Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO2. Strong SHG and THG outputs are observed by matching either in-coming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO2 RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm2. Without the textured TiO2 layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO2 and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.
© 2014 Optical Society of America
Nonlinear optical (NLO) harmonic generation [1,2] has attracted great attention due to the wide range of practical applications such as nonlinear signal data storage [3–5], bio-imaging [6,7], and straight frequency conversion [1,2,8]. In particular, frequency conversion based on nonlinear harmonic generation extends the coherent emission of infra-red (IR) laser to visible and near ultraviolet wavelengths. Several methods and geometries have been proposed to enhance the frequency conversion efficiency for specific applications. Examples of these include; phase matching [1,2], quasi-phase matching [9–11], slow light in photonic crystal (PhC) waveguides [12,13], high-Q PhC microcavities [14–16], planar PhC enhanced second-harmonic generation (SHG) [17–19], surface plasmons enhanced SHG [20,21], and grating-assisted enhanced SHG [22–24].
Resonant waveguide grating (RWG) structures [25–27] for enhanced SHG [28,29] and third harmonic generation (THG) are the focus of the present work. A RWG typically comprises a high refractive-index grating-waveguide layer and a low refractive-index supporting layer. This design can produce very sharp reflection and transmission resonances associated with coupling incident light into guided modes (GMR) [25–27]. As a GMR condition is reached, strong local electric fields are excited in the vicinity of the grating, which is useful for enhancing NLO interactions and the photoluminance of fluorescent dyes and quantum dots [27–31]. Recently, it has been demonstrated that SHG can be dramatically enhanced in an all-dielectric RWG comprising a low refractive index silicon dioxide grating with a square-wave profile, covered by a thick, high refractive index TiO2 layer [28,29]. The strong SHG enhancement was attributed to the strong interaction of the local electric field of the resonant waveguide mode with the inherent surface nonlinearity of the TiO2 layer [28,29]. The present work shows that by coating a textured TiO2 RWG structure with a strongly nonlinear azo polymer bulk second and third order nonlinearities (esu  and esu  at 1.5 μm), resonant SHG and THG conversion processes can be substantially increased over those of bare TiO2 covered grating structures. The NLO polymers offer flexible processing options , which facilitate easy integration with a pure TiO2 RWG.
In this work, we employ a pure TiO2 RWG covered with a poled azo-polymer layer to enhance nonlinear harmonic generation using normal incidence excitation near 1300 nm. A simple and low cost two-beam interference technique is used to fabricate the grating texture. Resonant SHG and THG are measured and compared to reference samples: the enhancement factor for SHG is up to 3560 times that observed from an untextured poled azo-polymer layer.
2. Sample preparation and measurement setup
2.1 Fabrication of the azo-polymer RWG
Figure 1(a) schematically shows the azo-polymer RWG structure studied in this work. The structure from top to bottom comprises a nonlinear azo-polymer layer, a 1D sinusoidal TiO2 waveguide grating, a SU8 bottom cladding layer and an indium-tin-oxide (ITO) glass substrate. The SU8 bottom cladding layer was obtained by spin coating of SU8 photoresist on the backside of the ITO glass substrate with a thickness (TSU8) of 1.1 µm. A 1D sinusoidal grating was patterned on the top of the SU8 bottom cladding layer by a two-beam interference technique [34,35]. The TiO2 waveguide grating layer with a thickness (TTiO2) of 180 nm was deposited on the top of SU8 grating by using an electron-beam deposition system. Figure 1(b) shows the scanning electron microscopy (SEM) image of the fabricated RWG structure. The SEM image indicates that TiO2 layer is uniformly coated on the top of SU8 grating with sinusoidal modulation. The area of the RWG is 6 × 6 mm2. From SEM measurement, the period (Λ) and depth (d) of grating were determined to be 840 nm and 230 nm, respectively. The refractive indices of SU8 (nSU8), TiO2 (nTiO2) and ITO glass substrate (ng) are 1.55, 2.1 and 1.48 at 1.3 μm, respectively (the first two were obtained by ellipsometry measurements and the latter was taken from Ref. 36). A DR1-PMMA copolymer thin film with 15% molar azo-dye concentration was chosen to increase the NLO response of the device . Its molecular structure is shown in Fig. 1(a). The azo-polymer was dissolved in chloroform and spin-coated on the top of the TiO2 RWG structure. The azo-polymer RWG was then baked for 1 hour at 70 °C on a hotplate before use. The thickness (Tazo) of azo-polymer layer was determined to be 650 nm by using AFM measurement and its refractive index (nazo) is 1.54 at 1.3 μm . A corona poling technique was used to align azo-polymer molecules to form a noncentro-symmetric distribution at 4.5 kV and T〜100 °C for 45 minutes. The uniform poling area is about 1 cm2, which covers the whole azo-polymer RWG structure.
