1. Introduction

Terahertz (THz) wave technology presents a fascinating benefits in communication, material science, biomedical diagnostics, and security checking [1–5]. Over the past decades, the wide applications in the ∼1–3 THz range, such as spectral fingerprinting [6] and molecular orientation [7,8], has led to a surge in the performance demand of THz wave detection techniques with high sensitivity, room-temperature operation, and a wide frequency band. Bolometers, Golay cells and pyroelectric detectors are popular broadband THz wave detectors. Bolometers have high sensitivity, but they require 4-K cryogenic cooling. Golay cells and pyroelectric detectors are room-temperature detectors, but their sensitivities are relatively low. Additionally, photoconductance and electro-optical sampling are common THz detection techniques and have been widely used in THz time-domain spectroscopy (TDS) systems in the ∼1–3 THz range, where high-power femtosecond lasers with high repetition rates are necessary [9,10]. Frequency upconversion detection has been proven to be a promising technique. It is based on the difference frequency generation (DFG) or stimulated polariton scattering mechanism of optical nonlinear crystals. Unlike TDS systems, THz waves detected through this technique are typically nanosecond pulses with a narrow linewidth. THz waves can be upconverted to near-infrared (NIR) or visible signals, which can be measured using high-performance detectors, resulting in high sensitivity at room temperature [11,12]. It also has the advantages of fast response, wide-range frequency tunability, and low cost.

Based on the THz frequency upconversion detection technique, various prominent studies that employ inorganic or organic optical nonlinear crystals have been reported. The highest detection sensitivity was achieved using LiNbO₃ crystal, which is an inorganic crystal commonly used in the THz region [13]. The minimum detectable energy reached 130 zJ at 1.05 THz pumped by sub-ns pulses. As another inorganic crystal, KTiOPO₄ (KTP) crystal has also been recently employed for THz wave detection, with a minimum detectable energy of 2.35 fJ at 4.85 THz [14]. Although the detection sensitivities based on these inorganic crystals are quite high, the noncollinear phase matching of these systems are not ideal for practical applications. Realizing the optical alignment and a broad working frequency band is more convenient with organic nonlinear crystals compared to inorganic crystals. The widest frequency range in the field of THz frequency upconversion detection of 1.85–30 THz was achieved using a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal [15]. The THz pulse and pump beam of frequency upconversion detection have narrow linewidths, thus, for detection in a wide frequency range, the upconverted signal should have a varying wavelength. Using filters with fixed parameters for extracting the signal beam in broadband frequency tuning detection is inconvenient. A scheme of two diffraction gratings has been used to realize frequency agility in a wide range with a fixed optical path [16]. Based on this crystal, a high sensitivity with a minimum detectable energy of about 10 aJ at 4.3 THz has been achieved. However, the efficiency of this crystal in the range below 3 THz is relatively low due to the large absorption peak around 1.1 THz [17–19].

In this study, to meet the increasing performance demands of detection in the ∼1–3 THz range, a frequency upconversion detection technique based on another nonlinear crystal known as 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1) is investigated. This crystal has the potential to realize highly efficient detection in the ∼1–3 THz range due to its large second order nonlinearity and low THz absorption [19–26]. The scheme of two diffraction gratings is used to realize frequency agility with a fixed optical path. For THz frequencies lower than 1.8 THz, the isolation between a weak upconverted signal beam and a strong pump beam is insufficient for yielding an ideal signal-to-noise ratio. An optical unit is proposed to increase the beam isolation, which is termed beam isolation enhancer (BIE). With BIE, a highly sensitive, frequency tunable detection system for the 1–3 THz range is realized. The proposed system can detect a minimum detectable pulse energy of about 100 aJ at the frequency of 1.9 THz. The performances of the THz detection system based on OH1 and DAST crystals are compared, and the results shows that the OH1 crystal has significant advantages in the1–2.4 THz range.

