Semiconductor lasers enable advances in nonlinear biophotonics

Abstract

The successful demonstration of two-photon excitation fluorescence imaging of biotissues,1 such as mouse kidney and brain cells, has spurred development of a variety of novel multiphoton imaging (MPI) technologies. For example, using near-IR optical pulses, MPI can peer deep into tissues for noninvasive profiling. Moreover, because the fluorescence occurs only at the focal point of IR optical pulses, very long-time imaging is possible. This minimizes the consumption of fluorescent material. Many MPI technologies depend on high peak-power (over a kilowatt) ultrashort optical pulse sources to efficiently induce various multiphoton, nonlinear optical effects inside biospecimens. But most current sources are mode-locked solid-state lasers, which are bulky, expensive, and require maintenance. Increasing the practical appeal of MPI will require ‘real-world’ alternatives. Semiconductor laser diodes (LDs) have enjoyed substantial success as dependable, low-cost, high-performance light sources in information and communication technologies. In addition to their usefulness as mass-produced IT devices, LDs also have potential applications in scientific and technological measurement systems. Here we report evidence of their relevance to nonlinear bioimaging. In our research, LDs serve as key, easy-tooperate devices for generating stable picosecond optical pulses. A subsequent increase in optical power using low-noise and low-nonlinear-effect amplifiers raises the optical pulse peak power to more than a kilowatt, making it suitable for MPI. Because of the high peak power, efficient wavelength conversions using second-order nonlinear materials (e.g., ferroelectric optical crystals) and third-order materials (e.g., optical fibers, liquids) are also possible. Figure 1 presents the basic concept of our optical pulse source in schematic form. Figure 1. Schematic for generating kilowatt peak power picosecond optical pulses using a semiconductor laser oscillator. LD: laser diode.