While grating based spectrometers have become more compact and inexpensive, handheld microchip spectrometers are more desirable configurations for applications requiring portability and handheld operation. Other groups have investigated single chip microspectrometers based on a CMOS detector and numerous Fabry-Perot etalons, a self-focusing waveguide and chirped grating prism fabricated on a silicon chip, and a tunable interference filter using micro-thermal actuators to tune across the visible spectrum. These techniques have failed to gain acceptance due to persisting costs and difficulty of fabrication. Here, we address a new technique for fabricating single panel microchip spectrometers utilizing liquid crystal technology.

Short pitch ferroelectric liquid crystals (FLC) in the vertically aligned (VA) configuration have shown potential as full spectrum tunable filters for microchip spectrometer applications. VA-FLC materials have been used in a variety of other applications including display technology, rotatable waveplates, and spatial light modulators. The narrow bandwidth (< 15 nm) coupled with fast response and full visible spectral coverage enable the use of a single FLC panel and a photodiode as a microspectrometer. These cells can be fabricated to be tuned either thermally or using an applied field. In the applied field case, an in-plane switching pattern of inter-digitated electrodes is created on an indium tin oxide (ITO) coated substrate and then coated with a homeotropic polyimide alignment layer. This substrate is sandwiched with another polyimide- coated substrate with glass fiber spacers included to predetermine the cell gap (~10 microns). The cell is subsequently heated and capillary filled with FLC material to create a VA-deformed helix FLC (VA-DHFLC). In the instance of the thermally tuned device, the fabrication procedure is the same except the ITO layer is omitted. Figure 1 shows the reflection notch of a single panel FLC device being tuned across the entire visible spectrum using thermal methods while the inset showing the peak reflection wavelength as a function of applied temperature.

Extending the detector from a single photodiode to a spatially resolved CCD detector, the microspectrometer can be expanded to a spectral imaging device. As opposed to scanning grating imaging devices, a DHFLC/CCD device is inexpensive, stable, and easy to fabricate. Although the uniformity in tuning across the panel clear aperture is still being addressed, particularly in the case of in-plane switching modes, this device has great potential as both a tool for display measurement and biomedical spectral imaging application. With a particular focus on near infrared breast tumor detection, a DHFLC spectral imager has potential to enhance the field of spectral imaging due to its comparatively compact size and inexpensive cost. Noninvasive NIR images of breast tissue can be spectrally decomposed to assess potential tumors as a result of varying hemoglobin and lipid density.