Irene Datskou

Photoinduced and thermal stress in silicon microcantilevers

The photogeneration of free charge carriers in a semiconductor gives rise to mechanical strain. We measured the deflection of silicon microcantilevers resulting from photoinduced stress. The excess charge carriers responsible for the photoinduced stress, were produced via photon irradiation from a diode laser with wavelength λ=780 nm. For Si microcantilevers, the photoinduced stress is of opposite direction and about four times larger than the stress resulting from only thermal excitation. In this letter we report on our study of the photoinduced stress in silicon microcantilevers and discuss their temporal and photometric response.

Ionization of Organic Molecules Using Coherent Vacuum Ultraviolet Light

Third harmonic generation in xenon was used to produce vacuum ultraviolet light at a fixed wavelength of 118 nm. These photons were able to ionize a number of organic molecules with negligible yields of fragmentation ions. This method yielded far simpler mass spectra than those utilizing electron impact ionization. Absolute photoionization cross sections were measured for six organic compounds at 118.2 nm.

Two-photon spectroscopy of molecular oxygen

Abstract Two-photon transitions are examined in molecular oxygen in the region of 8 to 9 eV. Two-photon spectra are presented and absolute two-photon absorption constants are tabulated.

Ultra-responsive thermal sensors for the detection of explosives using calorimetric spectroscopy (CalSpec)

We have developed a novel chemical detection technique based on IR micro-calorimetric spectroscopy that can be used to identify the presence of trace amounts of very low vapor pressure target compounds. Unlike numerous recently developed low-cost sensor approaches, the selectivity is derived from the unique differential temperature spectrum and does not require the questionable reliability of highly selective coatings to achieve the required specificity. This is accomplished by obtaining the IR micro-calorimetric absorption spectrum of a small number of molecules absorbed on the surface of a thermal detector after illumination through a scanning monochromator. We have obtained IR micro- calorimetric spectra for explosives such as TNT over the wavelength region 2.5 to 14.5 micrometers . Thus both sophisticated and relatively crude explosives compounds and components are detectable with this technique due to the recent development of ultra sensitive thermal-mechanical micro-structures. In addition to the above mentioned spectroscopy technique and associated data, the development of these advanced thermal detectors is also presented in detail.

Detection of infrared photons using the electronic stress in metal-semiconductor interfaces

It is well known that the work function of metals decrease when they are placed in a nonpolar liquid. A similar decrease occurs when the metal is placed into contact with a semiconductor forming a Schottky barrier. We report on a new method for detecting photon is using the stress caused by photon-electronics emitted forma metal film surface in contact with a semiconductor microstructure. The photoelectrons diffuse into the microstructure and produced an electronic stress. The photon detection results from the measurement of the photo-induced bending of the microstructure. Internal photo-emission has been sued in the past to detect photons, however, in those cases the detection was accomplished by measuring the current due to photoelectrons and not due to electronic stress. Small changes in position of microstructures are routinely measured in atomic force microscopy where atomic imaging of surface relies on the measurement of small changes in the bending of microcantilevers. In the present work we studied the photon response of Si microcantilevers with a thin film of Pt. The Si microcantilevers. In the present work we studied the photon response of Si microcantilevers with a thin film of Pt. The Si microcantilevers were 500 nm thick and had a 30 nm layer of Pt. Photons with high enough energies produce electrons from the platinum-silicon interface which diffuse into the Si and produce an electronic stress. Since the excess charge carriers cause the Si microcantilever to contact in length but not the Pt layer, the bimaterial microcantilever bends. In our present studies we used the optical detection technique to measure the photometric response of Pt-Si microcantilevers as a function of photon energy. The charge carriers responsible for the photo-induced stress in Si, were produced via internal photo-emission using a diode laser with wavelength (lambda) equals 1550 nm.

Optical readout of uncooled thermal detectors

We investigated microposition sensing of micro-electro- mechanical systems (MEMS) that is based on optical readout techniques. We determined the parameters that affect or limit the performance of optical readout techniques especially as they apply to detection of infrared radiation. Such microposition sensing schemes are very important as readout mechanisms for large arrays of microstructures which are required for imaging. In addition, we explored the performance of uncooled micromechanical IR sensors using Fresnel zone plates (FZP). This type of diffractive feature diffracts along the optical axis and not perpendicular to that axis. We found that temperature fluctuation noise and background fluctuation noise, are currently the limits to the performance of uncooled micromechanical IR detectors. The noise at the output of the optical readout includes amplified noise from the micromechanical structures and noise added by the optical readout itself. However, the added noise is negligible compared to the amplified temperature fluctuation noise inherent in the microstructures. In this context an optical readout is nearly an ideal, noiseless readout method.

