We fabricated terahertz (THz) wave generation devices using electro-optic (EO) polymer slab waveguides and cyclo-olefin polymer (COP) clads with very small absorption loss of the THz waves based on a novel device fabrication procedure involving bonding of the poled EO polymer layer to the COP substrates. We demonstrated THz wave generation from the EO polymer slab devices using a 1.55 µm-band femtosecond fiber laser and evaluated the THz wave generation properties of the devices. Our results will lead to the development of compact, highly efficient, and ultrabroadband THz devices using EO polymers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz (THz) waves (0.1-10 THz) have promising applications in fields such as material analysis , noninvasive / nondestructive sensing [2,3] and ultrahigh-speed wireless communication . To increase the use of THz waves in the real world, it is important to develop small size, highly efficient, and ultrabroadband THz sources. THz wave generation has been performed using second-order nonlinear optical (NLO) materials, such as lithium niobate (LiNbO3) [5,6], zinc telluride (ZnTe) , DAST (4-N,N-dimethylamino-4’-N’-methylstilbazolium tosylate) , DSTMS (4’-dimethylamino-N-methyl-4-stilbazolium 2,4,trimethylbenzenesulfonate) [9,10], and electro-optic (EO) polymers [11,12], photoconductive antennae , and laser-induced air plasma  with pulsed pump laser sources. As for THz wave generation using NLO crystals, because inorganic NLO crystals, such as LiNbO3 and ZnTe, have strong absorption in the THz region (the region above 2 THz for LiNbO3 and 3-7 THz for ZnTe ) due to lattice vibrations, the bandwidths for THz wave generation can be restricted. On the other hand, organic NLO crystals, such as DAST and DSTMS, have relatively small absorption coefficients in the broad THz range; thus, they are widely used as ultrabroadband THz emitters [8–10,15]. However, because DAST or DSTMS have specific absorption peaks at 1.1 THz or 1 THz, respectively, etc., there exist gaps in the obtained THz spectra [9,15]. Therefore, organic NLO crystals with improved absorption properties in the lower frequency region have been proposed . Although, until now, THz wave generation using regeneratively amplified femtosecond lasers has been performed, THz wave generation using compact fiber lasers, such as 1.55 µm-band erbium-doped fiber lasers, has become increasingly important for commercialization and miniaturization of THz equipment. To efficiently generate the THz waves using the compact and low power light sources, it is necessary to develop waveguide-type THz wave generation devices capable of increasing not only the light intensity in the NLO materials but also the interaction length between the pump light and the NLO materials .
Since the late 1990s, high-performance organic EO polymers with EO chromophores with large hyperpolarizability have been developed [17–19] and applied to fabricating high-speed optical modulators [20,21], optical phased arrays , electromagnetic field sensors , and THz emitters and detectors [11,12,24]. EO polymers can have large EO coefficients (> 100 pm/V) and large figure-of-merit (FOM) values for THz wave generation (FOMTHz = no6r2/16nTHz , where r is the EO coefficient, and no and nTHz are the refractive indices at the pump and THz frequencies, respectively) (FOMTHz > 8900 for EO polymers) compared with LiNbO3 (FOMTHz = 1500), ZnTe (FOMTHz = 160), and DAST (FOMTHz = 5600). In addition, since the EO polymers have relatively small absorption coefficients over a wide THz range without strong absorption peaks , ultrabroadband and spectral gap-free THz wave generation is possible [11,12]. Another advantage of EO polymers over NLO crystals is that they show very small dispersion of the refractive index between optical frequencies and THz frequencies . The large coherence length enables THz wave generation under a collinear phase-matching condition. Furthermore, EO polymers have excellent processability compared with NLO crystals. Thin films of EO polymers are easily formed by spin-coating, and the films can be microfabricated to produce waveguide structures using normal photolithographic techniques .
