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In the broadest sense a spectrometer is any instrument that is used to measure the variation of a physical characteristic over a given range; i.e. a spectrum. This could be a mass-to-charge ratio spectrum in the case of a mass spectrometer, the variation of nuclear resonant frequencies in an NMR spectrometer or the change in the absorption and emission of light with wavelength in an optical spectrometer.
Examples of spectrometers are devices that separate particles, atoms, and molecules by their mass, momentum, or energy. These types of spectrometers are used in chemical analysis and particle physics.
NMR Spectrometer
The NMR Spectrometer observes and measures the interaction of nuclei spins when the sample is placed in a strong, constant magnetic field. The NMR signal is produced when the nuclei interacts with the magnetic field at a frequency that resonates with the frequency of the nuclei.
The intramolecular magnetic field surrounding the atom in a molecule changes with the resonance frequency, therefore revealing the molecular structure of the sample.
Mass Spectrometer
A mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. A Mass Spectrometer measures the mass-to-charge ratio of ions and identifies the composition of elements present in a sample. This works by ionizing a sample, which causes some of the molecules to become charged and separate according to their mass-to-charge ratio. These ions are then detected by a device that can detect charged particles.
Explosive detention using Mass Spectrometry
The threats of terrorism and criminal bombing are becoming ever more serious problems for all countries. To find hidden explosives, various types of detectors for explosives have been developed. They are classified into two categories: bulk detection and trace detection. Bulk detection includes X-ray imaging, nuclear quadrupole resonance (NQR), and neutron techniques that are used to determine the shape and size of suspicious objects in luggage, letters, packages, etc. However, bulk detection is not capable of easily identifying what the suspicious objects are.
Trace detection, on the other hand, includes chemical-analysis methods such as chemical luminescence, ion mobility spectroscopy, and mass spectrometry,1 has been applied to the detection of trace contaminants that are present on a passenger’s body, clothes, and luggage. Trace detection permits specific chemicals to be identified, and its selectivity is higher than that of bulk detection. However, it cannot determine the actual amount of chemicals in a suspicious item such as luggage. As mentioned above, the characteristics of bulk detection and trace detection are different. To improve security at important facilities, therefore, the combined use of bulk detection and trace detection is recommended.
Optical Spectrometer technology
Optical spectrometers (often simply called “spectrometers”), in particular, show the intensity of light as a function of wavelength or of frequency. An Optical Spectrometer measures the properties of light, usually near the optical region in the electromagnetic spectrum, i.e. ultraviolet, visible and infrared light.
In visible light a spectrometer can separate white light and measure individual narrow bands of color, called a spectrum. The different wavelengths of light are separated by refraction in a prism or by diffraction by a diffraction grating. Ultraviolet–visible spectroscopy is an example.
The change in the absorption and emission of the light intensity with wavelength allows for materials to be identified.
The goal of any optical spectrometer is to measure the interaction (absorption, reflection, scattering) of electromagnetic radiation with a sample or the emission (fluorescence, phosphorescence, electroluminescence) of electromagnetic radiation from a sample. Optical spectrometers are concerned with electromagnetic radiation that falls within the optical region of the electromagnetic spectrum which is light spanning the ultraviolet, visible and infrared wavelength regions of the spectrum.
A spectrometer consists of three main components – entrance slit, grating and detector.
Entrance Slit
Light from the source enters the entrance slit and the size of the slit determines the amount of light that can be measured by the instrument. The slit size also affects the optical resolution of the spectrometer, where the smaller the slit size, the better the resolution.
The beam becomes divergent after passing through the slit and by reflecting the divergent beam on a collimating mirror, the beam becomes collimated. Collimated rays are then directed towards a diffraction grating. The grating acts as a dispersive element and splits the light into its constituent wavelengths.
Grating
A monochromator uses a phenomenon of optical dispersion in a prism or diffraction from diffraction gratings to select a particular wavelength of light. In traditional spectrometers, prisms were used to disperse light. However, with the invention of the diffraction grating, it became the most used monochromator in modern spectrometers as it has more advantages over the prism.
