Electrically tunable liquid crystal laser using a nanoimprinted indium-tin-oxide electrode as a distributed feedback resonator Kyung Won Yoon1 and Na Young Ha1,2,* 1
Department of Energy Systems Research, Ajou University 16499, Suwon, South Korea 2 Department of Physics, Ajou University 16499, Suwon, South Korea * [email protected]
Abstract: We demonstrated electrical tunability of a liquid crystal (LC) laser using a nanoimprinted indium-tin-oxide (ITO) film as a distributed feedback (DFB) resonator, a transparent electrode, and an alignment layer for LCs. From the field-induced reorientation of LCs and changes in effective refractive indices of guided laser modes, lasing emission is tuned by 6 nm at low applied voltage of 8.0 V. This is because the LC laser with the nanoimprinted ITO electrode has no additional insulating layers for lasing performance. The present system is based on the functional electrode and its active control provides various applications and advances in laser technology. ©2016 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (160.3710) Liquid crystals; (310.7005) Transparent conductive coatings; (220.4241) Nanostructure fabrication.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
H. Kogelnik and C. V. Shank, “Stimulated emission in a periodic structure,” Appl. Phys. Lett. 18(4), 152–154 (1971). I. D. W. Samuel and G. A. Turnbull, “Organic semiconductor lasers,” Chem. Rev. 107(4), 1272–1295 (2007). G. A. Turnbull, P. Andrew, M. J. Jory, W. L. Barnes, and I. D. W. Samuel, “Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser,” Phys. Rev. B 64(12), 125122 (2001). G. Tsiminis, Y. Wang, A. L. Kanibolotsky, A. R. Inigo, P. J. Skabara, I. D. W. Samuel, and G. A. Turnbull, “Nanoimprinted organic semiconductor laser pumped by a light-emitting diode,” Adv. Mater. 25(20), 2826– 2830 (2013). S. Riechel, U. Lemmer, J. Feldmann, S. Berleb, A. G. Mückl, W. Brütting, A. Gombert, and V. Wittwer, “Very compact tunable solid-state laser utilizing a thin-film organic semiconductor,” Opt. Lett. 26(9), 593–595 (2001). M. Stroisch, T. Woggon, C. Teiwes-Morin, S. Klinkhammer, K. Forberich, A. Gombert, M. Gerken, and U. Lemmer, “Intermediate high index layer for laser mode tuning in organic semiconductor lasers,” Opt. Express 18(6), 5890–5895 (2010). R. Ozaki, T. Shinpo, K. Yoshino, M. Ozaki, and H. Moritake, “Tunable liquid crystal Laser using distributed feedback cavity fabricated by nanoimprint lithography,” Appl. Phys. Express 1(1), 012003 (2008). S. M. Jeong, N. Y. Ha, F. Araoka, K. Ishikawa, and H. Takezoe, “Electrotunable polarization of surface-emitting distributed feedback laser with nematic liquid crystals,” Appl. Phys. Lett. 92(17), 171105 (2008). S. Klinkhammer, N. Heussner, K. Huska, T. Bocksrocker, F. Geislhoringer, C. Vannahme, T. Mappes, and U. Lemmer, “Voltage-controlled tuning of an organic semiconductor distributed feedback laser using liquid crystal,” Appl. Phys. Lett. 99(2), 023307 (2011). K.-Y. Yu, S.-H. Chang, C.-R. Lee, T.-Y. Hsu, and C.-T. Kuo, “Thermally tunable liquid crystal distributed feedback laser based on a polymer grating with nanogrooves fabricated by nanoimprint lithography,” Opt. Mater. Express 4(2), 234 (2014). B. H. Wallikewitz, G. O. Nikiforov, H. Sirringhaus, and R. H. Friend, “Nanoimprinted, optically tuneable organic laser,” Appl. Phys. Lett. 100(17), 173301 (2012). D. R. Cairns, R. P. Witte II, D. K. Sparacin, S. M. Sachsman, D. C. Paine, G. P. Crawford, and R. R. Newton, “Stain-dependent electrical resistance of tin-doped indium oxide on polymer substrate,” Appl. Phys. Lett. 76(11), 1425–1427 (2000). S. H. Lee and N. Y. Ha, “Nanostructured indium-tin-oxide films fabricated by all-solution processing for functional transparent electrodes,” Opt. Express 19(22), 21803–21808 (2011). S. Y. Chou, P. R. Krauss, and P. J. Renstrom, “Imprint lithography with 25-nanometer resolution,” Science 272(5258), 85–87 (1996).
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 516
15. N. Y. Ha, S. M. Jeong, S. Nishimura, G. Suzaki, K. Ishikawa, and H. Takezoe, “Simultaneous red, green, and blue lasing emissions in a single-pitched cholesteric liquid-crystal system,” Adv. Mater. 20(13), 2503–2507 (2008). 16. M. Berggren, A. Dodabalapur, R. E. Slusher, and Z. Bao, “Light amplification in organic thin films using cascade energy transfer,” Nature 389(6650), 466–469 (1997). 17. http://www.lumerical.com.
