An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1–20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.
©2009 Optical Society of America
Terahertz (THz) technology has recently undergone considerable development , and THz radiation has attracted a great deal of interest for a growing number of applications, including biomedical imaging, security, medicine, art conservation, and nondestructive testing [2–6]. A monochromatic THz-wave source [7–10] using a nonlinear optical (NLO) process was employed in developing many of these applications, but the advancement of broadband sources and sources that possess some degree of frequency agility is desirable.
We previously demonstrated an ultra-broadband capability using difference-frequency generation (DFG) in an organic crystal 4-dimethylamino-N-methyl-4-stilbavolium tosylate (DAST) [11–15]. The sources described in Refs. 14 and 15 had the important characteristic of frequency-agile operation. The tuning ranges of monochromatic THz-wave sources using organic materials were much wider than those of inorganic materials, and the range of the DAST-DFG THz-wave source was ultra-wideband (1.5–40 THz). Even so, we developed another wideband THz-wave source using the organic solid N-benzyl-2-methyl-4-nitroaniline (BNA) . BNA, which was invented by Hashimoto [17–19], is a promising material for wideband, efficient, and high-power THz-wave generation because of its large second-order optical nonlinearity. The BNA-DFG monochromatic THz-wave was tunable over the range 0.1–15 THz, which is a different frequency spectrum compared to that of DAST-DFG.
Our aim was to produce an effective and widely tunable THz-wave source using BNA-DFG. The BNA is preferable for THz-wave generation due to the large second-order optical nonlinearity. In previous work, however, the full potential of BNA had not been realized because knowledge of the refractive index was limited to the 0.1–4 THz range and the estimation of coherence length distribution was poor at high frequencies. Additionally, the pump wavelengths were not optimized: they were not controlled independently and the λ1 was fixed. In this study, we calculated the wideband refractive index based on the measured Fourier transform infrared (FTIR) transmittance spectrum and the wideband coherence-length distribution. The pump wavelengths were controlled independently to determine the optimum phase-matching condition of the BNA-DFG from a comparison of calculated and experimental results.
2. Properties of N-benzyl-2-methyl-4-nitroaniline (BNA) crystals
The NLO coefficient d33 of a BNA crystal is 234 pm/V, the largest ever reported among yellow-colored NLO materials . We generated the wideband monochromatic THz-wave in the BNA crystal using a type-0 collinear phase-matching configuration (whereby the polarization of all waves involved in the nonlinear wavelength conversion is parallel to the c-axis) by maintaining the crystal angle . The refractive index is very important in determining the phase-matching condition for THz-wave generation. We estimated the refractive index in the range 0.1–22 THz based on the absorption coefficient , which was determined from the FTIR transmittance spectrum (see Figs. 1 and 2 , which show the transmittance spectrum and absorption coefficient, respectively). The thickness of the crystal, which was polished and thinned, was 0.3 mm. BNA crystals have another favorable characteristic in that they are not hygroscopic and are thus stable during water-based mechanical processing. The dots in Fig. 2 show the experimental absorption coefficient measured by FTIR spectroscopy, with the red dots showing the results of spectral simulations using a Lorentz-type oscillator model. Several absorption peaks are observed and the maximum absorption coefficient is around 15 THz.
We calculated the refractive index at the optical (nopt) and wideband THz (nTHz) region in the  direction, which is shown in Fig. 3 . The blue line indicates nopt, which can be determined from the Sellmeier equation . The red line is our simulation data of refractive index in wideband THz region. The nTHz of this work is slightly different from the data in Ref , which has been measured using THz time-domain spectroscopy. In the previous work, nTHz was simulated in the range 0.1–4 THz; however, for efficient and wideband THz-wave generation, we need to know the refractive index over a wider frequency range.
Using this wideband refractive index data, one can more accurately estimate the coherence length distribution shown in Fig. 4 . The horizontal axis shows the THz-wave frequency; the vertical axis is the pump wavelength λ1 (< λ2). The red level shows the magnitude of the coherence length, which is greater than 1 mm. We achieved collinear phase-matching for the type-0 configuration because the nopt and nTHz are almost the same. A noteworthy point, revealed in coherence length calculations, arises in considering the phase-matching condition in BNA crystal for high-efficiency frequency conversion. The between 700 and 1100nm band is very significant pump tuning range for THz generation on BNA-DFG. This range will be enabled to efficient and widely tunable THz generation, concurrently using the independently controlled dual pump wavelength suitable for the long coherence length at each THz frequency.
3. Widely tunable THz-wave sources using BNA crystals
An independently controlled dual pump source is necessary for THz-wave generation using BNA-DFG. A double-crystal KTiOPO4 (KTP) optical parametric oscillator (OPO) was used to excite the BNA-DFG, utilizing an experimental setup almost identical to that originally developed for DAST-DFG [14,15]. The resonator that was used differs, however, and a schematic diagram of the coherent system is shown in Fig. 5 .