2. Experimental design

Figure 1 displays the experimental setup. The system comprises a THz generation part and a detection part, both of which are pumped by a 532 nm laser with a pulse width of 10 ns and a repetition rate of a 100 Hz. The green laser is divided into two beams with a controllable separation ratio by using the combination of a half-wave plate (HWP) and a polarizing beam splitter (PBS). The THz generation part mainly includes a dual-KTP optical parametric oscillator (KTP OPO) and an OH1 crystal. Two NIR beams (λ₁ and λ₂) that are used to pump the OH1 crystal are generated by the KTP OPO, whose wavelengths could be tuned from 1250 to 1650 nm. A THz wave is generated based on the DFG mechanism of the OH1 crystal with a wavelength of λ_T = λ₁ − λ₂. For the THz wave detection part, the THz wave to be detected is guided to the second OH1 crystal by a pair of off-axis parabolic mirrors (OAPs, Thorlabs). Furthermore, an NIR pump beam of the detection part with the wavelength of λ₁ is generated by the single KTP-OPO and then focused on the second OH1 crystal. Based on the DFG mechanism in the OH1 crystal, a frequency upconverted signal with a wavelength of λ₂ is generated. After filtering the unexpected beam, the signal is detected by a home-made avalanche photo diode (APD). The active area diameter, the 3 dB bandwidth, and the typical maximum responsivity of the APD are 200 µm, 500 MHz, and 10⁵V/W at 1550 nm, respectively.

 

Fig. 1. Schematic of the frequency upconversion detection system based on OH1 crystal. HWP, half-wave plate; PBS, polarizing beam splitter; OAPs, off-axis parabolic mirrors; DL, diverging lens; FL, focal lens; BIE, beam isolation enhancer.

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The upconverted signal is collinear with the residual NIR pump beam, and thus, the signal beam must be extracted from the residual pump beam. A pair of reflective ruled diffraction gratings is used to separate the two beams in space, instead of using combinations of longpass and bandpass filters, which cannot achieve sufficiently large optical density (OD) values over a broad frequency band. In the frequency tuning process of the THz wave detection, the paths of the wavelength tunable upconverted signals are fixed, while those of the residual NIR pump beams are shifted by synchronously rotating the grating pair with the corresponding angles. Based on the nature of diffractive grating, the smaller the wavelength difference between the upconverted signal and the NIR pump beam, the closer are the paths of the two beams. Limited by the dispersion power of commercial gratings, the isolation between the two beams is insufficient for producing an ideal signal-to-noise ratio when the THz frequency is lower than 1.8 THz.

To reduce the optical noises, an optical unit comprising a diverging lens, an adjustable iris, and a focal lens is proposed to increase the separation angle, which increases the isolation between the signal and residual pump beams, as shown in Fig. 2(a). The BIE is placed between the two gratings (Thorlabs GR25-1210). The diverging lens is placed 75 cm behind the first grating. The adjustable iris is placed about 9.5 cm behind the diverging lens, and it serves as a spatial filter by blocking the residual pump beam. The focal lens is placed about 10 cm behind the diverging lens. The focal lengths of the diverging and focal lenses are 100 and 200 mm, respectively. Both optical axes of these lenses coincide with the signal beam path, and therefore, the signal beam slightly expands without any changes to the propagation path during the frequency tuning process. The residual pump beam becomes non-collinear with the signal after diffraction by the first grating; the beam could further stray from the signal path after crossing the diverging lens.

 

Fig. 2. (a) Schematic of the beam isolation enhancer (BIE). (b) Relative isolation versus the THz wave frequency.

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Herein, the distance between the inner edges of the signal and residual pump beams at the position of the iris is defined as relative isolation, and the values are normalized based on the distance at 1.8 THz, where the upconverted signal can be exactly extracted without detection noise using a pair of gratings without the BIE component. The spot diameter of the residual pump beam before passing through BIE is about 2 mm, which is measured by the knife-edge method. As shown in Fig. 2(b), using gratings only, the relative isolations in the 1–1.8 THz range are lower than 1, indicating that the optical noise levels caused by the residual pump beams are too high to make the signal unreadable. In contrast, with the BIE component between two gratings, the relative isolations are larger than 1 in the 1–1.8 THz range. Therefore, continuous frequency tuning can be realized.

3. Results and discussion

To experimentally verify the role of the BIE in expansion on lower frequency bands, the optical noises caused by the residual pump beams from 1 to 3 THz were measured using a PIN detector (Thorlabs PDA 10CF-EC). As shown in Fig. 3, the optical noise amplitudes in the frequency upconversion detection system with BIE for extracting signal beams were low enough for the signal reading in the 1–3 THz range. The optical noise amplitudes with only gratings were significantly higher than those with BIE between the gratings, especially in the frequency range below 1.5 THz. The equivalent voltages without BIE in this frequency range were calculated using the oscilloscope (Teledyne Lecroy HDO6104, 1 GHz) display amplitudes and transmittances of the neutral density filters used to attenuate the optical noise. The results exhibit the significance of the proposed BIE component in terms of optical noise reduction.