1. Introduction An organic DFB laser is a promising approach for compact tunable laser sources in the visible spectral region due to a broad and continuous tunability, an intrinsic low threshold, and an easy processing [1–11]. Particularly, tunability of lasing emissions from organic DFB lasers has been demonstrated with various approaches based on modulations of effective refractive indices by applying voltages [7–9], temperatures [10], and lights [11]. The emission wavelength λlaser of the DFB laser depends on the grating period Λ, the diffraction order md, and the effective refractive index neff of the laser mode according to the Bragg equation: mdλlaser = 2Λneff. For md = 2, the second-order diffracted light makes the counter-propagating mode, which results in an optical feedback for lasing action. In this case, laser light can be out-coupled to the surface normal direction by first-order Bragg diffraction, and is highly TE polarized because a mode propagating perpendicular to the grating line (TE mode) is effectively Bragg scattered [2,3,8]. The ITO has been the most commonly used transparent electrode material for various photonic applications such as flat-panel displays, touch screens, and solar cells. Particularly, a sputtered ITO electrode is attractive from high transmission in visible region and good electrical conductivity although the ITO contains expensive rare elements and has poor mechanical flexibility [12]. Recently, we have fabricated nanostructured ITO films by allsolution processing of ITO nanoparticles and a nanoimprint lithography (NIL) technique, and demonstrated the electro-optic performance of LC devices with the nanostructured ITO films [13]. The NIL technique can quickly and simply reproduce feature sizes in the submicron range [14]. In this study, we use this nanoimprinted ITO film as a DFB resonator, a transparent electrode, and an alignment layer for LCs, and present a successful tuning of lasing wavelength from an organic dye-doped LC laser with the nanoimprinted ITO electrode by applying external voltage. On the basis of the voltage-induced reorientation of LC molecules, we can control the refractive index of a LC layer nLC and the resultant neff of the DFB laser with the nanoimprinted ITO electrode. For the DFB laser with a 12 μm LC layer, the spectral position of lasing emission is tuned by 6 nm at very low applied voltage of 8.0 V, compared with those of other tunable DFB lasers with LCs [7,9]. This is because the present system has no additional insulting layers for the lasing performance. Hence, the nanoimprint ITO film is a functional electrode with an appropriate structure for lasing emission. In addition, we numerically simulate the laser waveguide modes to calculate nLC and neff for tuning range of the lasing emission and, the obtained values are in close agreement with experimental results. 2. Nanoimprinted ITO electrodes for tunable LC lasers To fabricate nanoimprinted ITO electrodes for tunable LC lasers, we used an ITO ink (Ulvac Materials), which was formulated by dispersing ITO nanoparticles of less than ~20 nm in diameter into a cyclododecene solvent [13]. For a grating structure, a Si master mold with period Λ = 379 nm and grating depth dgrating = 151 nm was prepared by e-beam lithography method. In the NIL process, the master molds were converted to poly(dimethylsiloane) (PDMS; RT-601, Wacker) molds by drop-casting the PDMS materials and curing at 70°C for 30 min. Next, the ITO ink was spin-coated on the ITO-sputtered glass (ITO glass), and the patterned PDMS mold was pressed onto it at 230°C for 30 min. Figure 1(a) shows an atomic force microscopy (AFM; XE-100, Park Systems) image at the edge area of the nanoimprinted ITO electrode with Λ = 364 ± 6 nm and dgrating = 94 ± 18 nm. This AFM image clearly indicates that the grating structure of the maser mold was replicated on the ITO layer with a thickness dITO = 256 ± 10 nm. Here, we employed the ITO glass with dITO = 149 ± 5 nm and a
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 517
sheet resistance Rs = 10.7 ± 0.2 Ω/sq as a substrate. The sheet resistance Rs = 15.5 ± 0.9 Ω/sq of the nanoimprinted ITO electrode, consisting of the nanoimprinted ITO film and the sputtered ITO layer, is similar to that of the conventional ITO and is suitable for LC devices, OLEDs, and polymer solar cells. The thickness and the sheet resistance of each ITO layer were measured by using a surface profiler (P-10, KLS-Tencor) and a four-point probe meter (Model280, Four Dimensions), respectively. (c)
(a)
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(b) 100
50
ITO glass Nanoimprinted ITO film/ ITO glass
0 300 400 500 600 700 800 Wavelength [nm]
Fig. 1. (a) AFM image at the edge region of the nanoimprinted ITO electrode with Λ = 364 nm and dgrating = 94 nm. (b) Transmission spectra of the ITO glass and the nanoimprinted ITO film on the ITO glass. (c) Photographs of diffracted reflection lights from the nanoimprinted ITO electrode of (a) for various diffraction angles.
