A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers. A three-dimensional model is formulated within the framework of geometric optics. The model includes various launching conditions, input and output end-face Fresnel transmission losses, multiple Fresnel reflections, bulk absorption, and evanescent-wave absorption. An evanescent-wave sensor response is analyzed as a function of externally controlled parameters such as coupling angle, f number, fiber length, and diameter. Conclusions are drawn for several experimental apparatuses.

We present transport measurements on surface superlattices fabricated on a GaAs͞AlGaAs twodimensional electron gas. Significant suppression of the conductance is found with increasing electric fields and with increasing temperature T. We attribute these effects to e-e scattering, which can significantly affect the resistance in the presence of a spatially modulated static potential.[S0031-9007(96)02082-0] PACS numbers: 73.20.Dx, 73.50.Fq The concept of a lateral surface superlattice (SSL) dates back almost two decades [1][2][3][4]. The idea is to impose a periodic potential on a two-dimensional electron gas (2DEG). This can be achieved, e.g., by a periodic array of surface electrodes, or gates, to which a voltage V g is applied with respect to the 2DEG. A variety of alternative techniques are known to induce a SSL [5][6][7][8][9][10]. SSLs are of great experimental interest due to the geometric control and the amplitude tunability of the periodic potential.Our work was originally motivated by the notion that a periodic potential should induce a spectrum of Bloch minibands and minigaps, which depends on the gate geometry and on V g . Furthermore, because of their low scattering rates, SSLs should offer a favorable testing ground to study Wannier-Stark (WS) localization and Bloch oscillations in the presence of longitudinal electric fields [3]. The observation of minigaps in the electronic spectrum of SSLs via transport has long been an important, yet elusive, goal. Indeed, there have been several early reports of nonmonotonic dependence of the resistance on V g , which were interpreted as resulting from the Fermi energy, E F sweeping through the miniband spectrum [11][12][13]. Unfortunately, these results have been difficult to reproduce, despite subsequent improvement in fabrication technology and sample quality. On the other hand, the presence of the periodic potential in the 2DEG leads to distinct features in the magnetoresistance of SSLs [5][6][7]14].In this paper we report the results of transport measurements in various SSL devices. We study the dependence of the current I on the drain-source voltage V ds , on the gate voltage V g and on temperature T . We find a rapid drop in the differential conductance G ϵ dI͞dV ds as we increase V ds . We also find a significant decrease in conductance as T is increased in the range of 2-10 K. We interpret these results in terms of electron-electron ͑e-e͒ scattering in the presence of the modulated gate potential.The samples were fabricated on high mobility 2DEGs formed by GaAs͞AlGaAs heterojunctions, with Schottky gate electrodes patterned via electron beam lithography. The SSLs had a period a 100, 200, or 300 nm. We have studied devices of several different wafers and geometries, including short gratings as well as long Hall bars covered entirely by a grating gate. Figure 1 shows a short grating sample. We also had control samples, which were Hall bars covered with a single uniform gate but otherwise identical geometry. In all measurements the current flows along th...

Fiberoptic evanescent wave spectroscopy (FEWS) based on AgClBr fibers and a Fourier transform infrared (FTIR) spectrometer was used for the first time to measure chlorinated hydrocarbons (CH) in water. A minimum detection limit lower than 10 mg/l was achieved by coating the fiber with low density polyethylene (LDPE), which shows reversible enrichment of CH. The response of the sensor to CH diffusion through the polymer layer was analyzed theoretically and the results were found to be in good agreement with the experiments.

A novel fiber-optic evanescent wave sensor is presented. The sensor, composed of a lead salt tunable infrared laser and a multimode uncladded silver halide infrared optic fiber, was used for rapid measurement of sulfur hexafluoride pressures at a few-percent detection limit. High-precision detection and identification of a large variety of gases are possible.

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