Prism coupled Terahertz wave-guide sensors
Surfaces providing total internal reflection (TIR) may serve for efficient coupling of radiation into wave-guides by means of evanescent waves. The coupling is frequency selective and determined by the mode spectrum of the wave-guide. Depending in particular on the thickness and the phase shifts upon reflection from wave-guide surfaces, the behaviour of coupled frequencies can be utilized to detect adsorbates on the surface and even determine them: by measuring the caused shifts. Simple sensors for terahertz (THz) range utilizing the attenuated total reflection (ATR) scheme in combination with a wave-guide were designed and realized for detecting thin layers and adsorbed molecules on surfaces.
The condition for transverse resonance in a symmetric slab optical wave-guide of thickness d and refractive index n is:
with: m - order number; Θ - internal incidence angle; Δφfilm - difference between the two values of phase shift acquired by reflection of the beam from one or the other surface: for ideally symmetric wave-guide Δφ = 0.
Prism coupled wave-guide
A simple way to couple the THz beam into a planar wave-guide and couple out from it is to position the wave-guide close enough and parallel to a surface of TIR. A high-resistivity silicon prism of 90° apex angle with the hypotenuse edge serving as a TIR surface and GaAs or Si wafers as wave-guide are used in our measurements (the choice conditioned by the best THz-beam transparency). A pair of thin plastic spacers keep the wave-guide at desired distance from the TIR surface and parallel to it. The whole sensor is a "plug-and-play" module, thanks to two aluminium plane mirrors used to direct the THz beam into the prism normal to its input edge and redirect the output beam further to the detector (shown in the figure above).
If a thin layer of liquid is applied to the wave-guide surface, the symmetry is violated and it can cause an additional phase shift Δφfilm. The latter results in shift of the mode frequency ν which can be derived from the formula above. Shifts of ν can be caused by d or Θ too but only the one caused by the Δφfilm is not dependent on the order m and suits best for a sensor.
Modified prism scheme
The scheme of the sensor can be modified so that the wave-guide modes be measured directly: a folded aperture made of aluminium foil is attached to the apex of the prism to optically block the direct way for the THz beam through the prism alone. In the presence of the wave-guide the excited modes travel further and couple back into the prism behind the central line of TIR surface. Those rays leave the prism out of the blocked area and when detected compose a spectrum of emission lines on the mode frequencies.
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