2.2 Experimental setup
Figure 2 shows the experimental setup to measure THG and SHG spectra of the pure TiO2 and azo-polymer RWGs. The signal output from an optical parametric oscillator (OPO), with wavelength tuning range from 1200 to 1380 nm, 5 ns pulse width, and 10 Hz repetition rate, was used as the fundamental beam. The power of the fundamental beam was controlled by the combination of a half-wave plate and a polarizer. The fundamental beam was collimated by a pair of lenses (L1 and L2) with a beam diameter of 2 mm. The fundamental beam was normally incident to the sample with 3 mW of average power and its peak intensity was 2 MW/cm2. The SHG and THG output from the sample were detected by a grating spectrometer combined with a photomultiplier (PMT). The THG and SHG output detected by the PMT were integrated by a boxcar integrator. To characterize the GMR properties of the azo-polymer RWG, a transmission spectrum measurement setup, including: a halogen white light source and a grating spectrometer (Andor Shamrock SR-500i) combined with an InGaAs detector, was used.
3. Experimental results and discussion
3.1 Transmission spectra of the azo-polymer RWG
The azo-polymer RWG was designed to exhibit resonant waveguide modes in the near IR for both transverse-magnetic (TM) and transverse-electric (TE) polarizations. Figure 3 shows the experimental and calculated transmission spectra of the pure TiO2 and azo-polymer RWGs at normal incidence. Figures 3(a) and 3(b) are the experimental transmission spectra of the pure TiO2 and azo-polymer RWGs for both TM and TE polarizations, and Figs. 3(c) and 3(d) show the corresponding spectra obtained using the rigorous coupled-wave analysis (RCWA) simulation [37,38], respectively. In the simulation, one unit cell is defined as a single sinusoidal period of both pure TiO2 and azo-polymer RWG structures with periodic boundary conditions in x direction [see Fig. 1(a)]. The RWG structures were sliced into 404 layers with a thickness of 6.56 nm. All diffraction orders were calculated by default . To reach convergence, we used 25 Fourier harmonics for the construction of the periodic boundary condition. The illumination field consisted of a plane wave at normal incidence for both TE and TM polarizations, and a non-conical incident geometry was considered in the simulation. Other simulation parameters, shown in the caption of Fig. 3, were chosen based on refractive indices and thicknesses of the designed structure.
Table 1 summarizes the measured and simulated TE and TM resonance wavelengths for the two textured waveguide samples. The calculated spectra of both pure TiO2 and azo-polymer RWGs are close to those of experimental results, except the dip positions and transmission minimum values of resonant modes. The difference in dip position probably results from minor differences between the real grating structure and the ideal sinusoidal grating assumed in the calculation, and the use of wavelength-independent refractive indices of nazo, nTiO2 and nSU8 in the calculation. The difference in transmission minimum values might be due to the low resolution (0.5 nm) of the spectrometer used and inhomogeneous broadening.
Figure 4 shows the visible-harmonic generation spectra obtained from the pure TiO2 and azo-polymer RWGs, where THG and SHG are in the wavelength ranges of 400-460 nm and 600-690 nm, respectively. In contrast to previous studies [28,29], there is no observable SHG from the pure TiO2 waveguide structures. This may be in part due to the fact that the peak intensities used in Refs [28,29] were approximately 100 times larger than in the present work, and in part due to the square-wave, rather than sinusoidal grating profile. There is a resonant THG signal in the TE polarization from the bare TiO2 RWG structure at a wavelength very close to 1/3 of the wavelength of the main TE resonance measured in that sample, near 1300 nm.
In the azo-polymer RWG sample, several THG and SHG resonances are observed. For the TM polarization, THG at 447.3 nm and SHG at 671.0 nm were simultaneously produced by setting the fundamental wavelength at 1342 nm matching with the GMR at 1343.8 nm. Another SHG at 633.0 nm corresponds to the resonant excitation of the GMR at 1266.7 nm. Note that there are two smaller SHG resonant modes (at 666.5 nm and 631.0 nm) adjacent to the two SHG modes mentioned above. These are attributed to out-going resonant extraction of the SHG via high-order GMRs, as evident in the visible transmission spectrum of the azo-polymer RWG, highlighted by the arrow in the inset of Fig. 4(a). For the TE polarization, THG at 451.3 nm and SHG at 677.0 nm result from the resonant excitation of the GMR at 1352.5 nm by the fundamental beam [Fig. 4(b)]. The SHG at 635.5 nm, is related to the fundamental resonant excitation of the GMR at 1267.4 nm, while the SHG at 632.0 nm is attributed to the resonant extraction of SHG coupled with the high-order GMR resonance highlighted by the other arrow in the inset of Fig. 4(b). When the azo-polymer RWG sample is not poled, the THG resonances are observed, but no SHG, is generated.
