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The term spectroscopy denotes methods where the interaction of light with matter is utilized. Usually, the strength of some interaction is measured as a function of the wavelength or optical frequency; i.e., the optical spectrum plays an important role.
There exists a very wide range of very different spectroscopic methods; this article can give only a course overview. Many of the modern spectroscopic methods involve one or several lasers and are then called laser spectroscopy. Due to the enormous potentials of lasers in terms of temporal and spatial coherence, narrow linewidth and wavelength tunability, optical power (particularly peak power), ultrashort pulse generation etc., the field of spectroscopy has been widened very substantially since the advent of lasers. Even before that time, spectroscopy has delivered invaluable insight into many phenomena; for example, it allowed researchers to study the interior of the sun and discover helium there, even before it was found on earth.
Exploited Physical Effects
Absorption of Light
Different interactions between light and matter can be utilized in spectroscopy. The most commonly used interaction is the absorption of light (→ absorption spectroscopy). For example, atoms and molecules exhibit distinct absorption features, so that different atoms or molecules can easily be distinguished if the absorption versus wavelength is measured. Particularly in the mid-infrared spectral region, molecules have relatively strong and narrow absorption lines related to their vibrational and rotational modes. This allows one to detect a wide range of substances with a very high sensitivity. An example for an application is the detection of tiny concentrations of air pollutants.
As molecules can have many different absorption lines, some of which may overlap for different species, detection of a single line is often not sufficient for discriminating between different molecules. Recording the absorption spectrum for some sufficiently wide wavelength range, however, often results in a clear spectroscopic finger print. One may also distinguish different isotopes.
The mid-infrared spectral region is quite ideal for sensitive spectroscopy e.g. of many gases, such as air pollutants. Unfortunately, mid-infrared laser sources are typically less powerful and/or more complicated and expensive than near-infrared laser sources. For example, one may apply difference frequency generation in nonlinear crystal materials, requiring two input wavelengths and often leading to quite low optical powers. Other challenges are that it is more difficult to obtain optical materials with high transmission at long wavelengths and also sensitive photodetectors. Therefore, one often exploits overtones, i.e., absorption lines at higher optical frequencies, corresponding to shorter wavelengths, where suitable laser sources are more easily realized and better photodetectors are available (in the extreme case, even for single photon counting). However, such overtone absorption bands are weaker and thus often result in a lower sensitivity.
A low absorption coefficient can still allow for a high sensitivity if the pass length of light is increased accordingly. Therefore, one uses multi-pass absorption cells, where a long path length can be realized in a still relatively compact cell. There are also resonant techniques, where one uses absorption inside a passive optical resonator (e.g., cavity ring-down spectroscopy) or within the laser resonator (intracavity laser absorption spectroscopy). In the latter case, one may e.g. use a broadband fiber laser, where the output spectrum is detected for some time after applying a short pump pulse. Even weak absorption features can lead to pronounced dips in the measured spectrum as that absorption applies in many subsequent resonator round-trips.
Spectroscopy is not only applied to microscopic particles, but also to macroscopic objects such as optical resonators. For example, a high-finesse stable optical resonator can be used as an optical frequency standard and probed with spectroscopic methods.
Other Effects of Absorption
Usually, the measured effect in spectroscopy is the reduction in optical power due to the absorption. However, there are also other possibilities. For example, in photoacoustic spectroscopy one exploits the generation of sound by absorption of light from a power-modulated source. The absorbed light leads to some heating, which causes expansion of the gas and thus generates a sound wave, which can be detected with a sensitive microphone.
Emission or Scattering of Light
Matter can also be exited to emit or scatter light with characteristic features. This is already possible with simple heating or by illumination with sun light. For example, a spectrometer on a satellite may record spectra of scattered light from the earth surface in order to distinguish rocks, plants, lake surfaces, etc. Many advanced spectroscopic methods use the illumination of a sample with a laser source. An example is laser-induced breakdown spectroscopy, where one excites a sample with intense laser pulses and detects light then emitted by the sample in the visible and/or infrared spectral region.
The absorbed light may not only lead to heating, but also excite atoms or molecules, which then subsequently emit fluorescence light. One may record the spectrum of the emitted light and/or the strength of excitation as a function of the wavelength of the pump light; both methods can lead to highly specific detection of certain substances or allow one to measure certain microscopic properties of materials.
