1. Introduction

Organic DFB lasers have received considerable attentions because of their attractive characteristics such as spectral tunability in the visible region, low threshold energy, and easy processability [1–13]. The peak emission wavelength λ_laser of the DFB laser is determined by the Bragg condition: m_dλ_laser = 2n_effΛ, where m_d is the diffraction order, n_eff is the effective refractive index of the guided mode, and Λ is the grating period. For m_d = 2, the second-order diffracted light makes the counter-propagating mode, resulting in an optical feedback for the laser action at λ_laser. Laser light can be out-coupled to the surface normal direction by first-order Bragg diffraction. According to this relation, the lasing wavelength in organic DFB lasers can be tuned by adjusting n_eff, which can be performed by external voltages [6–9], lights [10], temperatures [11], chemicals [12], and variable thickness [13].

An ITO film deposited by sputtering has been extensively used as a transparent conducting electrode. Recently, we have fabricated a LC laser by using a nanostructured ITO film with one-dimensional (1D) periodicity as a DFB resonator, a transparent electrode, and an alignment layer for LCs [9].The nanostructured ITO film was fabricated by all-solution processing of ITO nanoparticles and a nanoimprint lithography (NIL) method [14]. From the voltage-induced reorientation of nematic LCs and resultant modulations of n_eff, lasing emission could be tuned by 6 nm at very low applied voltage of 8.0 V. However, to obtain broad tunability in such LC DFB lasers [7–9], exceptionally large changes, that is, enormous birefringence of nematic LCs are required.

To overcome these difficulties, we introduce a 2D periodically nanostructured ITO film into a DFB resonator, and demonstrate surface-emitting lasing with wide-range tuning of ~26 nm in a single LC laser. We design a wedge-shaped LC laser with gradually increasing thickness because the value of n_eff depends on the thickness as well as refractive indices of the layers comprising the LC laser. The thickness of LC layer in the DFB resonator are modulated by changing the pumping position spatially. The values of n_eff, obtained by numerical simulations of the laser waveguide modes, are in close agreement with experimental results. In addition, the lowering of the lasing threshold is achieved by introducing Förster type energy transfer between two fluorescence dyes [15, 16] in the gain medium and highly ordered dye molecules along the directors of nematic LCs. Our laser system with wide spectral tunability and low threshold can provide expansion of applications in tunable laser technology such as laser displays, recoding and reading optical storage media, and laser spectroscopies.

2. LC DFB lasers with 2D nanostructured ITO films

To obtain enhanced, 2D photonic confinement [3–5], we introduced a 2D periodically nanostructured ITO film into a DFB resonator. The nanostructured ITO film was fabricated on a quartz substrate by the NIL process of liquid suspension of ITO nanoparticles with diameter below ~20 nm (Ulvac Materials). For the two crossed grating structure (2D square lattice), a Si mater mold with period Λ = 373 nm and grating depth d_grating = 134 nm was prepared by an e-beam lithography method. The detailed experimental conditions for the NIL process is described in [9, 14]. Figure 1(a) shows an atomic force microscopy (AFM; XE-100, Park Systems) image of the 2D nanostructured ITO film with Λ = 374 ± 11 nm and d_grating = 83 ± 14 nm. This AFM image clearly indicates that the 2D periodic nanostructure of the master mold was replicated onto the ITO film. For the optical characterization of the 2D nanostructured ITO films, we measured the transmission spectrum of the 2D nanostructured film on the quartz with a spectrometer (V-670, Jasco). At the normal incidence of unpolarized light, a clear dip in the transmission spectrum of the light diffraction corresponding to the wavelength of ~532 nm was observed [black solid line of Fig. 1(b)]. The transmittance in the visible region is more than ~85% for the ITO film with the thickness d_ITO = 102 ± 15 nm [dashed line of Fig. 1(b)]. Here, the ITO film without the periodic nanostructure was also prepared by all-solution processing of the ITO suspension, whose thickness was measured by using a surface profiler (P-10, KLS-Tencor).

 

Fig. 1 (a) AFM image of the 2D nanostructured ITO film with Λ = 374 nm and d_grating = 83 nm. (b) Transmission spectra of the quartz substrate, the ITO film on the quartz, and the nanostructured ITO film on the substrate. (c) Schematic illustration of the LC DFB laser fabricated with the nanostructured ITO film. The rubbing direction of the PI film is defined as the x-axis, and the z-axis is perpendicular to the x-axis in the plane of the substrate. (d) Polar plots of polarized absorption of the LC DFB lasers doped with 1.0 wt % of DCM and 0.5 wt % DCM and 0.5 wt % C460 dyes at the absorption peak of DCM (~475 nm), respectively. 0° – 180° direction is the x-direction of the samples. (e) Chemical structures of DCM and C460 dyes.

