Kashma Rai

Holographically formed polymer dispersed liquid crystal films for transmission mode spectrometer applications

We show proof of concept of a transmission-mode wavelength filtering device consisting of layered holographically formed polymer dispersed liquid crystal (H-PDLC) cells. H-PDLC cells were fabricated from a thiolene based polymer composite to have transmission notches in the near-IR wavelength range. Wavelength filtering was achieved by stacking four H-PDLC cells with transmission notches spaced at 10 nm intervals. Results show a broad transmission notch spanning the spectral width of the constituent cells. With bias applied to an individual cell within the stack, the transmission notch of the cell inverts and the overall transmission envelope changes shape. Using a transmitted energy sensing device and a lineshape mapping algorithm, spectral content can be determined to a resolution of 0.1 nm for narrow banded signals. Applications for this switchable wavelength filtering device include serial detection of spectral content for telecom data signals or chemical and biological sample identification through absorption or emission spectroscopy.

Order and Dynamics Inside H-PDLC Nanodroplets: An ESR Spin Probe Study

We have performed a detailed study of the order and dynamics of the commercially available BL038 liquid crystal (LC) inside nanosized (50-300 nm) droplets of a reflection-mode holographic-polymer dispersed liquid crystal (H-PDLC) device where LC nanodroplet layers and polymer layers are alternately arranged, forming a diffraction grating. We have determined the configuration of the LC local director and derived a model of the nanodroplet organization inside the layers. To achieve this, we have taken advantage of the high sensitivity of the ESR spin probe technique to study a series of temperatures ranging from the nematic to the isotropic phase of the LC. Using also additional information on the nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross section, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi-monodomain aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep their average orientation even when the layers are perpendicular to the magnetic field, suggesting that the molecular organization is dictated mainly by the confinement. This result might explain, at least in part, (i) the need for switching voltages significantly higher and (ii) the observed faster turn-off times in H-PDLCs compared to standard PDLC devices.

EPR Study of Order and Dynamics of the 5CB Liquid Crystal in an H-PDLC Device

We have studied the effects of confinement on the order and dynamics of the 5CB liquid crystal (LC) inside nanosized droplets of a reflection-mode holographic-polymer dispersed LC (H-PDLC) device, consisting of alternating LC nanodroplets and polymer layers, forming a diffraction grating. Here we have investigated, taking advantage of the high sensitivity of the EPR spin probe technique, a series of temperatures spanning the nematic and isotropic phase of the LC. The occurrence of phase separation and the consequent formation of a diffraction grating was revealed by SEM images of the H-PDLC cross section and by the presence of a reflection peak around 565 nm. Differently from the case of BL038 LC based H-PDLCs, the results here indicate the absence of an ordered fraction of mesogens throughout the analysed temperature range. Taking into account a previous model of LC molecules arrangement in the same kind of device [Bacchiocchi, C., et al. (2009). J. Phys. Chem. B 113, 5391], we postulate the presence of very small droplets in which the surface anchoring constraints represent the dominant effect. This results in the hindrance of the LC uniform macroscopic alignment, providing a plausible explanation for the malfunctioning of these devices.

Multilayer stacking technique for holographic polymer dispersed liquid crystals

We demonstrate an alternate method of stacking holographic polymer dispersed liquid crystal (HPDLC) reflection gratings on substrates coated with indium tin oxide on both sides allowing independent switching of each grating in the stack. Successive layers of the stack are formed by switching existing layers, while exposing the subsequent layer to an interference pattern. Wavefront analysis based on wavefront propagation through HPDLC with electric field on and off is used to substantiate the improvement in the reflection efficiency of the layers in the stack. Results show an optical path length reduction due to elimination of substrate layers at each grating.

Hydrostatic pressure response of polymer-dispersed liquid crystal gratings

Experimental analysis showed shifts in Bragg wavelength when examining the effects of applied hydrostatic pressure (0–10psi above ambient) on the reflection spectrum of holographic polymer-dispersed liquid-crystal Bragg gratings. With increased pressure, a spectral blueshift was observed, suggesting applications in optical pressure sensing. To analyze and quantify the observations, a Gaussian curve was fitted to the reflection spectrum of the gratings at each pressure interval. The spectral dependence on applied pressure is explained by elastic compression of the polymer sections of the Bragg planes in the reflection grating. The presented theory shows that the response of the gratings to the applied pressure is independent of the probe light incidence angle, but is linearly dependent on the ambient pressure reflection wavelength of the gratings.

Optimization of Pressure Response in HPDLC Gratings Based on Polymer Composition

This paper reports on the pressure response of Bragg gratings formed by reflective Holographic Polymer Dispersed Liquid Crystals (HPDLCs) as a function of polymer composition. Pressure response is determined by measuring the variation in reflected wavelength intensity in response to hydrostatic pressure applied parallel to the HPDLC grating vector. The pressure response of di- and tri-functional urethane polymer HPDLCs is reported here. These polymer gratings demonstrate a correlation between pressure response sensitivity and polymer functionality and polymer composition. A maximum response of 1.6 nm of wavelength shift over 20 psi of pressure variation is recorded for a di-functional polymer HPDLC.