ОПТИКА И СПЕКТРОСКОПИЯ, 2010, том 108, № 6, с. 890-893
ТЕРАГЕРЦОВАЯ ОПТИКА И СПЕКТРОСКОПИЯ =
УДК 535.14
NONLINEAR OPTICAL TERAHERTZ WAVE SOURCES
© 2010 г. K. Kawase, K. Suizu, S. Hayashi, and T. Shibuya
Nagoya University, Ecotopia, Furocho, Nagoya 4648603, Japan RIKEN, 519-1399 Aramakiaoba, Aoba, Sendai 9800845, Japan E-mail: kawase@nuee.nagoya-u.ac.jp Received December 7, 2009
We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.
INTRODUCTION
Only few terahertz sources bring together qualities such as room temperature operation, compactness, and ease of use. The THz-wave parametric generation is based on an optical parametric process in a nonlinear crystal. The principles of the terahertz wave parametric generator (TPG) and the terahertz wave parametric oscillator (TPO) allow building systems that are not only compact but also operate at room temperature, making them suitable as practical sources [1— 3]. Both a narrow linewidth and a wide tunability are possible in injection-seeded TPG (is-TPG) with single-longitudinal-mode near-infrared lasers as seeders. On the other hand, a difference frequency generation (DFG) was used to generate widely tunable monochromatic coherent THz waves using nonlinear optical crystals [4, 5]. In general, however, nonlinear optical materials have high absorption coefficients in the THz-wave region, which inhibits efficient THz-wave generation. Avetisyan et al. proposed surface-emitting THz-wave generation using the difference frequency generation technique in a periodically poled lithium niobate (PPLN) waveguide to overcome the problem [6]. A surface-emitted THz wave radiates from the surface of the PPLN and propagates perpendicular to the direction of the pump beam. The absorption loss is minimized because the THz wave is generated from the PPLN surface. Moreover, the phase-matching condition can be designed using PPLN with an appropriate grating period [7]. Surface-emitted THz-wave devices have the potential for high conversion efficiency, and continuous wave THz-wave generation has been successfully demonstrated [8]. Unfortunately, the tuning range of the THz wave is limited to about 100 GHz by the nature of PPLN, thus a wide tuning
range cannot be realized using the quasi-phase-matching method.
We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses producing very high peak power [9], these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz waves using a quasi continuous wave nanosecond pulsed laser source.
CHERENKOV TYPE PHASE MATCHED MONOCHROMATIC THZ-WAVE DIFFERENCE FREQUENCY GENERATION USING A BULK LITHIUM NIOBATE CRYSTAL
The Cherenkov phase-matching condition is satisfied when the velocity of the polarization wave inside the nonlinear crystal is greater than the velocity of the radiated wave. Lithium niobate (LiNbO3) has 2.2 and 5.2 of refractive index at near infrared and THz-wave region, respectively, which results in 65° of Cherenkov angle in the crystal. The radiation angle hardly changes during THz-frequency tuning because the silicon prism coupler used has low refractive index dispersion in the THz-wave region and the optical wavelength requires only slight tuning. The change in radiation angle is less than 0.01° for a fixed pumping wavelength. The actual angle change of the THz wave is significantly better than that of the THz parametric oscillator (TPO) with a Si prism coupler, which has an angle change of about 1.5° in the 0.7—3 THz tuning range.
First, we demonstrated Cherenkov phase-matched monochromatic THz-wave generation using differ-
Fig. 1. Experimental setup for Cherenkov phase-matching monochromatic THz-wave generation with a bulk lithium niobate crystal.
Si-prism coupler Turupica f = 45 mm MgO:LN (5 mol%) 65 mm
THz-wave
Si-Bolometer
ence frequency generation with a bulk lithium niobate crystal [10] using the experimental setup shown in Fig. 1. The frequency-doubled YAG:Nd laser, which has pulse duration of 15 ns, a pulse energy of 12 mJ, and a repetition rate of 50 Hz, was used as the pump source for a dual-wavelength optical parametric oscillator (OPO) using potassium titanium oxide phosphate (KTP). The KTP-OPO, which consists of two KTP crystals with independently controlled angles, is capable of dual-wavelength operation with independent tuning of each wavelength [11]. The OPO has a tuning range from 1300 to 1600 nm. The maximum output energy of 2 mJ was obtained by the pumping energy of 12 mJ. The size of 5 mol% MgO-doped lithium niobate crystal (LiNbO3:MgO) used in the experiment was 5 x 65 x 6 mm. The surfaces of the crystal were mirror-polished. An array of seven Si-prism couplers was placed on the y-surface of the LiNbO3:MgO crystal. The pump beam diameter was reduced to 0.3 mm to increase the power density. The polarizations of the pump and THz waves were both parallel to the Z-axis of the crystals. The THz-wave output was measured with a fixed 4 K Si-bolometer.