As shown in Fig. 1(b), we measured transmission spectra of the ITO glass and the nanoimprinted ITO film on the ITO glass by a spectrometer (V-670, Jasco). All transmission spectra for normal incidence of unpolarized light show more than ~80% transmittance in the visible region except for a characteristic dip (solid line of Fig. 1(b)) near ~593 nm by light diffraction through the grating structure of the nanoimprinted ITO electrode. In this sense, Fig. 1(c) displays photographs of diffracted reflection lights as well as high transparency from the nanoimprinted ITO electrode with the patterned area of ~10 mm × 10 mm. Here, as the diffraction angle decreased, the colors (or spectral positions) of the reflected diffraction lights changed from red (upper of Fig. 1(c)) to blue (lower of Fig. 1(c)) because of the decrease in path difference. 3. LC lasers with nanoimprinted ITO electrodes as DFB resonators We fabricated a surface-emitting DFB laser with this nanoimprinted ITO electrode as a resonator and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM) doped LCs (ZLI2293, Merck) as an active layer. The extraordinary (ne) and ordinary (no) refractive indices of the nematic LCs, ZLI2293 were 1.63 and 1.50, respectively [15]. The nanoimprinted ITO film on the ITO glass and the other ITO glass, coated with polyimide (PI; AL22620, JSR) and rubbed unidirectionally at room temperature, were stacked face-to-face with a spacer in between, and then sealed to fabricate an empty resonator, as shown in Fig. 2(a). The resonator gap was ~12 μm and the grating direction of the nanoimprinted ITO film was set parallel to the x-axis, corresponding to the rubbing direction of the PI film and the alignment direction of the LCs. Then, DCM-doped LCs (1 wt. %) was introduced into the DFB resonator by capillary action at 88 °C in the isotropic phase. In our previous work, we confirmed that the nanoimprinted ITO film could act as an alignment layer for LCs and the
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 518
electro-optic performance of LC devices with nanoimprinted ITO films was successfully demonstrated [13]. (a)
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Fig. 2. (a) Schematic illustration, (b) transmission and emission spectra at different pumping energies, (c) threshold behavior of lasing emission intensity, (d) polarized lasing emission spectra at the pumping energy of 11.6 μJ/pulse, and (e) photograph of far field emission pattern of the surface-emitting DFB laser with the nanoimprinted ITO electrode and dye-doped LCs.
For lasing experiments, a third-harmonic light from a Q-switched Nd:YAG laser (Minilite II, Continuum) was used as the optical pumping source. The wavelength, pulse width and the repetition rate were 355nm, 4ns and 10 Hz, respectively, and the spot size of the pumping beam was about 100 μm. The 355 nm light was incident on the imprinted side of the cell, as shown in Fig. 2(a). The input beam intensity was controlled by using neutral density filters and measured with a pyroelectric energy meter (PEM100, LTB). The emission from the sample was collected by a lens along the direction normal to the substrate and then detected by an optical fiber coupled to the spectrometer (HR4000, Ocean Optics). Figure 2(b) represents transmission and emission spectra from the LC laser with the nanoimprinted ITO electrode for different pumping energies. Transmission spectra of Fig. 1(b) and 2(b) exhibit the spectral red-shift of the dip positions from ~593 nm for air-filled ITO grating to ~610 nm for LCs-filled ITO grating due to the increase of refractive index in the grating trench. Below the excitation energy 8.3 μJ/pulse, the emission spectra show the clear dip near 600 nm and a small peak at the edge of the long-wavelength band. With increasing the pumping energy, a very narrow lasing peak begins to grow at 607 nm, thus indicating the onset of photonic band-edge laser operation [2,3]. Single mode lasing was clearly observed being attributed to a DFB by second-order Bragg diffraction. Next, the emission intensity at 607 nm is plotted as a function of pumping energy in Fig. 2(c). At a pumping energy about 10.1 μJ/pulse, there is a distinct kink in the curve where the LC laser with the nanoimprinted ITO electrode reaches lasing threshold followed by a linear increase in output emission with pumping energy. This threshold energy of our LC laser is slightly high than those of other efficient LC lasing systems [7,8]. We think that lower threshold energy can be achieved from the LC laser with the nanoimprinted ITO electrode, by changing a pumping wavelength into the absorption peak region of DCM [7,8] or by introducing Föster-type energy transfer into active materials [15,16]. We also observed that lasing emission from the sample is highly TE polarized, as shown in Fig. 2(d). TE- and TM-polarized emission spectra were obtained under the pumping energy of 11.6 μJ/pulse. Figure 2(e) shows a photograph of far field emission pattern of a LC laser with the nanoimprinted ITO electrode at pumping energy above the lasing threshold. The
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 519
