Throughout recent times, the only effective sources for spectroscopy were discharge lamps, dye lasers, and optical parametric oscillators. Supercontinuum lasers, sensor plasma sources, and light-emitting diodes are a few examples of new broadband light sources that are now accessible. This article examines the potential of these choices for spectroscopic applications.
Since spectroscopic systems depend on light sources, new broadband light sources present spectroscopists with fresh prospects. For most of the previous century, only limited broadband light sources were accessible.
A new generation of robust broadband light sources, however, is gradually taking over the spectroscopist’s toolkit. Superluminescent diodes, also known as SLEDs, have now replaced supercontinuum lasers, plasma sources powered by lasers, and other laser-driven devices in spectroscopic applications.
A new crop of sources that are comparatively tough is now gaining a place in the spectroscopist’s toolbox. Spectroscopic applications include supercontinuum lasers, laser-driven plasma sources, and high brightness LEDs in a variety of colors. Let’s discuss each of them quickly.
Supercontinuum Lasers for Spectroscopy
A supercontinuum is produced when a laser pulse’s frequency spectrum is broadened through one or more nonlinear processes. This phenomenon can also occur when spatial processes cause an electron beam’s frequency range to expand, leading to the creation of a supercontinuum. The mechanisms behind supercontinuum formation are complex and vary depending on the medium in which the process occurs.
In optical fibers, for instance, the interaction of stimulated Raman scattering and soliton effects enables the creation of a supercontinuum to the red of the pump beam using a spectrally wide pump beam. These nonlinear processes contribute to the broadening of the light spectrum, allowing for a wider range of applications in spectroscopy.
These broadband light sources, although distinct from traditional lasers, share similarities with lasers made from broadband lamps. Their capabilities are leveraged in several advanced spectroscopic techniques, such as innovative optical coherence tomography, visible cavity-enhanced spectroscopy, and mid-infrared absorption spectroscopy setups. These applications highlight the versatility of supercontinuum lasers in scientific research.
The key characteristics that set supercontinuum light sources apart are their single-mode beam quality, laser-like aiming stability, and high brightness. These qualities make them highly valuable in spectroscopy, resembling the output of lasered broadband lights but with much greater precision and control.
Laser-Driven Light Sources for Spectroscopy
Although laser plasmas have historically been used as broadband light sources for spectroscopy, they have often been deemed ineffective for analytical spectroscopy due to their instability or weak output. However, recent advancements have improved their performance, making them more viable for various spectroscopic applications.
These laser-driven broadband light sources enhance spectroscopy across wavelengths from near-infrared to ultraviolet. Significant improvements, particularly in the UV spectrum, have been reported, as predicted by referenced studies. For instance, advancements in UV microscopy now allow for the quantitative analysis of biomolecules’ spectra, where the UV wavelength facilitates sub-cellular spatial resolution. This enables scientists to gain deeper insights into biological structures at a molecular level.
The method used by Energetic to produce these light sources involves focusing a continuous wave (CW) laser inside a light bulb filled with a noble gas, such as xenon. This setup differs from traditional lamps, as the electrodes in conventional lamps act as heat sinks, helping maintain the lamp’s temperature. However, these electrodes degrade over time, causing electrical contact to spray onto the lamp’s surface, which results in the need for frequent bulb replacements. This design improvement mitigates the typical limitations of older systems.
These specialized broadband light sources are also making significant contributions to spectroscopy from the near-IR to UV range. For example, scattering-type scanning near-field optical microscopy (s-SNOM) is often used to investigate chemical compositions and nanoscale photonic phenomena. With infrared light, it achieves spatial resolutions several orders of magnitude below the diffraction limit. Yet, as Wagner and colleagues noted in recent research, the use of s-SNOM has been constrained by the lack of affordable broadband infrared sources, a challenge that laser-driven light sources are beginning to address.
Superluminescent Diodes for Spectroscopy
Superluminescent light-emitting diodes, also known as SLEDs, are the latest in broadband light sources, having only recently become available across a wide spectral range. SLEDs have brought significant advancements, especially in spectroscopy, but they also face unique challenges in different applications. For instance, in commercial LED lighting, the primary difficulty has been achieving high-power, high-efficiency LEDs with color combinations that are visually appealing.
In spectroscopy, however, the focus is different. Instead of eye-pleasing color combinations, tunable light sources with high spectral purity, repeatability, and stability are preferred. One of the key challenges for LEDs in spectroscopy is their performance stability as a function of temperature. Maintaining spectral stability and intensity over varying temperatures is particularly crucial in precise measurements. A specific difficulty has been accessing electronic transitions of environmentally significant gas molecules, especially with UV light sources. This is because many gas molecules have transitions that fall in the ultraviolet spectrum, making it vital to have stable, tunable UV sources.
In addition to SLEDs, traditional broadband light sources such as conventional plasmas, globars, and even sunlight have been widely used in spectroscopy. However, newer, high-brightness sources are now expanding the range of possibilities for successful spectroscopic applications. These applications span from microscopy to gas sensing, and from industrial measurements to biological research, providing unprecedented accuracy and flexibility. As these modern broadband sources become more accessible, they are driving innovation across a range of industries.
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