Until now, THz wave generation using EO polymers has been performed using EO polymer films and regeneratively amplified femtosecond lasers with a wavelength of 800 nm  or 1300 nm . In contrast, although waveguide-type EO polymer THz emitters have been numerically modeled, in which THz waves propagate in the clads made of THz-wave low-loss materials , actual devices have not been realized so far. In the conventional fabrication processes for EO polymer waveguide devices, conductive cladding materials (for example, sol-gel) and electrodes are embedded into the devices to apply a voltage for poling the EO chromophores in the EO polymer waveguide core [20,22]. The conductive clads are necessary to efficiently apply a voltage to the EO polymer waveguide core while preventing a voltage drop in the clad layer parts . However, since the conductive cladding materials strongly absorb the generated THz waves, the devices cannot be used for THz emitters. Therefore, the development of device fabrication processes that can exclude the conductive cladding materials and incorporate THz-wave low-loss cladding materials, which ordinarily have a very high resistivity, is required.
In the present study, we fabricated THz wave generation devices consisting of an EO polymer slab waveguide and clads made of a cyclo-olefin polymer (COP) (a material with very small absorption loss of the THz waves)  based on a novel device fabrication method involving bonding of the poled EO polymer film with the COP substrates. We demonstrated THz wave generation from the EO polymer slab waveguide devices using a 1.55 µm-band compact femtosecond fiber laser and evaluated the THz wave generation properties of the devices.
The EO polymer slab waveguide devices were fabricated according to the processes outlined in Figs. 1(a)-1(f). A solution of an EO polymer was spin-coated on a substrate with a bottom electrode, then the sample was thermally annealed. A top electrode composed of indium-doped zinc oxide (IZO) was formed by sputtering. The EO polymer film was poled by applying a voltage (100 V/µm) at 150 °C which is close to the glass transition temperature of the EO polymer. The applied voltage was maintained during a cooling process and stopped after dropping to room temperature to maintain the orientation of the poled EO chromophores. Then, the IZO electrode was removed by wet etching. The poled EO polymer film was transferred to the COP substrate after surface treatment using O2 plasma , with another COP substrate bonded to the EO polymer surface in the same way. The sample was diced to obtain the desired device lengths.
Figure 2 shows a schematic illustration of the experimental setup used for the THz time domain spectroscopy (THz-TDS), which detects THz signals using an EO sampling technique . A femtosecond fiber laser (BS-60-STD, IMRA, 1.56 µm center wavelength, < 100 fs, 50 MHz) was focused onto the EO polymer slab devices by two orthogonally arranged cylindrical lenses (f = 100 mm and f = 12 mm) to obtain an elliptical focused spot. To confirm the guiding
of the pump light in the slab waveguide, the end face of the device was observed using an infrared camera with a microscope objective lens, as shown by the image in the inset of Fig. 2. A-0.5-mm-thick Ge window was used to block the 1.56 µm pump light. The probe beam with a wavelength of 780 nm obtained using a periodically poled LiNbO3 (PPLN) crystal was focused onto a 1-mm-thick <110> ZnTe crystal. The probe beam whose polarization state was changed by the THz wave electric field was passed through a quarter waveplate and Wollaston prism and then detected using a balanced photodetector (2007 Nirvana, Newport) connected to a lock-in amplifier. The probe beam power before the balanced photodetector was 60 µW. The measurement was performed in atmosphere.
For the measurement of the absorption coefficients and refractive indices of the COP in the THz region, THz-TDS spectrometers (TAS7500TS with TAS1120 and TAS1230 modules, TAS7500SP, and TAS7500SU, Advantest) were used . The absorption coefficients in a wide THz range (0.4-20 THz) were measured using a Fourier transform far-infrared spectroscopic apparatus (VIR-F, JASCO).
3. Results and discussion
We evaluated the absorption losses and refractive indices of the COP in the THz region. Figure 3(a) shows the absorption coefficients of the COP (ZEONEX 480R, ZEON) in the THz region. The absorption coefficients of the COP are less than 1 cm−1 and 5 cm−1 in the range of 0.1-4 THz and 0.1-11 THz, respectively, which are much smaller than those of commonly used polymers, such as polymethyl methacrylate (PMMA) (less than 70 cm−1 in 0.1-4 THz ) and amorphous polycarbonate (APC) (less than 25 cm−1 in 0.1-4 THz ). It seems that the COP consists of cyclic alkanes without the functional groups such as methyl groups or carbonyl groups, that are contained in the PMMA or APC and show strong absorption in the THz region . The small absorption coefficients of the COP cladding material will lead to a large device length that can increase the THz wave generation efficiency. Figure 3(b) shows the refractive indices of the COP in the THz region. The spectrum of the refractive indices is flat, with a value of 1.54 in the range of 0.2-8 THz. The very small dispersion of the refractive indices overa wide THz range is desirable for THz devices operating in a broadband THz range. Therefore, the COP will be a most suitable cladding material for polymer-based THz waveguide devices.