output beam from this surface-emitting DFB laser is emitted as a divergent strip, parallel to the direction of the grating. Here, we used a 355 nm-cut filter to remove the pumping beam after the sample and obtain clear photographs. 4. Electrically tunings of lasing emissions from LC lasers with nanoimprinted ITO electrodes The possibility of tuning the lasing emissions from the LC laser via reorientation of LCs in the DFB resonator was investigated. To adjust the refractive indices of the LC layer, an external voltage (1 kHz, square wave) was applied across the DFB laser or to the nanoimprinted ITO and the other ITO electrodes. Figure 3(a) and 3(b) show lasing emission spectra from the LC laser with nanoimprinted ITO electrode for various voltages. From 0.0 V (no external voltage applied) to 7.0 V, intensities of lasing emissions at 607 nm decreased, whereas the spectral positions of the shark peaks were unchanged. At 7.5 V (Fig. 3(b)), two lasing peaks appeared at both 607 nm and 613 nm simultaneously. Also, from 8.0 V on, the peak wavelength switched up to 613 nm with increasing intensities. It is noted that the spectral shift of lasing peaks occurs at very low applied voltage of 8.0 V for a 12 μm LC layer in comparison with those of other tunable DFB systems based on LCs [7,9]. This is because we successfully demonstrate fabrication of the DFB resonator without introducing any additional insulating layers for the lasing performance. In other words, the nanoimprinted ITO film is not only a functional layer contributing to the lasing operation but also an electrode to obtain voltage-induced reorientation of LCs. This tuning behavior of Fig. 3(a) and 3(b) results from the fact that the effective refractive index of the guided mode, that is, neff increases by the voltage-induced reorientation of LCs. Hence, the external voltage across the DFB resonator reorients LCs originally aligned along the x-axis to the z-axis (Fig. 2(a)), decreasing nLC from ne to no for TE-polarized modes. With decreasing of nLC in the DFB resonator, the lasing wavelength can shift continuously to shorter wavelengths in the same waveguide mode for the decrease of neff, or can also switch to longer wavelengths in lower waveguide modes for the increase of neff [5,6]. Thick thickness of the LC laser with the nanoimprinted ITO electrode allows high-order TE modes in the DFB resonator so that we can obtain 6 nm switch of lasing peak into a longer wavelength of a lower-order TE mode by increasing the effective refractive index, as shown in Fig. 3(a) and 3(b). Emission spectra at 7.0 V, 7.5 V, and 8.0 V in Fig. 3(b) clearly indicate not continuous shift but the switch of lasing emissions, corresponding to a change in the order of guided modes, by applying external voltage. From the Bragg condition with md = 2 and Λ = 364 nm, experimentally determined from the AFM image, we obtained neff = 1.667 for λlaser = 607 nm and neff = 1.684 for λlaser = 613 nm, respectively. Numerical simulations were also carried out to calculate neff of laser waveguide modes by using commercial software MODE Solutions [17]. Figure 3(c) shows simulated intensity profiles of the TE1 and TE0 modes for five-layered slab waveguide consisting of a substrate with refractive index ns = 1.500, an ITO layer (nITO = 1.850, dITO = 150 nm), a LCs layer (nLC, dLC = 12 μm), an ITO layer (nITO = 1.850, dITO = 200 nm), and a substrate (ns = 1.500). Only difference between two waveguide modes of Fig. 3(c) is the refractive index of the LC layer, nLC, where the change of nLC corresponds to reorientation of LCs induced by external voltages. In the Fig. 3(c), the decrease of nLC in the DFB resonator from 1.607 to 1.517 results in increasing neff from 1.667 for the TE1 mode to 1.686 for the TE0 mode. The tuning range of lasing wavelength with Λ = 364 nm is 6.9 nm calculated from the Bragg equation. These calculated values of nLC are in close agreement with experimental parameters, ne = 1.63 and no = 1.50, considering slight differences of the layer thickness due to the nanoimprinting process, the dispersion relations of the refractive indices, and the orientation defects of the LCs in the DFB resonator.
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 520
Fig. 3. (a), (b) Lasing emission spectra of the LC laser with the nanoimprinted ITO electrode as the DFB resonator for various applied voltages. Colors represent the emission intensity obtained from the LC laser. Squares and circles correspond to lasing peaks at 607 nm and 613 nm, respectively. (c) Simulated intensity profiles of the TE1 mode with neff = 1.667 and the TE0 mode with neff = 1.686 (solid lines). The refractive indices n for each material in the device (dashed lines) are shown in the left axis.
5. Conclusion In summary, we fabricated the surface-emitting DFB laser with the nanoimprinted ITO electrode and dye-doped LCs. Because the reorientation of LCs by applying voltages across the DFB resonator brought changes of the effective refractive indices of the guided mode, lasing emission from the LC laser with the nanoimprinted ITO electrode was tuned by 6 nm at low voltage of 8.0 V. From these results, it is shown that the nanoimprinted ITO film acts as a DFB resonator, a transparent electrode, and an alignment layer for LCs simultaneously. This tunable lasing device with functional electrode give rise to various opportunities and advances in tunable laser technology. Acknowledgments This work was supported by the Basic Science Research Program through the NRF funded by the Ministry of Science, ICT & Future Planning (2012R1A1A1014948) and the Ministry of Education (2009-0094046).
