Although there are many designs, the core of any optical spectrometer is some component that separates light by wavelength. Most commonly this component is a diffraction grating – a sheet of material etched with repeating grooves that causes light reflected from it or transmitted through it to diffract and bend at an angle proportional to its wavelength. Prisms – or some combination of gratings and prisms – can also be used to bend and separate light in this way. This bending and separation by wavelength is referred to as dispersion. If a camera is placed after the dispersive element, some geometry can be used to determine where on the camera different wavelengths will fall.
The type of light source input to the spectrograph is highly dependent on the experimental needs (lasers, HgAr calibration lamps, deuterium lamps, the light emission of samples, etc).
Used to switch between direct and side inputs and outputs when quick switching between multiple experimental setups is desired. (Choice of mirror material/coating can affect light throughput efficiency of the spectrograph).
Used to collimate the light from a slit or pinhole and direct it to a diffraction element as well as to focus dispersed light from the diffraction element onto wavelength dependent locations on the detector. (Choice of mirror material/coating can affect light throughput efficiency of the spectrograph).
Diffraction Element (Such as a grating, a prism, or a grism)
Used to disperse light to different angles based on the wavelength of the light. (Choice of diffraction element material/coating can affect light throughput efficiency of the spectrograph).
Detector / Camera
Once light has been dispersed by wavelength in a spectrograph, it is usually desirable to measure that light with a detector or camera. The choice of detector/camera is highly dependent on the experimental needs and often is best decided through discussion between an expert at Andor and a customer based on the customer’s experimental requirements. Factors that can influence the decision of detector/camera choice include light sensitivity, time gating, speed of acquisition, spectral range, detector width/height, customer budget, etc.
The Czerny-Turner spectrometer is one of the most robust and mature spectrometer designs. A curved (toroidal shaped) mirror is used to collect light from a source and collimate (or make parallel) the emission and reflect it to a planar diffraction grating. This grating, mounted on a precision rotation stage, disperses the light from the source and reflects it to a second curved (toroidal shaped) mirror that focuses the previously collimated light onto a different position on a detector for each wavelength of light. By rotating the grating, different ranges of wavelength can be brought to bear on the detector. By switching between gratings or focal lengths of the spectrometers, light can be dispersed to a greater or lesser extent to achieve different wavelength resolutions.
Most spectrometer designs use a single axis of dispersion – light is separated by wavelength only in one direction. When trying to measure very wide wavelength ranges, multiple diffraction orders can overlap. However, if a second dispersive element is added to the design these orders can be separated, creating a 2D “echellogram” that combines wide bandwidth and high resolution. Echelle spectrometers are excellent choices where a very broad wavelength range, ultraviolet to near infrared, must be measured simultaneously without sacrificing spectral resolution.
The Spectroscopy Process
A transmission optical spectrometer is used to measure the absorption (or reflectance/transmission/emission) of a sample in the near UV throughout the visible region of the spectrum. The spectrometer consists of a light source, a dispersion element, a sample chamber, and a detector. Broadband light produced by the light source is transformed into monochromatic light by the dispersion element and passed through the sample. The intensity of the light is measured after passing through the sample by a detector. By scanning the dispersion element, wavelengths from the near UV throughout the visible may be produced and measured. A transmission spectrum is created by comparing the intensity of light after passing through a sample to the intensity of light before it passes through the sample. Early light sources, consisting of arc lamps or metal filaments have recently been replaced by light-emitting diodes (LEDs). Dispersion elements can either be a prism or a grating in a monochromator. Scanning monochromators use single photodiode detectors and photomultiplier tubes while fixed monochromators use CCDs or photodiode arrays. CCDs and photodiode arrays can measure multiple wavelengths of light simultaneously, resulting in faster measurements.
Optical spectrometers can be used for a range of different spectroscopy techniques ranging from the UV to the NIR and SWIR over a wide range of size regimes and timescales. In the following sections we will introduce different spectroscopy techniques including:
Raman spectroscopy is a non-destructive and (typically) electrically non-resonant technique where the vibrational modes of a sample are measured. The resulting spectrum is incredibly specific to the chemical composition, molecular environment, and temperature of the sample. Some applications of Raman spectroscopy include positive material identification of chemicals used in pharmaceutical QC, forensics, and remote explosive and narcotics detection, as a sensitive probe of molecular dynamics in condensed and gas phase systems, monitoring defects and strain in low dimensional materials, and medical diagnostics. Subclasses of Raman spectroscopy include surface -enhanced Raman (SERS) resonance Raman, tip-enhanced Raman, polarized Raman, and hyper Raman.
Luminescence is the spontaneous emission of light by a substance at a low temperature (not generated by heat). Some examples of luminescence include chemiluminescence, bioluminescence and electroluminescence. Photoluminescence spontaneous emission of light by a substance as result of absorbing photons. It is broken down into to sub classes; fluorescence, which is involves singlet-singlet electronic relaxation occurring on nanoseconds, and the longer-lived phosphorescence, resulting from triplet-triplet electronic relaxation, which can last from microseconds to hours. Phosphorescence is widely used in characterizing the optoelectronic properties of semiconductors, material purity and crystalline quality, carrier lifetime, and effects of strain. Phosphorescence is also used to investigate carrier dynamics in low dimensional materials such quantum confinement effects in nanocrystals.