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Figure 1(c) shows a schematic view of the LC DFB laser structure with this nanostructured ITO film. First, the 2D nanostructured ITO film on the quartz substrate and the other quartz, coated with polyimide (PI; AL22620, JSR) and rubbed unidirectionally, were stacked face-to-face with a spacer of ~13.5 μm in between, and sealed to fabricate a resonator. The rubbing direction of the PI film is defined as the x-axis, with the y- and z-axes shown in Fig. 1(c). Then, nematic LCs (MDA-99-3996, Merck) doped with fluorescence dyes were introduced into the DFB resonator at the temperature of the isotropic phase of LCs. The extraordinary (n_e) and ordinary (n_o) refractive indices of the nematic LCs, MDA-99-3996 were 1.63 and 1.50, respectively. In order to demonstrate DFB lasing with a lowered threshold, we considered the non-radiative transfer of excitation (Förster type) between fluorescence dyes in the active layer. Here, we employed two well-known dyes, 7-(diethyl-amino)-4-methyl-2H-benzopyran-2-one (Coumarin 460, C460) and 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) as a sensitizer and an emitter, respectively. The nematic LC mixture doped with 0.5 wt % of C460 and 0.5 wt % of DCM was prepared for the gain medium of the LC DFB laser with the 2D nanostructured ITO film. As a control, another LC DFB laser was fabricated with the DCM-doped LCs (1.0 wt %) instead of the C460- and DCM-doped LCs.

In order to identify the direction of the nematic LC director into the LC DFB laser with the 2D nanostructured ITO film (two crossed grating structure), we measured polarized absorption at the wavelength corresponding to the emitter absorption peak (~475 nm). Figure 1(d) shows polar plots of the rotation angle dependences of the emitter, DCM dyes in two ITO DFB resonators with the C460- and DCM- doped LCs and the DCM-doped LCs. For both samples, the directions of the maximum absorption are same as the x-axis, indicating that the LC directors (0°, x-direction), that is, the directions of LC alignment, are parallel to the rubbing direction of the PI film. The axial order parameter S [17] estimated from the dichroism of DCM in the LCs is about 0.27 for the DFB laser with the DCM-doped LCs and 0.35 for that of the C460- and DCM-doped LCs. Here, two order parameters are similar to values reported in systems fabricated using conventional alignment layers [17], and generally decreasing the concentration of the dye can induce the increase of order parameters as is observed for DCM from 0.27 for 1.0 wt % to 0.35 for 0.5 wt %. In addition, the high alignment of the fluorescence dye with the nematic LC director can provide an ideal optimization for laser action [18].

Next, we performed lasing experiments pumped by a third-harmonic light of 355 nm from a Q-switched Nd:YAG laser (Minilite II, Contimum). The pulse width and the repetition rate were 4ns and 10 Hz, respectively and the spot size of the pumping beam was about 100 μm. The emission from the sample was collected by a lens along the direction normal to the substrate [z-direction in Fig. 1(c)]. Details of the experimental setup for lasing were described in [9]. Figures 2(a) and 2(b) display transmission and lasing emission spectra from ITO DFB resonators with the DCM-doped LCs at the pumping energy of 6.9 μJ/pulse and the C460- and DCM-doped LCs at 3.1 μJ/pulse, respectively. Two transmission spectra represent the spectral shifts of the dip positions from ~532 nm for the air-filled ITO nanostructure to ~591 nm for LCs-filled nanostructures, compared with that of Fig. 1(b) (black solid line). This behavior comes from the increase of refractive index in the trench region of two crossed grating structures. In both LC lasers, single-mode lasings at the wavelength of 602 nm were clearly observed, being attributed to DFBs by second-order diffractions. From the Bragg condition with m_d = 2 and Λ = 374 nm, experimentally determined from the AFM image of Fig. 1(a), we obtained n_eff = 1.609 for λ_laser = 602 nm.

 

Fig. 2 Transmission and emission spectra at (a) 6.9 μJ/pulse and (b) 3.1 μJ/pulse and (c) threshold behavior of lasing emission intensity of two LC DFB lasers with 2D nanostructured ITO films and dye-doped LCs. (d) Photograph of far field emission pattern from the surface-emitting DFB laser of (b). (e) Simulated intensity profiles of the TE₀ mode with n_eff = 1.616 and the TE₁ mode with n_eff = 1.607 (solid lines). The refractive indices n for each material in the device (dashed lines) are shown in the left axis.