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 521
Department of Energy Systems Research, Ajou University 16499, Suwon, South Korea 2 Department of Physics, Ajou University 16499, Suwon, South Korea * [email protected]
Abstract: We demonstrated electrical tunability of a liquid crystal (LC) laser using a nanoimprinted indium-tin-oxide (ITO) film as a distributed feedback (DFB) resonator, a transparent electrode, and an alignment layer for LCs. From the field-induced reorientation of LCs and changes in effective refractive indices of guided laser modes, lasing emission is tuned by 6 nm at low applied voltage of 8.0 V. This is because the LC laser with the nanoimprinted ITO electrode has no additional insulating layers for lasing performance. The present system is based on the functional electrode and its active control provides various applications and advances in laser technology. ©2016 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (160.3710) Liquid crystals; (310.7005) Transparent conductive coatings; (220.4241) Nanostructure fabrication.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
H. Kogelnik and C. V. Shank, “Stimulated emission in a periodic structure,” Appl. Phys. Lett. 18(4), 152–154 (1971). I. D. W. Samuel and G. A. Turnbull, “Organic semiconductor lasers,” Chem. Rev. 107(4), 1272–1295 (2007). G. A. Turnbull, P. Andrew, M. J. Jory, W. L. Barnes, and I. D. W. Samuel, “Relationship between photonic band structure and emission characteristics of a polymer distributed feedback laser,” Phys. Rev. B 64(12), 125122 (2001). G. Tsiminis, Y. Wang, A. L. Kanibolotsky, A. R. Inigo, P. J. Skabara, I. D. W. Samuel, and G. A. Turnbull, “Nanoimprinted organic semiconductor laser pumped by a light-emitting diode,” Adv. Mater. 25(20), 2826– 2830 (2013). S. Riechel, U. Lemmer, J. Feldmann, S. Berleb, A. G. Mückl, W. Brütting, A. Gombert, and V. Wittwer, “Very compact tunable solid-state laser utilizing a thin-film organic semiconductor,” Opt. Lett. 26(9), 593–595 (2001). M. Stroisch, T. Woggon, C. Teiwes-Morin, S. Klinkhammer, K. Forberich, A. Gombert, M. Gerken, and U. Lemmer, “Intermediate high index layer for laser mode tuning in organic semiconductor lasers,” Opt. Express 18(6), 5890–5895 (2010). R. Ozaki, T. Shinpo, K. Yoshino, M. Ozaki, and H. Moritake, “Tunable liquid crystal Laser using distributed feedback cavity fabricated by nanoimprint lithography,” Appl. Phys. Express 1(1), 012003 (2008). S. M. Jeong, N. Y. Ha, F. Araoka, K. Ishikawa, and H. Takezoe, “Electrotunable polarization of surface-emitting distributed feedback laser with nematic liquid crystals,” Appl. Phys. Lett. 92(17), 171105 (2008). S. Klinkhammer, N. Heussner, K. Huska, T. Bocksrocker, F. Geislhoringer, C. Vannahme, T. Mappes, and U. Lemmer, “Voltage-controlled tuning of an organic semiconductor distributed feedback laser using liquid crystal,” Appl. Phys. Lett. 99(2), 023307 (2011). K.-Y. Yu, S.-H. Chang, C.-R. Lee, T.-Y. Hsu, and C.-T. Kuo, “Thermally tunable liquid crystal distributed feedback laser based on a polymer grating with nanogrooves fabricated by nanoimprint lithography,” Opt. Mater. Express 4(2), 234 (2014). B. H. Wallikewitz, G. O. Nikiforov, H. Sirringhaus, and R. H. Friend, “Nanoimprinted, optically tuneable organic laser,” Appl. Phys. Lett. 100(17), 173301 (2012). D. R. Cairns, R. P. Witte II, D. K. Sparacin, S. M. Sachsman, D. C. Paine, G. P. Crawford, and R. R. Newton, “Stain-dependent electrical resistance of tin-doped indium oxide on polymer substrate,” Appl. Phys. Lett. 76(11), 1425–1427 (2000). S. H. Lee and N. Y. Ha, “Nanostructured indium-tin-oxide films fabricated by all-solution processing for functional transparent electrodes,” Opt. Express 19(22), 21803–21808 (2011). S. Y. Chou, P. R. Krauss, and P. J. Renstrom, “Imprint lithography with 25-nanometer resolution,” Science 272(5258), 85–87 (1996).
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 516
15. N. Y. Ha, S. M. Jeong, S. Nishimura, G. Suzaki, K. Ishikawa, and H. Takezoe, “Simultaneous red, green, and blue lasing emissions in a single-pitched cholesteric liquid-crystal system,” Adv. Mater. 20(13), 2503–2507 (2008). 16. M. Berggren, A. Dodabalapur, R. E. Slusher, and Z. Bao, “Light amplification in organic thin films using cascade energy transfer,” Nature 389(6650), 466–469 (1997). 17. http://www.lumerical.com.
