טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentWolowelsky Karni
SubjectDynamic Spectral Response
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Carmel Rotschild
Full Thesis textFull thesis text - English Version


Abstract

In this thesis we studied, and experimentally demonstrated, methods to control spectral responses by applying electrical field. We explored the feasibility for two new applications:  (i) a dynamic color changeable pixel for displays - by controlling the concentration and energy transfer in fluorescent dyes solution. (ii) switchable notch filter (NF) for gas detection -  by modulating the absorption properties of a liquid crystal cell within the thermal infrared (IR)  spectral region in the mid and long wavelengths (MWIR, LWIR).  

The major motivation for developing a VIS color changeable pixel is the disadvantage of a mono color pixel technology which limits display's quality. Today, each pixel emits light at a fixed spectral band, while the construction of the entire visible spectrum is achieved by the combination of pixels, which restricts the spatial resolution.  In this thesis we suggest to increase the resolution by increasing the spectral dynamic range of each pixel. We experimentally demonstrate an electrical color-control method based on capacitive deionization (CDI), which changes the concentration of charged photoluminescent (PL) dyes within a liquid solution pumped by an external light source. In a single-dye solution we demonstrate modulation of the PL intensity by a factor of eight. In a two-dye solution, consist of a neutrally charged dye, and a charged one, we modulate the latter's concentration, and demonstrate a 4-fold change in the colors intensity ratio.  We also experiment on controlling the Forester energy transfer (FRET) efficiency between an excited donor (D*) and an acceptor (A) by modulating the D*- A distance.  The FRET efficiency has power 6 dependency on the inverse D*- A distance, which suggest on its effectiveness color control. However, we find that attractive forces between the dyes arrest FRET modulation. We envision future displays with higher resolution based on these proposed methods.

The motivation for developing a fast MWIR/LWIR NF comes from the field of optical gas Imaging (OGI), utilized in environmental applications, where most of optical devices are currently limited by complexity and cost. Here we experimentally demonstrate a low-cost, alternating bispectrality IR NF filter, based on liquid crystal's (LC) absorption-lines, which overlap those of hydrocarbon gases, and their extinction coefficients depend on the orientation of the elongated aligned liquid crystal molecules. Alternating voltage modulates the orientation and thus the transmittance in the absorption bands, giving rise to different On-to-Off intensity-ratio for Gases, and any clutter emission. Our demonstration of refrigerant gas R134a detection shows high Gas to Clutter contrast and opens the way for using this method in many low-cost applications.