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Figure 2(c) exhibits emission intensities at 602 nm as functions of pumping energies for the DCM-doped LC laser and the C460- and DCM-doped LC laser with 2D nanostructured ITO films, respectively. These two DFB lasers clearly display threshold values: 2.22 μJ/pulse for the C460- and DCM-doped LC laser, which is lower than that for the DCM-doped LC lasers, i.e., 4.31 μJ/pulse. In the present gain medium, the energy transfer between fluorescence dyes and highly ordered dye molecules along the director of a nematic LC host, as seen in Fig. 1(d), provide a significant decrease in lasing thresholds. Moreover, the 2D DFB lasers, used in this study, are expected to exhibit a lower operation threshold than their 1D counterparts, since the optical feedback in 2D DFB resonator is stronger [3–5]. Our previous system consisting of a nanostructured ITO film with 1D periodicity (grating structure) and a DCM-doped LCs showed higher threshold energy of 10.1 μJ/pulse [9].

At pumping energy above the lasing threshold, the far field emission pattern of a LC DFB laser with the 2D nanostructured film was visualized in Fig. 2(d). The output beam from this surface-emitting DFB laser is emitted as a cross shape with a bright center spot, parallel to the directions of two perpendicular gratings. Interestingly, the emission intensity of a divergent strip, parallel to the x-axis (corresponding to the direction of the nematic LC director), is stronger than that of another strip parallel to the y-axis, as expected from the DCM dichroisms of Fig. 1(d). Experimental deviations of grating periods for the x- and y-axes, the orientation defects of the LCs, and phase retardations of the LC layer give rise to these characteristics of lasing emissions from the LC DFB laser, although anisotropic alignments of the LCs in the DFB resonator can cause different lasing wavelength for each axis.

To calculate n_eff of laser waveguide modes, we also carried out numerical simulations by using commercial software MODE Solutions [19]. Figure 2(e) shows simulated intensity profiles of the TE₀ and TE₁ modes for five-layered slab waveguide consisting of a substrate with refractive index n_s = 1.440, a PI layer with refractive index n_PI = 1.500 and thickness d_PI = 100 nm, a LCs layer with refractive index n_LC = 1.607 and thickness d_LC = 13.5 μm, an ITO layer (n_ITO = 1.890, d_ITO = 80 nm), and a substrate (n_s = 1.440). It is shown that the waveguide light of the TE₀ mode is confined in the ITO layer, whereas light intensity of the TE₁ mode is confined in the LC layer. In each case, the calculated values of n_eff are 1.616 for the TE₀ mode and 1.607 for the TE₁ mode, respectively. Here, the value of n_eff = 1.607 for the TE₁ mode is good agreement with the experimental result of n_eff = 1.609 for λ_laser = 602 nm [Fig. 2(b)].

3. Spectral tunings of lasing emissions from wedge-shaped resonators with nanostructured ITO films

To demonstrate widely tunable lasing in a single LC laser with the 2D nanostructured ITO film, we consider a wedge-shaped resonator with gradually increasing thickness. Fabrication of the C460-and DCM-doped LCs-filled DFB resonator, shown in Fig. 3(a), follows the same procedure employed to make the LC lasers of Fig. 2, except that two kinds of spacers, 8.6 μm and 13.5 μm are used to obtain the wedge-shaped structure. Hence, the thickness of LC layer in the ITO DFB resonator can be adjusted by changing the pumping position spatially. For lasing experiments, we use the same pulsed laser at 355 nm and the experimental setup as those of Fig. 2.

 

Fig. 3 (a) Structure of the wedge-shaped DFB laser with the 2D nanostructured ITO film and the C460- and DCM-doped LCs. (b) Lasing emission spectra and (c) wavelength of lasing peaks for various thickness of LC layer of the LC DFB laser. Square and circle correspond to lasing peaks at 616 nm for d_LC = 11.3 μm and 601 nm for d_LC = 12.1 μm, respectively. (d) Simulated intensity profiles of the TE₀ mode with n_eff = 1.642 and d_LC = 11.3 μm (upper) and the TE₁ mode with n_eff = 1.607 and d_LC = 12.1 μm (lower). The refractive indices n for each layer in the device (dashed lines) are shown in the left axis.