1. Introduction An organic DFB laser is a promising approach for compact tunable laser sources in the visible spectral region due to a broad and continuous tunability, an intrinsic low threshold, and an easy processing [1–11]. Particularly, tunability of lasing emissions from organic DFB lasers has been demonstrated with various approaches based on modulations of effective refractive indices by applying voltages [7–9], temperatures [10], and lights [11]. The emission wavelength λlaser of the DFB laser depends on the grating period Λ, the diffraction order md, and the effective refractive index neff of the laser mode according to the Bragg equation: mdλlaser = 2Λneff. For md = 2, the second-order diffracted light makes the counter-propagating mode, which results in an optical feedback for lasing action. In this case, laser light can be out-coupled to the surface normal direction by first-order Bragg diffraction, and is highly TE polarized because a mode propagating perpendicular to the grating line (TE mode) is effectively Bragg scattered [2,3,8]. The ITO has been the most commonly used transparent electrode material for various photonic applications such as flat-panel displays, touch screens, and solar cells. Particularly, a sputtered ITO electrode is attractive from high transmission in visible region and good electrical conductivity although the ITO contains expensive rare elements and has poor mechanical flexibility [12]. Recently, we have fabricated nanostructured ITO films by allsolution processing of ITO nanoparticles and a nanoimprint lithography (NIL) technique, and demonstrated the electro-optic performance of LC devices with the nanostructured ITO films [13]. The NIL technique can quickly and simply reproduce feature sizes in the submicron range [14]. In this study, we use this nanoimprinted ITO film as a DFB resonator, a transparent electrode, and an alignment layer for LCs, and present a successful tuning of lasing wavelength from an organic dye-doped LC laser with the nanoimprinted ITO electrode by applying external voltage. On the basis of the voltage-induced reorientation of LC molecules, we can control the refractive index of a LC layer nLC and the resultant neff of the DFB laser with the nanoimprinted ITO electrode. For the DFB laser with a 12 μm LC layer, the spectral position of lasing emission is tuned by 6 nm at very low applied voltage of 8.0 V, compared with those of other tunable DFB lasers with LCs [7,9]. This is because the present system has no additional insulting layers for the lasing performance. Hence, the nanoimprint ITO film is a functional electrode with an appropriate structure for lasing emission. In addition, we numerically simulate the laser waveguide modes to calculate nLC and neff for tuning range of the lasing emission and, the obtained values are in close agreement with experimental results. 2. Nanoimprinted ITO electrodes for tunable LC lasers To fabricate nanoimprinted ITO electrodes for tunable LC lasers, we used an ITO ink (Ulvac Materials), which was formulated by dispersing ITO nanoparticles of less than ~20 nm in diameter into a cyclododecene solvent [13]. For a grating structure, a Si master mold with period Λ = 379 nm and grating depth dgrating = 151 nm was prepared by e-beam lithography method. In the NIL process, the master molds were converted to poly(dimethylsiloane) (PDMS; RT-601, Wacker) molds by drop-casting the PDMS materials and curing at 70°C for 30 min. Next, the ITO ink was spin-coated on the ITO-sputtered glass (ITO glass), and the patterned PDMS mold was pressed onto it at 230°C for 30 min. Figure 1(a) shows an atomic force microscopy (AFM; XE-100, Park Systems) image at the edge area of the nanoimprinted ITO electrode with Λ = 364 ± 6 nm and dgrating = 94 ± 18 nm. This AFM image clearly indicates that the grating structure of the maser mold was replicated on the ITO layer with a thickness dITO = 256 ± 10 nm. Here, we employed the ITO glass with dITO = 149 ± 5 nm and a
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 517
sheet resistance Rs = 10.7 ± 0.2 Ω/sq as a substrate. The sheet resistance Rs = 15.5 ± 0.9 Ω/sq of the nanoimprinted ITO electrode, consisting of the nanoimprinted ITO film and the sputtered ITO layer, is similar to that of the conventional ITO and is suitable for LC devices, OLEDs, and polymer solar cells. The thickness and the sheet resistance of each ITO layer were measured by using a surface profiler (P-10, KLS-Tencor) and a four-point probe meter (Model280, Four Dimensions), respectively. (c)
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(b) 100
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Fig. 1. (a) AFM image at the edge region of the nanoimprinted ITO electrode with Λ = 364 nm and dgrating = 94 nm. (b) Transmission spectra of the ITO glass and the nanoimprinted ITO film on the ITO glass. (c) Photographs of diffracted reflection lights from the nanoimprinted ITO electrode of (a) for various diffraction angles.