Figures 4(a) and 4(b) show a schematic illustration and the appearance of the fabricated EO polymer slab waveguide device, respectively. The poled EO polymer layer is located between the COP substrates, and the device does not contain conductive clads or electrodes. The thicknesses of the EO polymer layer and each COP substrate were 4.5 µm and 1 mm, respectively. Figure 4(c) shows a scanning electron microscope (SEM) image (reflected electron image) of the cross section of the EO polymer slab waveguide. It is confirmed that the EO polymer layer is tightly bonded to the COP substrates after the dicing process. Figure 4(d) shows the chemical structure of the side-chain type EO polymer [18,19] with a dicyclopentanyl methacrylate (DCPMA) backbone used for the device. Since the absorption peak of the EO polymer is ~755 nm, pumping of the EO polymer is performed under a nonresonant condition. One of the advantages of the present device fabrication procedure is that we can accurately evaluate the EO coefficient (r33) of the EO polymer part in the device since the EO polymer film is transferred after evaluation of the EO coefficients of the poled EO polymer film. The r33 of the EO polymer film at a wavelength of 1.55 µm was evaluated to be 46 pm/V using a transmission ellipsometric method . Under lossless and nonresonant conditions, the second-order NLO susceptibility for optical rectification χijk(2)(0, ω, -ω) can be assumed to be equal to that for EO effect χkji(2)(ω, ω, 0) and χijk(2)(ω, ω, 0), based on Kleinman symmetry condition . In our case, χ333(2)(0, ω, -ω) ≈χ333(2)(ω, ω, 0) ≈no4r33/2.
To characterize the optical properties of the EO polymer slab waveguide, optical modes of the slab waveguide were calculated using a 2D finite-difference time-domain (FDTD) method. Figure 4(e) shows the effective refractive indices (no_eff) of the transverse magnetic (TM) modes versus the EO polymer slab thickness (d) at a wavelength of 1.55 µm. We assumed that the EO polymer has a refractive index of 1.63 for TM polarization at a wavelength of 1.55 µm after the poling process. A value of 1.51 was used for the refractive index of the COP (ZEONEX 480R), which was measured using a prism coupler (2010/M, Metricon) at a wavelength of 1.532 µm. Although the result shows that the EO polymer slab waveguide has several modes, we can assume that the Gaussian beam would efficiently excite the fundamental mode rather than the higher order modes having multiple nodes in the mode profile. The group index [ng(λ)]  of the optical pulse for the fundamental mode was calculated to be 1.635 using the relation ng(λ) = no_eff(λ) - λ dno_eff (λ)/dλ, where λ is the optical pump wavelength. In analogy to the case of photomixing, the coherence length (lc) for THz wave generation is defined asFig. 4(f). The THz wave generation efficiency for each THz frequency can be maximized with a device length equal to the coherence length at that frequency.
Figure 5(a) shows the temporal waveform for a THz pulse generated from an EO polymer slab waveguide device with a length of 0.5 mm at a pump power of 20 mW after passing through the optical chopper. The THz signal obtained by EO sampling using the balanced photodetector is shown as the difference in the signal intensities of the two detectors normalized by the incident probe intensity ΔI/I0, which should be directly proportional to the THz electric field . It appears that efficient THz wave generation is observed due to the good phase matching between the optical pulse in the EO polymer slab waveguide and the generated THz waves in the COP clads. Figure 5(b) shows the Fourier-transformed spectrum of the temporal THz waveform. As mentioned above, the THz spectra generated by DAST or DSTMS crystals show gaps at 1.1 THz or 1 THz, respectively, etc. due to the strong absorption and dispersion of the crystals [9,15], which will cause a decrease in sensitivity in the low-frequency region in THz spectroscopy. In contrast, the spectrum from the EO polymer slab waveguide shows no such remarkable gaps due to absorption by the material. The gap appearing around 1.7 THz is mainly attributed to the absorption by water vapor in the atmosphere [3,31]. Although we can expect that the actual THz spectra generated from the EO polymer device are broadband (> 4~5 THz),the obtained spectra show a decrease in the amplitude spectrum above ~3 THz. This behavior is accounted for by absorption of the THz waves by the ZnTe detection crystal, which shows strong absorption in the THz region from 3 to 7 THz . Figure 5(c) shows the pump-power dependence of the peak-to-peak amplitude of the THz signals for the EO polymer slab device with a length of 0.5 mm. The linear relation agrees with that for the THz wave generation process due to optical rectification of the second-order NLO effect. In addition, the result also indicates that the effect of photobleaching on the EO chromophores due to the pump pulse is small up to a pump power of 20 mW.
