KEYWORDS: Terahertz radiation, Optical filters, Signal detection, Spectroscopy, Antennas, Manufacturing, Electronic filtering, Signal to noise ratio, Terahertz spectroscopy, Free space
We have developed electronic circuits that generate and detect subpicosecond shock-waves. We have used these circuits withintegrated antennas in a system capable of free space spectroscopy with greater than 10 dB foreground to backgroundradiation from 6.4 GHz to 1 THz. This system was used to characterize a submillimeter wave filter. In particular, 250-375GHz measurements are demonstrated with 2 dB accuracy over a 20 dB dynamic range.
Inorganic semiconductor nanocrystals, also known as quantum dots (QDs), are interesting as as fluorescent labels in biological studies. We have found that introduction of CdSe QDs to the vicinity of black lipid membranes (BLMs) results in current bursts through the membranes with bias voltage. These current bursts resemble those of the peptaibol class of antibiotics such as alamethicin and trichorzins, and are dependent both on voltage level and on concentration of the QDs applied to the membrane. Our data suggest that QDs with dipole moments similar to alamethicin are influenced by an external electric field, which creates a torque promoting insertion into the BLM, and a simple model predicts that at least three QDs can aggregate to form a pore leading to a macroscopic conductance.
We are developing a new type of detector for observational cosmology and astrophysical research. Incoming radiation from the sky is coupled to a superconducting microstrip transmission line that terminates in a thin film absorber. At sub-Kelvin temperature, the thermal isolation between the electrons and the lattice makes it possible for the electrons in the small absorber (100's of cubic micro-meter) and superconducting bilayer (Transition Edge Sensor) to heat up by the radiation absorbed by the electrons of the normal absorbing layer. We call this detector a Transition-edge Hot-electron Micro-bolometer (THM). THMs can be fabricated by photo lithography, so it is relatively easy to make matched detectors for a large focal plane array telescope. We report on the thermal properties of Mo/Au THMs with Bi/Au absorbers.
The terahertz regime (0.1 to 10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing bacterial spores and chemical material using field-deployable electronic terahertz techniques that use short-pulse generation and coherent detection based on nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving
greater sensing-at-a-distance (stand-off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of materials such as dimethyl methylphosphonate (DMMP) and several variants of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that indicate
specificity of the target. Although more work needs to be done to reduce the effects of standing waves through time gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.
We announce a new technique for the detection of changes in the conformation of small globular proteins in solution. We employ a coaxial-fed slot antenna with resonant frequencies in the 2-5 GHz range. This antenna detects changes in the dielectric properties of water. All proteins are surrounded by one or more shells of bound water. The dielectric properties of this 'bound' water are distinguishable from those of bulk water. As a protein changes it conformation, complementary changes occur in the three-dimensional arrangement of the 'bound' water. Thus, water can be used as a reporter for changes in protein conformation. Our technique has two advantages over conventional methods for microwave spectroscopy. First, unlike time-domain dielectric spectroscopy, data is measured in the frequency domain, so that time-to-frequency conversions are not necessary. Second, slot antennas may be affixed to the exterior of conventional fused-quartz cuvettes, so that simultane-ous measurements can be obtained using the antenna and conventional optical methods such as UV/VIS spectroscopy. When the unfolding of bovine pancreatic ribonuclease A (RNase A) is monitored at microwave frequencies, peak shifts in the antenna's resonant frequencies reflect changes in the protein's conformation. These peak shifts are sigmoidal with respect to temperature, and fit well to a two-state reversible unfolding model. Such sigmoidal peak shifts are not present when non-protein solutions are heated.
KEYWORDS: Near field optics, Diodes, Silicon, Sensors, Atomic force microscopy, Aluminum, Near field scanning optical microscopy, Waveguides, Microfabrication, Optical resolution
We report on the application as well as microfabrication process of batch-fabricated optical near-field sensors using cantilevered scanning force microscopy tips. The process includes implementation of a coaxial conductive geometry into a silicon sensor tip, along with electrical connections on the cantilever and chip body. The coaxial guide structure is used as electric lead to a sub-micron Schottky photodetector at the end of the tip, formed at the junction of the protruding silicon core and a recessed aluminum coating. The I-V curves of these sensors are consistent with numerical studies for such constricted geometries. Optical near-field data gathered by this sensor in topography-following mode is presented.
We report gas absorption spectra and energetic material reflection spectra measured with an all-electronic terahertz (THz) spectrometer. This instrument uses phase-locked microwave sources to drive picosecond GaAs nonlinear transmission lines, enabling measurement of both broadband spectra and single lines with hertz-level precision, a new mode of operation not readily available with optoelectronic THz techniques. We take two approaches to coherent measurements: (1) spatially combining the freely propagating beams from two coherent picosecond pulse generators, or (2) using a more conventional coherent source/detector arrangement with sampling detectors. The first method employs a dual-source interferometer modulating each harmonic of one source with a precisely-offset harmonic from the other source - both sources being driven with stable phase-locked synthesizers - the resultant beat frequency can be low enough for detection by a standard composite bolometer. Room-temperature detection possibilities for the DSI include antenna-coupled Schottky diodes. Finally, we have recently introduced a reflectometer based on serrodyne modulation of a linearized delay line, using a technique that is process-compatible with pulse generator circuits.
We have developed all-electronic integrated circuits that generate and detect picosecond pulses. We have used these circuits with integrated antennas in a system capable of free-space spectroscopy in the terahertz (THz) regime. With this system, we have measured magnitude and phase transmission characteristics for a variety of samples in the 200 GHz - 1 THz frequency range.
We have developed all-electronic integrated circuits that generate and detect picosecond pulses. We have used these circuits with integrated antennas in a system capable of free- space spectroscopy in the THz regime. With this system, we have measured magnitude and phase transmission characteristics for a variety of samples in the 200 GHz - 1 THz frequency range.
We report the first sub-picosecond shock-waves ever generated and measured by electronic circuits. We have used these circuits with integrated antennas to generate freely-propagating THz radiation. The circuits are monolithic nonlinear transmission lines fabricated on GaAs and operating at T = 77 K. Nonlinear transmission lines (NLTL's) have been used by several researchers for generating electronic shock-waves with picosecond transition times by compressing the wavefront of a microwave power source.1 While the literature reflects continual decreases in these transition times, as measured both by diode sampling bridges and by electro-optic sampling, the fastest reported 10%-90% fall time thus far has been 1.4 ps, with a -5 V amplitude.2 However, generating and measuring a sub-picosecond transition with an all-electronic device has been an elusive goal because a fundamental limitation has been the NLTL diode series resistance. By immersing a packaged NLTL into liquid nitrogen, we were able to lower this resistance significantly, thereby producing voltage shock-waves with 880 fs fall times and 3.5 V amplitudes, as measured by an on-chip diode sampling bridge. We have used these circuits with integrated magnetic dipole (slot) antennas to generate freely propagating THz radiation, and we have observed measurable radiation beyond 3 THz.
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