Absorption/Transmission spectroscopy refers to the wavelength (or frequency) dependent absorption of radiation by a sample. Absorption/Transmission spectroscopy can be performed throughout the entire electromagnetic spectrum from high energy x-rays which drive excitations of inner shell electrons to low energy radio wave radiation where electronic and nuclear spins can be excited. Absorption/Transmission spectroscopy is both specific and quantitative and is especially useful in chemical analysis and quantifying the number of species in a sample. It is also used in remote sensing applications such as astronomy where chemical composition of interstellar molecular clouds. Or as a sensitive probe of the electronic structure of atoms and molecules and can be used to determine atomic molecular masses and geometries.
Sum-frequency generation (SFG) is a nonlinear process where two photons with angular frequencies ω1 and ω2 interact in a medium and produce a photon with angular ω3. Due to the fact that the intensity of the signal depends on the product of the input fields, lasers having high intensity peak electric fields are typically used. Second-harmonic generation (SHG) is specific case of SFG where ω1 = ω2 and is the most common type of SFG. Since SFG can only happen where matter is asymmetric, it is especially useful for characterizing the properties of surfaces and interfaces. SFG is also used to measure electronic and vibrational dynamics at surfaces. SHG is a common technique to make novel lasers. It is also used in charactering ultra-short laser pulses (below 1 ps). Research applications using SHG include detecting non-symmetric species in high-resolution optical microscopy and the characterization of crystalline materials.
Optical emission spectroscopy (OES) is a technique where a sample is heated to a high temperature where electrons in the sample are excited to a high energy state. As the sample cools, the electrons relax and emit radiation in the visible region of the spectrum (OES). The emitted radiation has a frequency that is characteristic of the atomic identity of the sample and can be used to determine the elemental composition of a material. There are several ways to heat sample; inductively coupled plasmas (ICP), flame ionization, arcs and sparks. Laser induced break down spectroscopy is a specific subclass of emission spectroscopy where a high intensity laser is focused on a sample to form a plasma which atomizes and excites the samples. Electrons relax from their excited states and emit radiation on the order of a few microseconds after excitation. The resulting spectrum can be analyzed to determine the elemental content of the sample. LIBS has found applications in characterizing metal alloys, detection of hazardous materials, standoff chemical detection, detection of explosive residues, and the detection of lead in paint and soils.
Material Science Spectroscopy
The manipulation and/or characterization of solids at the nano-, micro-, or macro-scale to develop new materials useful for solar/photovoltaic cells, energy storage, LEDs, novel catalysts, chemical detectors, biomedical devices, and many other applications. Many researchers in this area characterise the chemical, structural, electronic, and/or optical properties of a wide range of materials down to the nanoscale with high accuracy and repeatability often with probing techniques including Raman, Photoluminescence / Fluorescence / Cathodoluminescence, Absorption, Optical Emission Spectroscopy and LIBS, Second Harmonic Generation or Darkfield Scattering. Researchers in this area often measure structural changes, photonics properties, presence of defects, behaviour under strain/stress, electronics behaviour, and many other physical material properties. Some examples of materials of recent interest include: low dimensional nanocrystals, Transition metal dichalcogenide (TMDs), organic semiconductors (e.g. OLEDs), and plasmonic metamaterials.
Chemistry & Catalysis Spectroscopy
There are many niche research applications in the area of chemistry and catalysis with a wide variety of specialised spectroscopic techniques that often require specialised high-end spectroscopic instrumentation to efficiently collect sufficient spectral data. Below are a few examples techniques used for chemistry and catalysis applications.
Infrared (IR) Spectroscopy can be used to monitor the progress of chemical reactions as the IR spectrum of a molecule is highly dependent on both atomic content and molecular structure. In addition, IR spectroscopy can be used to monitor isotopic substitution in molecules, as the force constant (and thus vibrational frequency) is a function of the reduced mass of the system.
Chemical reaction dynamics and mechanisms can be measured by UV/VIS (electronic) spectroscopy and are being applied to understand mechanisms of CO2 activation and water splitting.
Raman spectroscopy can be used to address the intermolecular modes of samples such as fluids and liquids. These modes, centred in the low frequency (0-200 cm-1) region of the spectrum result from the collective motion of a sample, are especially sensitive to the intermolecular forces between molecules and are thought to play a pivotal role in condensed phase chemical reaction kinetics and dynamics.
Surface Enhanced Raman Spectroscopy (SERS) Technique
Surface enhanced Raman Spectroscopy relies on the ability of metallic nanoparticles (or rough metal surfaces) to produce localized electric field enhancements when driven by an external light source. Enhancements on the order of 1010-1011 have been observed, increasing the sensitivity of this technique towards single molecule detection. SERS can detect low concentration biomolecules in bodily fluids and is being explored as a next generation platform for medical diagnostics and early disease detection.
