This work demonstrates a thermally tunable dye-doped liquid crystal (DDLC) distributed feedback (DFB) laser based on a polymer grating. The surface emitting DDLC DFB laser is supported by the second-order Bragg grating with nanogrooves of periodicity of 360 nm which is pre-fabricated using the UV nanoimprint lithography (UV-NIL) method. The lasing wavelength of the DFB laser can be tuned from 625.1 nm to 606.35 nm if the temperature increases from 10 °C to 50 °C. The tunability of the laser is attributable to the temperature-sensitive feature in the effective refractive index of the LCs. Such a tunable laser can be used in the fabrication of tunable optical sources and biosensors, and in integrated photonic circuits, optical communication, and laser displays.
© 2014 Optical Society of America
In 1971, distributed feedback (DFB) lasers were first demonstrated by Kogelnik and Shank in experiments and theoretical simulation based on the couple-wave theory . In mirrorless lasers, the optical feedback supported by the diffraction of Bragg grating relies on the periodic modulation of either the effective refractive index and/or the optical gain coefficient of the grating. The couple of the forward and backward waves is continuous throughout the DFB resonator, and the Bragg wavelength is determined by the Bragg condition,
In the last decade, tunable DFB lasers have attracted significant attention because of their great promise in the application of modern technologies such as tunable optical sources, bio-sensors and optical communication. According to the Bragg condition, the emission wavelength can be tuned by adjusting the periodicity or effective refractive index of the grating. Several DFB lasers which are tunable by changing the grating period have been demonstrated using methods of electrically or mechanically stretching the grating periodicity [2–7] and transient two-beam holography [8,9], and in a spatial gradient of film thickness or grating period [10–12].
Because of the high modulability of the refractive index of liquid crystals (LC), incorporating LC material into the DFB resonant cavity is one of the most promising ways to effectively tune the emission wavelength of the DFB laser. In the past, a few groups have investigated electrically tunable DFB lasers based on various configurations of the DFB cavity system by electric-field-induced LC reorientation [13–15]. So far, however, no one has yet developed a thermally tunable DFB laser based on the temperature sensitivity of the refractive index of the incorporated LCs. This work successfully develops a thermally tunable dye-doped LC (DDLC)/polymer-grating DFB laser based on an LC-covered polymer grating with nanogrooves which is fabricated using UV nanoimprint lithography (UV-NIL). The lasing wavelength of the surface-emitting DFB laser can be tuned from 625.1 nm to 606.35 nm by varying the temperature from 10 °C to 50 °C. This thermal tunability of the laser is attributable to the temperature sensitivity of the effective refractive index of the LCs in the laser.
2. Sample fabrication and experimental setup
Figure 1(a) shows the schematic structure of the DDLC/polymer-grating DFB laser. The sample mainly consists of three layers: the polymer grating mold, DDLC, and rubbed-polyimide (PI) from the top to the bottom between two glass substrates (ng ~1.52). The UV-NIL method used to fabricate the polymer grating mold is described as follows. First, a grating, with nanogrooves of periodicity of Λ = 360 nm, a depth of h = 150 nm and a duty cycle of 0.5, is pre-patterned onto a silicon wafer. The total area of the grating is 600 μm × 600 μm. For grating replication, a UV-curable monomer (NOA81, Norland Co., np ~1.56 after UV-polymerization) is injected and diffuses into the entire assembled cell of the pre-patterned silicon mold and a bare glass substrate. The monomer in the cell is cured to photo-polymerize under UV irradiation of 30 mW/cm2 for 10 mins, and the formed polymer grating and the silicon mold are then separated. The surface profile of the polymer grating measured by atomic force microscopy (AFM) is shown in Fig. 1(b), in which the depth of the formed polymer grating is quite uniform and is around 150 nm. Based on Berreman’s groove theory , the anchoring energy of the polymer grating is of the order of 10−4 J/m2, the energy of which is strong enough to induce a uniform alignment in a thin LC layer (e.g., a 7-μm-thick LC layer in the present work) parallel to the grating trench. A uniform alignment of LCs is necessary for obtaining good quality wave guiding and coupling and thus of the lasing emission of the DDLC/polymer-grating DFB laser .
