2.1.2. Raman Spectroscopy

Excitation light sources for Raman spectroscopic systems are typically high-power monochromatic laser sources, operating with more than 50 mW output power in the visible or in the near-infrared, typically at 785 nm or 1064 nm wavelengths. While external laser sources have primarily been used for Raman spectroscopy, recent advances in silicon photonics have made possible a wide variety of compact and chip-based lasers [46], which would ultimately pave the way for an integrated Raman system. However, to date, there are only a few examples of miniaturized laser sources combined with on-chip Raman systems. The difficulties in limiting the realizations of such systems arise from the requirement of a high-power monochromatic light source, and the difficulty in separating the pump and the scattered Raman signal with a high-enough extinction ratio on a chip. The strict

requirement of high laser power is motivated by the inherent weak scattering efficiency of the Raman signal [47]. Several strategies have been proposed to increase the strength of the Raman signal, such as confining the light to a small volume, particularly through the use of nanophotonic platforms, such as enhanced hotspot formation through the employment of metallic nanostructures in surface-enhanced Raman configuration [48,49] or through the use of slot waveguide platforms [50,51]. Working in a lower wavelength regime also helps to increase the scattered signal intensity since the scattering varies inversely as the fourth power of wavelength. However, this is often accompanied by the presence of unwanted fluorescence background [52].

Large-throughput spectrometers have traditionally been a major and integral part of a Raman spectrometer setup (see Section 2.4). A recent work by Atabaki et al. has demonstrated the concept of using tunable lasers as a way of eliminating the spectrometer from a Raman setup, through a method called swept-source Raman spectroscopy (SRSS) [53]. In this case, an excitation laser with only a few mWs of power was tuned in combination with a narrow bandpass filter on the detector side, which resulted in significantly high optical throughput compared to benchtop and compact handheld dispersive Raman spectrometers. A MEMS tunable laser was used, based on the concept of vertical-cavity surface-emitting lasers (VCSELs), thus demonstrating the suitability of VCSEL lasers for use in miniature Raman sources, as was also proposed earlier [54,55]. This work represents a major step toward the realization of miniature light sources for Raman spectroscopy.
