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A surface-emitting THz parametric oscillator is set up to generate a narrow-linewidth, nanosecond pulsed THz-wave radiation. The THz-wave radiation is coherently detected using the frequency up-conversion in MgO: LiNbO3 crystal. Fast frequency tuning and automatic achromatic THz-wave detection are achieved through a special optical design, including a variable-angle mirror and 1:1 telescope devices in the pump and THz-wave beams. We demonstrate a frequency-agile THz-wave parametric generation and THz-wave coherent detection system. This system can be used as a frequency-domain THz-wave spectrometer operated at room-temperature, and there are a high possible to develop into a real-time two-dimensional THz spectral imaging system.
© 2010 Optical Society of America
1. Introduction
It is well known that Terahertz (THz) parametric generators and oscillators (TPG/TPO), using non-collinear phase-matching [1–3] or quasi phase matching scheme [4] in MgO:LiNbO3, are widely used as tunable THz-wave sources for generating nanosecond pulse THz-wave radiation. In addition, nanosecond near-monochromatic THz-wave pulses can be produced by mixing two near-infrared (NIR) laser beams in nonlinear crystals using Difference-Frequency Generation (DFG) [5–10].
On the detection side, monochromatic THz-wave radiation is generally measured using a high-sensitivity bolometer system that needs liquid-He for cooling. Pyroelectric detectors and Golay cells can be used at room temperature, but have lower sensitivities. All of these detectors have slow response times, with a time constant of several hundred microseconds to milliseconds.
By mixing a weak, pulsed THz signal with a synchronized NIR pump pulse in a nonlinear crystal, such as GaSe, PPLN, bulk-LiNbO3 or DAST [11–14], researchers have observed a difference frequency up-conversion signal. This up-converted signal is captured directly using a high-speed optical photodiode. This process has the potential to be used to develop a fast and very sensitive THz-wave detector running at room temperature.
Up-conversion process can be considered as a narrowband tunable detector because the phase-matching condition must be maintained for effective nonlinear frequency conversion. An achromatic phase-correcting device must be considered to realize sensitive THz-wave detection with a broad frequency range. This restriction is usually the main obstruction in developing a practical THz-wave detection system based on frequency conversion [15–17].
In this letter, a Surface-Emitting THz Parametric Oscillator [2] (SE-TPO) is used to generate a widely tunable, narrow-linewidth nanosecond THz-wave radiation. The THz radiation from SE-TPO was detected using a frequency up-conversion detection unit (“the detection unit” in the following text) [13], which consisted of MgO: LiNbO3 nonlinear crystals and an optical InGaAs-based photodiode. Special optics design in the SE-TPO and the detection unit realized fast frequency tuning and automatic achromatic THz-wave detection in a wide frequency range. A frequency-agile, monochromatic THz-wave generation and detection system, operating at room temperature, has been demonstrated.
The SE-TPO consisted of a resonator for the idler beam, an equilateral trapezoid and two rectangular MgO: LiNbO3 crystals. The trapezoid crystal configuration allows THz-wave to be emitted perpendicular to the output surface of the MgO: LiNbO3 crystal. THz-wave absorption in the crystal was greatly reduced and high power THz-wave, with good beam quality, was radiated. We achieved rapid frequency tuning (pulse to pulse), with random frequency accessibility in the present experiment by changing the angle θ between the pump beam and idler beam using a variable-angle mirror and a telescope. This differs from the conventional TPO cavity rotation tuning method [1,2].
The same nonlinear crystals, as the ones that were used in SE-TPO, consisting of an equilateral trapezoidal and two rectangular MgO: LiNbO3 crystals, were used in the detection unit. This resulted in a simple optical design for THz-wave frequency tuning and achromatic THz-wave detection.
