380
Electromagnetic Fields in Biological Systems
electronic devices. Electronic devices typically consist of a microwave synthesizer or
oscillator, and a frequency multiplier element, which consists of an array of schottky
barrier diodes (SBDs). In brief, the oscillator functions to generate “seed” microwave
radiation, and the SBD array functions to multiply the frequency of the incoming microwave radiation to THz frequencies (i.e., frequency up-conversion).
Several features contribute to the utility of electronic sources. First, they provide high
average output power (typically, ~100 mW at lower THz frequencies). Second, they generate narrow line-width (10 −6 ), cw THz radiation. Finally, electronic sources are rugged,
compact, and operate at room temperature. Due to these properties, solid state electronic THz sources are commonly used in both basic and applied research. However,
despite their incredible efficiency at lower THz frequencies, such approaches are limited
and are only capable of generating a few mWs of power at higher frequencies. In fact, the
output power of electronic sources has been reported to drop off between 1/f 2 and 1/f 3
with increases in frequency. Thus, significant advances must be made in order to create
electronic sources that operate more efficiently at these higher frequencies.
In recent years, several groups have addressed these fundamental challenges. A few
notable examples of recent progress include: InP and GaAs Gunn diodes (Eisele and
Kamoua 2006; Eisele 2010), frequency multiplier units based on SBDs, impact ionization avalanche transit-time devices (IMPATTs) (Mukherjee et al. 2007; Mukherjee
and Roy 2010), tunneling transit-time diodes (TUNNETT) (Buniatyan et al. 2004;
Nishizawa et al. 2008; Ryzhii 2009), and resonant tunneling diodes (RTDs) (Asada
2008). Figure 7.7a contains an image of the InP Gunn diode created by Dr. Eisele, a professor from the University of Leeds in the United Kingdom (Eisele and Kamoua 2006;
Eisele 2010). This particular system uses a 160 GHz oscillator seed and up-converts it to
produce a third harmonic at 0.48 THz. This source generates greater than 100 mW of
power at 0.1 THz and 0.1 mW at 0.48 THz (Figure 7.8). Varactors, varistors, and other
advanced frequency-multiplier systems are also being developed at NASA Jet Propulsion
Laboratory (JPL) and Virginia Diodes, Inc. (Figure 7.7a). These systems provide nearly
a milliwatt of power at frequencies greater than 1 THz. Overall, electronic solid-state
devices continue to be reliable sources for generation of low-frequency THz radiation.
With future advances in fabrication techniques, such sources may find increased use in
THz bioeffects studies.
7.2.3 Accelerating Electron-Based Terahertz Sources
Several THz sources use electron accelerators and periodic beam undulation techniques
to generate bright THz radiation. Common examples include backward wave oscillators
(BWOs), travelling wave tubes (TWTs), klystrons, gyrotrons, and free-electron lasers
(FELs). Interestingly, despite their striking differences in appearance and size, all of
these devices function using the same general operation principle. Basically, an electron
beam is accelerated, collimated, and modulated using magnets and an external structure (i.e., comb grating in a BWO and a wiggler in a FEL). The external structure functions to create a periodic acceleration of the electrons in the beam, which directly results
in the generation of THz radiation. Further details regarding the specific elements and
operation principles of BWOs and FELs are provided below.
Electromagnetic Fields in Biological Systems
electronic devices. Electronic devices typically consist of a microwave synthesizer or
oscillator, and a frequency multiplier element, which consists of an array of schottky
barrier diodes (SBDs). In brief, the oscillator functions to generate “seed” microwave
radiation, and the SBD array functions to multiply the frequency of the incoming microwave radiation to THz frequencies (i.e., frequency up-conversion).
Several features contribute to the utility of electronic sources. First, they provide high
average output power (typically, ~100 mW at lower THz frequencies). Second, they generate narrow line-width (10 −6 ), cw THz radiation. Finally, electronic sources are rugged,
compact, and operate at room temperature. Due to these properties, solid state electronic THz sources are commonly used in both basic and applied research. However,
despite their incredible efficiency at lower THz frequencies, such approaches are limited
and are only capable of generating a few mWs of power at higher frequencies. In fact, the
output power of electronic sources has been reported to drop off between 1/f 2 and 1/f 3
with increases in frequency. Thus, significant advances must be made in order to create
electronic sources that operate more efficiently at these higher frequencies.
In recent years, several groups have addressed these fundamental challenges. A few
notable examples of recent progress include: InP and GaAs Gunn diodes (Eisele and
Kamoua 2006; Eisele 2010), frequency multiplier units based on SBDs, impact ionization avalanche transit-time devices (IMPATTs) (Mukherjee et al. 2007; Mukherjee
and Roy 2010), tunneling transit-time diodes (TUNNETT) (Buniatyan et al. 2004;
Nishizawa et al. 2008; Ryzhii 2009), and resonant tunneling diodes (RTDs) (Asada
2008). Figure 7.7a contains an image of the InP Gunn diode created by Dr. Eisele, a professor from the University of Leeds in the United Kingdom (Eisele and Kamoua 2006;
Eisele 2010). This particular system uses a 160 GHz oscillator seed and up-converts it to
produce a third harmonic at 0.48 THz. This source generates greater than 100 mW of
power at 0.1 THz and 0.1 mW at 0.48 THz (Figure 7.8). Varactors, varistors, and other
advanced frequency-multiplier systems are also being developed at NASA Jet Propulsion
Laboratory (JPL) and Virginia Diodes, Inc. (Figure 7.7a). These systems provide nearly
a milliwatt of power at frequencies greater than 1 THz. Overall, electronic solid-state
devices continue to be reliable sources for generation of low-frequency THz radiation.
With future advances in fabrication techniques, such sources may find increased use in
THz bioeffects studies.
7.2.3 Accelerating Electron-Based Terahertz Sources
Several THz sources use electron accelerators and periodic beam undulation techniques
to generate bright THz radiation. Common examples include backward wave oscillators
(BWOs), travelling wave tubes (TWTs), klystrons, gyrotrons, and free-electron lasers
(FELs). Interestingly, despite their striking differences in appearance and size, all of
these devices function using the same general operation principle. Basically, an electron
beam is accelerated, collimated, and modulated using magnets and an external structure (i.e., comb grating in a BWO and a wiggler in a FEL). The external structure functions to create a periodic acceleration of the electrons in the beam, which directly results
in the generation of THz radiation. Further details regarding the specific elements and
operation principles of BWOs and FELs are provided below.