A biosensor is a device which detects the presence of antibodies or oligonucleotides (short DNA sequences). Such devices are important for medical diagnostics. For example, there exists a prostate cancer specific antibody, which if detected in a person's blood, is an early indicator of the cancer. Antibody sensing has also been applied to the detection of AIDS antibodies and to detection of food poisoning. Direct observation of a virus is a more challenging task and requires detection of short DNA sequences. DNA sensors fabricated to date are not sensitive enough to work without a preliminary chemical-amplification step, therefore we are working to develop a more precise, optically based sensor.
Tapered optical fibers are being used increasingly as light delivery and collection structures for optical biosensors. In these applications, it is desired to optimize the delivery and collection of light from a large surface area which has been made biochemically active, and where fluorescent molecules are used as the transduction mechanism for measuring biochemical activity. In these biosensor applications, an optical fiber is tapered from its normal diameter to a smaller diameter in order to increase the electromagnetic field intensity at the surface of the fiber where the optochemical transducing layer is present. This group of fiber-optic sensors rely on the evanescent field properties of the modes propagated within the wavegiude. The modal fields must be exposed in order for the intrinsic optical fiber sensor to interact with the surrounding environment. Among other advantages, the use of fiber in these applications permits the development of a compact system requiring no optical alignment and which is easy to use. Although multimode fibers are typically applied in these systems because of their larger power-transmitting capacity, they require rather bulky optical components which may easily become misaligned. We are developing single-mode fiber optic biosensors which promise to provide a more compact and robust sensing unit.
Tapered fiber
There are other interesting pictures which show light leaving the tip of an optical fiber as viewed from the side. The pictures were generated by submersing the fiber in a fluorescent dye solution and sending 785 nm laser light through the fiber. When the laser light leaves the fiber, it excites fluorescent dye molecules which re-emit light in all directions (including some toward our camera). The light from the fluorophores is emitted at 835 nm; the difference between the excitation and emission wavelengths is known as the Stokes shift.
Below you see light leaving the end of an untapered cleaved fiber. (The actual fiber optic cable is not visible in this picture. It ends where the light starts.)
Light leaving cleaved fiber
In the next picture you see light leaving the end of a tapered fiber. The beam spreads out with a wider angle from the tapered fiber than from the cleaved fiber because the effective Gaussian beam waist is smaller at the end of the tapered fiber. (Again, unfortunately, the fiber is not visible in this picture.)
These pictures highlight the Gaussian beam quality of laser light propagation.
To make a functional sensor we need to stick the fluorescent dye molecules on the tapered fiber surface. We have made one-inch long, hair-like strands of optical fiber for this purpose and shown is an image of one of these sensors as viewed from the side. The glowing along the fiber comes from dye molecules attached to the fiber surface. That is shown below.
We are currently engaged in the research and development of another highly sensitive evanescent field sensor using a biconical tapered single-mode optical fiber. The biconical tapered section of the fiber is formed by heating the fiber along its length and simultaneously pulling equally in opposite longitudinal directions . The fiber is pulled in such a way that the tapered fiber region has a waist diameter equal to or smaller than the original core diameter of the fiber (shown below). This ensures the efficient interaction of the evanescent field with the outside medium. It is important to mention that the tapered fiber should exhibit very low loss. This can be accomplished by carefully controlling the tapering angle and symmetry of the tapered portion during the pulling operation. The sensor made up of such tapered fibers are highly sensitive to very small changes in refractive index, chemical concentration, pH, fluorescence, etc.
Biconical Tapered Fiber
The goal for this research was to develop more sensitive and compact fiber-optic magnteic field sensors. If these devices can attain sensitivity on the order of 1pT/Hz1/2 and still remain compact, an array of medical applications, say, biological diagnostics and imaging become possible. The two sensor technologies studied offer different advantages. The Faraday effect sensors offer spatial resolution on the order of a few cubic millimeters with sensitivity in the low nT/Hz1/2 regime for measurements at frequencies over 500Hz and the magnetostrictive sensors offer sensivity below 1pT/Hz1/2 at frequencies over 500Hz but with typical spatial resolutions on the order of cubic centimeters.
Our early work involved research on magneto-optic activity and we performed measurements on diamagnetic glasses and recently produced ferrimagnetic materials for magnetic field detection. The ferrimagnetic materials are still being studied for their potential in fiber-optic magnetic field sensor market. These materials offer freat sensitivity for measurement of changes in applied magnetic fields but also partially depolarize light that passes through them.
We studied the sensitivity and the magnitude of the depolarization effects in a number of yttrium iron garnet crystals. We also used Computer simulations to determine the detection sensitivity enhancement using ferrite flux concentrators in an optical detection configuration. Also, the effect of the flux concentrator size, shape, permeability and axial separation has been studied with respect to the effect on magnetic field detection using small ferrimagnetic crystals. A new fiber-optic magnetic field detection enhancement technique has been developed using an external cavity fiber Fabry-Perot. We also conducted experiments in heterodyne interferometry with and without ferrite flux concentrators.