2. Optical design for fast frequency tuning and automatic achromatic THz-wave detection
Figure 1 shows the optical design used for realizing fast frequency tuning and automatic achromatic THz-wave detection. Figure 1(a) shows the optics in the pump beam. A variable-angle mirror, M0, mounted on a high speed optical beam scanner to control the reflection direction of pump beam, and a 1:1 telescope device with two lenses, were used. The diverging pump beam, from the mirror M0, was changed into a converging beam by the telescope. The converged pump beam was divided into two beams: one for pumping the SE-TPO and the other for pumping the detection unit. The two pump beams, incident into the SE-TPO and the detection unit, were tuned synchronously by controlling the reflection angle of M0.
Fig. 1 Optical design for fast frequency tuning and achromatic THz-wave detection. (a) The optics in the pump beam. A variable-angle mirror, M0, and a 1:1 telescope device with two lenses were used. (b) The optics in the THz-wave beam. A pair of off-axis parabolic gold mirrors (PG1 and PG2) formed a 1:1 telescope device in the THz-wave beam. The inset shows the phase matching condition for THz-wave generation and detection: .
Figure 1(b) shows the optics in the THz-wave beam. A pair of off-axis parabolic gold mirrors (PG1, PG2) formed the second 1:1 telescope device for the THz-wave beam. It collimates and steers the THz-wave beam towards the detection unit.
The inset in Fig. 1(b) shows the non-collinear phase matching condition for THz-wave generation and detection in MgO:LiNbO3:
where , and are the wave vector of the pump, THz beam, and idler (the up-converted signal), respectively.3. Experimental setup
Figure 2 shows the experimental setup for achieving a frequency-agile monochromatic THz-wave parametric generation and the achromatic THz-wave detection.
Fig. 2 The experimental setup used to achieve the frequency-agile, monochromatic THz-wave generation and detection.
A high-power Q-switched Nd:YAG laser (pulse energy: 42 mJ/pulse, pulse width: 25 ns, linewidth: <30 GHz, repetition rate: 50 Hz) was used to pump both the SE-TPO and the detection unit. Mirror M0 was mounted on a Galvano-optical beam scanner (Harmonic Drive Systems Inc., LSA-20A-30) to rapidly change the incident angle of the pump beam. It had a maximum angular deflection of ± 5° and had an angular deviation of less than ± 0.06%. The response time of the scanner was 1 ms for a 1° change in the angle of the mirror at the optimum inertia. A faster response time of the scanner than the pulse repetition rate of the pump laser resulted in the fast terahertz frequency tuning from pulse to pulse.
To reduce the total size of the setup, lenses 1 and 2, shown in Fig. 1 were replaced by a pair of concave mirrors (M1, M2) with a radius of curvature of 1000 mm. The diverged pump beam from the variable-angle mirror (M0) was also changed into a converging beam by the telescope. The converged pump beam was then split by the polarizing beam splitter (PBS) to pump the SE-TPO and the detection unit. The converged points of the pump beam were located at the positions where THz-wave radiation was emitted and collected by the SE-TPO and the detection unit.
The positions of the two parabolic mirrors PG1 and PG2 (focal lengths: 150 mm) were arranged carefully to form a 1:1 telescope as showed in Fig. 1. THz-wave radiation from the SE-TPO is collimated and focused onto the longer side of the parallel surfaces of the trapezoidal crystal in the detection unit.
The pump beam used in the detection unit was incident normal to one of the beveled edges of the trapezoidal crystal, and was totally reflected at the center of the longer side of the two parallel surfaces. The angle between the THz-wave beam and the pump beam inside the crystal was close to 65°, which was necessary to satisfy the phase-matching conditions in the MgO:LiNbO3 crystal. Mixing the THz-wave radiation and the pump beam in the trapezoidal crystal produced an up-converted difference-frequency signal. Two rectangular MgO: LiNbO3 crystals, with a total length of 130 mm, then parametrically amplified this signal which was later detected with a high-speed InGaAs-based photodiode. Our detection unit had extracted THz-wave information from the up-converted signal. Caution must be taken to separate the pump beam and the up-converted signal because they could easily be scattered and mixed together. The causes of the scattering and mixing are because the angle (about 1–3°: outside the crystal) and the frequency between the pump beam and up-converted beam were fairly close to one another. Two high-pass filters (OCJ/Optical Coatings Japan) were added to the detection unit to separate the up-converted signal from the pump.