Previously, the KTP OPO pump source had an L-shaped beam path with three oscillator mirrors. In this study, a new oscillator design consisting of two flat oscillator mirrors and a beam splitter [BS] was used. The pump laser was a frequency-doubled Nd:YAG laser (532 nm, 7-ns pulse duration at 100 Hz; Nano-L, Litron Lasers). This oscillator is a very simple type-II KTP OPO with a flat–flat mirror configuration. The input mirror, M1, had a reflectance, R, of >99% in the 1250–1500-nm range (idler wave of KTP OPO) and high transmittance at 532 nm (pump) and 750–950 nm (signal). Mirror M2 reflected the pump, signal, and idler waves with R > 99%. In addition, BS was transmissive at 532 nm and highly reflective (>99%) within a 750–950-nm range. The oscillator was double-pass and single-resonance for the signal wave and high-Q for the idler wave. The maximum measured energy was 4.0 mJ/pulse at a dual pump-wavelength of λ1 = 887 nm and λ2 = 902 nm, with a Nd:YAG laser energy of 30 mJ/pulse (~60 MW/cm2), which represents a fourfold improvement in output power over the previous oscillator. The conversion efficiency from the 532-nm pump to the near-infrared dual pump was 13.3%, corresponding to 22.6% quantum efficiency. The peak power was 570 kW, and the oscillation threshold was approximately 2 mJ/pulse (~4 MW/cm2). The pump wavelengths from the KTP OPO were controlled independently using a galvano-optical beam scanner (LSA-20B-30-SP; Harmonic Drive Systems).
The vertically polarized output of KTP OPO was focused onto the BNA crystal with a beam diameter of ~2 mm [full-width at half-maximum (FWHM)] using an f = 250-mm lens (T = 90% at 900 nm). The BNA crystal had an 8-mm cross section and 1-mm thickness. The pump beam and stray light were excluded from the detector using a Yoshinaga filter and a black polyethylene filter. Since the response time of the galvano scanners is less than 1 ms, any required frequency pair for THz-DFG was accessible within 1 ms; i.e., frequency-agile operation  was possible.
4. Results and discussion
We measured the THz-wave output power while independently controlling the optical pump-wavelengths λ1 and λ2 (λ1 < λ2). Frequency-agile technology allows measurement of the pump-wavelength dependence of the energy distribution (which was impossible for the previous system without control over λ1).
Figure 6 shows a two-dimensional map of the dependence of the output power on the pump wavelengths. The horizontal and vertical axes represent the frequency and pump wavelength λ1, respectively. The control range of λ1 and λ2 was 780–950 nm, corresponding to the THz frequency range from 0.1 to 20 THz. The mapping resolution was 200 × 200 pixels, and averaging over five measurements was used. The signal acquisition time was ~33 minutes, which could detect from pulse to pulse using synchronized control with laser rep. rate 100Hz. The incident energy was 3 mJ/pulse at dual pump-wavelengths (λ1 = 887 nm and λ2 = 902 nm). The intensity has been normalized to the maximum at each frequency. The red level shows the magnitude of the output THz energy at each frequency. The small dark stripe at the center of Fig. 6 shows that the λ1 output power was extremely strong and efficient compared to the flat operation of a KTP crystal angle for beam normal incident angle, and therefore, the THz output energy was higher than in other areas. It is not suitable for this research purpose; the obtained data have been cut out of this area. The other black area does not represent THz generation to limited to dual pump-wavelength tuning.
The effective THz generation is possible by selecting the proper pair of wavelengths of the two pump beams, λ1 and λ2. This proper selection is based on the very accurate data of the coherence length (or refractive index) of the organic NLO crystal used. Unfortunately, such accurate values of the refractive index are not easily obtained because of the difficulty in mechanical processing of organic crystals due to its fragility. Nevertheless the proper pair of the wavelengths can be experimentally determined by our frequency-agile technique. Our frequency-agile technique can potentially be applied to other NLO crystals for their effective THz-wave generation.
We determined the optimum pump-wavelength at each frequency using pump-wavelength dependence mapping in Fig. 6. One can make a lookup table of output power at various pairs of pump wavelengths. Figure 7 shows the maximum THz-wave output spectrum using this optimum lookup table with independently controlled λ1 and λ2. This THz-wave generation efficiency increases compared to the fixed wavelength λ1 at 812 nm. The dependency of the THz-wave output on the wavelength λ1 was clearly observed. The maximum output power obtained was about ten times at 11.6THz compared to when λ1 was fixed. The sensitivity of the bolometer used in this experiment is 142 pJ/pulse for 1 V output at 1.4 THz . It has the frequency dependency due to filters and window transmission characteristics. The output characteristics shown in Fig. 7 include this frequency dependency. The tuning range was 0.1–20 THz. This is of great interest because the BNA-DFG might play a key role as radiation source linking the sub-THz and THz regions. This source has the potential for higher output power and wideband THz generation, using the dual pump wavelength source with the tunability from about 700 to 1100 nm.
We have demonstrated that the optimum phase-matching condition of BNA crystal for the THz wave generation can be obtained by controlling the wavelengths of the DFG dual pump sources. The coherence lengths calculated from the experimentally determined refractive indexes have provided us with rough estimation of the proper set of the two pump wavelengths λ1 and λ2. On the basis of this rough estimation, we have obtained the pump-wavelength dependence mapping of BNA-DFG by scanning both λ1 and λ2, independently, which tells us the optimum set of λ1 and λ2 for efficient THz generation. In fact, we have simultaneously realized 10 times enhancement of the output power and wider frequency range up to 20THz with the optimized wavelengths. This frequency-agile technique with the two independently controlled pump sources is a key to the highly efficient and widely tunable THz-wave generation.
We thank C. Takyu and T. Shoji, who performed the dielectric coating and polishing of various optical components. This work was partly supported by a Grant-in-Aid for Scientific Research (A) (No. 19206009) from the Japan Society for the Promotion of Science (JSPS).
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