Holographic Polymer Dispersed Liquid Crystals (HPDLCs) have remarkable applications in switchable color filters, pressure sensors, reflective displays and other electro-optic elements due to their ability to reflect a particular wavelength under zero bias state and transmit that wavelength with an applied voltage. A specific application that relies on the optical switching of an H-PDLC film is a compact transmission mode spectrometer. The principle of operation of a thin film transmission mode spectrometer involves using the H-PDLC as a filter in conjunction with a detector to deconstruct the incident light spectrum. In order to fabricate a more complete system for transmission mode spectrometry, multiple wavelengths can be detected simultaneously using an H-PDLC stack. In this implementation, the H-PDLC stack consists of multiple film layers with sequential wavelength reflection characteristics adhered to form a filtration stack. Each layer maintains its individual wavelength switchability. When used to filter a transmission spectrum, the resulting optical element will scan by individually switching each layer while a detector senses the presence or absence of expected wavelengths. Time sequencing the filtration stack will result in a complete spectroscopic measurement. The complete thin film spectrometer is envisioned to have space-borne applications in sensing water vapor, carbon dioxide, and other life compounds as well as compact in-situ medical applications.

H-PDLC stacks were fabricated using a tri and hexafunctional blend of monomer, a photoinitiator, and liquid crystal mixed with a surfactant used to lower the switching voltage. The homogenous blend was spaced with 5um glass beads and pressed onto indium-tin-oxide coated glass or polyethylene terephthalate (PET). Forming films on flexible PET substrate has advantages in high optical transmittance, low sheet resistance, and narrow substrate width approximately 15% thinner than glass.

The monomer/LC mixture was exposed to a two-beam interference pattern using a Coherent Verdi 532nm laser with a power density of 64mW/cm2. The angle of the incident interference pattern determined the reflection wavelength of the grating formed, and ranged from 525nm to 650nm. A sequence of gratings was formed with the intension of creating a multi-wavelength stack. The H-PDLC samples were post cured to polymerize any remaining monomer. Layers of the stack were adhered using an index matching optical adhesive to maximize reflection efficiency of the internal layers.

Preliminary results of a three-layer stack optically characterized in transmission mode show under zero bias conditions a broad gap representing the reflected wavelengths. Individually biasing each layer showed transmission of the wavelength being switched and demonstrated the stack's ability to filter selected wavelengths for spectrometer functionality. Further results to be presented in this poster include characterization of a 10 layer stack formed on ITO glass and PET substrates. Reflected wavelength separation of the 10 films will be the full-width half-maximum of the reflection peak, approximately 15nm. The expectation is, when in zero bias mode, the combined optical element will have greater than 100nm transmission notch. As each layer or combination of layers is switched, the transmission spectrum will adjust accordingly.