Infrared microcalorimetric spectroscopy using uncooled thermal detectors

We have investigated a novel IR microcalorimetric spectroscopy technique that can be used to detect the presence of trace amounts of target molecules. The chemical detection is accomplished by obtaining the IR photothermal spectra of molecules absorbed on the surface of an uncooled thermal detector. Traditional gravimetric based chemical detectors require highly selective coatings to achieve chemical specificity. In contrast, IR microcalorimetric based detection requires only moderately specific coatings since the specificity is a consequence of the photothermal spectrum. We have obtained IR photothermal spectra for trace concentrations of chemical analytes including diisopropyl methylphosphonate (DIMP), 2-mercaptoethanol and trinitrotoluene (TNT) over the wavelength region 2.5 to 14.5 micrometers . We found that in the wavelength region 2.5 to 14.5 micrometers DIMP exhibits two strong photothermal peaks. The photothermal spectra of 2-mercaptoethanol and TNT exhibit a number of peaks in the wavelength region 2.5 to 14.5 micrometers and the photothermal peaks for 2-mercaptoethanol are in excellent agreement with IR absorption peaks present in its IR spectrum. The photothermal response of chemical detectors based on microcalorimetric spectroscopy has been found to vary reproducibly and sensitively as a consequence of adsorption of small number of molecules on a detector surface followed by photon irradiation and can be used for improved chemical characterization.

Optimization of Micromachined Photon Devices

The Oak Ridge National Laboratory has been instrumental in developing ultraprecision technologies for the fabrication of optical devices. We are currently extending our ultraprecision capabilities to the design, fabrication, and testing of micro-optics and MEMS devices. Techniques have been developed in our lab for fabricating micro-devices using single point diamond turning and ion milling. The devices we fabricated can be used in micro-scale interferometry, micro-positioners, micro-mirrors, and chemical sensors. In this paper, we focus on the optimization of microstructure performance using finite element analysis and the experimental validation of those results. We also discuss the fabrication of such structures and the optical testing of the devices. The performance is simulated using finite element analysis to optimize geometric and material parameters. The parameters we studied include bimaterial coating thickness effects; device length, width, and thickness effects, as well as changes in the geometry itself. This optimization results in increased sensitivity of these structures to absorbed incoming energy, which is important for photon detection or micro-mirror actuation. We have investigated and tested multiple geometries. the devices were fabricated using focused ion beam milling, and their response was measured using a chopped photon source and laser triangulation techniques. Our results are presented and discussed.

Estimation of secondary waste loads generated during environmental restoration activities

The US Department of Energys (DOEs) Environmental Restoration (ER) program is responsible for remediating thousands of contaminated sites across the DOE complex. A major concern during remediation is the secondary waste generated as a result of ER site remediation. The treatment, storage and disposal (TSD) of secondary waste generated as a result of ER site remediation is the responsibility of the waste management (WM) program. In order to manage TSD operations on ER generated waste, WM needs accurate and consistent estimates of the volume of secondary waste generated. This paper discuss three remedial processes and presents estimates of waste volumes generated during these processes.

Novel magnetic, acoustic, and chemical microsensors for in-situ, real-time, and unattended use

Consistent with the underlying long-term objectives ofthe development ofthe Unattended Ground Sensors (UGS) program, we are developing a new planted ground sensor platform based on Micro-Electro-Mechanical Systems ( MEMS) which offers magnetic, chemical and acoustic detection. The envisioned micro-system will be low-power and low-cost, and it will be built around a single type ofmicrostructure element integrating a monolithic optical system and electronics package. In the first phase of our work supported by DARPA, we demonstrated that the microcalorimetric spectroscopy technique can be applied to detect and identify chemicals in the ppm level and that the studied magnetometer can provide sensitivities in the order of 1 T. Continuation studies will be discussed that aim to achieve chemical detection sensitivities in the order ofppb or better. We will present the methods to be used to achieve magnetic detection sensitivities in the order of ipT @ 1Hz. The studied acoustic detection system will act as the trigger for the overall system. Sensors ofthis type are required for the next generation of magnetic and chemical multisensor instrumentation. And will be useful in numerous commercial and military applications where small size, lowpower consumption, low cost, and ruggedness are prime requirements. Possible applications of such a system include: sensitive perimeter monitoring, such as minefields and military/nuclear bases, vehicle detection, airport security, and medical industry applications.