Both devices are capable of splitting light into several colours, but a diffraction grating can be made to spread the colours over a bigger angle than a prism. Prisms also have a higher dispersion only in the UV region while diffraction gratings have a high and constant dispersion across the UV, VIS and IR spectrum.
Once the light hits the diffraction grating, each wavelength is reflected at a different angle. Diffraction grating of different sizes are also used to determine different wavelength ranges. The beam becomes divergent again after being reflected from the grating, thus it hits a second mirror to focus and direct it towards the detector.
Detector
The detector captures the light spectra and measures the intensity of light as a function of wavelength. These data are then digitized and plotted onto a software as a graph.
Compact on Chip spectrometers
With potential applications that range from detecting greenhouse gases to making self-driving vehicles safer, there has been a great deal of interest in recent years in developing compact, on-chip spectrometers. An on-chip spectrometer would greatly expand the applications and accessibility of the technology.
The miniaturization of IR spectrometers could lead to their wider use in consumer electronics — for example, they could be implemented in a smartphone for monitoring food quality. They could be used to quickly detect certain chemicals without the need for laboratory equipment. Miniaturized spectrometers could also help users detect counterfeit medical drugs or greenhouse gases such as methane and carbon dioxide.
Ultracompact on-chip computational infrared spectrometer
In June 2021, a team of researchers in the US, Israel and Japan has developed an ultracompact mid-infrared spectrometer. The work is the result of a collaboration between Yale University, Bar-Ilan University, Israel and the National Institute for Materials Science, Japan.
The device incorporates black phosphorus (BP) for a spectrometer that is operational at a 2–9 µm wavelength range, based on a single tuneable photodetector. The material, which is about 10 nm thick, allows users to tune the light–matter interaction to capture the different spectral components—a key to the device’s success. Moreover, an advanced algorithm plays an equally important role in this spectrometer, partly shifting the innate complexity in spectroscopy from hardware to software.
The spectrometer’s dimensions of 9 × 16 µm are comparable to the wavelength of light that it measures. Even if it were possible to make the device smaller, it would not show much improvement due to the diffraction limit.
“It is very exciting to realize such a high-performance spectrometer with the ultimate compactness”, said Professor Doron Naveh of Bar-Ilan University. “We expect that the principle of leveraging advances in hardware and software simultaneously as shown in this work will lead to commercial applications in medicine, agriculture and food quality control.”
“This spectrometer shows an advantage over conventional light-splitting spectrometers because the light doesn’t need to be split into different parts spatially”, said Shaofan Yuan.
Unlike conventional spectrometers, the system does not rely on optical components such as interferometers or tuneable infrared lasers. That opens the possibility for an extreme miniaturisation of spectrometers and could enable on-chip, affordable mid-infrared spectroscopy and spectral imaging.
Ultra Compact IR spectrometer
In Oct 2022 it was reported that Scientists in Europe have collaborated to develop an ultracompact spectrometer design that offers large bandwidth, moderate spectral resolution, and a spectral sensitivity in the infrared (IR) region. According to the team, its design for a Fourier-transform waveguide spectrometer will allow optical measurement instruments to be integrated into compact devices such as consumer electronics and ultrasmall satellites.
Extreme miniaturization of IR spectrometers is critical for their integration into next-generation smartphones, wearables, and space devices. Though teams have demonstrated miniaturization efforts on various elements of spectrometers, such as dispersive elements, narrow band-pass filters, and Fourier-transform and reconstructive spectrometers, the scaling of spectrometers has traditionally required a trade-off between spectral bandwidth, resolution, and being limited to the visible spectral range.
Fourier-transform IR spectrometers combine large spectral bandwidth and resolution in the IR range and have yet to be fully miniaturized. Waveguide-based Fourier-transform spectrometers offer a low device footprint, but rely on bulky, expensive external imaging sensors such as InGaAs cameras. Currently, the size of the overall waveguide spectrometer cannot be smaller than commercially available detectors.
