This study shows the results of a photonic band-edge laser using dye-doped cholesteric liquid crystals (CLCs) combined with silver (Ag) nanoparticles. When the Ag nanoparticle surface plasmon resonance wavelength matched the excitation source wavelength, the large optical fields provided by surface plasmons increased the fluorescence of dye molecules by enhancing the molecular excitation rate, achieving a low lasing threshold and high pumping efficiency.
©2012 Optical Society of America
Cholesteric liquid crystals (CLCs) possess a helical arrangement of the directors by adding chiral molecules into nematic liquid crystal (LC) media. The ordered structure with a periodic dielectric constant causes a photonic bandgap, at edges of which the group velocity of light approaches zero and leads to the divergence of the density of photon states. As the embedded active dyes are excited, the rate of spontaneous emission is enhanced, and low-threshold lasing occurs at the bandgap edges . Strategies to improve the pumping efficiency and lower the lasing threshold of the CLC laser have been reported frequently [2–9]. Fluorescence enhancement may provide a potentially crucial solution. Metal nanoparticles have been found to enhance fluorescence because of the localized surface plasmon interaction with fluorescent molecules [10,11]. The enhancement phenomenon has many applications in biological sciences [12,13], light-emitting devices [14,15], and random lasers [16,17].
This study shows the results from a photonic band-edge laser using dye-doped CLCs combined with silver (Ag) nanoparticles. Light incident onto a Ag nanoparticle could induce localized surface plasmon resonance, enhancing the electromagnetic field on the Ag particle surface. When the Ag nanoparticle surface plasmon resonance wavelength matches the wavelength of the excitation source, the large optical fields provided by surface plasmons increases the fluorescence intensity of dye molecules by enhancing the molecular excitation rate, achieving a low lasing threshold and high pumping efficiency.
2. Sample fabrication
The dye-doped CLC samples used in this study were assembled with two indium tin oxide (ITO) glass substrates. One was coated with a polyvinyl alcohol (PVA) layer and rubbed for homogeneous alignment. The other was coated with a Ag film using a sputtering system to produce an average thickness of 71 nm. The Ag film was then nucleated by annealing to form Ag nanoparticles on the substrate surface. Table 1 lists four substrates at different annealing temperatures for 10 min. The surface status of fabricated Ag nanoparticles was confirmed using scanning electron microscopy (SEM). The CLC host was prepared by mixing 21.8 wt % of chiral agent S811 (Merck) in nematic LC host MDA3970 (Merck). The extraordinary and ordinary refractive indices of MDA3970 were 1.6309 and 1.4987, respectively, at a wavelength of 589.3 nm and a temperature of 20 °C. The helical twist power of S811 was 11.9 μm−1 at 20 °C for MDA3970. The active dye used to emit fluorescence in this study was the laser dye DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran, Exciton), dissolved into the CLC host at a concentration of 0.5 wt %. The mixture was capillary filled into empty cells with a 15 μm gap to form planar dye-doped CLC Samples A, B, C, and D with Substrates A, B, C, and D (Table 1), respectively. Another conventional dye-doped CLC Sample E without Ag nanoparticles was fabricated for comparison. Figure 1 schematically depicts the profile of the samples.
3. Results and discussion
Figure 2 shows the SEM images of the Ag nanoparticle layer. At a fixed deposited film thickness, the average diameter of the Ag nanoparticles expands and the density of particles decreases as the annealing temperature increases. Figure 3 presents the particle size distribution of Substrates A, B, C, and D. The average diameter and density of particles are presented in Table 2 . Figure 4 shows the absorption spectra of Ag nanoparticles measured with a spectrometer (Ocean Optics 2000) and the substrate appearance at various annealing temperatures. The surface plasmon resonance wavelength determined by the wavelength at the absorption peak was red-shifted with the increase in the size of Ag nanoparticles, being approximately 450, 500, 520, and 550 nm for Substrates A, B, C, and D, respectively. The plasmonic response of a metal nanoparticle to illumination can be varied as the dielectric property of the environment is altered . The surface plasmon resonance wavelengths were measured at approximately 480, 530, 550, and 580 nm, respectively, for Substrates A, B, C, and D in the surroundings of the CLC host used in this study.
Figure 5 shows the sample absorption spectra, which consist of the absorption caused by DCM and Ag nanoparticles and the Bragg reflection of the CLCs, located between 580 and 627 nm. The red-shift of the surface plasmon resonance wavelength caused the absorption in the Bragg reflection band to increase monotonically. The maximum absorption at an excitation wavelength of 532 nm occurred in Sample B, the surface plasmon resonance wavelength of which matched the excitation wavelength.
Figure 6 displays the fluorescence spectra of the samples pumped by a 532 nm continuous light from a diode-pumped solid-state laser, measured with a spectrometer by PVA-side excitation and PVA-side detection. Fluorescence was enhanced before reaching the maximum in Sample B, and then decreasing with the red-shift of the surface plasmon resonance wavelength of the Ag nanoparticles. The fluorescence emission rate γem for a dye molecule placed near a metal nanoparticle can be expressed as19]. The excitation light incident onto Ag nanoparticles induced the conduction electrons to oscillate coherently with the oscillating electric field. The collective oscillation of the electrons occurring at the particle surfaces resulted in the significant enhancement of the local electric field intensity near the surface. The enhanced local electric field intensified the excitation rate of laser dyes in proximity to the Ag nanoparticles, thereby increasing the fluorescence intensity . The Ag nanoparticles in Sample B induced the greatest enhancement of excitation light near the surface of the particles because the surface plasmon resonance wavelength matched the excitation wavelength, resulting in maximum fluorescence intensity.
The lasing performance of the samples was examined under optical excitation using a frequency-doubled pulsed Nd:YAG laser with a wavelength of 532 nm. The pulse duration was 8 ns with a 10 Hz repetition rate. A lens with a 20 cm focal length was used to focus the pumping energy on the PVA side of the samples at an incident angle of 25° from the surface normal. The relative lasing intensities were recorded from the PVA side of the samples using a spectrometer. Figure 7 shows the output lasing intensity at the long wavelength band edge of 627 nm from the samples as a function of the pumping laser energy. Pumping efficiency was increased in conjunction with the surface plasmon resonance wavelength, before reaching a maximum in Sample B because of the maximum fluorescence intensity, and then decreasing. The slope efficiency of Sample B increased by a factor of 1.6 when compared with that of Sample E. The lasing threshold decreased from 1.6 μJ/pulse for the conventional Sample E to 1.0 μJ/pulse for Sample B with the maximum fluorescence enhancement. The decrease in lasing threshold and the increase in pumping efficiency compared with the conventional CLC sample can be understood as the increase in the dye molecule fluorescence emission rate caused by a larger electric field of the excitation light resulting from the surface plasmon resonance . Figure 8 displays the CLC lasing patterns from Samples B and E on the screens placed at a distance of 7 cm behind the samples. The lasing brightness from Sample B with Ag nanoparticles was clearly greater than that from the conventional Sample E. The lasing beams exhibited a large divergence resulting from weak alignment. The half divergence angle of the lasing beam from Sample B was measured at 10°. The lasing spectra were shown in Fig. 9 .
This study shows the results of CLC lasing enhancement using surface plasmons. The lasing properties depend significantly on the surface plasmon resonance wavelength of the Ag nanoparticles on the substrate, determined by the size of Ag nanoparticles. The lasing threshold and pumping efficiency were reduced and increased, respectively, as the surface plasmon resonance wavelength of Ag nanoparticles matched the wavelength of the excitation source.
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Grant Number NSC 100-2221-E-327-023.
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