Biophotonics is a thriving research field with numerous groups now contributing to the steadily increasing numbers of subfields of the general subject. As the guest editor of this special section of Laser & Photonics Reviews, named Biophotonics – Techniques and Applications, I recall the early days of the emerging field, when the number of groups in the area (not including the pure surgical practitioners of lasers in medicine) could be counted on the fingers of one hand. We in the Lund University group were very proud when we published our first regular paper in biophotonics in 1984 , quickly followed by a letter paper , now almost 30 years ago. Numerous conferences are now dedicated to the field – its importance can, e.g., be illustrated by observing that Biophotonics is the largest subconference at the annual SPIE Photonics West conference in San Francisco, where this year almost 2000 presentations on Biophotonics were made. A broad stream of new textbooks, handbooks, and reviews on the topic is forcefully flowing. Academic front-line research is accompanied by product development with a strong and growing impact on today's clinical practice.
Laser & Photonics Reviews, being a key publication for authoritative reviews in a broad and quickly expanding field that now influences most aspects of human life, technology and industrialization, is certainly a most appropriate forum for presenting frontline developments in biophotonics. I certainly felt proud when asked to be guest editor for a focus section on this topic. But I also felt very humble in view of the challenge to make some selections that would illustrate the vigor of the field without being even close to any type of comprehensiveness. The eight reviews included in this issue illustrate both the status of some more established fields and also challenges addressed by new approaches.
Biophotonics relies heavily on the availability of efficient, reliable and cost-effective laser sources. Two reviews address this area. A. Müller, S. Marschall, O. Bjarlin Jensen, J. Fricke, H. Wenzel, B. Sumpf, and P. E. Andersen provide a review on Diode laser based light sources for biomedical applications [605–627], and H. Tu and S. A. Boppart discuss Coherent fiber supercontinuum for biophotonics [628–645].
Fluorescence and Raman techniques provide important possibilities for diagnostics and characterization of human tissues. Fluorescence has been utilized from the early days of biophotonics, and the field is steadily developing dynamically. M. Olivo, C. Jun Hui Ho, and C. Yaw Fu discuss Advances in fluorescence diagnosis to track footprints of cancer progression in vivo [646–662]. Endogenous as well as added sensitizing agents are utilized to find demarcation of malignant tissue. When using external marker chromophores in low concentrations, their characteristic signatures can be buried in strong autofluorescence. Here, upconverting nanoparticles can be a remedy. This emerging field is being treated by C. T. Xu, Q. Zhan, H. Liu, G. Somesfalean, J. Qian, S. He, and S. Andersson-Engels in their review Upconverting nanoparticles for pre-clinical diffuse optical imaging, microscopy and sensing: Current trends and future challenges [663–697]. Raman spectroscopy provides direct mapping of molecular bonds and has substantial potential for tissue characterization. I. Latka, S. Dochow, C. Krafft, B. Dietzek, and J. Popp in particular address how the techniques can be made clinically accessible in Fiber optic probes for linear and nonlinear Raman applications – Current trends and future development [698–731].
Scattering and absorption in human tissue limit the depth at which optical techniques can provide information and what image clarity can be obtained. Such issues are addressed in two reviews. Scattering is mostly due to varying optical indices of refraction for different organelles. A remedy for this is discussed by D. Zhu, K. V. Larin, Q. Luo, and V. V. Tuchin in Recent progress in tissue optical clearing [732–757]. Achieving spectroscopic contrast by using light absorbed by tissue structures, but detecting that specific absorption did happen using photoacoustic techniques is discussed by J. Yao and L. V. Wang in Photoacoustic microscopy [758–778]. Penetration and clarity are greatly enhanced.
Finally, a new technique that combines elements from tissue optics with atmospheric gas monitoring is treated by S. Svanberg in Gas in scattering media absorption spectroscopy – from basic studies to biomedical applications [779–796]. Sinus cavity diagnostics and neonatal children surveillance are emerging.
I hope this selection of reviews provides an interesting, albeit fragmentary insight into the current state of several biophotonic techniques and applications.