Biophotonics

The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics in that it is believed that photons will play a similar central role in future information technology as electrons do today.

Biophotonics has therefore become the established general term for all techniques that deal with the interaction between biological items and photons. This refers to emission, detection, absorption, reflection, modification, and creation of radiation from biomolecular, cells, tissues, organisms and biomaterials. Areas of application are life science, medicine, agriculture, and environmental science. Similar to the differentiation between "electric" and "electronics" a difference can be made between applications, which use light mainly to transfer energy via light (like Therapy or surgery) and applications which excite matter via light and transfer information back to the operator (like diagnostics). In most cases the term biophotonics is only referred to the second case.

Applications

Biophotonics can be used to study biological materials or materials with properties similar to biological material, i.e., scattering material, on a microscopic or macroscopic scale. On the microscopic scale common applications include microscopy and optical coherence tomography. On the macroscopic scale, the light is diffuse and applications commonly deal with diffuse optical imaging and tomography (DOI and DOT).

In microscopy, the development and refinement of the confocal microscope, the fluorescence microscope, and the total internal reflection fluorescence microscope all belong to the field of biophotonics.

The specimens that are imaged with microscopic techniques can also be manipulated by optical tweezers and laser micro-scalpels, which are further applications in the field of biophotonics.

DOT is a method used to reconstruct an internal anomaly inside a scattering material . The method is non invasive and only requires the data collected at the boundaries. The typical procedure involves scanning a sample with a light source while collecting light that exits the boundaries. The collected light is then matched with a model, for example, the diffusion model, giving an optimization problem.

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For the spontaneous low-level emission of photons from living tissues, see: Biophoton

Light Sources for Biophotonics

The most dominantly used light source are lasers. However also LED´s, SLED´s or lamps play an important role. Typical wavelengths, which are used in biophotonics are between 200 nm (UV) and 3000 nm (near IR).

Lasers

Lasers play a more and more important role in Biophotonics. Their unique intrinsic properties like precise wavelength selection, widest wavelength coverage, highest focussability and thus best spectral resolution, strong power densities and broad spectrum of excitation periods make them the most universal light tool for a wide spectrum of applications. As a consequence a variety of different laser technologies from a broad number of suppliers can be found in the market today.

Gas lasers

Major gas laser, which are used for biophotonics applications and their most important wavelengths are:

- Argon Ion laser: 457.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514,5 nm

- Krypton Ion laser: 350.7 nm, 356.4 nm, 476.2 nm, 482.5 nm, 520.6 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm, 752.5 nm, 799.3 nm

- Helium-Neon laser: 632.8 nm (543.5 nm, 594.1 nm, 611.9 nm)

- HeCd lasers: 325 nm, 442 nm

Other commercial gas lasers like carbon dioxide (CO2), carbon monoxide, nitrogen, oxygen, xenon-ion, excimer or metal vapor lasers have no or only very minor importance in biophotonics.

Diode Lasers

The most commonly integrated laser diodes, which are used for diode lasers in biophotonics are based either on GaN or GaAs semiconductor material. GaN covers a wavelength spectrum from 375 to 488 nm (commercial products at 515 have been announced recently) whereas GaAs covers a wavelength spectrum starting from 635 nm.

Most commoly used wavelengths from diode lasers in biophotonics are: 375, 405, 445, 473, 488, 640, 643, 660, 675, 785 nm.

Laser Diodes are available in 4 classes:

- Single edge emitter/broad stripe/broad area

- Surface emitter/VCSEL

- Edge emitter/Ridge waveguide

- Grating stabilized (FDB, DBR, ECDL)

For biophotonic applications the most commonly used laser diodes are edge emitting/ridge waveguide diodes, which are single transverse mode and can be optimized to an almost perfect TEM00 beam quality. Due to the small size of the resonator, digital modulation can be very fast (up to 500 MHz). Coherence length is low (typically < 1 mm) and the typical linewidth is in the nm-range. Typical power levels are around 100 mW (depending on wavelength and supplier).

Grating stabilized diode lasers either have an lithographical incorporated grating (DFB, DBR) or an external grating (ECDL). As a result, the coherence length will raise into the range of several meters, whereas the linewidth will drop well below picometers (pm). Biophotonic applications, which make use of this characteristics are Raman spectroscopy (requires linewidth below cm-1) and spectroscopic gas sensing.

Ultrachrome Lasers

Many advanced applications in biophotonics require individually selectable light at multiple wavelengths. As a consequence a series of new laser technologies has been introduced, which currently looks for precise wording.

The most commonly used terminology are supercontinuum lasers, which emit visible light over a wide spectrum simultaneously. This light is the filtered e.g. via acousto-optic modulators (AOM, AOTF) into 1 or up to 8 different wavelengths. Typical suppliers for this technology are Koheras or Fianium.

In another approach (Toptica/iChrome) the supercontinuum is generated in the infra-red and then converted at a single selectable wavelength into the visible regime. This approach does not require AOTF's and has a background-free spectral purity.

Since both concepts have major importance for biophotonics the umbrella term "ultrachrome lasers" is often used.

See also

• Kasper Roldsgaard
• Biology

Periodicals

• Photonics Spectra
• Laser Focus World
• Electro Optics
• Optics & Photonics Focus
• Nature Photonics
• Photonics news
• Industrial Laser Solutions
• Journal of Biophotonics

External links

• Biophotonics at the Fitzpatrick Institute at Duke University
• Biophotonics Laboratory at California Institute of Technology
• NSF Center for Biophotonics Science and Technology at UC Davis
• Centre for Biophotonics at University of Strathclyde, UK
• The Institute for Lasers, Photonics and Biophotonics University at Buffalo
• BIOP - Technical University of Denmark
• Biophotonics at Cedars-Sinai Medical Center, Los Angeles
• The University of St Andrews Biophotonics Collaboration, Scotland
• McMaster University Biophotonics Facility, Canada
• Biophotonics of the Blood
• Biophotonics International Magazine, published by Laurin Publishing Co. of Pittsfield, MA, USA
• PHOTONICS4LIFE, European Network of Excellence for Biophotonics
• Beckman Laser Institute at UC Irvine
• National Biophotonics and Imaging Platform Ireland
• Centre for Biophotonics at Lancaster University, UK

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