Joerg Martini

Hyperspectral imaging with a liquid crystal polarization interferometer.

A novel hyperspectral imaging system has been developed that takes advantage of the tunable path delay between orthogonal polarization states of a liquid crystal variable retarder. The liquid crystal is placed in the optical path of an imaging system and the path delay between the polarization states is varied, causing an interferogram to be generated simultaneously at each pixel. A data set consisting of a series of images is recorded while varying the path delay; Fourier transforming the data set with respect to the path delay yields the hyperspectral data-cube. The concept is demonstrated with a prototype imager consisting of a liquid crystal variable retarder integrated into a commercial 640x480 pixel CMOS camera. The prototype can acquire a full hyperspectral data-cube in 0.4 s, and is sensitive to light over a 400 nm to 1100 nm range with a dispersion-dependent spectral resolution of 450 cm(-1) to 660 cm(-1). Similar to Fourier transform spectroscopy, the imager is spatially and spectrally multiplexed and therefore achieves high optical throughput. Additionally, the common-path nature of the polarization interferometer yields a vibration-insensitive device. Our concept allows for the spectral resolution, imaging speed, and spatial resolution to be traded off in software to optimally address a given application. The simplicity, compactness, potential low cost, and software adaptability of the device may enable a disruptive class of hyperspectral imaging systems with a broad range of applications.

Flow Cytometry on a Chip

Flow cytometers are indispensable tools in medical research and clinical diagnostics for medical treatment, such as in diagnosing cancer, AIDS, and infectious diseases. The cost, complexity, and size of existing flow cytometers preclude their use in point-of-care (POC) diagnostics, doctor’s offices, small clinics, on-site water monitoring, agriculture/veterinary diagnostics, and rapidly deployable bio-threat detection. Here, we present a fundamentally new design for a flow cytometer for the POC that delivers high effective sensitivity without complex optics or bulky, expensive light sources. The enabling technology is spatially modulated emission, which utilizes the relative motion between fluorescing bioparticles and a selectively patterned environment to produce time-modulated signals that can be analyzed with real-time correlation techniques.

Opto-fluidic detection platform for pathogen detection in water

We prototyped a compact micro-fluidic based flow cytometer for pathogen detection in water. The system uses a pin-photodiode for detection and yielded a detection limit of ~200MEPE, sufficient to reliably identify and count specifically-tagged pathogens.

Opto-fluidic detection system enabling sophisticated point-of-care diagnostics

Most biomedical tests are performed at large clinical laboratories because compact, robust, and inexpensive instruments for point-of-care (POC) testing are simply not available. Yet there is a need for POC testing. Optofluidic systems for fluorescence detection of bio-particles offer high performance, but they cannot meet POC specifications. We have demonstrated, prototyped, and benchmarked against commercial systems a new optical detection approach that delivers high signal-to-noise discrimination — without complex optics or bulky excitation sources. It therefore enables a truly compact, low-cost, high-performance microfluidic-based instrument that can be used for CD4 diagnostics on whole blood and for other complex biological fluids.

Accurate glucose detection in a small etalon

We are developing a continuous glucose monitor for subcutaneous long-term implantation. This detector contains a double chamber Fabry-Perot-etalon that measures the differential refractive index (RI) between a reference and a measurement chamber at 850 nm. The etalon chambers have wavelength dependent transmission maxima which dependent linearly on the RI of their contents. An RI difference of ▵n=1.5·10-6 changes the spectral position of a transmission maximum by 1pm in our measurement. By sweeping the wavelength of a single-mode Vertical-Cavity-Surface-Emitting-Laser (VCSEL) linearly in time and detecting the maximum transmission peaks of the etalon we are able to measure the RI of a liquid. We have demonstrated accuracy of ▵n=±3.5·10-6 over a ▵n-range of 0 to 1.75·10-4 and an accuracy of 2% over a ▵nrange of 1.75·10-4 to 9.8·10-4. The accuracy is primarily limited by the reference measurement. The RI difference between the etalon chambers is made specific to glucose by the competitive, reversible release of Concanavalin A (ConA) from an immobilized dextran matrix. The matrix and ConA bound to it, is positioned outside the optical detection path. ConA is released from the matrix by reacting with glucose and diffuses into the optical path to change the RI in the etalon. Factors such as temperature affect the RI in measurement and detection chamber equally but do not affect the differential measurement. A typical standard deviation in RI is ±1.4·10-6 over the range 32°C to 42°C. The detector enables an accurate glucose specific concentration measurement.

Glucose concentration monitoring using a small Fabry-Pérot etalon

Accurate measurements of aqueous glucose concentrations have been made in a double-chamber Fabry-Perot etalon that can be miniaturized for subcutaneous implantation to determine the concentration of glucose in interstitial fluid. In general, optical approaches to glucose detection measure light intensity, which in tissue varies due to inherent scattering and absorption. In our measurements, we compare the spectral positions of transmission maximums in two adjunct sections of an etalon in order to determine the refractive index difference between these sections and therefore we can tolerate large changes in intensity. With this approach, we were able to determine aqueous glucose concentrations between 0 mg/dl and 700 mg/dl within the precision of our reference measurement (+/-2.5 mg/dl or 2% of the measurement value). The use of reference cavities eliminates interference due to temperature variations, and we show the temperature independence over a temperature range of 32 degrees C to 42 degrees C. Furthermore, external filters eliminate interference from large molecule contaminants.