The frequency of the THz-wave, from SE-TPO, was tuned by controlling the incident angle of the pump beam into the MgO: LiNbO3 crystal. It radiated in different directions at different frequencies due to the non-collinear phase-matching condition. Although the frequency of the idler was also tuned, the wave direction was fixed because of the TPO static cavity.
Using our design of two 1:1 telescopes for the pump beam and the THz-wave beam, the angles of incidence of both the pump beam and THz-wave beam were aligned between the SE-TPO and the detection unit. It made a fixed propagation path for the idler and the up-conversion beam. This design also fixed the position of the photodiode throughout the whole frequency tuning range that was critical in the measurement of the up-conversion signal.
4. Results and discussions
4.1. THz-wave up-conversion detection
The lowest pump energy required to generate a measurable up-conversion signal was 5 mJ/pulse. When the pump power was raised from 5 mJ/pulse to 7 mJ/pulse, a slightly stronger up-conversion signal was observed. However, the pump energy cannot be increased continuously because it will excite super-radiant fluorescence that yields a bad signal-to-noise ratio. So, we set the pump energy to 7 mJ/pulse for our experimental measurement. The red line in Fig. 3(a) shows the measured temporal profile of the up-converted signal with an input THz-wave radiation of 8pJ/pulse at 1.5 THz. The black line near 0 a.u shows the background noise from the residual pump scattering and parametric fluorescence. It was obtained by blocking the injected THz-wave beam with a transparent glass plate. Figure 3(b) shows the measured spectra of the up-converted signal using an optical spectrum analyzer. We had deduced the frequency of THz-wave beam from the frequency difference between the pump beam and the up-converted signal based on the energy conservation of parametric processes in nonlinear optics. The upper horizontal axis in Fig. 3(b) indicates the corresponding THz-wave wavelength.
Fig. 3 (a) The temporal profiles of the up-converted signal at pump energy 7 mJ/pulse (45 MW/cm2) and THz-wave energy 8 pJ/pulse at 1.5 THz (the red line). The black line is the noise baseline. (b) The measured spectra of the up-converted signal.
The lowest detectable THz-wave energy using this up-conversion detection unit was 0.1 pJ/pulse at 1.5 THz, which is approximately two orders of magnitude higher than that of a typical liquid-He-cooled Si bolometer for detecting nanosecond pulsed THz-wave beam. Compared to the single longitudinal-mode Q-switched YAG-pumped THz-wave detection scheme [13], the multi-mode Q-switched YAG laser pumped detection scheme has a higher conversion efficiency but with more noise. It requires less than 40% of the pump intensity than the single longitudinal-mode scheme to generate a measurable up-conversion signal. However, the detection sensitivity is about three orders of magnitude lower than that achieved using the single longitudinal-mode Q-switched YAG laser as a pump. This sensitivity is still better than a typical pyroelectric detector or a Golay-cell detector.
According to the temporal profile of the up-converted signal shown in Fig. 3(a), we find a Full-Width at Half-Maximum (FWHM) pulse width of 4.5 ns. If we assume that the NIR and THz signals have Gaussian temporal profiles, then the pulse width for the THz-wave signal works out to be 4.57 ns.
