Introduction to THz Wave Photonics

Various frequencies are spaced along the frequently used electromagnetic spectrum, including microwaves, infrared radiations, visible lights, and X-rays. Between the microwave and infrared frequencies lies terahertz (THz) radiation (Fig1.1). In the electromagnetic spectrum, radiation at 1 THz has a period of 1 ps, a wavelength of 300 μm, a wave number of 33 cm^–1, a photon energy of 4.1 meV, and an equivalent temperature of 47.6 K. In the same way that visible light can create a photograph, radio waves can transmit sound, and X-rays can see shapes within the human body, terahertz waves (also called as T-rays) can create pictures and transmit information.

Before discussing the nature of THz waves and their applications, it is suitable to introduce how THz waves are generated and detected. As mentioned in Chapter 1, this book will focus on pulsed THz technologies. A typical pulsed THz wave generation and detection system is a pump and probe setup as presented in Fig. 2.1. The most common way that pulsed systems work is by splitting a beam from a femtosecond (fs) laser into two beams: the probe and the pump beams. The pump beam is used to generate the THz pulse, while the probe beam is used to sample and obtain the pulse profile. Detecting of THz field is performed by modulating the probe pulse with the THz field or by accelerating free carriers induced by the probe pulse with the THz field. A mechanical delay line is used to change the time delay between THz pulse and the probe pulse. The THz waveform can be obtained by scanning this time delay. To increase the sensitivity, the pump beam is modulated by an optical chopper, and the THz-induced modulation on the probe beam is extracted by a lock-in amplifier. This pulse information acquired in the time domain is transformed to the frequency domain with a Fourier transform from which spectral information can be obtained.

Once appropriate THz wave sources and detectors become available, researchers will be interested in developing application technologies, in order to utilize the THz waves. By far, most of the applications using THz wave can be sorted into the following two categories, namely, THz wave spectroscopy and THz wave imaging. When a pulsed THz wave is used, the mainly employed spectroscopic technique is THz wave time-domain spectroscopy. In this chapter we briefly introduce the concept of THz wave time-domain spectroscopy and THz wave imaging. Then we discuss a few spectroscopic and imaging techniques.

To develop technologies utilizing THz waves, one needs to first understand the interaction between THz waves and materials. In this chapter we will discuss the dynamics of THz wave interaction with different kinds of materials.

THz wave spectroscopy and imaging technologies are promising in security inspection applications. However, the following hurdles prevent THz technologies to be used in in situ applications, especially when standoff detection is required. First of all, the attenuation of THz waves in the atmosphere is higher than 100 dB/km, so it was previously thought impossible to perform long distance broadband THz wave sensing and spectroscopy, due to severe water vapor attenuation. Secondly, pulsed THz wave emitters using either real or virtual photocurrents, saturate when high excitation intensities are used. Further increase of the excitation power may even cause damage to the emitter. The saturation and damage of THz wave emitter limits the strength of the THz fields that can be generated from such emitters. Additionally, although pulsed THz wave generation and detection systems provide broadband spectral coverage, the spectrum does not generally cover the entire terahertz band continuously. Semiconductors or nonlinear crystals usually have phonon modes in the THz band. Absorption and dispersion due to photons result in dark areas in the measured THz spectrum. Finally, the reflection of THz waves, by both surfaces of the emitter or sensor, generates interference patterns in the THz spectrum. Confronted by those hurdles, using ambient air as the THz wave emitter and sensor becomes more and more interesting. By using ambient air as THz wave emitter and sensor, one can generate and detect THz waves close to the sample. Sending an optical beam instead a of THz wave, benefits long-distance standoff detection due to the relatively low attenuation experienced in the atmosphere. Since air or other gases are easily replaceable, damage is not a concern even if a strong laser field is used to generate the THz pulses. As a result, it is preferable in the generation of high intensity THz pulses. Finally, dry air has neither phonon bands nor boundary reflection surfaces, and thus provides continuous coverage along the entire bandwidth.