As shown in Fig. 1(b), we measured transmission spectra of the ITO glass and the nanoimprinted ITO film on the ITO glass by a spectrometer (V-670, Jasco). All transmission spectra for normal incidence of unpolarized light show more than ~80% transmittance in the visible region except for a characteristic dip (solid line of Fig. 1(b)) near ~593 nm by light diffraction through the grating structure of the nanoimprinted ITO electrode. In this sense, Fig. 1(c) displays photographs of diffracted reflection lights as well as high transparency from the nanoimprinted ITO electrode with the patterned area of ~10 mm × 10 mm. Here, as the diffraction angle decreased, the colors (or spectral positions) of the reflected diffraction lights changed from red (upper of Fig. 1(c)) to blue (lower of Fig. 1(c)) because of the decrease in path difference. 3. LC lasers with nanoimprinted ITO electrodes as DFB resonators We fabricated a surface-emitting DFB laser with this nanoimprinted ITO electrode as a resonator and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM) doped LCs (ZLI2293, Merck) as an active layer. The extraordinary (ne) and ordinary (no) refractive indices of the nematic LCs, ZLI2293 were 1.63 and 1.50, respectively [15]. The nanoimprinted ITO film on the ITO glass and the other ITO glass, coated with polyimide (PI; AL22620, JSR) and rubbed unidirectionally at room temperature, were stacked face-to-face with a spacer in between, and then sealed to fabricate an empty resonator, as shown in Fig. 2(a). The resonator gap was ~12 μm and the grating direction of the nanoimprinted ITO film was set parallel to the x-axis, corresponding to the rubbing direction of the PI film and the alignment direction of the LCs. Then, DCM-doped LCs (1 wt. %) was introduced into the DFB resonator by capillary action at 88 °C in the isotropic phase. In our previous work, we confirmed that the nanoimprinted ITO film could act as an alignment layer for LCs and the
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 518
electro-optic performance of LC devices with nanoimprinted ITO films was successfully demonstrated [13]. (a)
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Fig. 2. (a) Schematic illustration, (b) transmission and emission spectra at different pumping energies, (c) threshold behavior of lasing emission intensity, (d) polarized lasing emission spectra at the pumping energy of 11.6 μJ/pulse, and (e) photograph of far field emission pattern of the surface-emitting DFB laser with the nanoimprinted ITO electrode and dye-doped LCs.
For lasing experiments, a third-harmonic light from a Q-switched Nd:YAG laser (Minilite II, Continuum) was used as the optical pumping source. The wavelength, pulse width and the repetition rate were 355nm, 4ns and 10 Hz, respectively, and the spot size of the pumping beam was about 100 μm. The 355 nm light was incident on the imprinted side of the cell, as shown in Fig. 2(a). The input beam intensity was controlled by using neutral density filters and measured with a pyroelectric energy meter (PEM100, LTB). The emission from the sample was collected by a lens along the direction normal to the substrate and then detected by an optical fiber coupled to the spectrometer (HR4000, Ocean Optics). Figure 2(b) represents transmission and emission spectra from the LC laser with the nanoimprinted ITO electrode for different pumping energies. Transmission spectra of Fig. 1(b) and 2(b) exhibit the spectral red-shift of the dip positions from ~593 nm for air-filled ITO grating to ~610 nm for LCs-filled ITO grating due to the increase of refractive index in the grating trench. Below the excitation energy 8.3 μJ/pulse, the emission spectra show the clear dip near 600 nm and a small peak at the edge of the long-wavelength band. With increasing the pumping energy, a very narrow lasing peak begins to grow at 607 nm, thus indicating the onset of photonic band-edge laser operation [2,3]. Single mode lasing was clearly observed being attributed to a DFB by second-order Bragg diffraction. Next, the emission intensity at 607 nm is plotted as a function of pumping energy in Fig. 2(c). At a pumping energy about 10.1 μJ/pulse, there is a distinct kink in the curve where the LC laser with the nanoimprinted ITO electrode reaches lasing threshold followed by a linear increase in output emission with pumping energy. This threshold energy of our LC laser is slightly high than those of other efficient LC lasing systems [7,8]. We think that lower threshold energy can be achieved from the LC laser with the nanoimprinted ITO electrode, by changing a pumping wavelength into the absorption peak region of DCM [7,8] or by introducing Föster-type energy transfer into active materials [15,16]. We also observed that lasing emission from the sample is highly TE polarized, as shown in Fig. 2(d). TE- and TM-polarized emission spectra were obtained under the pumping energy of 11.6 μJ/pulse. Figure 2(e) shows a photograph of far field emission pattern of a LC laser with the nanoimprinted ITO electrode at pumping energy above the lasing threshold. The
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 519
