Session 3: Optimizing Equipment for Optical Spectroscopy

In this session, the speaker, Adam Wise, explored the essential tools that drive this fascinating field to promote understanding of the equipment behind optical spectroscopy. Accompanied by a range of cameras optimised for spectroscopy and low-light imaging, the spectrometers offered by Andor empower researchers to explore the depths of the electromagnetic spectrum with unparalleled precision. Additionally, other instruments including optical cryostats to cool samples to liquid nitrogen or helium temperatures while maintaining optical accessibility and confocal microscope systems, exemplified by the dragonfly, that ensure high-resolution imaging and spectroscopic capabilities are offered. For this article, the focus turns to the fundamental equipment used in spectroscopy. Spectrometers and monochromators, understanding their basic design, the principles of dispersion, and how spectral resolution impacts data quality are crucial. Additionally, we'll delve into detectors, distinguishing between single and multi-channel systems and elucidating key performance parameters such as quantum efficiency, noise levels, and pixel size.

As a reminder, the terms spectrometer, spectrograph, and spectroscope are often used interchangeably, though you may hear the former more frequently nowadays. Essentially, these devices serve the purpose of separating light or electromagnetic radiation for measurement through a process known as dispersion. While spectrometers primarily utilise this method, other strategies such as interferometric methods or filters are also employed in some cases, like Fourier-transform infrared spectroscopy (FTIR). The essential components of a spectrometer include elements to control incoming light, a mechanism for dispersion or separating colours, a means to measure the light, optics for manipulation, and potentially motors for automation. These components work synergistically to facilitate accurate and precise measurements. Additionally, having a reference or standard for wavelength calibration is instrumental in ensuring the reliability of the obtained data.

Let's consider a simple yet instructive example from the 18th century. Picture a setup comprising an entrance slit, a prism for dispersion, and a white screen acting as the detector. As sunlight passes through the slit, the prism disperses the light, causing different colours to spread out vertically on the screen. This straightforward setup demonstrates the fundamental concepts of dispersion and color separation, showcasing how easily one can replicate a basic spectrometer at home with minimal equipment. Fast forward to modern times, and we see the evolution of spectrometers into sophisticated instruments like the Kymera 328i where advancements in technology have led to refinements and enhancements in performance.

Now, let's delve into an example of dispersion, focusing on the refraction of light by a prism. Snell's law defines how light bends when transitioning from one medium to another with a different refractive index. Continuing the exploration of dispersion in optical spectroscopy, it is critical to acknowledge the interplay between wavelength, refractive index, and dispersion. We often speak of refractive index as if it were a constant, but it varies significantly with wavelength. Considering two different colours, each experiencing a slightly different refractive index due to their distinct wavelengths, they will be bent through different angles, illustrating the concept of dispersion—spatially spreading light according to its wavelength, each experiencing its own unique dispersion.

Dispersion plays a pivotal role in spectroscopy, enabling the resolution of complex spectra like Joseph von Fraunhofer's iconic solar spectrum. Originally sketched in 1815, Fraunhofer's spectrum revealed hundreds of individual absorption lines, each representing a unique spectral feature. The significant dispersion present in his spectrum allowed for the clear delineation of these lines. However, when this spectrum is compressed, reducing the physical separation between colours, the individual lines become indiscernible. This lack of dispersion results in a loss of detail, rendering the spectrum less informative and challenging to analyse.

Spectral resolution refers to the ability of a spectrometer and its detector to distinguish between two closely spaced wavelengths. Imagine a scenario where our light source emits exactly two wavelengths, λ1, and λ2, corresponding to very red and very blue light, respectively (e.g., 650 nm and 450 nm). Our hypothetical spectrometer consists of an entrance slit, a dispersive element, and a detector, whether it's a piece of paper, film, or a camera. As light passes through the entrance slit, any light that makes it through is relatively well collimated, meaning it doesn't spread out much. The dispersive element then separates the wavelengths, with one bending up and the other bending down. Finally, the detector records the resulting spectrum. The distinct separation of the blobs corresponding to λ1 and λ2 on the detector allows for clear differentiation between the two wavelengths. This ability to resolve closely spaced wavelengths is crucial for accurately capturing fine spectral features and subtle shifts, such as Raman shifts or sub-nanometre variations. However, as the difference between wavelengths decreases, the challenge of spectral resolution intensifies.

Achieving high spectral resolution requires careful design and optimisation of the spectrometer's components, including the dispersive element, detector sensitivity, and signal processing algorithms. A strategy to enhance spectral resolution involves adjusting the size of the entrance slit through which light enters the spectrometer. By making the entrance slit smaller, the size of the spots on the detector decreases. This concept is akin to the design of the entrance slit on spectrometers like the Kymera, where two opposing jaws can be adjusted to control the size of the aperture. However, diminishing the entrance slit size too much can lead to diffraction which occurs when light passing through a small aperture spread out, akin to ripples in a pond, ultimately limiting the effectiveness of this approach.