 

Fig. 3. Comparison of the optical noises in THz frequency upconversion detection with and without BIE components.

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The performance of the THz detection system at 1.9 THz was comprehensively investigated. The pump power of the detection part was approximately 10 mW, as measured by a power meter (Thorlabs, S470C). Since the spot diameter focused on OH1 was 350 µm (measured by the CMOS beam profiler, WinCanmd), the energy density of the pump beam was about 0.1 J/cm². First, the pulse energy of the 1.9 THz wave emitted by the DFG THz generation part was measured using a Golay cell (Tydex GC-1D). A chopper was used to modulate the THz wave to 5 Hz with a duty cycle of 1:1 for energy calibration. Based on the responsivity of 110 kV/W at this modulation rate, the THz pulse energy was about 46 pJ. Then, the THz pulse was detected by the DFG THz detection part via the PIN detector. The equivalent signal voltage was 105 dBmV, which is five orders higher than the readout voltage of the Golay cell, indicating the higher sensitivity of the frequency upconversion detection system, as shown in Fig. 4. The equivalent voltages signify the calculation results of the oscilloscope display amplitudes and OD values of the neutral density filters used to attenuate the frequency upconversion signal.

 

Fig. 4. Comparison between THz wave frequency detection using Golay cell and frequency upconversion detection with PIN and APD. Straight lines are the linear fittings of the experimental data.

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Thereafter, THz attenuators were inserted in the THz wave propagation path to quantitatively control the energies of THz pulses. The THz attenuators were composed of polyethylene terephthalate (PET) plates with different thicknesses, corresponding to transmittances of 20%, 14%, and 4%. Arbitrary attenuation was achieved through various combinations of PET plates. For the 0.0054% transmission (attenuation of 43 dB), the readout voltage from the PIN diode was about 7 mV (17 dBmV). The fitting curve signifies good linearity, indicating the reliable upconversion detection of THz pulse. The home-made APD was used to detect the further attenuated THz pulses. The readout signal voltage from the APD was about 2 mV when the THz wave energy was attenuated to about 100 aJ. By switching the PIN detector and APD, the dynamic range was increased to larger than six orders.

Figure 5 displays a comparison of the detection power of the THz frequency upconversion detection based on OH1 and DAST crystals without THz energy attenuation. In the figure, blue and red lines denote upconverted signals from OH1 and DAST crystals, respectively. THz pulses in the 1–3 THz range were emitted by the DFG THz generation part based on the OH1 crystal. Therefore, the THz pulse energies for the detection part based on OH1 and DAST were completely equal. For the OH1-based detection system, the signal voltage peak was at the 1.9 THz frequency. In the 1–2.4 THz range, the signals from the OH1-based detector were higher than those from the DAST-based detector. Especially below 1.5 THz, the DAST-based detector hardly yielded upconverted signals, while the OH1-based detector yielded strong signals. To be noticed that the upconverted signals were too weak to be read out when the THz wave frequencies were tuned below 1.3 THz by the PIN detector due to the high THz wave absorption of DAST, as shown in Fig. 5. Therefore, the OH1 crystal exhibited better performance than the DAST crystal when serving as an optical nonlinear crystal in the THz frequency upconversion detection system in the 1–2.4 THz range. Particularly, when the frequency was lower than 1.9 THz, the advantages of the OH1 crystal were more prominent. The intensities of the frequency upconverted signals, which were generated during the THz wave frequency tuning process, significantly varied due to the frequency-dependent coherent lengths of the two crystals. Therefore, a PIN detector was used to receive the signals instead of APD to avoid photodetector damage. The employment of the PIN detector resulted in a decrease in the detection sensitivity. Hence, the experimental results in Fig. 5 do not represent the optimal sensitivity of the proposed detection system.

 

Fig. 5. Comparison of the THz frequency upconversion detection with PIN detector based on OH1 and DAST crystals.

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4. Conclusion

In summary, highly sensitive frequency upconversion detection from 1 to 3 THz was achieved with the OH1 crystal. An optical unit called BIE was proposed to reduce the optical noises for frequency agility. The minimum detectable THz pulse energy was about 100 aJ at 1.9 THz. The results showed that OH1 performed better than DAST when serving as an optical nonlinear crystal in the 1–2.4 THz range. Thus, OH1 is an advanced optical nonlinear crystal for THz detection in the 1–3 THz range.