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Figures 3(b) and 3(c) display lasing emission spectra and peak wavelengths of them from the LC DFB laser with nanostructured ITO film for various thickness of LC layers, that is, d_LC. The full width at half maximum (FWHM) of lasing peaks were measured to be ~1 nm, limited by the resolution of our spectrometer. Upon spatial changes of the pumping positions (corresponding to variations in thickness of the LC layer), a drastic spectral tuning in lasing peaks, as large as ~26 nm, occurs from 590 nm for d_LC = 12.3 μm to 616 nm for d_LC = 11.3 μm. In our electrically tunable system with the nanostructured ITO film and dye-doped LCs, lasing emission was tuned ~6 nm at low applied voltage of 8.0 V [9]. From d_LC = 10.8 μm to d_LC = 11.3 μm, the lasing wavelength go through a continuous red-shift of ~9 nm and at d_LC = 11.4 μm, the peak wavelength switches down to 602 nm, and then, between d_LC = 11.4 μm and d_LC = 12.2 μm, the spectral shift is smooth. Also, the peak wavelength switches down to 591nm at d_LC = 12.3 μm, from which the spectral red-shift of lasing emissions is smooth again. It is clear that the increase in the thickness of the LC layer results in continuous spectral red-shifts as well as switches of lasing emissions. In addition, we could observe almost same tuning characteristics from five LC DFB lasers with same experimental conditions.

This tuning behavior comes from the change in n_eff of the guided mode by adjusting thickness of the LC layer. With increasing d_LC in the wedge-shaped DFB resonator, the lasing wavelength can shift continuously to longer wavelengths in the same waveguide mode for the increase of n_eff, or can also switch to shorter wavelength in lower waveguide mode for the decrease of n_eff [3, 13]. Hence, continuous red-shifts and switches of lasing peaks in Fig. 3(c), correspond to increases of n_eff in the same waveguide mode and decreases of n_eff in lower waveguide mode, respectively. Thick thickness of the LC laser above ~8.6 μm allows high-order TE modes in the DFB resonator so that we can obtain 14 nm (from 616 nm to 602 nm) and 12 nm (from 603 nm to 591 nm) shifts of lasing peaks into lower wavelengths of higher-order TE modes by decreasing n_eff. This switching of waveguide modes or lasing wavelengths results from increases of the optical confinements and decreases of the material gain (DCM) for the long wavelength side. Furthermore, the lasing threshold energy and its slope efficiency for each lasing emission can also be determined by the material gain and the optical confinements of the given DFB resonator [13]. In our LC system, these lasing characteristics match well with the previously measured DCM gain spectrum [16]. From the Bragg condition with m_d = 2 and Λ = 374 nm, we obtained n_eff = 1.647 for λ_laser = 616 nm [d_LC = 11.3 μm, square in Fig. 3(c)] and n_eff = 1.608 for λ_laser = 601 nm [d_LC = 12.1 μm, circle in Fig. 3(c)].

Next, Fig. 3(d) shows intensity profiles of the TE₀ and TE₁ modes simulated for the slab waveguide structure with d_LC = 11.3 μm and d_LC = 12.1 μm, respectively. Here, the other simulation conditions are the same as those of the LC DFB laser in Fig. 2(e) and differences are d_LC and a slight increase in the thickness of the ITO layer (d_ITO = 110 nm) due to the nanoimprinting process. In Fig. 3(d), the only increase of d_LC in the DFB resonator from 11.3 μm to 12.1 μm results in decreasing n_eff from 1.642 for the TE₀ mode to 1.607 for the TE₁ mode. These calculated values are in close agreement with experimental results such as n_eff = 1.647 for λ_laser = 616 nm and n_eff = 1.608 for λ_laser = 601 nm in Fig. 3(c), considering slight deviations of the layer thickness mentioned above and the dispersion relations of refractive indices in the LC laser with the nanoimprinted ITO film.

4. Conclusion

In summary, we investigated the surface emitting DFB lasers with the 2D nanostructured ITO films and dye-doped LCs. From the optical measurements for the LC lasers, it was proven that the energy transfer process between two organic dyes and highly ordered dye molecules in the LC host, together with strong optical feedbacks in 2D DFB resonators, reduce the lasing threshold. Moreover, we successfully demonstrated wide spectral tuning of laser emissions of ~26 nm in a single LC laser with a wedge-shaped ITO resonator. From these results, it is shown that the LC DFB laser with the nanostructured ITO film is an important system for practical applications, which provides various possibilities in tunable laser technology.