Alternatively, we can increase the dispersion angle within the spectrometer to achieve better separation between wavelengths. By widening the dispersion angle, even a small wavelength difference of 1 nanometre could result in two clearly resolved spots on the detector, rather than merging. One way to achieve this is by using a different dispersive element, such as a diffraction grating, which offers advantages over prisms in terms of flexibility and performance. Unlike prisms, which are limited by the specific properties of the materials they are made from, diffraction gratings can be engineered to provide precise and customisable dispersion characteristics. Additionally, advancements in fabrication techniques have made diffraction gratings more accessible and cost-effective, further enhancing their appeal in spectroscopic applications.

Exploring the workings of diffraction gratings sheds light on their efficacy in optical spectroscopy. Unlike prisms, which exploit the dispersion of light through refraction, diffraction gratings operate based on the principle of diffraction. To grasp this concept, recall the interference pattern observed when light passes through a narrow slit, generating a characteristic diffraction pattern. In the case of a single slit, light passing through produces a diffraction pattern characterised by a central peak flanked by smaller side lobes. However, what makes diffraction gratings powerful is their ability to employ arrays of slits. By arranging thousands of slits in concert, diffraction gratings can diffract light of different wavelengths in distinct directions, thereby achieving dispersion. This dispersion results from the angular separation of wavelengths induced by the diffraction pattern. What makes diffraction gratings particularly versatile is their ability to function in both transmission and reflection modes. In transmission mode, light passes through the grating, while in reflection mode, light is bounced off the grating surface, still producing diffraction and dispersion.

The behaviour of light diffracted by a grating is governed by the grating equation, which describes how the angle of diffraction (θD) depends on parameters such as the wavelength of incident light, the order of diffraction, and the spacing between the grating's grooves. By manipulating these parameters, researchers can tailor the diffraction angle to achieve specific spectral resolutions or bandwidths. Diving deeper into the grating equation offers insights into why diffraction gratings are favoured over prisms in spectrometers. Let's break down the equation to understand its significance further. The grating equation states that the product of the order of diffraction (N) and the wavelength of light (λ) equals the groove spacing of the grating (d) times the sum of the sine of the incident angle (θ) and the sine of the diffractive angle (θD). Simplifying the equation for a straightforward scenario where the incident angle is 0 (i.e., light hitting the grating straight on) and considering the first-order diffraction (N = 1), we arrive at a simplified form: λ = d * sin(θD). Now, if we aim to solve for the angle of diffraction (θD), which is typically our goal, the equation becomes θD = arcsin(λ / d).

This simple relationship illustrates how changing the groove spacing of the grating directly impacts the angle of diffraction. Increasing the groove spacing results in a larger diffraction angle, leading to greater dispersion, while decreasing the groove spacing narrows the angle, reducing dispersion. This flexibility allows for precise control over dispersion without the need for different types of glass, as required with prisms.

Gratings can be fabricated with varying densities of lines per millimetre, allowing for customisation of dispersion characteristics. Tighter slit spacing corresponds to more lines per millimetre, resulting in higher dispersion, whereas less tightly spaced lines yield lower dispersion. By replacing the diffraction grating with one containing more lines per millimetre, we increase dispersion. This concept is exemplified by spectrometers like the Kymera 328, Shamrock 500, and Shamrock 750, each offering varying levels of dispersion proportional to their size. Echelle spectrometers, like the Mechelle 5000, offer a unique approach to spectroscopy by employing two dispersive elements: a prism and a grating. These elements are oriented perpendicular to each other, dispersing light in both the Y and X directions. The result is an "Echellogram," resembling a page in a book, where spectral data is arranged in a spiral pattern across the sensor. This design enables the capture of a vast amount of information in a single shot, making it particularly suitable for applications such as optical emission spectroscopy and laser-induced breakdown spectroscopy (LIBS), where wide spectra with numerous narrow peaks are common.

Traditionally, photographic film was employed for this purpose, although its usage has waned with the advent of digital technology. Nowadays, detectors play a pivotal role in converting light into electrical signals for analysis. While photographic film has largely been replaced, the fundamental requirements for detectors remain constant: high sensitivity, spatial resolution, dynamic range, and for certain applications, high-time resolution

It's important to distinguish between point detectors and array detectors. Point detectors, such as photodiodes, photomultiplier tubes (PMTs), and avalanche photodiodes (APDs), consist of a single photosensitive element or pixel. In contrast, array detectors, which are more commonly used today, feature a vast array of photosensitive elements arranged in a two-dimensional grid. Examples include charge-coupled devices (CCDs), electron-multiplying CCDs (EMCCDs), intensified CCDs (ICCDs), complementary metal-oxide-semiconductor (CMOS) and scientific CMOS (sCMOS) cameras. These array detectors offer high spatial resolution and can contain millions of pixels, enabling the capture of detailed spatial and spectral information across a wide range of applications.