Figure 6(a) shows the temporal waveforms of the THz pulses generated from the EO polymer slab devices with lengths (l) of 0.125, 0.25, 0.5, 0.75, and 1 mm at a pump power of 15 mW. The difference in the shapes of the waveforms can be explained by the difference in frequency components included in the waveforms, because of the different coherence lengths for each THz frequency. Figure 6(b) shows the peak-to-peak amplitudes of the THz pulses generated from the EO polymer slab devices with different device lengths. The peak-to-peak amplitude increased with increasing device length up to 0.5 mm. The Fourier-transformed amplitude spectra of the THz pulses for different device lengths are shown in Fig. 6(c).
To understand the device length dependence in the experimental results, we calculated the THz amplitude spectra for each device length. Since the absorption coefficients of the EO polymer at the pump wavelength and those of the cladding material at the THz frequencies are very small, the absorption loss of the pump light and generated THz waves can be ignored. The spectra of the generated THz electric field under zero-absorption and nondepleted pump approximations can be given by [8,9,16,34]31]. The full-width at half-maximum (FWHM) of the optical pulse intensity I0 (t) is expressed as using the typical value τ. The effective generation length lgen under the zero-absorption approximation is given byFigure 6(d) shows the calculated THz spectra for the different device lengths, where the optical pulse width τp was set to 100 fs. In the THz region below ~1.6 THz, the amplitude of the calculated THz spectra increased with increasing device length up to 1 mm. In the region above ~1.6 THz, the amplitude did not simply increase with increasing device length and decreased above a certain device length. This behavior is consistent with the result that the calculated coherence lengths are less than 1 mm in the THz region above ~1.6 THz [Fig. 4(f)]. As seen in Fig. 6(c), a similar tendency that the THz amplitude in the lower frequency region (< ~0.8 THz) increased with increasing device length up to 1 mm was observed in the experimental spectra. Due to the cutoff frequency in our THz detection system caused by the THz absorption in the ZnTe crystal, the amplitudes in the higher frequency region (> 3 THz) were uniformly small regardless of the device length. On the other hand, the increase rate of the THz amplitude versus the device length observed in the experimental spectra was smaller than that in the calculation. In addition, the upper limit of the THz frequency (~0.8 THz) at which the amplitude monotonically increases as the device length increases in the experimental spectra was also smaller than that in the calculation (~1.6 THz). These observations suggest that the apparent coherence lengths of the fabricated devices are smaller than the calculated values. This can be responsible for optically induced inhomogeneity in the refractive index (due to a slight photobleaching effect), which affects the optical propagation property in the transverse direction of the slab waveguide as well as for device lengths comparable to the Rayleigh length (~1 mm). Improvement of the waveguide structure and photostability of the device will further increase the THz wave generation efficiency.
By using a novel device fabrication procedure, we successfully fabricated THz wave generation devices consisting of an EO polymer slab waveguide and THz-wave low-loss cyclo-olefin polymer clads, which demonstrated gap-free THz wave generation from the EO polymer device using a 1.55 µm-band compact femtosecond fiber laser. It seems that the present results are the first demonstration of THz wave generation from an EO polymer waveguide structure using a compact fiber laser. Our results allow for the fabrication of various THz devices using EO polymers and THz-wave low-loss materials and will lead to the realization of compact, highly efficient, and ultrabroadband THz wave generation and detection devices using EO polymers.
Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan Grants-in-Aid for Scientific Research (18K04274, 17K06411).
We thank Dr. Maya Mizuno for assistance with the measurements of the absorption coefficients and refractive indices in the THz region.
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