Holographic Polymer-dispersed Liquid Crystal Transmission Research Articles

Single-mode lasing from dye-doped holographic polymer-dispersed liquid crystal transmission gratings

We demonstrate single-mode laser operation in dye-doped holographic polymer-dispersed liquid crystal (HPDLC) transmission gratings. The gratings are fabricated in cells made from specifically chosen glass substrates to decrease the refractive index difference between the waveguide core layer and cladding layer. The phase separation degree of liquid crystal after holographic recording is further optimized to confine only the lowest propagation mode in the device. The mode selection mechanism is explained under the framework of the waveguide distributed feedback (DFB) theory. The wavelength of single-mode lasing can be tuned between 620 and 660 nm by varying the grating period. Our results show the HPDLC technique could provide single-mode organic DFB lasers in a tunable, simple, and large-area manner.

Low threshold of distributed feedback lasers based on scaffolding morphologic holographic polymer dispersed liquid crystal gratings: reduced losses through Forster transfer

The threshold of distributed feedback (DFB) lasing from dye-doped holographic polymer dispersed liquid crystal (HPDLC) gratings was reduced using a guest–host (G–H) system. Different doping concentrations of the guest emitter 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) in the host dye 1,3,5,7,8-pentamethyl-2,6-diethylpyrromethene-difluoroborate (PM567) were dispersed in the HPDLC transmission gratings. DFB lasing was observed in the pure host, the pure guest and in the G–H system. With the increase in doping concentration of the guest, the gain spectrum was redshifted. Through effective Forster energy transfer, we found that the lasing threshold was lowered to 1.35 μJ/pulse, which reduced by factor 2 compared to that without energy transfer process. This effect is attributed to the suppression of self-absorption at the lasing wavelength.

Characterization of optically switchable holographic polymer-dispersed liquid crystal transmission gratings

This study characterizes the all-optical switching effect in holographic polymer-dispersed liquid crystal transmission gratings. The light-induced switching behaviors of these structures are due to the doped azobenzene-derived LC (azo-LC), which changes the refractive index of phase-separated LC within the polymer composite. This study also optimizes the polymer-dispersed liquid crystal formulation containing 15 wt.% azo-LC and 35 wt.% nematic LC to achieve a grating performance with a tunable diffraction efficiency of 78% and a fast switching-on time (0.5 s) with a relatively small light stimulus of 9 mW/cm 2.

Distributed-feedback lasing from dye-doped holographic polymer dispersed liquid crystal transmission grating

In this paper,we report the fabrication of dye-doped holographic polymer dispersed liquid crystal(HPDLC) transmission grating and the property of distributed-feedback lasing from dye-doped HPDLC transmission grating. Under the excitation of a frequency-doubled Nd:yttrium-aluminum-garnet laser operating at a wavelength of 532 nm, optically pumped lasing with narrow linewidth and low threshold was observed from a DCM dye-doped HPDLC transmission grating with the grating period of 586 nm. The results showed that the emitted lasing peak centered at about 603 nm with a full width at half maximum (FWHM) of only 1.4 nm, and the threshold pumping intensity was about 17.3 μJ, which is evidently lower than the previously reported values.

Effect of liquid crystal concentration on the lasing properties of dye-doped holographic polymer-dispersed liquid crystal transmission gratings

Optically pumped single-mode lasing was achieved from a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye-doped holographic polymer-dispersed liquid crystal transmission grating with various liquid crystal concentrations, which played an important role in the lasing generation and wavelength selection. With the decrease of the liquid crystal concentration, under the excitation of a frequency-doubled Nd:yttrium aluminum garnet laser operating at 532nm, the lasing wavelength was blueshifted, and the full width at half maximum of the lasing peak became narrower. The lowest threshold pumping energy was found to be about 5μJ∕pulse at a liquid crystal concentration of 19.7wt%. The lasing emission was thermally switchable due to the change of the refractive index modulation.