output beam from this surface-emitting DFB laser is emitted as a divergent strip, parallel to the direction of the grating. Here, we used a 355 nm-cut filter to remove the pumping beam after the sample and obtain clear photographs. 4. Electrically tunings of lasing emissions from LC lasers with nanoimprinted ITO electrodes The possibility of tuning the lasing emissions from the LC laser via reorientation of LCs in the DFB resonator was investigated. To adjust the refractive indices of the LC layer, an external voltage (1 kHz, square wave) was applied across the DFB laser or to the nanoimprinted ITO and the other ITO electrodes. Figure 3(a) and 3(b) show lasing emission spectra from the LC laser with nanoimprinted ITO electrode for various voltages. From 0.0 V (no external voltage applied) to 7.0 V, intensities of lasing emissions at 607 nm decreased, whereas the spectral positions of the shark peaks were unchanged. At 7.5 V (Fig. 3(b)), two lasing peaks appeared at both 607 nm and 613 nm simultaneously. Also, from 8.0 V on, the peak wavelength switched up to 613 nm with increasing intensities. It is noted that the spectral shift of lasing peaks occurs at very low applied voltage of 8.0 V for a 12 μm LC layer in comparison with those of other tunable DFB systems based on LCs [7,9]. This is because we successfully demonstrate fabrication of the DFB resonator without introducing any additional insulating layers for the lasing performance. In other words, the nanoimprinted ITO film is not only a functional layer contributing to the lasing operation but also an electrode to obtain voltage-induced reorientation of LCs. This tuning behavior of Fig. 3(a) and 3(b) results from the fact that the effective refractive index of the guided mode, that is, neff increases by the voltage-induced reorientation of LCs. Hence, the external voltage across the DFB resonator reorients LCs originally aligned along the x-axis to the z-axis (Fig. 2(a)), decreasing nLC from ne to no for TE-polarized modes. With decreasing of nLC in the DFB resonator, the lasing wavelength can shift continuously to shorter wavelengths in the same waveguide mode for the decrease of neff, or can also switch to longer wavelengths in lower waveguide modes for the increase of neff [5,6]. Thick thickness of the LC laser with the nanoimprinted ITO electrode allows high-order TE modes in the DFB resonator so that we can obtain 6 nm switch of lasing peak into a longer wavelength of a lower-order TE mode by increasing the effective refractive index, as shown in Fig. 3(a) and 3(b). Emission spectra at 7.0 V, 7.5 V, and 8.0 V in Fig. 3(b) clearly indicate not continuous shift but the switch of lasing emissions, corresponding to a change in the order of guided modes, by applying external voltage. From the Bragg condition with md = 2 and Λ = 364 nm, experimentally determined from the AFM image, we obtained neff = 1.667 for λlaser = 607 nm and neff = 1.684 for λlaser = 613 nm, respectively. Numerical simulations were also carried out to calculate neff of laser waveguide modes by using commercial software MODE Solutions [17]. Figure 3(c) shows simulated intensity profiles of the TE1 and TE0 modes for five-layered slab waveguide consisting of a substrate with refractive index ns = 1.500, an ITO layer (nITO = 1.850, dITO = 150 nm), a LCs layer (nLC, dLC = 12 μm), an ITO layer (nITO = 1.850, dITO = 200 nm), and a substrate (ns = 1.500). Only difference between two waveguide modes of Fig. 3(c) is the refractive index of the LC layer, nLC, where the change of nLC corresponds to reorientation of LCs induced by external voltages. In the Fig. 3(c), the decrease of nLC in the DFB resonator from 1.607 to 1.517 results in increasing neff from 1.667 for the TE1 mode to 1.686 for the TE0 mode. The tuning range of lasing wavelength with Λ = 364 nm is 6.9 nm calculated from the Bragg equation. These calculated values of nLC are in close agreement with experimental parameters, ne = 1.63 and no = 1.50, considering slight differences of the layer thickness due to the nanoimprinting process, the dispersion relations of the refractive indices, and the orientation defects of the LCs in the DFB resonator.
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 520
Fig. 3. (a), (b) Lasing emission spectra of the LC laser with the nanoimprinted ITO electrode as the DFB resonator for various applied voltages. Colors represent the emission intensity obtained from the LC laser. Squares and circles correspond to lasing peaks at 607 nm and 613 nm, respectively. (c) Simulated intensity profiles of the TE1 mode with neff = 1.667 and the TE0 mode with neff = 1.686 (solid lines). The refractive indices n for each material in the device (dashed lines) are shown in the left axis.
5. Conclusion In summary, we fabricated the surface-emitting DFB laser with the nanoimprinted ITO electrode and dye-doped LCs. Because the reorientation of LCs by applying voltages across the DFB resonator brought changes of the effective refractive indices of the guided mode, lasing emission from the LC laser with the nanoimprinted ITO electrode was tuned by 6 nm at low voltage of 8.0 V. From these results, it is shown that the nanoimprinted ITO film acts as a DFB resonator, a transparent electrode, and an alignment layer for LCs simultaneously. This tunable lasing device with functional electrode give rise to various opportunities and advances in tunable laser technology. Acknowledgments This work was supported by the Basic Science Research Program through the NRF funded by the Ministry of Science, ICT & Future Planning (2012R1A1A1014948) and the Ministry of Education (2009-0094046).
#255012 (C) 2016 OSA
Received 2 Dec 2015; revised 29 Dec 2015; accepted 31 Dec 2015; published 7 Jan 2016 11 Jan 2016 | Vol. 24, No. 1 | DOI:10.1364/OE.24.000516 | OPTICS EXPRESS 521
