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Electromagnetic Fields in Biological Systems
source for THz bioeffects studies. (Zenil et al. 2007; Scarfi et al. 2003; Doria et al. 2004;
Grosse 2002; Berns et al. 1990; Berns et al. 1994; Berns and Bewley 1987.)
7.2.4 Nonlinear Optical Terahertz Sources:
Frequency Down-Conversion
Today’s most ubiquitous pulsed THz sources use laser-driven emitters (i.e., diodepumped fiber lasers) and nonlinear optical effects to down-convert frequencies from
the optical region. These compact systems are attractive sources because they provide
monochromatic THz radiation with a narrow line width, efficiently operate at room
temperature, and are broadly tunable. Pulsed THz sources typically exploit one of the
following second-order (χ 2 ) nonlinear optical processes to generate THz radiation:
(1) difference frequency generation (DFG), (2) optical rectification (OR), and (3) optical parametric oscillation (OPO). In this section, we provide a brief overview of each
of these systems. For a more comprehensive review, we refer the reader to the following
review articles (Suizo and Kawase 2008; Vodopyanov 2008).
7.2.4.1 Difference Frequency Generation
DFG sources were first demonstrated in the 1960s using a quartz crystal placed inside
the cavity of a ruby laser (Zernike and Berman 1966). These studies demonstrated that
THz radiation could be generated through the interaction of two optical laser beams
with nonlinear crystals. The optical sources typically used in DFG systems are narrowband IR (λ = 0.8–2.1 μm) laser beams with slightly different frequencies (ω 1 and ω 2 ).
When these sources interact with nonlinear crystals, most commonly gallium selenide
or arsenide (GaSe or GaAs), they generate emission at a beat frequency equal to the difference between the two input frequencies (ω THz = ω 1 − ω 2 ). DFG systems are attractive
THz sources for spectroscopy applications because they are easily tunable, provide THz
radiation with frequencies ranging from 0.2 to 6 THz, and provide high-peak powers
ranging from 10 2 to 10 5 mW (Shi et al. 2002). However, due to their low average power,
DFG sources are not commonly used for THz bioeffects studies. The primary challenge
with DFG techniques is that the nonlinear optical materials have very high absorption
coefficients, which make THz generation inefficient. In recent years, several methods
have been developed to overcome this limitation. Among these, quasi-phase-matching
(QPM) techniques show tremendous promise (Suizo and Kawase 2008).
7.2.4.2 Optical Rectification
In OR sources, THz radiation is generated using nonlinear media and difference frequency mixing approaches between the Fourier components of an optical pulse. OR
systems typically consist of an optical pump source, most often a femtosecond (fs) optical laser source, and thin electro-optic nonlinear crystals. OR sources have been shown
to generate broadband THz radiation with pulse energies of 10 μJ and a peak power of
100 μW. Such systems are commonly used in THz spectroscopy systems.
7.2.4.3 Optical Parametric Oscillation
Optical parametric generation is a second-order (χ 2 ) nonlinear optical process where
the photon of a pump pulse is converted into two photons with lower quantum energies.
Electromagnetic Fields in Biological Systems
source for THz bioeffects studies. (Zenil et al. 2007; Scarfi et al. 2003; Doria et al. 2004;
Grosse 2002; Berns et al. 1990; Berns et al. 1994; Berns and Bewley 1987.)
7.2.4 Nonlinear Optical Terahertz Sources:
Frequency Down-Conversion
Today’s most ubiquitous pulsed THz sources use laser-driven emitters (i.e., diodepumped fiber lasers) and nonlinear optical effects to down-convert frequencies from
the optical region. These compact systems are attractive sources because they provide
monochromatic THz radiation with a narrow line width, efficiently operate at room
temperature, and are broadly tunable. Pulsed THz sources typically exploit one of the
following second-order (χ 2 ) nonlinear optical processes to generate THz radiation:
(1) difference frequency generation (DFG), (2) optical rectification (OR), and (3) optical parametric oscillation (OPO). In this section, we provide a brief overview of each
of these systems. For a more comprehensive review, we refer the reader to the following
review articles (Suizo and Kawase 2008; Vodopyanov 2008).
7.2.4.1 Difference Frequency Generation
DFG sources were first demonstrated in the 1960s using a quartz crystal placed inside
the cavity of a ruby laser (Zernike and Berman 1966). These studies demonstrated that
THz radiation could be generated through the interaction of two optical laser beams
with nonlinear crystals. The optical sources typically used in DFG systems are narrowband IR (λ = 0.8–2.1 μm) laser beams with slightly different frequencies (ω 1 and ω 2 ).
When these sources interact with nonlinear crystals, most commonly gallium selenide
or arsenide (GaSe or GaAs), they generate emission at a beat frequency equal to the difference between the two input frequencies (ω THz = ω 1 − ω 2 ). DFG systems are attractive
THz sources for spectroscopy applications because they are easily tunable, provide THz
radiation with frequencies ranging from 0.2 to 6 THz, and provide high-peak powers
ranging from 10 2 to 10 5 mW (Shi et al. 2002). However, due to their low average power,
DFG sources are not commonly used for THz bioeffects studies. The primary challenge
with DFG techniques is that the nonlinear optical materials have very high absorption
coefficients, which make THz generation inefficient. In recent years, several methods
have been developed to overcome this limitation. Among these, quasi-phase-matching
(QPM) techniques show tremendous promise (Suizo and Kawase 2008).
7.2.4.2 Optical Rectification
In OR sources, THz radiation is generated using nonlinear media and difference frequency mixing approaches between the Fourier components of an optical pulse. OR
systems typically consist of an optical pump source, most often a femtosecond (fs) optical laser source, and thin electro-optic nonlinear crystals. OR sources have been shown
to generate broadband THz radiation with pulse energies of 10 μJ and a peak power of
100 μW. Such systems are commonly used in THz spectroscopy systems.
7.2.4.3 Optical Parametric Oscillation
Optical parametric generation is a second-order (χ 2 ) nonlinear optical process where
the photon of a pump pulse is converted into two photons with lower quantum energies.