To acquire the tunable DFB laser, an empty cell is pre-fabricated by combining one glass substrate pre-coated with a polyimide (PI) layer and the other with the polymer grating which is pre-obtained through the steps described above. Two plastic spacers with a thickness of d = 7 μm are placed between the two substrates to control the cell gap. The rubbing direction (R) of the PI layer on the glass substrate is set parallel to the nanogrooves of the polymer grating. The uniform mixture of DDLC is then injected and diffuses into the entire empty cell via capillary action. The image of the DDLC/polymer-grating DFB resonator observed under the POM with crossed polarizers is shown in Fig. 1(c), in which the transmission axis of one of the polarizers is set parallel to R and perpendicular to the grating vector (G). The dark state in a part of the grating region within the dotted rectangle indicates that a uniform alignment of LCs in the DFB resonator can be obtained under the anchoring of the two boundary surfaces with the rubbed-PI layer and the nanogrooves of the polymer grating. The abovementioned DDLC mixture includes a laser dye (DCM, Exciton) and a nematics (E7, Merck). The extraordinary and ordinary refractive indices for E7 at the wavelength of 620 nm and 26 °C are 1.732 and 1.519, respectively. The nematic phase of E7 is in the temperature range from 10 °C to 60 °C. The concentration of the laser dye (4-Dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), Aldrich) in the DDLC is about 1.0 wt%.
The pump source to excite the DDLC/polymer-grating DFB laser is a second-harmonic-generation (SHG) Q-switch Nd:YAG pulse laser (Surelite II, Continuum Co.). The incident pumped pulse laser beam with a linear polarization parallel to the groove trench (parallel to the y-axis) and a wavelength, a pulsed duration and a repetition rate of 532 nm, 6 ns and 10 Hz, respectively, is focused by a lens (focal length: 10 cm) to stimulate the grating region of the DFB laser from the side of the polymer grating. The incident angle of the pump beam to excite the DDLC/polymer-grating cell is around 20°. The pumped energy of the pump beam can be measured by an energy meter (NOVA II, Ophir). The second-order DFB lasing signal from the pumped DFB laser along the cell normal (Fig. 1(a), parallel to x-axis) is collected by an optical fiber based spectrometer (USB4000, Ocean Optics, optical resolution: ~1.0 nm). The DDLC/polymer-grating DFB laser sample is set up on a hot-stage (T95-PE, Linkam Scientific Instruments) for examining the associated temperature-dependent lasing experiments.
3. Results and discussion
Before the performance of the lasing-associated experiments, the fluorescence emission and absorption spectra of the laser dye in isotropic E7 were measured. The experimental results are shown in Fig. 2. The peak for the fluorescence emission spectra (red curve) is located at around 600 nm, and the absorption band of DCM in the absorption spectrum (blue curve) at the long-wavelength side extends to about 560 nm.
In lasing-associated experiments, the lasing features of the DDLC/polymer-grating DFB laser at room temperature (~26 °C) are first measured. Figure 3(a) presents the obtained fluorescence emission spectrum of the DFB laser at E = 25 μJ/pulse. The emission spectrum shows a lasing peak with a narrow full-width at half-maximum (FWHM) of ~1.3 nm and a central wavelength of ~619.7 nm. The inset in Fig. 3(a) presents the variations in the fluorescence emission intensity of the DFB laser with the pumped energy, which implies the lasing threshold (Eth) is about 14 μJ/pulse. Because the pumped energy of 25 μJ/pulse is higher than Eth, the lasing as shown in Fig. 3(a) can be acquired. In addition, the experimental results also show that the polarization of the lasing output is linear parallel to the grating nanogrooves.
Based on the waveguide theory , the DDLC/polymer-grating DFB resonator may guide multiple modes of light to propagate in the LC guiding layer. The lasing-associated experimental result shown in Fig. 3 presents that only a TE guided mode is involved in the single-peak lasing occurrence. The lasing peak must be a fundamental guided mode because the losses for the higher-order modes are all higher than those for the fundamental mode. The calculated value of the effective refractive index of the fundamental guided mode in the LC/polymer-grating waveguide can be obtained to be around 1.732 based on the waveguide theory. In addition, substituting the lasing wavelength of 619.7 nm into the Bragg condition and setting m = 2, the real value of the effective reflective index is around 1.72. The discrepancy between the calculated and real values of neff is attributable to the reason that the LCs in the waveguide are not very well-aligned along the y axis, especially those regions far away from the boundary surfaces of the waveguide. Figure 3(b) further shows the surface-emitting red lasing emission of the DFB laser along the cell normal after the pumped pulses excite the DDLC/polymer-grating DFB sample. The lasing emission displays a narrow oval-shaped pattern with a long-axis parallel to the grating nanogrooves. The oval-shaped lasing spot is generated due to the resultant emission of lasing peaks for the guided modes not only propagating along the grating vector but also propagating laterally at oblique angles with respect to the grating vector .
