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A frequency-agile ring-cavity Terahertz-wave parametric oscillator

Hiroaki Minamide

Random frequency access and rapid frequency tuning are of great importance in coherent tunable THz-wave sources because they allow for rapid spectroscopic measurements, spectroscopic imaging, and background-free spectroscopy. Recently, THz waves have attracted great interest for a growing number of applications, such as biomedical imaging, security, and illicit-drug detection. For these applications or to explore new applications, the development of a frequency-agile THz-wave parametric source is desired.
We developed a ring-cavity THz-wave parametric oscillator (ring-cavity TPO) as a frequency-agile THz-wave parametric source, with a simple, unique tuning method that involves rotating a single mirror incorporated in a cavity. The ring-cavity TPO possesses the superior features of random frequency access, rapid frequency tunability, high output, high coherency, and small system size. Moreover, the TPO was applied to THz-wave spectroscopy and imaging. The spectra of biomolecules were measured within a few seconds and images of human liver cancer were acquired within a few minutes.
An innovative tuning technique with a ring-cavity optical configuration was used as a frequency-agile THz-wave source. TPOs are based on stimulated polariton scattering in a nonlinear LiNbO₃ crystal pumped using a Q-switched Nd:YAG laser. In the polariton scattering process, the noncollinear phase-matching condition, k_p = k_T + k_i, and the conservation of energy, ω_p = ω_T+ω_i, are satisfied (p = pump, i = idler, T= THz-wave). Frequency tuning can be achieved by varying the phase-matching angle between the idler and pump beams slightly. In practice, the angle outside the crystal is varied from 1 to 3 degrees, which corresponds to a frequency range from about 1 to 3 THz. In the TPO, only the idler beam is resonated between the flat high-reflection (HR) mirrors that constitute the ring-cavity.
Figure 1 shows a schematic of the ring cavity configuration. In a ring-cavity TPO, the incident angle of the pump beam to the nonlinear crystal is fixed. The phase-matching angle is controlled by changing the path of the idler beam, so that THz-wave frequency tunability is obtained. The ring resonator for the idler beam is formed from three mirrors: M₁, M₂, and M₃. M₁ and M₂ are fixed mirrors and M₃ is the angle-tuning mirror, which is located at point O. P₁ and P₂ are the images of point P seen through M₁ and M₂, respectively. The length of arc P₁O equals that of arc P₂O. From the figure, the resonating beam path from P₁ to P₂ via M₃ can be selected by controlling M₃. For random or rapid tuning, M₃ is mounted on a Galvano optical beam scanner.

Fig. 1: Schematic of the design for the frequency-agile ring-cavity TPO.

Figure 2 shows the experimental setup. The ring-cavity consisted of three mirrors (M₁, M₂, M₃) for the idler wave (1.067 - 1.075 µm) based on the above design theory. Mirrors M₁ and M₃ were super mirrors made by Optoquest (http://www.optoquest.co.jp/en/index.html). These mirrors provided an excellent alternative for selecting the wavelength, and had transmittance characterized by a 2-dB reduction at 1.067 µm (0 dB at 1.064 µm) when the incident angle of the idler beam was about 30 degrees. A super mirror was used to separate the pump and idler beams. Half-area HR-coated mirrors were used initially; the pump beam passed through the uncoated part of the mirrors and the idler beam was reflected at the HR-coated part. However, difficulty with separation restricted the development of a more compact system. Therefore, the use of a special mirror that was developed with recent advances in optical technology was proposed. M₃ was mounted on a Galvano-optical beam scanner (LSA-20B-30-SP, Harmonic Drive Systems) for THz-wave frequency tuning. A Galvano-scanner with excellent thermal drift stability was introduced because control of the phase-matching angle within 1 degree was required. Then, the scanner was set on a movable x-stage in order to adjust the rotation center of M₃ to the appropriate position. The angle was controlled using the output voltage from a computer via a digital-analog converter board.

Fig. 2: Photographic image of the ring-cavity TPO using high-performance mirrors; the 1.064µm pump beam was transmitted, while the idler beam around 1.07µm was reflected.

A 5-mol% MgO:LiNbO₃ crystal (63 × 4 × 5 mm, long × wide × high) was used as the nonlinear optical crystal. The x-surfaces at both ends were mirror polished and coated with antireflection coating centered at 1.064 µm. The y-surface was also polished, in order to minimize the coupling gap between the Si prism base and the crystal surface, and to prevent scattering of the pump beam, which excites a free carrier at the Si prism base. An array of six Si prism couplers was placed on the y-surface of the crystal for efficient coupling of the THz wave [6]. The right-angle prisms were fabricated from high resistivity Si (ρ> 10 kΩ cm); each was cut from a bulk Si crystal to triangular dimensions of 10 × 5 mm (base × thickness) with angles of 50, 40, and 90 degrees.
The pump source used in this experiment was an LD-pumped Q-sw Nd:YAG laser (wavelength: 1.064 µm, pulse width: 17 ns, repetition rate: 500 Hz, longitudinal mode: multi-mode). The laser was a high-power, high-repetition-rate, pump laser developed in collaboration with the RIKEN Teraphotonics team and Megaopto.
Consequently, a compact ring-cavity TPO measuring 140 x 180 mm was developed and a tabletop-sized pump source was prepared.

Figure 3 shows the relationship between the THz-wave output pulse energy and wavenumber; the upper x-axis shows the frequency. The THz-wave output was detected using a 4.2-K Si bolometer. The noise level of the detection system was about 0.01 pJ/pulse. Consequently, a widely tunable range from 0.9 to about 3.0 THz was obtained when the pump input energy was 24 mJ/pulse. A THz wave synchronized with the pump repetition rate of a maximum of 500 Hz was generated and the THz-wave frequency could be random accessed from pulse to pulse under computer control.

Fig. 3: Tunable range of the ring-cavity TPO. The THz-wave frequency was controlled by applying a voltage to the Galvano optical scanner.

The THz-wave spectra of biomolecules were measured to confirm the frequency tuning. Many molecules have a unique spectrum in the THz-wave region called a THz fingerprint spectrum (see the section on THz-wave spectroscopy). Figure 4 shows the spectrum of maltose (a disaccharide) measured using the ring-cavity TPO. A sample of powdered maltose and polyethylene was prepared containing 15% maltose by weight and formed into a pellet. Consequently, absorption peaks at 55 and 70 cm^-1 were detected, which were the same as those measured using FT-IR.

Fig. 4: THz-wave spectroscopy of maltose (a disaccharide) using the ring-cavity TPO.