We propose a surface-emitted cavity configuration for a terahertz-wave parametric oscillator that allows THz wave emission perpendicular to the crystal surface without any output coupler. The oscillating idler and pump waves are reflected at the surface of a nonlinear crystal in a single resonance cavity, satisfying the noncollinear phase-matching condition. The radiated THz wave has a Gaussian profile. The measured beam quality factors (M2) were 1.15 and 1.25 in the horizontal and vertical directions, respectively. The measured tunable range was 0.8-2.74 THz. A test of transmission imaging using a test pattern was demonstrated.
©2006 Optical Society of America
The generation of widely tunable monochromatic terahertz-wave (THz-wave) radiation is of great interest for a variety of applications in basic and applied physics, communications, the life sciences, and security applications [1, 2]. THz-wave generation using the nonlinear optical characteristics of phonon polaritons in LiNbO3 crystal, a well-understood nonlinear crystal, is a convenient technique for the development of a practical THz-wave source [3–6]. However, the LiNbO3 crystal absorbs THz waves strongly and therefore, a special extraction technique is necessary to avoid absorption by the crystal and total internal reflection within it, caused by the high absorption coefficient (10–100 cm-1) and refractive index (~5.2) of LiNbO3.
We studied the THz-wave parametric oscillator (TPO) as a practical, widely tunable and monochromatic THz-wave source. The TPO consists of the LiNbO3 or MgO:LiNbO3 and a resonator for the idler wave. The THz wave is extracted as the signal wave using the noncollinear phase matching condition. In this TPO, a Si prism array coupler is used to extract the THz-wave generated in the crystal and to obtain higher THz-wave output by increasing the coupling area. The unidirectional radiation was also achieved by using the arrayed Si prism coupler . However, the wave front of the extracted THz-wave is not uniform due to multiple prisms.
In contrast, the surface emission configuration in which the beam is emitted perpendicularly to the surface of the crystal has considerable advantages for THz-wave generation in the LiNbO3 crystal because the THz-wave is extracted easily without any output coupler, and damping is low, due to the short path length within the LiNbO3 crystal. Surface emitted generation has been also demonstrated using the quasi-phase matching DFG technique in the PPLN crystal [8–11]. In the surface emission configuration using PPLN, the THz frequency can be tuned and the bandwidth varied by changing the period and number of domain pairs, respectively. Either change requires a new crystal.
In this letter, we propose the use of the surface emission configuration in the widely tunable TPO, characterized by the perpendicular emission of THz-wave radiation from the crystal, with high beam quality. In this TPO, the pump wave and resonated idler wave are reflected completely at the THz-wave output surface of a MgO:LiNbO3 crystal. By setting the incident angles of the pump and resonated idler above the critical angle, perpendicular emission of THz wave radiation is achieved. Also, the generation area for THz-wave, the polariton excited area, is located directly at the THz-wave output surface, so damping of the THz wave by absorption in the crystal is greatly reduced and the surface-emitted TPO produces high output and high beam quality THz-wave.
2. Design and experimental setup
TPOs are based on stimulated polariton scattering in a nonlinear LiNbO3 or MgO:LiNbO3 crystal pumped by a Q-switched Nd:YAG laser . In the polariton-scattering process, idler (Stokes) and signal (THz) waves are generated from the pump (near-infrared) wave in a direction consistent with noncollinear phase-matching, kp = ki + kT, where kp, ki, and kT are the wave vectors of the pump, idler, and THz waves, respectively (Fig. 1(a)). The conservation of energy is also satisfied, ωp = ωi + ωT, (p = pump, i = idler, T = THz-wave). In the case of a 1.064 μm pump, the idler wavelength is typically generated in the range from 1.067 to 1.076 μm, which corresponds to a THz-wave frequency range from about 0.8 to 3 THz. Under these conditions, the phase-matching angle, θ, between the pump and idler waves is much smaller (≈0.35-1.45°) than the phase-matching angle, δ, between the THz wave and idler (64.4-67.4°) in the crystal.
THz-wave generation perpendicular to the output surface is achieved by setting of the incident angles of pump and idler beams to the crystal surface as shown in Fig. 1(a). For perpendicular emission at 1.5 THz, the incident angle of the idler wave is set at 65°, i.e., the phase match angle, δ1.5THz, at 1.5 THz generation, and the pump beam is set at 64.3°, i.e., δ1.5THz - θ1.5THz, as shown in Fig. 1(a). These angles exceed the critical angle (θ3. = 27.7°) of the interface between the MgO:LiNbO3 crystal (np,i≈2.15) and air (n = 1). So, the idler and pump waves are totally reflected at the surface. By varying the pump incident angle from 64.6 to 63.5°, the THz-wave frequency tuning in the range 0.8–3 THz is obtained; the THz-wave generation direction changes by about 15.9° to the crystal surface.