The following measurement shows the temperature dependence of the lasing wavelength of the DFB laser in the temperature range from 10 °C to 50 °C, which is ensured LC to be in nematic phase. Figure 4(a) presents the thermal tuning property of the DFB laser at E = 25 μJ/pulse. When increasing the temperature from T = 10 °C to T = 50 °C, the lasing peaks have blue-shift from λ = 625.1nm to λ = 606.35nm. Figure 4(b) shows the variation of the lasing wavelength of the DFB laser with the temperature, based on the summary of the experimental data in Fig. 4(a). The thermally tunable range of the laser in the lasing wavelength is around 19 nm if the temperature increases from 10 °C to 50 °C. The obtained result for the negative dλ/dT of the lasing peak shown in Fig. 4 is attributed to the decrease in the effective refractive index in the waveguide due to the decrease in the extraordinary index of the LCs with increasing temperature ; that is, negative dne/dT and dneff/dT. The negative dneff/dT is confirmed by the simulation based on waveguide theory . The simulation of the variation of the neff for fundamental TE mode in LC guiding layer with temperature from 10 °C to 50 °C is displayed in Fig. 5.Apparently, the theoretical value of neff of LC layer (without the dopant of laser dye) with 7μm-thickness decreases ideally from 1.750 to 1.684 with increasing temperature from 10 °C to 50 °C. In addition, the relative intensity (energy threshold) of the lasing emission increases (decreases) from 10 °C to 30 °C and then decreases (increases) from 30 °C to 50 °C. This phenomenon can be explained by two dominant effects. The first effect is that the fluorescence emission intensity increases with the decreasing of the lasing emission wavelength corresponding to the temperature from 10 °C to 30 °C (Fig. 4(a)). The second effect is that the absorption of laser dye in E7 decreases with the increasing temperature from 30 °C to 50 °C. To confirm the second effect, the measurements of absorption and fluorescence emission intensity of the laser dye (1.0 wt%) doped in E7 from T = 10 °C to T = 60 °C are performed, as shown in Fig. 6.Apparently, the fluorescence emission intensity decreases as the temperature increases in the entire emission wavelength range, especially, from 30 °C to 50 °C. This is mainly due to the evident decreasing of absorption of the laser dye doped in E7 with the increasing temperature from 30 °C to 50 °C because of the decreasing of molecular order of LC and thus of the laser dye with the increasing temperature.
Figure 7 shows the simulation of the intensity distribution of the electric field for the fundamental TE resonant mode at the 2nd Bragg grating condition in the structure of glass/DDLC/polymer grating/glass at ne = 1.736 (T = 10 °C), Λ = 360 nm, h = 150 nm, d = 7 μm and a duty cycle of 0.5. This simulation is based on the Finite-Difference Time-Domain (FDTD) method with Bloch periodic boundary condition in the horizontal direction and perfect matching layers in the top and bottom region. In the spectral analysis (not shown herein), other higher order TE waveguide resonant modes at the same 2nd Bragg grating condition also exist at shorter wavelengths. The allowed mode number is bounded by the cutoff condition determined by the multilayered waveguide width. The intensity distribution reveals that the fundamental resonant mode is almost confined in the DDLC layer and suffers less grating scattering loss than other higher order modes. Thus, the fundamental mode has the lowest lasing threshold. As the temperature is increasing from 10 °C to 50 °C, ne decreases from 1.736 to 1.684 in the DDLC layer; only little change with slightly less mode confinement will be occurred in the intensity distribution of the fundamental TE resonant mode. However, the lasing wavelength for the fundamental TE resonant mode at the 2nd Bragg grating condition shows the blue-shift due to the ne-dependent waveguide dispersion relation. Based on the above discussion, it might also lead to the variation of the lasing intensity and the energy threshold for the laser performance when the temperature is changed.
This work has developed a thermally-tunable DDLC/polymer-grating DFB laser. The nanogrooves of the polymer grating mold are fabricated using UV-NIL. A surface-emitting lasing output with TE-polarization can be obtained due to the amplification and DFB effect of the fundamental TE-mode guided wave in the LC guiding layer. By varying the temperature of the sample from 10 °C to 50 °C, the lasing wavelength of the DFB laser can be tuned from 625.1 nm to 606.35 nm. The negative dλ/dT is attributable to the negative dne/dT and thus the negative dneff/dT. The tunable laser can be used in the fabrication of tunable optical sources, biosensors, integrated photonic circuits, optical communication, and laser displays.
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract Nos. NSC 102-2112-M-110-007 and NSC 100-2112-M-006-012-MY3.
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