6. Conclusion

devices that exploit transitions between conduction band states. The conduction band offsets between neighboring materials in the superlattice create a series of quantum wells and barriers. The most widely used QCL designs are based on GaAs/AlGaAs superlattices. The word cascading means that one electron can produce many photons in the superlattice periods. A significant advantage of THZ radiation is that it can easily inspect sealed packages for contrasting metal and plastic objects, testing pharmaceutical tablets for integrity, detecting skin

Figure 17. Experimental setup of CW THz-DH: OC is optical chopper and M is a mirror.

Figure 16. Spectrum of waves showing the location of THz radiation.

288 Holographic Materials and Optical Systems

In Section 5, we present the usage of THz radiation for 3D surface characterization via a digital holography (DH) technique [11]. Since the usual DH uses a visible light, it is difficult to visualize 3D internal structure of visibly opaque objects due to their limited penetration depth. The compelling advantage of THz radiation is that it has a good penetration capability; thus, 3D visualization of both surface shape and internal structure in visibly opaque object can be achieved [12]. We constructed off-axis THz digital holography (THz-DH) system equipped with CW-THz radiation generated by QCL, and the THz digital hologram is captured by a

Figure 17 illustrates a schematic diagram of off-axis THz-DH system. An optical chopper (OC) is positioned in front of the QCL to reduce the noise. The radiation was collimated by a Teflon lens (f = 300 mm). The collimated THz beam of diameter around 60 mm is divided into a signal THz beam and a reference one by a silicon beam splitter (BS). Figure 18(a) shows 2D intensity distribution of the THz beam without the sample. Figure 18(b) shows the dark frame captured

cancers, etc.

THz camera.

In conclusion, we have presented the recent developments of digital holography techniques for surface characterization. In this chapter, the principle of digital holography with focus on numerical reconstruction algorithms is presented. Also, influence of slightly imperfect collimation of the reference wave in off-axis DH is discussed. Finally, we have described two different DH techniques for surface characterization: the first technique by using short coherent length, namely, high-brightness DHM, and the second technique by using long coherent length, namely, THz-DH. Experimental results are presented to verify the principles.