Since the finesse of the Fabry-Perot is much higher than the Mach-Zehner we were able to improve the sensitivity from 42pT/Hz1/2 to approximately 2.6pT/Hz1/2. This was carried out in a frequency range beyond mechanical resonance (~20kHz).
Remote measurement of electric fields with hybrid sensors using
electro-optic sensor elements at the 1V/m/Hz1/2 level. These sensors
can operate in adverse environments of high RFI, chemical pollution,
or in the presence of other high strength electromagnetic fields.
The common-mode rejection capabilities of our dual-frequency,
orthogonal-polarization, heterodyne sensors permits the operation
of these sensors at the remote end of several kilometers of single-mode
fiber.
Remote measurement of magnetic fields with hybrid sensors using magneto-optic materials. These sensors can operate at the uT/Hz1/2 level when using FR-5 Faraday glass, but much higher sensitivities will be possible with the new generation of substituted YIG materials that are beginning to become available. These sensors also possess the desirable properties of the electric field probes mentioned above in terms of their non-invasive ability to function in adverse environments. jmp Non-invasive, non-contact mapping of the birefringence pattern induced in an active GaAs device when voltages are applied. The birefringence pattern, which can be mapped with sub-mum precision, can show-up processing defects, material inhomogeneities, crystal defects, and possible or actual device failure locations. With these scanning heterodyne probes imagery can be obtained that cannot be obtained by conventional techniques, such as SEM.
Non-contact mapping of the surface profile of delicate, and high precision devices and components - such as precision UV and XUV optical components, using a vibrating sample, homodyne or heterodyne interferometric technique. The average surface height variation over regions of about 1 micrometer can be measured with a sensitivity of 0.01 picometer per root Hertz. The device or sample under study need only be vibrated with an amplitude of 10nm at an audio frequency to allow the surface slope map to be obtained.
Injection-locking of a near-IR broad-area diode laser (BAL) from a fiber-coupled master oscillator and the generation of several hundreds of milliwatt of power in a diffraction limited lobe.
Development of single-ended, single-and multi-mode fiber sensors that can monitor the electrocardiagram (EKG) in a minimally inasive way. These sensors use a voltage-sensitive dye that is placed in the tisse under study. Laser light is directed down a fiber and excites fluorescence from the dye. The returning fluorescence is monitored through a suitable filter. The EKG modulates the fluorescence intensity. These probes are being evaluated for studies of irregular heart behavior under conditions of defibrillation. Once again the ability of the probe to operate in a high strength electromagnetic field environment is advantageous.
Detection
of trace molecules in the atmosphere at the sub-ppb level. Our
system for doing this uses a photothermal technique in which absorption
of laser energy by trace species produces minute local heating,
expansion, and a subsequent refractive index change. This refractive
index change is detected at the photon-noise limit in a homodyne
interferometer. The equivalent sensitivity for measuring a refractive
index change is about 1 part in 1014/Hz1/2. These trace detection
techniques are rapid (about 1s), are relatively molecule specific,
and hold promise in such commercially important areas as pollution
monitoring and control, general atmospheric monitoring, explosives
detection, and waste disposal. In the global atmospheric monitoring
scenario, our current attention is directed at an ammonia detector
that will operate down to part-per-trillion levels.
Our coherent sensors can be used for studying defects in semiconductor devices. This will permit the study of processing techniques, material characterization, and device failure modes. In addition our ongoing work on high resolution scanning microscopy using fiber probes holds great promise in a host of practical applications. Not least of these are applications in biology. Scanning electron microscopy is widely used, but cannot study biological molecules, cells, or animals, except in a freeze-dried, metallic-coated state. STOM, and our proposed variants on it, can probe biological entities in an in-vitro state.
Our work on new, highly stable solid-state laser sources should provided improved sources for sensor applications. Injection-locked diode lasers, or diode-pumped Nd:YAG lasers are probably going to be the sources of choice in inter-satellite, and deep-space optical links at both very high data rates (>1Gbit/s) in near-Earth orbit, and at lower data rates for deep space communications.
Direct-write lithography is a straitforward application of Near-field microscopic technique. By placing a small aperture at a small distance from a surface, it is possible to create lithographically structures that are significantly smaller than the wavelength. In addition, a shear-force technique used for the tip-sample distance control allows to detect UV light induced changes in the photoresist film before photoresist development. An example of such undeveloped directly written pattern is shown in the figure below. Letters "UMCP" are the logo of the University of Maryland. This work is described in detail in Applied Physics Letters, 67, 3859-3861 (1995)
Direct-Write Lithography Image
Surface plasmons are electromagnetic waves wich propagate along some interfaces. The most known example are plasmons at the interface metal-vacuum. Near-field optical microscope is the most appropriate tool to study two-dimensional optics of surface plasmons. We use our lithographic technique to create some simple prototype plasmon optics devices, such as plasmon "flashlights" shown in the figure. This work is described in a Physical Review Letter which will be published soon.
Plasmon "Flashlights"
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