The experimental setup of our surface-emitted TPO is illustrated schematically in Fig. 1b. Two rectangular and one trapezoidal MgO:LiNbO3 crystals were used for the surface-emitted TPO, and two rectangular crystals were placed on both sides of the trapezoidal crystal. Two rectangular crystals were cut into 4 × 50 × 5 mm (a × b × c-axis). The end surfaces that transmit the pump and idler waves were antireflection-coated for 1.064 μm. The polarization of pump beam is parallel to the c-axis of the crystals. The pump laser was a multi-longitudinal mode Q-switched Nd:YAG laser (1.064 μm, 25 ns, 50 Hz, TEM00). The pump beam was collimated by a lens pair to a diameter of about 1.2 mm [full width at half maximum (FWHM)]. The idler generated in the crystal and the pump waves were totally reflected at the THz-wave output surface of the trapezoidal crystal. The polarization of the generated idler and THz wave are parallel to that of pump beam. The idler and pump beams interacted in a 109-mm-long crystal in a symmetric-geometry resonator, consisting of two flat cavity mirrors for idler wave resonance. These cavity mirrors, M1 and M2, were specially fabricated with high transmittance at 1.064 μm and a reflectivity of >90% above 1.068 μm. These high-performance mirrors were key to the setup. The resonant mirrors were placed so that the incident angle of the resonant idler to the output surface was 65°, satisfying the condition for perpendicular generation of 1.5 THz. The surface-emitted TPO was fixed on a rotating stage, and continuous frequency tuning was achieved by varying the phase-matching angle between the resonated idler and pump beams by rotating the stage.
3. Output characteristics
Figure 2 shows the THz-wave output energy at a pump input of 20.3 mJ/pulse. The THz-wave frequency is tuned by rotating the stage. Wide range tunability from 0.8 to 2.74 THz was observed for a pump energy of 20.3 mJ/pulse. The decreased parametric gain  and the reflectivity reduction of the high-performance mirrors used for the resonant idler produced the lower oscillation frequency limit. The frequency of the THz wave was calculated using the law of energy conservation with a pump wavelength of 1.0647 μm and the measured idler wavelength 1.0675-1.0734 μm.
The measured maximum THz-wave output was 104 pJ/pulse at 1.46 THz. The minimum sensitivity of the Si bolometer is about 10 fJ/pulse. The Si bolometer became saturated at 5 pJ/pulse for the pulsed THz-wave, so we used calibrated thin metalized Mylar film was used as a THz-wave attenuator.
The measured maximum THz-wave energy and tuning range was closed to the conventional TPO with Fabry-Perot cavity. However, the output energy in the high frequency region, especially above 1.5 THz, was enhanced due to the smaller absorption in the crystal . In the surface emitted TPO, the THz-wave generation area that comprises the area of interaction between the idler and pump beams is located at the crystal surface. Therefore, the THz-wave energy can be further increased by simultaneous up-scaling the pump energy and pump beam size in order to keep the pump intensity below the crystal damage threshold .
4. Beam quality and transmission imaging
To evaluate the beam quality, we measured the THz-wave beam profile using two-dimensional scanning with a 400 μm diameter pinhole. The THz wave that passed through a pinhole was measured by a Si bolometer. Figure 3(a) shows the far-field two-dimensional THz-wave pulse energy distribution at a distance of 22 mm from the output surface. Figure 3(b) shows the output THz-wave beam profile in the horizontal (x) and vertical (y) directions. The measured THz-wave beam profiles had a Gaussian distribution in both the x and y directions. The measured data fit well to a Gaussian function (solid lines in Fig. 3(b) lower and 3(b) upper). Half maximum (FWHM) beam diameters of 3.1 and 6.1 mm were observed in the x and y directions, respectively. The measured beam profile did not have a uniform spatial distribution but showed a highly asymmetric elliptical distribution.
