized frequency conversion process, ultrahigh sensitivity has been achieved. Direct comparisons with a 4 K bolometer were implemented. By using a simple positive intrinsic negative (PIN) diode without either electrical amplification or optical amplification, frequency upconversion detection can compete with the commercial 4 K bolometer, while by replacing the PIN diode with an avalanche photo diode (APD), it performs more than three orders better than the 4 K bolometer. Based on power calibration, the minimum detectable THz pulse energy is in the order of 10 aJ (9-25 aJ) at 4.3 THz, with a pulse duration of 6 ns. Thus, the minimum number of THz photons that can be detected is down to the order of 10(3) at room temperature. The current THz detection system gives a noise equivalent power (NEP) in the order of 100  fW/Hz(1/2) (50-128  fW/Hz(1/2)). Moreover, by switching current optical detectors, the dynamic range is over six orders.

Long-range parametric amplification of THz wave with absorption loss exceeding parametric gain

Optical parametric mixing is a popular scheme to generate an idler wave at THz frequencies, although the THz wave is often absorbing in the nonlinear optical material. It is widely suggested that the useful material length for co-directional parametric mixing with strong THz-wave absorption is comparable to the THz-wave absorption length in the material. Here we show that, even in the limit of the absorption loss exceeding parametric gain, the THz idler wave can grows monotonically from optical parametric amplification over a much longer distance in a nonlinear optical material until pump depletion. The coherent production of the non-absorbing signal wave can assist the growth of the highly absorbing idler wave. We also show that, for the case of an equal input pump and signal in difference frequency generation, the quick saturation of the THz idler wave predicted from a much simplified and yet popular plane-wave model fails when fast diffraction of the THz wave from the co-propagating optical mixing waves is considered.

Dual-frequency picosecond optical parametric generator pumped by a Nd-doped vanadate bounce laser

Dual-frequency MHz-repetition 1.5-μm optical parametric generation is demonstrated from a tandem system pumped by a picosecond Nd:YVO₄ bounce laser. An average power of 1.0 W is obtained over the range 1571-1630 nm, corresponding to an optical-optical conversion efficiency and slope efficiency of 16% and 23%, respectively. Terahertz-wave with a frequency of 3.2 THz is also generated from the system in combination with an organic nonlinear DAST crystal.

Optical properties of DAST in the THz range

We report the far-infrared properties of the organic crystal DAST, a popular terahertz emitter, from 0.6-12 THz through use of a THz spectrometer incorporating air-plasma THz generation and electro-optic (EO) sampling in a poled EO polymer. We identify absorption features at 1.1, 3.1, 5.2, 7.1, 8.4, 11, and 12.3 THz and at 1.1, 1.3, 1.6, 2.2, 3, 5.2, 7.2, 9.6 and 11.7 THz for a-axis and b-axis polarized THz radiation respectively. These results allow for more accurate prediction of the optimum crystal thickness for broadband THz emission via optical rectification and difference frequency generation.