In order to further understand the results shown in Fig. 4, RCWA simulations were performed to calculate the electric-field intensity distributions of the azo-polymer RWG at resonant wavelengths. Figures 5(a)-5(d) display electric-field intensity profiles normalized to that of the incident light of four resonant modes in the azo-polymer RWG; in sequence are TM resonant modes at 1247.3 nm (TM, λ = 1247.3 nm) and 1337.7 nm (TM, λ = 1337.7 nm), TE resonant modes at 1250.4 nm (TE, λ = 1250.4 nm) and 1367.6 nm (TE, λ = 1367.6 nm). For short-wavelength resonant modes (TM, λ = 1247.3 nm) and (TE, λ = 1250.4 nm), strong localized electric-fields appear in the region between the SU8 and glass substrate. In contrast, for long-wavelength resonant modes (TM, λ = 1337.7 nm) and (TE, λ = 1367.6 nm), the strong electric fields are located at the borders between the azo-polymer, TiO2 and SU8 layers. The electric-field intensity distributions of these two resonant modes are slightly different; for the (TM, λ = 1247.3 nm) mode, the electric-field intensity maximizes at azo-polymer/TiO2 interface, but the electric-field intensity of the (TE, λ = 1367.6 nm) mode maximizes at the TiO2/SU8 interface. The electric-field intensity at the NLO azo-polymer layer is very crucial for the SHG output because it has the largest second order nonlinearity in the azo-polymer RWG structure. The maximum enhancement factors of electric-field intensity at the azo-polymer layer are 700 for (TM, λ = 1337.7 nm), 230 for (TM, λ = 1247.3 nm), 40 for (TE, λ = 1250.4 nm), and 10 for (TE, λ = 1367.6 nm). Their trend qualitatively agrees with that of SHG outputs associated with these four resonant modes as shown in Fig. 4; from high to low are (TM, λ = 1337.7 nm), (TM, λ = 1247.3 nm), (TE, λ = 1250.4 nm), and (TE, λ = 1367.6 nm). This indicates that the SHG outputs observed in Fig. 4 mainly result from strong localized electric field in the azo-polymer layer produced by the fundamental resonant excitation of GMRs.
Finally, we employed the comparison method used in Ref  to determine the SHG enhancement factor of the azo-polymer RWG compared with an azo-polymer reference sample which is identical to the azo-polymer RWG except that there was no grating texture. Since the SHG output of the azo-polymer reference sample is too weak to detect by using the collimated-beam setup shown in Fig. 2, instead, a focused-beam setup was used. A LBO nonlinear crystal with thickness of 2 mm was used to link results obtained from the collimated- and focused-beam setups. In the collimated-beam setup samples were placed at the center of the rotational stage of the SHG measurement setup shown in Fig. 2, while in the focused-beam setup samples were placed at the focus position of L1. The diameter of the focusing spot is about 50 μm. The LBO crystal can produce detectable SHG but no detectable THG using both setups. We only determined the enhancement factor of SHG for the long-wavelength TM resonate mode of the azo-polymer RWG. The SHG ratio of the azo-polymer RWG to the LBO crystal obtained in the collimated-beam setup is 8.9, and the SHG ratio of the LBO crystal to the azo-polymer reference to that obtained in the focused-beam setup is 400. Accordingly, compared to the azo-polymer reference, the SHG output of the azo-polymer RWG for the long-wavelength TM resonate mode is enhanced by 3560 times.
This work presents a promising method to promote harmonic conversion in NLO polymers by incorporating a poled azo-polymer thin film on the top of a pure TiO2 RWG to form an azo-polymer RWG. By arranging either in-coming fundamental or out-going harmonic wavelengths to match with the wavelengths of GMRs of the azo-polymer RWG, strong THG and SHG are observed. The azo-polymer RWG possesses two GMRs for both TM and TE polarizations at wavelength range between 1200 nm and 1400 nm, and all these four GMRs result in resonant-enhanced SHG. This is very different to the result of the pure TiO2 RWG, in which no SHG signal is detected under the same excitation conditions. The long-wavelength TM resonate mode produces ~3560 times more SH signal than a reference sample that does not include the RWG structure. The electric-field intensity distributions of the GMRs obtained by the RCWA calculation reveal that a strong local electric field is generated at the interface of the TiO2 grating and azo-polymer which facilitates its strong interaction with the azo-polymer and results in large enhancement of SHG and THG outputs.
The authors gratefully acknowledge financial support from the National Science Council, Taiwan, under grant Nos. NSC 98-2112-M194-008-MY3, NSC 99-2112-M194-008-MY3 and NSC 98-2811-M194-007. J. H. Lin acknowledges the support of postdoctoral fellowship from National Science Council, Taiwan. J. F. Young acknowledges the financial support of the Natural Sciences and Engineering Research Council in Canada. The authors are grateful to Dr. G. W. Rieger, for his assistance to this work.
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