This asymmetricity is caused by the elliptical radiation cross-section. We estimated the width of the radiation cross-section based on the diffraction theory. The measured far-field divergence angles at 1.5 THz were 3.15 and 7.73° in the x and y directions, respectively. The far-field divergence angle, θdiv., increased in inverse proportion to the width of the radiation cross section, which is θdiv.≈λ/W when, θdiv«1, where λ is THz-wave wavelength and W is the width of the emission cross section on the output surface. The larger divergence angle along the y direction was caused by the smaller emission cross section width in the y direction on the output surface. The estimated width of the emission cross section were 4.4 and 1.88 mm for the x and y directions, respectively. Then, the calculated width of the oblique reflection area of the pump beam at the incident angle of 65° was 4.8 and 2.0 mm for the x and y directions, respectively. The similar aspect ratio of the THz-wave radiation cross section, 1:2.34, was measured compared with the oblique reflection area of the pump, 1:2.4. This asymmetric radiation pattern might be improved by reshaping the pump beam.
To evaluate the THz beam quality, we measured the beam quality factor (M2) from the optical axis (z) dependence of the focused beam spot size. For this measurement, the knife-edge method was used to measure the beam spot size. The THz wave emitted from the surface-emitted TPO was collimated using a 30-mm-focal length cylindrical lens in the vertical (y) direction and was focused using a 30-mm-focal length Tsurupica lens (plastic lens for THz use). The beam diameter was defined as twice the edge translation distance between the 15.9 and 84.1% translation points (this reduces to the 1/e2 diameter for Gaussian input) . The beam spot sizes were measured between the two 30-mm-focal length lenses along the optical propagation direction. The result is shown in Fig. 4. The measured beam waist sizes were 607 and 410 μm in the horizontal (x) and vertical (y) directions, respectively. From the theoretical calculation using the measured waist size, the estimated beam quality factor (M2) was 1.15 and 1.25 for the horizontal and vertical directions, respectively. The excellent beam quality of the THz-wave from the surface-emitted TPO allows focusing the beam to very small areas, and thus, this THz-wave source is practical for many applications that use THz waves, such as imaging.
As an evaluation of the property of the THz-wave imaging system using the surface-emitted TPO, we measured the transmission imaging of a test sample consisting of lines and spaces. The imaging system to which we had access relied on transmission optics (a plastic aspherical lens for THz use). The beam divergence angle of the THz wave in the y direction was matched with that in the x direction using a cylindrical lens (focal length: 50 mm) in order that the THz wave in x and y is focused at the same point along the optical axis z. The THz wave is focused on the sample by a plastic aspherical lens, and the transmitted THz wave was detected by a Si bolometer. In this imaging system, measured beam spot sizes (FWHM) were 265 and 318 μm in the x- and y-axes, respectively. The test sample was placed on the THz-wave focus point on the optical axis. For image acquisition, the sample was x, y scanned with a computer-controlled scanning stage synchronized with the detection of the THz-wave energy transmitted through the sample. The test sample was made from silver salt plating. Figure 5(a) shows the layout of the test sample. The silver salt attenuates the THz wave sufficiently to allow visual checking of the spatial resolution. The THz-wave image at 1.9 THz (λ=157 μm) is shown in Fig. 5(b). We measured lines spaced from 200 to 1000 μm. The image contrast decreased as the spacing narrowed, and the result for 200-μm spacing is depicted. We conclude that the THz-wave image using surface-emitted TPO has the capability to depict objects at close to wavelength size.
We report a TPO with a surface-emitted cavity configuration, in which the THz-wave radiates in a direction perpendicular to the output surface. First, we designed the configuration to produce total pump and idler reflection in the crystal. We obtained a THz-wave energy exceeding 1 pJ/pulse over a wide range from 0.8 to 2.74 THz, with a maximum energy of 104 pJ/pulse. A near-Gaussian beam profile was produced without using an output coupler to extract the THz wave from the nonlinear crystal. The measured beam quality factor was 1.15 and 1.25 along the horizontal (x) and vertical (y) directions, respectively. The initial THzwave transmission imaging system presented here suggests the possibility of using a surface-emitted TPO as the source in a THz-wave imaging system.
Xianbin Zhang expresses his thanks for his stay at PDC RIKEN to Shaanxi Higher Education Project supported by Japan Bank for International Cooperation and Shaanxi People's Provincial Government, China.
We thank Mr. C. Takyu and Mr. T. Shoji of the RIEC at Tohoku University for their excellent coating and polishing work. Part of this study was supported by a Grant-in-Aid (#16760336) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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