THz waves are transparent to most of the dry dielectric materials. This property makes THz wave a promising candidate for nondestructive evaluation of the internal structures of targets. THz wave time-of-flight imaging method is one of the techniques to extract the information about the layered structures of a target. If there is no layer structure within the target, or if the interesting features are not located on those layer structures, one needs to use tomographic imaging techniques to extract those interesting information [1].

THz waves offer innovative imaging and sensing capabilities for applications in material characterization, microelectronics, medical diagnosis, environmental control, and chemical and biological identification. However, the spatial resolution of conventional THz imaging technique is limited by diffraction of THz waves to be in the same order as THz wavelength (1 THz = 300 μm). This diffraction limit is an obstacle for using THz technology in probing the electronic and optical properties of semiconductor and bimolecular nanostructures. Several approaches have been used to obtain a sub-wavelength spatial resolution based on near-field techniques. One way to overcome diffraction is to use a sub-wavelength size aperture to limit the detection or generation area. This technique is known as apertured THz wave near-field microscopy. The aperture could be a static aperture made on a metallic screen or a dynamic one excited by an optical beam. Localized THz wave emitter or sensor based on real or virtual instant photocurrent excited by a highly focused optical beam can also provide spatial resolution much finer than THz wavelength. Another way, called apertureless THz near-field microscopy, use a sharp tip as local field enhancer which scatters the evanescent light in the near-field region of the target to make it detectable in the far field, and provide a spatial resolution well below the diffraction limit. Last but not least, THz wave emission microscope based on STM technique can achieve a nanometer resolution. A pulsed laser is used to generate photo-carriers on the semiconductor surface and a biased scanning-tunneling-microscope (STM) needle is used to modulate the localized electric field in the Schottky barrier under the tip. The transient photo-carriers driven by the modulated field emit THz waves, which can be detected at the modulated frequency in the far field. THz wave near-filed microscopy described above represents a milestone toward THz wave spectroscopic imaging of materials and devices at nanometer, sub-nanometer, and even atomic scales.

The Nondestructive Evaluation and Testing, in short NDE, discipline includes technologies and methods with the goal to examine objects and materials (samples) without impairing their future use. For example, ultrasounds and X-rays have been used in NDT applications for a long time such as material inspection, medical diagnostics, manufacturing, and quality control. On the other hand, in destructive testing, the sample is damaged during testing process. The destructive testing could be an extreme testing, where the selected samples are tested up to a failure point, then, the behavior of similar samples is statistically extrapolated. Or it can be a nonextreme test, where the sample is dissembled for a better investigation. Examples of destructive testing are found in mechanical elasticity and stress, heat insulation, and corrosion resistance measurements. NDE involves mechanical, optical, or chemical analysis, by use of ultrasonic waves, thermal waves, and electromagnetic waves. The results of applying NDE have a very broad impact on many fields, such as helping the aeronautics industry to ensure the integrity and reliability, and supporting cancer research by finding tumors The implementation of NDE techniques must include, at least, the following components

The development of techniques for inspection of explosives and other hazardous materials has become more and more attractive as concerns about public security have increased considerably in the past years 1–3. Among all explosive devices, landmine is the most demanding target to be detected. Landmines were widely used in all kinds of battlefields and they are very difficult to eliminate once a conflict ends. The remaining landmines represent an enormous danger for the people, both military and civilian, that occupy the terrain affected by the presence of landmines. As of today, more than 100 million mines remain active and undetected in many fields around the world. Those mines claim more than 30,000 lives or injures each year. Although antilandmine technologies already exist and are being used in minefields, most of these technologies tend to give high false-positive results due to the presence of other objects present in the area.

As THz waves interact with vibration and rotation transitions of organic molecules, they can be used to identify specific molecules based on their spectral features. In this way, THz technology can be used as a complement to other electromagnetic spectroscopy methods, such as visible and infrared. As THz photons have lower energy, they are unable to ionize biological samples under normal conditions. This makes THz spectroscopy an ideal tool for the examination of active biomedical samples. Due to the complexity of working with biological samples, biological applications are considered a mid- to long-term goal of THz research.