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

Zeptojoule detection of terahertz pulses by parametric frequency upconversion

We combine parametric frequency upconversion with the single-photon counting technology to achieve terahertz (THz) detection sensitivity down to the zeptojoule (zJ) pulse energy level. Our detection scheme employs a near-infrared ultrafast source, a GaP nonlinear crystal, optical filters, and a single-photon avalanche diode. This configuration is able to resolve 1.4 zJ (1.4 × 10-21 J) THz pulse energy, corresponding to 1.5 photons per pulse, when the signal is averaged within only 1 s (or 50,000 pulses). A single THz pulse can also be detected when its energy is above 1185 zJ. These numbers correspond to the noise-equivalent power and THz-to-NIR photon detection efficiency of 1.3 × 10-16 W/Hz1/2 and 5.8 × 10-2%, respectively. To test our scheme, we perform spectroscopy of the water vapor between 1 and 3.7 THz and obtain results that are in agreement with those acquired with a standard electro-optic sampling (EOS) method. Our technique provides a 0.2 THz spectral resolution offering a fast alternative to EOS THz detection for monitoring specific spectral components in spectroscopy, imaging, and communication applications.

Highly sensitive frequency upconversion detection from 1 to 3 THz with OH1 crystal

The wide applications of terahertz (THz) wave technology in the ∼1-3 THz range has resulted in a surge in the demand for the performance improvement of THz wave detection technique. In this study, a frequency tunable, highly sensitive frequency upconversion detection based on a 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1) crystal at room temperature is demonstrated. Moreover, to effectively increase the signal-to-noise ratio in the low frequency range, a beam isolation enhancer is proposed and its effect is verified. The minimum detectable THz pulse energy reaches about 100 aJ at 1.9 THz. The frequency tuning ranging from 1 to 3 THz. Sensitivity comparison with a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal system shows that OH1 is a more suitable nonlinear crystal in the 1-2.4 THz range.

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/Hz^1/2. 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.

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.

Efficient terahertz detection in a sheet cavity using a nonlinear optical parametric process

We designed and simulated a GaP-based sheet cavity structure for room-temperature THz wave detection via a nonlinear optical (NLO) parametric process, using photonic conversion from THz frequencies to the optical domain. The sheet cavity structure consisted of a GaP rectangular waveguide for THz waves. Pump enhancement in the cavity and strong confinement of the THz wave in the waveguide resulted in a high power conversion efficiency of 1% (detection at 5 THz). The noise equivalent power for THz wave detection using an optical single-photon detector was estimated to be in the order of a few fW Hz(-1/2), which was higher than that obtained using room-temperature bolometers, field-effect transistors, and other NLO processes.

Diffraction-limited real-time terahertz imaging by optical frequency up-conversion in a DAST crystal

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and the arts. This report describes real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in an organic 4-dimethylamino-N'-methyl-4'-stilbazolium tosylate (DAST) crystal, with high resolution reaching the diffraction limit. THz-wave images were converted to the near infrared region and then captured using an InGaAs camera in a tandem imaging system. The resolution of the imaging system was analyzed. Diffraction and interference of THz wave were observed in the experiments. Videos are supplied to show the interference pattern variation that occurs with sample moving and tilting.