371
Terahertz Radiation
it into a logical structure that provides readers with a framework to better understand
THz bioeffects. The need to fill this void prompted us to write this book chapter.
The purpose of this chapter is twofold. First, to provide readers with a common reference that contains the necessary background concepts in biology, physics, and THz
technology, which are required to both conduct and evaluate THz biological research.
Second, to provide a review of the THz bioeffects reports that have been published in
the literature.
The contents of this chapter are divided into six sections. The first section provides a
general introduction to the THz spectral region. The second and third sections then provide an overview of THz sources and applications, respectively. Following the description of THz sources and applications, the fourth section then provides an overview of
the fundamental concepts that are required to understand the interaction of THz radiation with biological systems. This section begins with details regarding the composition
and function of skin, the primary biological target for THz radiation. This discussion
is followed where emphasis is placed on the fundamental biophysical mechanisms that
govern THz-skin interactions, including a discussion on energy deposition, temperature transients, and conventional thermal effects. The concepts described in this section are particularly valuable for the proper analysis of the pleotropic effects observed
in THz bioeffects studies. The fifth section then provides a comprehensive review of
THz bioeffects studies performed to date. We end this chapter with a conclusion section
where we address challenges and future opportunities in this field.
7.1.1 The Terahertz Region: Fundamental Units with
Biological Relevance
The THz spectral band is a region of the EM spectrum that neighbors the infrared
(IR) region on the high-frequency side and the microwave (MW) region on the lowfrequency side (Figure 7.1). This region is defined as frequencies ranging from 0.1 to
10 THz (1 THz = 10 12 Hz), or the wavelengths ranging from 30 to 3000 μm. The size of a
THz wave has biological relevance in regards to the following general concepts: spatial
resolution of imaging applications, scattering properties in biological tissues, and direct
biomolecular excitation.
First, modalities that employ THz wavelengths can provide images with higher spatial
resolution than counterpart systems, which use longer wavelengths, such as millimeterwave imaging systems (i.e., Rayleigh criterion). The practical value of this property was
first demonstrated in 1995 at Bell Laboratories (Hu and Nuss 1995). In this study, Hu
and Nuss illustrated that THz waves could be collimated and focused down to the diffraction limit of a few hundred microns at the surface of a biological sample. This finding was particularly compelling because it demonstrated that THz-imaging approaches
permit the accurate measurement of biochemical compositions.
In addition to resolution, the size of a THz wave is also relevant to the scattering processes associated with imaging applications. This feature stems from the fact that THz
waves are larger in size than many biological structures: mammalian cells (10–25 μm),
arterioles (30 μm), collagen fibrils (20–50 μm), human hair (50–70 μm), and the thickness of epidermis (∼500–1500 μm). Because photons are known to be scattered most
Terahertz Radiation
it into a logical structure that provides readers with a framework to better understand
THz bioeffects. The need to fill this void prompted us to write this book chapter.
The purpose of this chapter is twofold. First, to provide readers with a common reference that contains the necessary background concepts in biology, physics, and THz
technology, which are required to both conduct and evaluate THz biological research.
Second, to provide a review of the THz bioeffects reports that have been published in
the literature.
The contents of this chapter are divided into six sections. The first section provides a
general introduction to the THz spectral region. The second and third sections then provide an overview of THz sources and applications, respectively. Following the description of THz sources and applications, the fourth section then provides an overview of
the fundamental concepts that are required to understand the interaction of THz radiation with biological systems. This section begins with details regarding the composition
and function of skin, the primary biological target for THz radiation. This discussion
is followed where emphasis is placed on the fundamental biophysical mechanisms that
govern THz-skin interactions, including a discussion on energy deposition, temperature transients, and conventional thermal effects. The concepts described in this section are particularly valuable for the proper analysis of the pleotropic effects observed
in THz bioeffects studies. The fifth section then provides a comprehensive review of
THz bioeffects studies performed to date. We end this chapter with a conclusion section
where we address challenges and future opportunities in this field.
7.1.1 The Terahertz Region: Fundamental Units with
Biological Relevance
The THz spectral band is a region of the EM spectrum that neighbors the infrared
(IR) region on the high-frequency side and the microwave (MW) region on the lowfrequency side (Figure 7.1). This region is defined as frequencies ranging from 0.1 to
10 THz (1 THz = 10 12 Hz), or the wavelengths ranging from 30 to 3000 μm. The size of a
THz wave has biological relevance in regards to the following general concepts: spatial
resolution of imaging applications, scattering properties in biological tissues, and direct
biomolecular excitation.
First, modalities that employ THz wavelengths can provide images with higher spatial
resolution than counterpart systems, which use longer wavelengths, such as millimeterwave imaging systems (i.e., Rayleigh criterion). The practical value of this property was
first demonstrated in 1995 at Bell Laboratories (Hu and Nuss 1995). In this study, Hu
and Nuss illustrated that THz waves could be collimated and focused down to the diffraction limit of a few hundred microns at the surface of a biological sample. This finding was particularly compelling because it demonstrated that THz-imaging approaches
permit the accurate measurement of biochemical compositions.
In addition to resolution, the size of a THz wave is also relevant to the scattering processes associated with imaging applications. This feature stems from the fact that THz
waves are larger in size than many biological structures: mammalian cells (10–25 μm),
arterioles (30 μm), collagen fibrils (20–50 μm), human hair (50–70 μm), and the thickness of epidermis (∼500–1500 μm). Because photons are known to be scattered most