4.2. Performing achromatic THz-wave detection in a wide frequency range
Frequency tuning was achieved by scanning the input voltage of the Galvano-optical beam scanner. Figure 4(a) shows the measured spectra of the up-converted signal (from left to right) for varying applied voltages of 0.4 V, 0.6 V, 0.8 V, and 1 V to the Galvano-optical beam scanner. Figure 4(b) shows the idler signal. The center frequency and the FWHM of the spectral line half-width can be obtained from a nonlinear least-square Gaussian fit of the profiles. As shown in Fig. 4, the center frequencies of idler and up-converted signals maintained exact synchronization at the same input voltage of the Galvano-optical beam scanner. We can deduce the frequency of THz-wave beam from either the idler beam or the up-converted optical signal. The linewidth of the idler signal was approximately 70 GHz, and that of the up-converted signal was approximately 130 GHz. The spectral broadening of the up-converted signal is attributed to the frequency-mixing process between the THz-wave and the pump beam (which has a FWHM of around 30 GHz).
Fig. 4 (a) shows the measured spectra of the up-converted signal with the applied voltages of 0.4 V, 0.6 V, 0.8 V, and 1 V (from left to right) at the Galvano-optical beam scanner. The top horizontal axis shows the corresponding THz-wave frequency. (b) Spectra of the idler signal at the same applied voltages.
Furthermore, as shown in Fig. 4, compared to the idler output, the relative spectral intensities of the up-converted signal showed a weaker intensity for the higher THz-wave frequency region. Same phenomena has been observed in the THz-wave detection using Si-bolometer [2]: Although the intensity of the THz-wave signal generated in the nonlinear crystal is always related to the intensity of the idler signal, THz-wave intensity decreases rapidly due to the increase in the absorption coefficient as we approach the soft-mode resonant frequency of LiNbO3 [18]. This indicates that the intensity of the up-converted signal can reflect the real intensity of the THz-wave signal but not the idler beam. In other words, the intensity, frequency, and linewidth of the THz-wave signal can all be determined from the up-conversion optical signal.
When the input voltage of the Galvano-optical beam scanner is varied from –0.5 V to 2.5 V, we observed the center wavelength of the up-conversion signal changed from 1068.99 nm to 1074.24 nm, corresponding to a change in the phase-matching angle from 1.25° to 2.85° (outside the crystal). The frequency of THz-wave signal was tuned from 1.26 THz to 2.63 THz. Since the pump beam positions moved when we changed the voltage applied at the Galvano-optical beam scanner, a broader frequency tuning range from 0.5 to 3 THz-wave is possible if we use mirrors and PBS with the diameters >1.5 inch in the pump beam (at present, they are 1 inch and 15 mm, respectively).
4. Conclusion
Our system requires a single fixed-frequency optical pump source to realize a frequency-agile, monochromatic THz-wave generation and detection system at room temperature. The position of all of the key components, including the TPO cavity, the nonlinear crystals in the SE-TPO and the detection unit, and the photo detector in the detection unit, are fixed. By controlling the angle of the mirror M0, the frequency of the THz-wave radiation can be rapidly tuned or randomly accessed across a range of 1.26–2.63 THz, pulse by pulse at a rate of 50 Hz (The pulse-repetition rate of the pump laser limited the pulse rate).
The THz-wave and up-conversion signals were synchronized using a Q-switched nanosecond pulsed laser. A very strong nanosecond up-conversion signal at an average power of several tens of microwatts (sub-µJ/pulse) was observed. This up-converted optical pulse can be detected by a high speed photodiode or a high-speed CCD array camera. Fast THz-wave spectroscopy and real-time THz-wave spectra imaging are possible with this system.
This frequency-agile THz-wave generation and detection system is capable of being operated at room temperature as a frequency-domain THz-wave spectrometer. There is also a high possibility of being developed into a real-time, two-dimensional, THz spectral imaging system if the photo detector is replaced by image optics and a high-speed CCD array camera. Thus, this system could be very useful in exploring THz-wave imaging and spectroscopy technology.
Acknowledgements
Polishing and dielectric coating of the crystals were performed by T. Shoji and C. Takyu, technicians at RIKEN and at Tohoku University, respectively. Mechanical processing was performed by A. Kudo at PAX. We thank them all for their helpful cooperation. This study was supported in part by a grant-in-aid for exploratory research from the Japan Society for the Promotion of Science (No. 19656102).
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