Page 97 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

90 Chapter Four

light over as large a solid angle as possible. This either requires large detectors
or very compact systems. As usual, this leads to difficult design trade-offs.
Nevertheless, fluorescence can deliver superb performance. In liquid chro-
matographic detection a mass LOD of a few femtogram (fg) of material is pos-
sible, about 100 times better than using absorption detection. See for example
Skoog et al. (1998) for comparisons.

4.6 Parametric Detection
When we want to detect tiny changes in optical transmission in materials that
are largely transparent, we have extra difficulties. This is a small change on a
large background, and we must first contend with the shot noise of the full-
scale transmitted signal. Second, as discussed in Chap. 10, the best sensitivity
is often limited not by measurement noise but by variations in source intensity.
This problem can be ameliorated by using techniques that measure not the
transmitted light but some light-influenced property. The most widely studied
methods belong to a family of photothermal techniques.
One approach is to detect the light actually absorbed by the sample via the
heat generated in absorption. We could use any type of thermometer, but for
solid samples a more sensitive technique might be to measure thermal expan-
sion using a sensitive interferometer (Fig. 4.10a). If the “absorbed beam” is
slowly scanned in wavelength through an absorption feature, the interferome-
ter will in principle read out the absorption through changes in temperature
and expansion. The absorbed beam here performs the job of the light source of
a spectrometer. However, in doing this we have suppressed the large signal when
absorption is lowest. With the expansion readout, no absorption means zero
signal. We have also separated the excitation and readout optics. To get high
sensitivity we can increase the intensity of the probe beam to improve the shot-
noise limited S/N, and operate in a wavelength region with low-noise photo-
detectors. For liquid samples we can additionally make use of a hydraulic gain.
Figure 4.10b shows a system in which the sample expansion is detected via
movement of a liquid meniscus, a liquid version of the famous Golay detector.
A near-infrared-sourced single-mode fiber interferometer was used to sensi-
tively detect the liquid expansion caused by UV and blue light absorption
(Hodgkinson, 1998). As long as absorption and heat generation in the trans-
parent windows and at the cell wall are minimized, this technique also allows
us to separate absorption from scattering. In a conventional spectrometric
transmission measurement these two are mixed and cannot be directly
separated.
Temperature changes can also be transduced into refractive index changes,
which can be read out to high resolution using beam deflection techniques.
Figure 4.10c shows detection of the refracted beam caused by a hot spot in the
sample cell using a split photodiode. Without absorption the probe beam passes
straight through to impinge symmetrically on the photodiodes. With absorption
the beam is deflected through a small angle to unbalance the two photocur-

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