Second Harmonic Generation (SHG) is a spectroscopic technique that, due to symmetry constraints, can be uniquely sensitive to measuring analytes at surfaces. SHG is used to investigate the surfaces of liposome biolayers and supported bilayers on solid substrates, allowing the study of molecular interactions of biomolecules at membrane surfaces and kinetics of molecular transport across liposome bilayers.
Multidimensional, Ultrafast Spectroscopy Technique
Ultrafast spectroscopy is used to study processes that typically occur on the timescale of 100’s of Femtoseconds (10-15), with cutting edge experiments now moving to study processes on the order of Attoseconds (10-18). Ultrafast spectroscopy is typically used to study photochemical reactions, which is when chemistry is initiated by light. Ultrafast chemical reactions can be initiated using a laser and the behaviour of the molecules monitored using infrared or UV-Vis spectroscopy.
The ultrafast in ultrafast spectroscopy refers to the pulse duration of the light used to illuminate the sample and initiate or detect reaction; whilst the detector used to acquire the absorption spectra operate on 1 – 100 kHz frequency.
A typical application of this research involves studying the behaviour of molecules in sunscreens to understand changes in isomerisation or breakdown on light exposure. Alternatively photochemical reactions can be used to better understand energy coupling in natural photosynthetic systems. These natural systems have incredibly high quantum efficiencies, and a detailed understanding of their origins are contributing to the design of robust, artificial photosynthetic devices.
Multidimensional, ultrafast spectroscopy can also be used to understand energy transfer dynamics, for example in 2D Infrared experiments that study how vibrational states interact and how energy is quenched.
Steady State Fluorescence Spectroscopy Technique
Steady state fluorescence spectroscopy can be used to study the local conformation changes of RNA-protein interactions as the intensity and shape of the fluorescence spectrum is highly dependent on the local environment and can be an extremely sensitive probe of these interactions.
Combustion/Fluid Dynamics Spectroscopy
The study of how flowing gas and liquids behave in (sometimes) reactive environments. In these fields it is often important to understand the flow of molecular energy and composition in order to design more efficient engines, vehicle exterior designs, turbines, reactors, and many other engineering devices that need to consider operation with/in fluid environments. Often measurements like Molecular Tagging Velocimetry (MTV) or Particle Imaging Velocimetry (PIV) are used to characterise turbulent fluctuations in velocities in a fluid flow field. These techniques are often used in conjunction with other techniques such as (planar) laser induced fluorescence ((P)LIF) to measure fluid temperatures and monitor chemical reactions in the flow. Other techniques such as Coherent Anti-Stokes Raman Spectroscopy (CARS) and Chemiluminescence are also useful optical diagnostic techniques for fluid flows.
Optical Spectrometers are not inherently expensive – a simple spectroscope can be made with a compact disc as the dispersive element, held with a cardboard frame. However, constructing a high-precision spectrometer requires implementing the best optical and mechanical parts. Mirrors must be shaped without error to bring light to an optimal focus, and polished to a fine smoothness to avoid bright signals scattering and overwhelming weak signals. Diffraction gratings must be precisely ruled with hundreds to thousands of lines per millimetre, with each line equally spaced and uniform. Motors must move precisely and repeatably under computer control. All of this must be securely mounted and aligned within a suitable chassis, often with additional motorized parts – turning mirrors, adjustable slits, filter wheels etc, to maximise the usability of the system
No single component will dominate production costs, but a fully featured high-precision optical spectrometer is like other metrology capital equipment – it requires skill, knowledge, and precision parts to build well.
In addition, an optical spectrometer has the potential for a broad range of analytical measurements. When compared to other measurement techniques which offer similar material characterisation capabilities, the optical spectrometer may compare favourably.
The Andor Kymera 193i, Kymera 328i, Shamrock 163, Shamrock 500i, Shamrock 750 are spectrometers suitable for UV, NIR and SWIR measurement when used with the appropriate diffraction gratings and detectors.
Andor optical spectrometers provide high resolution, high throughput, high modularity, ease of use from the UV to the NIR and SWIR, from macro- to nano-scale, with fluxes down to single photon and time-resolution down to nanosecond. Features include high modularity, intelligent motorisation, TruRes™ – Highest spectral resolution, Adaptive Focus™ technology for applications such as Raman, Luminescence/PL, Absorption/Transmission, SFG/SHG, OES and LIBS, Material Science, Chemistry & Catalysis and Life Science/Biomedical.
Spectroscopy-based diagnostics in the fields of Material Science, Chemistry, Life Science or Fundamental Physics & Optics rely on the capture and analysis of optical and chemical signatures with a high degree of precision.
Andor’s range of spectroscopy cameras and detectors offer a wide range of sensitivity, time-resolution and sensor formats to best suit specific experimental conditions from UV to SWIR, nanosecond to hours time resolution, high photon flux to single photon with super dynamic range and resolution.
The original article published here: https://www.oxinst.com/learning/view/article/what-is-an-optical-spectrometer