Electromagnetic fields

Gamma rays define the highest frequency, highest energy photons known to man ($E=h\nu =hc/\lambda$, with $h$ Planck's constant). The wavelength is $\lambda \lt 0.01\,{\rm{nm}}$ and has no lower limit. They are produced, and thus used to study, high energy phenomena in nuclear physics. Gamma rays are also used in astronomy as they can report on high-energy interaction deep into the stars.

In medicine, gamma rays are commonly used for diagnostic imaging, for example, in positron emission tomography. In recent years, gamma radiation has been increasingly used for cancer radiation therapy.

X-ray radiation has lower wavelengths, but is still capable of penetrating large objects without significant absorption. Telescopes based on x-ray radiation are broadly used for studying high-energy processes in, for example, black holes, neutron stars, and coronas of stars. Gamma and x-ray telescopes must operate in space due to the high absorption in the atmosphere.

X-rays are routinely used in medicine for radiography and computed tomography. This radiation is not significantly absorbed in soft tissues (without contrast agents), but provides excellent contrast of bone. X-ray wavelengths are above $\lambda \,10\,{\rm{nm}}$ and overlap with gamma rays at the short frequency end.

Ultraviolet (UV) is below x-ray in energy, but still energetic enough that it can ionize atoms, as can x-rays and gamma rays. As a result, UV can be harmful to living tissue. Even at frequencies below the threshold for ionization, UV can damage chemical bonds and turn molecules into reactive compounds, which are also potentially harmful. Such radiation can induce, for example, irreparable damage to the DNA of living cells. Exposure to UV is strongly correlated with skin cancer. Because UV is commonly used as the excitation light in fluorescence microscopy, cell viability is often a concern when studying live specimens.

UV wavelength is roughly within the range $10\lt \lambda \lt 400\,{\rm{nm}}$. Vacuum UV (VUV) defines the spectral range below $200\,{\rm{nm}}$, while extreme UV (EUV) covers the end of the high energy spectrum, $10\lt \lambda \lt 121\,{\rm{nm}}$. The longer wavelength range is separated into UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm).

The atmosphere blocks approximately 75%–80% of UV radiation. The portion that reaches the surface of the Earth consists of >95% UVA, <5% UVB, and no UVC. The sunlight reaching the Earth consists only of approximately 3% UV. Despite its potential harmful effects to living organisms, UVB is involved in metabolizing vitamin D, which is crucial for developing bone strength.

The visible (VIS) spectrum covers roughly the 380–760 nm range, where the human retina exhibits its highest sensitivity. The colors of surrounding objects are due to their absorption and reflection properties. For example, plants look green because chlorophyll absorbs strongly in blue and red. Thus, they reflect the 'left-over light', the green. A prism separates the sunlight into colors because the glass has dispersion, i.e. its refractive index depends on wavelength, and the refraction angle at the prism surface is different for different colors. As described later, in chapter 4, the human retina reaches a maximum sensitivity around $550\,{\rm{nm}}$, corresponding to the green color.

Infrared (IR) covers a broad range of wavelengths that are larger than that of the red color, 0.75–1.000 μm. This range is further split into three regions: near infrared (NIR, 0.75−2.5 μm) mid-infrared (MIR, 2.5–10 μm), and far-infrared (FIR, 10–100 μm). NIR radiation is involved in similar electronic charge interactions as visible light and can be detected by solid-state sensors (see chapters 12–13). MIR is a spectral range in which bodies at room temperature can radiate. Because MIR contains fine spectral structures due to specific molecular vibrations, it is sometimes called the fingerprint region. The FIR region contains radiation absorbed (and generated) by the rotational modes of molecules. For example, water molecules in the atmosphere absorb strongly in this range and render the atmosphere opaque to these frequencies, except for $\lambda \gt 200\,{\rm{\mu }}{\rm{m}}$. The NIR spectral region is broadly employed in biophotonics, particularly due to the tissue penetration, which is typically deeper that in the VIS range. IR is also used to study tissues with chemical specificity.

Terahertz (THz) (also submillimeter) radiation covers the wavelength region 0.1–1 mm. Due to the significant absorption by the atmosphere, THz cannot be used for long-range communications. With recent improvements in sources and detectors, THz radiation has received significant attention of late for imaging applications.

Microwaves span the wavelengths $1\,{\rm{mm}}\lt \lambda \lt 100\,{\rm{mm}}$. They are typically generated by antennas (e.g. in cell phones). However, microwaves can interact with rotational and vibrational modes of molecules, eventually converting the energy into heat. Heating food by microwave ovens is a notorious example, where energy is transferred to the rotational modes of water and, finally, into heat. The microwave spectrum is broadly used in satellite communication, radar, and wireless connections. Microwaves have been used for the thermal treatment of tumors.

Radio frequency waves are emitted and detected by antennas of various dimensions, depending on the wavelength. They cover the broad range of wavelengths $\lambda \gt 10\,{\rm{cm}}$. Radiowaves are by far the most commonly used region of the electromagnetic spectrum for long distance communication: radio, television, global positioning systems, radar, etc. As a result, the frequency bands are highly regulated.

Biophotonics encompasses the interaction of tissue with the electromagnetic fields, generally, from IR to UV. The type of interaction between the field and specimen determines what information can be extracted from the tissue. The subsequent volumes discuss in detail the principles of various methods that have a particular field–matter interaction as the starting point. The particular frequency range used determines whether the object of interest is:

• dispersive, that is, with a wavelength-dependent refractive index;
• spatially inhomogeneous, that is, with a spatially-dependent refractive index;
• anisotropic, that is, with a polarization-dependent refractive index;
• nonlinear, that is, with an irradiance-dependent refractive index.

For now, we postpone the rigorous discussion of these phenomena. Instead, we briefly describe next the spectral absorption of water and hemoglobin, as they are crucial contributors to the overall light attenuation in tissues.