375
Terahertz Radiation
Several studies have also shown that DNA nucleobases (i.e., A, T, C, and G) exhibit
resonances between 33.4 cm –1 and 117 cm –1 (Fischer et al. 2002; Xie 2002; Fischer 2005),
whereas DNA films exhibit modes from 300 to 500 cm –1 (Globus et al. 2002; Figure 7.2b).
A growing number of research groups are using THz spectroscopic approaches to investigate the important role that water plays at the surface of biomolecules. Most notably,
Havenith’s group recently demonstrated that water molecules actually “dance” with
proteins during solvation (Figure 7.2c). Finally, transmembrane proteins have also been
shown to exhibit collective modes in the THz region (~ at 115 cm –1 ) (Xie 2002). In summary, water molecules, biomolecules, and water–protein solvation processes all exhibit
dynamics that occur on the picosecond timescale. Therefore, THz spectroscopic tools
are uniquely suited to probe these dynamics.
7.2 Terahertz Sources: Conventional
and State-of-the-Art
Historically, fewer sources have been available for the THz region than for neighboring
millimeter-wave and IR spectral bands. For many years, the lack of suitable sources
enticed scientists from both the millimeter wave and IR research communities to
develop sources for this uncharted territory. Developing bright THz sources has been a
challenging task for two fundamental reasons. First, conventional photonic approaches
are limited because few materials exist that have a small enough band gap to directly
generate THz radiation. A band gap is the difference in energy between the valence and
conduction band of a solid, and band-gap spacing is directly correlated with the output
wavelength of laser sources. The second principle is that the intrinsic transit time and
resistance–capacitance effects of conventional electronic devices causes their output
power to roll off with increases in frequency (i.e., power ∼ 1/f 2 –1/f 3 ) (Siegel 2002). As a
result, the output power of conventional electronic sources drops off at THz frequencies.
Although an extensive review on THz sources is beyond the scope of this chapter, a
general understanding of modern sources is necessary for subsequent discussions on
THz-induced bioeffects. For further details about THz sources, we refer the reader to
several excellent reviews and books (Ferguson and Zhang 2002; Siegel 2002; Wolbarst
and Hendee 2006; Hosako et al. 2007; Lee and Wanke 2007; Liu et al. 2007; Tonouchi
2007; Williams 2007; Lee 2009; Sirtori 2009). THz sources are typically categorized
by their principle operational scheme. The most commonly used schemes are the following: (1) direct generation laser sources; (2) solid-state electronic devices (frequency
up-conversion); (3) accelerating electron-based sources; and (4) nonlinear optical effect
sources (frequency down-conversion).
7.2.1 Direct Generation Laser Sources
Far-IR gas lasers (FIR), electrically pumped solid-state lasers, and quantum cascade
lasers (QCLs) are the three most common types of direct generation THz laser sources.
The FIR laser is the oldest THz source and has initial uses dating back as early as the
1960s (Crocker et al. 1964; Dodel 1999; Figure 7.3a). FIR systems, also referred to as
optically pumped molecular gas lasers, consist of a tunable CO 2 optical pump laser
Terahertz Radiation
Several studies have also shown that DNA nucleobases (i.e., A, T, C, and G) exhibit
resonances between 33.4 cm –1 and 117 cm –1 (Fischer et al. 2002; Xie 2002; Fischer 2005),
whereas DNA films exhibit modes from 300 to 500 cm –1 (Globus et al. 2002; Figure 7.2b).
A growing number of research groups are using THz spectroscopic approaches to investigate the important role that water plays at the surface of biomolecules. Most notably,
Havenith’s group recently demonstrated that water molecules actually “dance” with
proteins during solvation (Figure 7.2c). Finally, transmembrane proteins have also been
shown to exhibit collective modes in the THz region (~ at 115 cm –1 ) (Xie 2002). In summary, water molecules, biomolecules, and water–protein solvation processes all exhibit
dynamics that occur on the picosecond timescale. Therefore, THz spectroscopic tools
are uniquely suited to probe these dynamics.
7.2 Terahertz Sources: Conventional
and State-of-the-Art
Historically, fewer sources have been available for the THz region than for neighboring
millimeter-wave and IR spectral bands. For many years, the lack of suitable sources
enticed scientists from both the millimeter wave and IR research communities to
develop sources for this uncharted territory. Developing bright THz sources has been a
challenging task for two fundamental reasons. First, conventional photonic approaches
are limited because few materials exist that have a small enough band gap to directly
generate THz radiation. A band gap is the difference in energy between the valence and
conduction band of a solid, and band-gap spacing is directly correlated with the output
wavelength of laser sources. The second principle is that the intrinsic transit time and
resistance–capacitance effects of conventional electronic devices causes their output
power to roll off with increases in frequency (i.e., power ∼ 1/f 2 –1/f 3 ) (Siegel 2002). As a
result, the output power of conventional electronic sources drops off at THz frequencies.
Although an extensive review on THz sources is beyond the scope of this chapter, a
general understanding of modern sources is necessary for subsequent discussions on
THz-induced bioeffects. For further details about THz sources, we refer the reader to
several excellent reviews and books (Ferguson and Zhang 2002; Siegel 2002; Wolbarst
and Hendee 2006; Hosako et al. 2007; Lee and Wanke 2007; Liu et al. 2007; Tonouchi
2007; Williams 2007; Lee 2009; Sirtori 2009). THz sources are typically categorized
by their principle operational scheme. The most commonly used schemes are the following: (1) direct generation laser sources; (2) solid-state electronic devices (frequency
up-conversion); (3) accelerating electron-based sources; and (4) nonlinear optical effect
sources (frequency down-conversion).
7.2.1 Direct Generation Laser Sources
Far-IR gas lasers (FIR), electrically pumped solid-state lasers, and quantum cascade
lasers (QCLs) are the three most common types of direct generation THz laser sources.
The FIR laser is the oldest THz source and has initial uses dating back as early as the
1960s (Crocker et al. 1964; Dodel 1999; Figure 7.3a). FIR systems, also referred to as
optically pumped molecular gas lasers, consist of a tunable CO 2 optical pump laser