Quasi-near field terahertz spectroscopy

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Abstract

Everyday we see around us many materials that are different from one another. We identify them on the basis of their shape, texture, smell, taste, color, etc with the help of our senses. Many times it is almost impossible to identify materials only with our senses. Then, one has to look into more fundamental aspects of the materials such as their atomic or molecular constituents. Different techniques exist to identify and characterize materials. Spectroscopy is one such technique. Spectroscopy relies on the frequency selective emission or absorption of electromagnetic radiation by the materials to get information about their physical/chemical properties. There are different spectroscopic techniques to study materials and their interaction with electromagnetic radiation. Terahertz (THz) radiation is the part of electromagnetic spectrum, which lies between the microwave and infrared regions of the electromagnetic spectrum. It is loosely defined as the frequency range from 0.1 to 10 THz. Terahertz radiation can penetrate a wide range of materials: paper, wood, plastics, fabric, ceramics, semiconductors, and many others that are often opaque to visible and near-infrared (NIR) radiation. Many materials have characteristic absorption bands in the THz region. Thus, in THz imaging applications, apart from getting a THz image, the measurement can also give spectroscopic information on the samples under study, which can be used to identify the materials. In principle, this spectroscopic imaging makes it possible to identify the contents inside a package without even opening it. There exist various techniques to generate terahertz (THz) radiation. In photo-conductive antennas (PCAs), a time-dependent polarization is formed when charge carriers, created by a femtosecond laser pulse, are accelerated in an externally applied electric field. PCAs are capable of generating broadband pulses with a fairly high power. The THz generation and detection setup with the photo-conductive antenna as the THz source and electro-optic sampling as the THz detector as described in Chapter 2 gives a very high SNR of ?15000 in a measurement time of 10 ms. Even though our femtosecond laser pulses have a very high peak power, the generated THz power could not be increased further with increasing laser pump power on the emitter, because of emitter saturation. Increasing the laser spot size on the emitter gives a higher THz peak power than in the case of a tightly focused pump beam. Among the different techniques to generate THz radiation, the technique using the optical conversion of extremely short pulses of light into THz pulses at a high repetition rate is very popular. These short laser pulses are partially converted into THz light in certain non-linear optical media. By using coherent detection techniques, the amplitude and phase of the THz pulse can be detected in the time domain. This spectroscopic technique is called terahertz time domain spectroscopy (THz-TDS). A typical THz-TDS setup has a long THz beam path. Atmospheric water vapor, present also in the beam path, has many strong absorption bands in the THz region, which makes it difficult to perform spectroscopy on samples. Such a setup should therefore be flushed with dry N2 gas to reduce absorption of the THz radiation by water vapor molecules in the atmosphere. Typically, a THz-TDS setup also requires parabolic mirrors to collimate, steer and focus the THz radiation onto the detection crystal which is complicated by the fact that THz radiation is invisible. Furthermore, the THz beam diffraction and absorption at the reflecting surfaces in the THz beam path will lead to reduction of the THz power. One way to overcome all these problems is by placing the THz source and detector very close to each other. Apart from its simplicity, the advantage of such a quasi-near field terahertz spectrometer is that it can also provide a broad bandwidth (0.5-7 THz) and a good signal-to-noise ratio even without the use of lock-in detection. THz radiation is generated by optical rectication of 50 fs, 800 nm pulses from a Ti:sapphire oscillator in suitable nonlinear optical crystals such as GaP. The shape and bandwidth of the spectra of the THz pulses generated in this way, are explained with a simple model, which takes into account the effects of phase-matching and absorption of THz radiation in the generation and detection crystals. If one wants to increase the generated bandwidth to frequencies above the phonon resonance frequency, then very short laser pulses (?10 fs) and very thin crystals (?50 ?m) should be used. An important point to note is that the probe and the THz pulses are initially counter propagating and, after reflecting, co-propagating in the detection crystal. Both the calculations and the experimental results show that the effect of counter propagation of the THz pulse and the probe pulse in the detection crystal is negligible at higher frequencies, above a few hundred GHz. Samples can be inserted between the generation and detection crystals, and their absorption spectra can be measured. As the THz beam propagates, it is expected to show a frequency-dependent divergence. This changes when a sample is placed between the generation and detection crystals, making it more difficult to obtain the absolute absorption coefficient. However, this has no effect on the ability of the setup to identify materials by their spectral fingerprint. This is shown in Chapter 4, which contains measured absorption spectra of, D-tartaric acid, certain amino acids, sugars and metal oxides, in the frequency range of 0.5-7 THz. Another exciting application of THz-TDS, as described in this thesis, is the identification of polymorphs in freeze dried mannitol. Freeze drying is very commonly used in the pharmaceutical industry to increase the shelf life and dry state stabilization of the therapeutic agents, where polymorphism is identified as a serious problem. These polymorphs often can have unique physical and chemical characteristics that can influence their stability, solubility and other performance characteristics. It is important for the pharmaceutical industry to know which polymorph is formed during the freeze drying process. The measured THz absorption spectra of the ? and ? polymorphs of mannitol from 0.5 THz to 7 THz, have distinct THz absorption spectra as shown in Chapter 5, especially between 2.5 and 6 THz. Because THz-TDS can be used to identify the polymorphs of mannitol, we have subsequently used this technique to study the effect of various freeze drying techniques on the formation of these polymorphs. The results show that, for mannitol, changes in the way the material is frozen can result in the formation of different polymorphs or a mixture of polymorphs, as supported by X-ray diffraction measurements performed on these samples. The THz-TDS has the added advantage that it may be relatively easy to employ as an in-line and, almost, real time monitoring tool, unlike X-ray diffraction analysis.