1. Optical biosensing for water quality control
This activity involves search for improved performance of optical biosensors in water. Our research is focused on plasmonic nano-photonic structures that exhibit enhancement of the electromagnetic field near the sensor-water interface. As an example we have recently demonstrated surface Plasmon resonance (SPR) sensor with enhanced sensitivity by adding a nano dielectric layer with high dielectric constant to the metal (see Lahav et.al., Optics Letter 2008). Another type of nano-photonic structures being investigated in our group is the metallic sculptured thin films (STFs) which are made of nano-columns of metal with controlled porosity and orientation. As nano-rods like structures the surface Plasmon wave is localized near their tips and hence the electromagnetic wave density is enhanced. As a result we have demonstrated recently the enhancement of fluorescence signal from fluorophores near these structures (see Abdulhalim et.al., Appl.Phys.Lett., 2009). Raman signal enhancement is another promising technique we are working on for sensing because it provides specificity, that is it can tell also the type of the pollutant not only its concentration. Since the STFs are porous then when used as SPR sensors in the Krechmann configuration they exhibit enhanced sensitivity due to the increase of the surface to volume ratio (see Shalabney et.al., J. Photonics and Nanostructures, 2009). Another promising structure for sensing is the use of resonant enhanced transmission through nano-holes in metals. Presently we are optimizing one-dimensional array of metal nanoslits for Biosensing in water (see Karabchevsky et.al., J. Photonics and Nanostructures, 2009). Our main goal in these studies is to come up with a highly sensitive and reliable sensor that can be easily integrated in water to monitor small quantities of pollutants. In parallel to the fundamental studies that we are performing, we also design and build prototype sensors that will be used in a true water purifying system.
2. Optical Bioimaging:
This activity involves developing new improved imaging methods for human tissue. Presently it is divided into three main sub-activities:
2.1 Polarimetric imaging
2.2 Spectral imaging
2.3 Full field optical coherence tomography
Skin spectro-polarimetric imaging:
For the first two sub-activities we are developing fast and miniature liquid crystal devices (LCDs) and integrating them into imaging setups. The idea behind this concept is that imaging through scattering medium such as biological tissue depends strongly on the polarization and wavelength. A cancerous tissue scatters and absorbs light differently from its surrounding. The image obtained depends both on the cancerous tissue, its surrounding, its shape and how deep it is inside the tissue layers. Therefore in order to find the best image that differentiates the cancerous tissue from its surrounding normal tissue it is important to scan all the possible polarization states and wavelengths in short time before any movements of the person or bio-changes take place. Liquid crystal devices allow this to happen within less a second. The grabbed images are processed and the resulting images with useful information are displayed. Initial setup was built including incorporation of liquid crystal retarder and tunable filter (see Aharon et.al., 2008, Abdulhalim et.al., 2007). Recently we have developed a new tunable filter with extended dynamic range that covers both the visible and the near infrared ranges (Aharon et.al., submitted to Opt.Commun. 2009) and a new polarization controller (Safrani et.al., submitted to Opt.Lett., 2009) that performs both as a linear rotator and as a tunable polarizer. The integration of these two devices into a miniature setup is going on and will be used to image skin cancer in the Department of plastic surgery at Soroka.
Full field optical coherence tomography:
In this project our goal is to build a multimodal three dimensional imaging system in real time. The system uses interference microscopy with low coherence to allow discriminating layers through tissue with high resolution. The fact that it is full field, it gives high resolution two dimensional image without xy scanning while the depth information will be obtained using the spectral domain approach in which the interference images at each wavelength are obtained and then Fourier transformed to get the depth information. There are two main problems that need to be solved before we achieve our goal: fast tunable light source, and extended depth of focus. To resolve the first problem we shall use our liquid crystal tunable filters that we are developing. We invented fast, miniature and polarization independent tunable filter (see patent by Abdulhalim, WO/2008/068753, PCT/IL2007/001497, 2007). To resolve the 2nd problem we are working on the use of annular apertures that extend the depth of focus without significant deterioration of the resolution (see Abdulhalim et.al., SPIE 2007). Another possibility is the use of liquid crystal SLMs for modifying the pupil functions to extend the depth of focus. Presently we have designed and built three types of improved FF-OCT systems in the time domain and used them for cell profiling: the Linnik system, the Mirau common path setup and the Michelson near common path setup. The next step is the incorporation of LC tunable filters to perform the spectral scanning instead of the mechanical depth scanning. The incorporation of LC devices into such setup will also allow performing different imaging modes using the same system such as polarimetric, hyperspectral, fluorescence in addition to the low coherence 3D images. This will allows gathering large amount of information on the tissue being images in the cell or even sub-cellular level thus providing a high potential pathological tool.
3. Liquid crystal devices:
Liquid crystal (LCs) devices have proved to give high quality images in display applications and as a result, interest has emerged recently to use them in non-display imaging applications. Due to their high birefringence, LCs exhibit large electro-optic effects which make them useful for a variety of other non-display applications as fast, compact tunable spectral filters, phase modulators, polarization controllers and optical shutters. They can be miniaturized thus have a great potential to be used with miniature optical imaging systems. Several novel designs for polarization independent filters and achromatic polarization controllers and their incorporation into imaging systems are issued in our group (graduate students: Ofir Aharon, Avner Safrani and Shahar Mor) and being built in our lab for incorporation into skin spectro-polarimetric imaging setup. Our main goal is the development of such improved devices in terms of speed, dynamic range, polarization independence, etc., and their incorporation into biomedical optical imaging modalities. Presently we are focused on single pixel nematic devices but we plan also to extend our activity for pixellated devices (Spatial light modulators-SLMs) and the use of ferroelectric LCs which exhibit higher speed. High dynamic tunable filters were developed (Aharon et.al., Opt.Lett. 2009, Opt. Express 2009) and polarization controllers (Safrani et.al. Opt.Lett. 2009).