The THz-wave output map for various pumping wavelengths and corresponding THz-wave frequencies is shown in Fig. 2. The magnitude of the map denotes the output voltage of a Si-bolometer with a gain of 200. The noise level of the bolometer was about 10 mV and is shown as the blue region in the figure. The red indicates the region where over 2 V of output voltage was obtained. As seen in the figure, wide tun-ability in the range 0.2—3.0 THz was obtained by
choosing the proper pumping wavelength. This method was efficient for frequency region below 1.0 THz, compared to our TPO system.
EXTREMELY FREQUENCY WIDENED CHERENKOV TYPE PHASE MATCHED TERAHERTZ WAVE GENERATION USING A LITHIUM NIOBATE WAVEGUIDE
Next, we introduced a waveguide structure in Cherenkov radiation to achieve efficient and extremely wide tunable THz-wave source [12]. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation. A Y-cut 5 mol% MgO doped lithium niobate crystal on a thick congruent lithium niobate substrate was polished down to 3.8 ^m. A thin MgO doped lithium niobate layer worked as an optical slab waveguide, because the refractive indexes of 5 mol % MgO-doped lithium niobate and congruent lithium niobate at 1300 nm are 2.22 and 2.15, respectively. The waveguide device was 5-mm wide and 70-mm long (X-axis direction). Each X-surface facet was mechanically polished to obtain an optical surface. As shown in Fig. 3a, a thin (3.4-^m thick) polyethylene terephthalate (PET) film was slipped between the array of Si-prism couplers and the y-surface of the MgO-doped lithium niobate crystal. Directly placing an array of Si-prism couplers on the y-surface of the MgO-doped lithium niobate will inhibit the function of the waveguide for pumping waves, because the refractive index of Si in the near-infrared region is higher (about 3.5) than that of lithium niobate (about 2.2). A PET, in contrast, has a lower re-
ОПТИКА И СПЕКТРОСКОПИЯ том 108 № 6 2010
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KAWASE et al.
Frenquency, THz
1300 1400 1500
nm
Fig. 2. THz-wave output mapping for various pumping wavelengths and corresponding THz-wave frequencies. The magnitude of the map values indicates the output voltage of the detector.
Fig. 3. (a) Schematic ofthe lithium niobate waveguide device with Si-prism array coupler. (b) THz-wave detection experimental setup.
fractive index in that region (about 1.3), so adding a thin PET film does not inhibit the function of the crystal as a waveguide. An array of Si-prism couplers on a PET film can work as a coupler for THz-frequency waves because the PET film is thin compared to the wavelength of a THz-wave.
We demonstrated difference-frequency generation using the experimental setup shown in Fig. 3b. A dual-wavelength potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO) with a pulse duration of 15 ns, a pulse energy of 1 mJ and a
1300- to 1600-nm tunable range was used as a pumping source. To couple the pumping waves, the beam was reduced to few micrometers in the X-axis direction by a 3-mm diameter glass rod lens. The width of the pumping beams in the Z-direction was about 1.9 mm. The waveguide power density was about 50 MW cm-2, estimated from the pump wave pulse energy after waveguide propagation (about 60 J). We did not observe or calculate the waveguide mode of the structure in which a thin MgO-doped lithium niobate layer was sandwiched by a thick congruent lithium
OOTHKA H CnEKTPOCKOnHa tom 108 № 6 2010
niobate layer and a thin PET film. It remains an area of future work to optimize the waveguide structure. The pump wave and THz-frequency wave polarizations were parallel to the crystal's Z-axis.
As shown in Fig. 4, a high-frequency THz-wave output ranging to about 7.1 THz was confirmed. We were unable to observe THz-wave generation above 7.2 THz due to very strong THz-wave absorption around 7.5 THz by the LO-phonon mod
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