Position sensitive photodetector

Instrumentation. A cantilever with a sharp tip interacting with the surface under investigation is used. The actual bending of the cantilever is measured with a laser beam deflected from a mirror-like surface spot on the back of the cantilever towards a position-sensitive photodetector. The measured signal is used to control the piezo actuators. A constant force mode in which the cantilever-surface distance is kept at a preset interaction force and a constant height mode of scanning operation are possible. The principle of operation is schematically outlined in Fig. 7.9. 

FIGURE 24.1 Schematic representation of the atomic force microscope. In this configuration, the sample would be mounted on the scanner and scanned under a fixed AFM tip. The laser is focused on the AFM tip and the position of the reflected laser spot is monitored on a position-sensitive photodetector (PSPD). 

Similar to the thermal lens effect is exploitation of the fact that a narrow probe laser beam exhibits deflection as a consequence of a refractive index gradient again, the amount of deflection can be measured using a position-sensitive photodetector. 

Signal transduction from microcantilevers is based on optical, piezoelectric, and piezoresistive methods. Optical transduction is based on measurement of displacement of reflected light from the surface of microcantilever with the help of a position-sensitive photodetector [95]. However, this is not suitable for POC devices, because a larger path length might be required, which precludes portable instrumentation. Piezoelectric and piezoresistive detection involves integration of a piezoresistive (poly silicon) or piezoelectric material, respectively, on the microcantilever. In this case, the instrumentation can obviously be miniaturized. However, the requirement for a mechanically stable environment, difficulty in functionalization of a large number of cantilevers at the same time, fabrication losses, etc. are obstacles to commercialization of devices based on microcantilevers as POC products. 

To date, various methods have been proposed for cantilever deflection measurement Among the most widely used methods is the OBD method, owing to its simple setup and high sensitivity. In the method, a focused laser beam is irradiated to the cantilever backside and the reflected beam is detected with a position-sensitive photodetector (PSPD). The PSPD is t3q)ically made of a two- or four-divided-photodiode array. 

In this case, the position of the sample is controlled and varied by means of a piezoscanner and the corresponding movement of the tip, which is positioned at the end of a cantilever, is measured optically by means of a laser beam and a position-sensitive photodetector. There are many ways of controlling the relative movement of sample and tip and physicists are continually developing increasingly sophisticated variations. Such complexities are dealt with in many research papers, reviews and books and need not concern us here. However, it is interesting to note the typical form of a cantilever and a tip, as shown in the electron microscope images of Figure 6.2. 

The excitation volume generated by a Gaussian profile laser beam constitutes a thermal lens, and hence a probe beam focuses or defocuses on passage through the thermal lens volume. Therefore, a different spot size of the probe laser beam is observed at the location of a photodetector the diameter change can be measured directly with a position-sensitive detector. 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

Pulse oximeter sensors consist of a pair of small and inexpensive red and infrared LEDs and a single, highly sensitive, silicon photodetector. These components are mounted inside a reusable rigid spring-loaded clip, a flexible probe, or a disposable adhesive wrap (Figure 6.6). The majority of the commercially available sensors are of the transmittance type in which the pulsatile arterial bed, for example, ear lobe, fingertip, or toe, is positioned between the LEDs and the photodetector. Other probes are available for reflectance (backscatter) measurement where both the LEDs and photodetectors are mounted side-by-side facing the skin [7,8]. 

As AFM has matured, a number of more sophisticated imaging modes have been developed. In lateral force mode (Fig. 3B) the sample is scanned sideways relative to the long axis of the cantilever. Torsional forces exerted on the tip cause the cantilever to twist and consequently to deflect the optical beam horizontally on the photodetector. Recording the lateral deflection or twisting of the cantilever as a function of X-Y position gives a lateral force map. Lateral force images are particularly sensitive to friction force between the tip and sample, and therefore these images are also called friction force maps. 

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