Low Driving Voltage and Analysis of Azobenzene Polymer DopedLiquid Crystal Grating

We mix azobenzene polymer and liquid crystal incertain ratio. Then the mixture is injected into cells. Nonlinearly photoinducedbirefringence takes place when linearly polarized ultraviolet is applied with thepattern photomask covering on the cells, which results in the formation ofazobenzene polymer doped liquid crystal grating. The obtained grating ischaracterized by an optical microscope and a He–Ne laser. The results indicate thatthe samples have clear grating structure, and the diffraction efficiencies can bemodulated by electric field. The sample driving voltage is 0.6 V/μm. It islower than the driving voltage of holographic polymer dispersed liquid crystaltransmission grating and could be matched with the driving integrated circuit.

Low-threshold and narrow-linewidth lasing from dye-doped holographic polymer-dispersed liquid crystal transmission gratings

Optically pumped lasing with low threshold and narrow linewidth was observed in a 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran dye-doped holographic polymer-dispersed liquid crystal transmission grating structure. The results showed that the lasing peak centered at about 609 nm, with a full width at half maximum of about only 1.8 nm. The threshold pumping intensity was about 120μJ under the excitation of a frequency-doubled Nd:yttrium–aluminum–garnet laser operating at a wavelength of 532 nm. Theoretical calculation showed that the lasing from this structure happened at the band edge of the photonic band gap. The lasing modes were also investigated. The transmission grating investigated enjoys a much larger gain length compared to the reflection one.

High contrast switching of distributed-feedback lasing in dye-doped H-PDLC transmission grating structures

Electrically switched distributed-feedback (DFB) lasing action is presented in a Pyrromethene 580 lasing dye-doped holographic polymer dispersed liquid crystal (H-PDLC) transmission grating structure. This design, when compared with the previously utilized H-PDLC reflection grating structure, has the advantage of a greatly enlarged gain length (10 mm) and a low concentration of liquid crystal (20%) while maintaining sufficient refractive index modulation. The experimental results demonstrate that the emitted laser bandwidth (~5 nm) can be obtained with a pump energy threshold of ~0.3 mJ at three different wavelengths, 561 nm, 569 nm and 592 nm, corresponding to three different grating spacings. The near- and far-field measurements have shown a high directionality of the lasing output. The lasing can be electrically switched off by an applied field of 30V/mum. The temporal, spectral, and output/input properties of the laser output are also presented.

Structural transitions in holographic polymer-dispersed liquid crystals.

Dynamic light scattering was used to analyze the structural and dynamic properties of nematic director field within liquid crystal domains formed in holographic polymer-dispersed liquid crystal transmission gratings. Samples prepared from two different types of prepolymer mixture: one curable with visible (VIS) and another curable with UV light were investigated. In both formulations a critical slowing down of thermal director fluctuations, signifying the second-order structural transition of the nematic director field was observed in the vicinity of some critical external electric field as well as close to some critical temperature. For VIS samples also the size and the shape of phase separated droplets and viscoelastic and surface anchoring parameters of the liquid crystalline (LC) material forming the droplets were deduced. The viscoelastic constants were found to significantly deviate from the viscoelastic parameters of the pure LC mixture.

Influence of partial matrix fluorination on morphology and performance of HPDLC transmission gratings

The morphology and the electro-optical performance characteristics were investigated in holographic polymer-dispersed liquid crystal (HPDLC) transmission gratings with partially fluorinated polymer matrices. HPDLC transmission gratings were prepared using standard UV curable monomer mixtures along with mono-functional fluorinated acrylate monomers and a nematic liquid crystal, TL203. Partial fluorination of the host polymer matrices by incorporating hexafluoroisopropyl acrylate (HFIPA) or trifluoroethyl acrylate (TFEA) in the standard formulation has been found to influence the morphological and the electro-optical properties of the resulting HPDLC transmission gratings. Significant decrease in switching voltages and higher relaxation times were observed in fluorinated HPDLCs. Conversely, an addition of methyl acrylate (MA), a non-fluorinated monomer with a similar structure in the standard formulation, resulted in an increase in the switching voltage and produced no significant change in the relaxation time in the HPDLC gratings. Presence of fluorine atoms at the polymer-liquid crystal (LC) interface not only decreased the surface anchoring strength but also influenced the orientation of LC droplet directors.