Infrared thermo-electric photodetectors

During the last decade pulsed laser radiation has gained interest by the material processing industry and the medical sector. For a growing set of applications (laser drilling, laser marking, laser surgery, semiconductor doping profiling, micro-structuring, layer deposition, etc.) it is advantageous to use pulsed laser radiation instead of continuous wave illumination as time limited exposure often results in reduced collateral damage and more precise processing (Phipps, 2007).
In laser ablation for example, one aims to put an intense laser pulse on the surface of a target material in an as short time as possible. This short exposure time, which limits thermal diffusion inside the material, together with a carefully selected wavelength with a minimal absorption depth is required to ensure energy deposition in a small volume of the target material. Hence, for laser pulses which meet the ablation requirements, one can evaporate material in a very controlled fashion. Different methods exists as Q-switching, mode-locking and cavity dumping for achieving the required pulse characteristics for laser ablation.
Laser pulses span an enormously large parameter space in terms of wavelength, repetition rate, pulse duration and pulse energy, further referenced as pulse-parameters. Each of these pulse-parameters can be optimized for a given application and target material. Besides its dependence on the temporal characteristics of pulsed laser radiation, for some applications, the processing quality is also strongly dependent on the transverse laser beam profile. Consequently, there is a growing interest in detecting the spatiotemporal behavior of laser pulses.
In this chapter we briefly describe the most common infrared detector principles for measuring laser pulses and point out their respective advantages and disadvantages with respect to different pulse-parameters. Next, we show that Seebeck-effect based thermo electric photodetectors can be designed to cover a relatively broad range of pulse parameters. (Stiens, 2006) Further, we discuss the working principle and operation regimes of the thermo-electric photodetector and explain the corresponding theoretical background in detail. Experimental results concerning short laser pulse induced thermo voltages in n GaAs are presented. This chapter is also concerned with the possibility of using the thermo-electric effect to measure the spatio-temporal behavior of laser pulses by means of linear focal plane arrays (LFPA). Certain related issues will be highlighted such as thermal cross talk between pixels in case of pulse durations approaching continuous wave illumination. A lock-in method is proposed to reduce the cross-talk level. The chapter will conclude with describing the future directions of research.

ISBN:978-953-307-405-4

Publication year:2010

Keywords:Seebeck, thermo-electric detector, photodetector