The experimental setup for the compact Fourier-transform waveguide spectrometer, which, according to its designers and developers, will allow optical measurement instruments to be integrated consumer electronics and ultrasmall satellites. The team used a red alignment laser to visualize the beam path from the fiber into the optical waveguide and its reflection at a gold mirror. Two microprobes were used to contact the photoconductor, the size of which is in the subwavelength range. Courtesy of Empa.
The experimental setup for the compact Fourier-transform waveguide spectrometer, which, according to its designers and developers, will allow optical measurement instruments to be integrated in consumer electronics and ultrasmall satellites. The team used a red alignment laser to visualize the beam path from the fiber into the optical waveguide and its reflection at a gold mirror. Two microprobes were used to contact the photoconductor, the size of which is in the subwavelength range. Courtesy of Empa.
The research team — scientists from Swiss Federal Laboratories for Materials Science and Technology (Empa), ETH Zurich, École Polytechnique Fédérale de Lausanne (EPFL), the University of Salamanca, the European Space Agency (ESA), and the University of Basel — built a proof-of-concept, miniaturized Fourier transform waveguide spectrometer that incorporated a subwavelength photodetector as a light sensor. The photodetector was based on colloidal mercury telluride quantum dots (HgTe CQDs) and was CMOS-compatible. The room-temperature-operated photodetector exhibits a spectral response up to a wavelength of 2 μm.
In addition, the wire-shaped, subwavelength-size photodetector was monolithically integrated with an optical waveguide. The monolithic integration of the photodetector downscaled the thickness of the imaging sensor by a factor of 1000. The result was a large-bandwidth, ultracompact (below 100 × 100 × 100 μm) IR micro-spectrometer with a spectral resolution of 50 cm−1.
IR photodetectors based on solution-processable QDs offer several advantages: They can be fabricated on various substrates, and their spectral response can be tuned by the size and composition of the QDs. For example, the absorption spectrum of HgTe QDs can be extended to cover the visible and IR regions and approach the THz region by varying the QD size. Subwavelength IR photodetectors traditionally rely on nonscalable device fabrication or require cryogenic cooling. The scaling of commercial IR detectors such as InGaAs and mercury cadmium tellurides down to subwavelength dimensions and their integration with optical waveguides are challenging.
The device’s photodetector, fabricated on top of a surface optical waveguide, consists of a bottom gold electrode at the bottom functioning as a scattering center, a photoactive layer consisting of colloidal mercury telluride (HgTe) quantum dots, and a top gold electrode. By moving the mirror, the measured photocurrent maps the light intensity of the standing wave, i.e., the IR light. A Fourier transformation of the measured signal gives the optical spectra.
The device’s photodetector, fabricated on top of a surface optical waveguide, consists of a gold electrode at the bottom functioning as a scattering center, a photoactive layer consisting of colloidal mercury telluride (HgTe) quantum dots, and a top gold electrode. By moving the mirror, the measured photocurrent maps the light intensity of the standing wave, that is, the IR light. A Fourier transformation of the measured signal gives the optical spectra. Courtesy of Lars Lüder.
HgTe QD-based photodetectors are typically fabricated either as photoconductors or photodiodes. To the best of the team’s knowledge, HgTe QD-based photodetectors have not been monolithically integrated into waveguide spectrometers until now.
According to Empa researcher Ivan Shorubalko, the demonstrated scaling could also be of interest to the development of miniaturized Raman spectrometers, biosensors, lab-on-a-chip devices, and high-resolution hyperspectral cameras. In addition, femtosatellites — space devices with a maximum weight below 100 g — will require ultracompact spectrometers.
The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01088-7).
References and Resources also include:
https://www.photonics.com/Articles/Miniaturized_Spectrometer_Proves_a_Fit_for/a68498
https://www.spectroscopyeurope.com/news/ultracompact-chip-computational-infrared-spectrometer
