Photonics is the science and technology of generation, manipulation, and detection of light. The
field uses the quantum-like particles of light, i.e., the photons, instead of electrons to transmit,
process, and store information. Biophotonics is recently emerged from the applications of photonics
in the fields of biology and medicine. The invention of lasers in the 1960s revolutionized photonics,
and made rapid technological advancements that produced useful tools, such as bar code scanners,
CD players and laser pointers that are already playing an important part in our daily life. The
fluorescence microscope is the first taste of the power of biophotonics that brought us the important
molecular information within cells in almost all biological laboratories. Today, biophotonics is
widely regarded as the key science upon which the next generation of clinical tools and biomedical
research instrumentation will be based. Although nature has used the principle of biophotonics to
harness light for photosynthesis, it wasn’t until about 10 years ago that a substantial translation of
photonics technologies to biological applications began to transform medical and life sciences.
The knowledge of biophotonics essentially includes the fundamentals of many interdisciplinary
fields and how they are uniquely related to each other. Researchers and students who are interested
in biophotonics should have a solid understanding of the physics of light, and the engineering of
devices and instruments that are used to generate, modify, and manipulate light. On the other hand,
they must also understand the fundamentals of biology and medicine, such as the molecular and
cellular processes that occur in living systems to properly and meaningfully utilize the biophotonics
techniques to address their biological questions. Healthy and diseased tissues have differing biolog-
ical processes in different states; thus, it is also important to have a fundamental understanding of
pathophysiology, and common disease states such as cancer, cardiovascular disease, neurodegener-
ative disease, and infectious dis ease, to name just a few.
Although still “young, biophotonics is now steadily becoming an important discipline that in-
vestigates fundamental principles and develops new optical technology for the interaction of light
or photons with biological organisms, tis sues, cells, and molecules. One of the most important
elements in this discipline is the rapid developments of innovative bio-imaging technologies that
brought us the opportunities to visualize tissue organization, biochemical compositions, and func-
tional information about tissue without any harm to its native state. The light with wavelengths be-
tween visible and near-infrared ranges is highly scattered within a turbid medium, such as biological
tissue. Therefore, the bio-imaging methods that attempt to form images from light passi ng through
tissue can be classified into two categories—ballistic (minimally scattered) optical microscopy and
diffuse (multiply scattered) optical tomography. The former provides fine resolution but with a shal-
low imaging depth in tissue—up to about 1–2 mm, as defined by the optical diffusion limit. The
representative technologies in this category include confocal microscopy, multiphoton microscopy,
optical coherence tomography, and others.
When incident photons reach their diffusion limit, most of them have undergone tens of scat-
tering events, making the ability to focus the light extremely difficult. Fortunately, diffuse optical
tomography can effectively utilize the multiply scattering photons to provide an image that repre-
sents information centimeters into tissue, albeit with poor spatial resolution—roughly about one-
third of the imaging depth. However, randomized paths of the diffuse photons render the image
reconstruction mathematically ill-posed. It still remains a challenge for pure optical imaging to
attain fine spatial resolution at depths beyond the optical diffusion limit. Until recently, photoacous-

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