Dielectric filters

The 4 3/2 4 9/2 transition was utilized and dielectric reflectors were necessary to suppress the 1.06 fi oscillation. Mauer (127) then produced oscillation at 1.37 /x in a borate glass again dielectric filters were essential. 

This leads to a substantial increase in the coupling of laser radiation into the sample with a corresponding boost to the overall Raman signal. In practical terms, the method requires that the sample surface is not excessively curved as dielectric filters are typically only readily available as flat optical elements, although this is not a fundamental obstacle. 

It should be noted that the transmission of the acousto-optic filter is only about 50%, while the transmission of the liquid crystal filter is about 20-40%. By contrast, a dielectric filter passes 80-90% of the incident light. The differences arise because both the AOTF and LCTF operate on linearly polarized light. In most Raman microprobes, both polarization components of the Raman scatter are collected, even when the exciting laser is linearly polarized. 

The method utiHzes the angular dependence of the dielectric filter on impacting photon direction, with its transmission spectral profile shifting to the blue, in fine with the increase in the deviation of photons away from normal incidence. This feature enables the filter to serve as a unidirectional mirror, passing a semi-colhmated laser beam through unhindered from one side while at the other side reflecting any photons emerging from the sample predominantly at random directions, back into the sample. 

Beger MG, Arens-Fischer R, Thoenissen M, Krueger M, Billat S, Lueth H, Hilbrich S, Theiss W, Grosse P (1997) Dielectric filters made of PS advanced performance by oxidation and new layer structures. Thin Solid Films 297 237-240... 

Much of solar radiation lies in the infrared part of the spectrum and is of too low energy to be utilized in conventional PV cells (or in photo-electrochemical reactions see below) so that it is wasted. To harness this thermal energy and thereby improve the efficiency of solar energy conversion, Licht has proposed the use of dielectric filters to separate the radiation received by the solar tower into an infrared component to heat pressurized water to at least 300 °C and an optical/ultraviolet component to generate electricity through PV (or photo-electrochemical) cells. The electricity would then be used to split water by high-temperature steam electrolysis (see above) with the entropy term Th.S of Figure... 

Microscope with Multichannel Detection In this scheme, an unfocused laser beam illuminates the area of the sample. An optical filter isolates the selected Raman line, and the multichannel detector produces a picture of the sample. At the time of this publication, the optical filter was a monochromator used as a filter. [Since then, this same concept has been implemented with dielectric filters [57], AOTFs (acousto-optic tuning filter) [58], and LCTFs (liquid-crystal tuning filter) [59,60] these technologies will certainly be covered in later chapters on microscopy and imaging.]... 

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