An example for such methods is the investigation of laser gain media. A laser beam with suitable optical frequency can excite laser-active ions into metastable states, from which they may be further excited (for sufficiently high optical intensities) or get back to the ground state, emitting fluorescence light. The fluorescence spectrum is related to the wavelength-dependent emission cross sections, which are relevant for the achievable laser gain. One may also detect the decay of fluorescence intensity after excitation with a short laser pulse in order to determine the upper-state lifetime. With a combination of spectroscopic techniques, one may achieve a comprehensive characterization of the physical details of laser gain media such as laser crystals or rare-earth-doped fibers . One may even study subtle effects such as interactions between different laser-active ions, leading to energy transfer phenomena.
Another example is Raman spectroscopy, where a substance is irradiated with narrow-band light at a high optical power level, and one detects the weak emission of light with slightly longer wavelengths, which arises from spontaneous and/or stimulated Raman scattering, a kind of inelastic scattering. The spectrum of Raman-scattered light (which is separated from the pump light with a special narrow-band dichroic optical filter such as Rugate filters) contains information on molecular vibrations.
High-energy Lidar systems, as used e.g. in atmospheric research, allow remote spectroscopic measurements over large distances. Here, one may e.g. exploit Doppler shifts of back-scattered light, which reveal longitudinal wind velocities. A variety of methods has been developed which allow the remote measurement of many properties such as temperature, pressure, trace gas concentrations and cloud particle densities.
In some spectroscopic methods, one exploits changes of the optical phase. Typically, an interaction of interest occurs in one arm of an interferometer. The resulting phase changes can be sensitively detected because they are turned into power changes at the interferometer output. Phase changes resulting from absorption lines are also relevant in frequency modulation spectroscopy.
Methods of Spectroscopy
Spectral Decomposition of Transmitted Light
One may use a broadband light source (white light source) and then apply a spectral decomposition in the photodetection system. Often, a high spectral sensitivity is obtained by combining the photodetector with some kind of monochromator. A high-resolution spectrometer, for example, allows measurements for many different narrow wavelength bins without precisely controlling a light source. The measurement time may be long if each wavelength is treated separated and there are many narrow wavelength bins, but some spectrometers can simultaneously record many wavelengths, e.g. when they contain a CCD array instead of a single photodiode.
A special way of obtaining spectral resolution is the use of an interferometer where an arm length is scanned during the measurement. With a narrow-band light source, that would result in a simple sinusoidal oscillation of the output power. With a broadband source, however, one obtains a superposition of many different oscillations. Using a Fourier transform, one can retrieve the spectrum. (Interferometric wavemeters have a similar working principle.) Such methods are called Fourier transform spectroscopy. This is often applied in the infrared spectral region and then also called Fourier transform infrared spectroscopy (= FTIR spectroscopy). Many modern infrared spectrometers are based on such techniques.
Variation of the Source Wavelength
In many cases, spectroscopy is done with a wavelength-tunable light source. (Various such sources are discussed below.) One may then, for example, measure the absorption of a sample as a function of the emission wavelength of the laser. As it is more difficult to make a widely tunable narrow-band laser source, that technique often leads to a smaller tuning range – smaller than the range possible with a simple broadband light source. However, the method offers superior resolution and also rapid data acquisition with low noise, as one can obtain a relatively strong detector signal. Note that all available optical power is concentrated on a particular wavelength of interest, rather than spread over the whole measurement range.
As the optical power often somewhat varies with wavelength (and possibly also with time), a high precision requires monitoring of that power. This can be done, for example, in two-beam spectrometers, where the light from the source is split into two beams. Only one beam (the sample beam), but not the reference beam, is sent through the medium under investigation, and the powers or intensities of both beams are measured. Particularly high sensitivities can be obtained with balanced photodetectors, where one directly measures the difference of photocurrents from the two beams.
Exploitation of Nonlinear Effects
When high-intensity light sources (normally lasers) are used, one may exploit various nonlinear optical effects for spectroscopy. Raman spectroscopy has already been mentioned above. Two-photon absorption has been utilized for a long time . Another important example is coherent anti-Stokes Raman spectroscopy (CARS), where two input waves generate a detected signal with slightly higher optical frequency via four-wave mixing. Various other nonlinear effects, such as Brillouin scattering, can be exploited with other methods.
Elimination of Doppler Effects
Atoms and molecules in gases can exhibit substantial Doppler broadening of their absorption lines due to their thermal motion. There are, however, various methods of Doppler-free spectroscopy. For example, this can involve counterpropagating laser beams, where a probe beam in one direction selects a certain velocity class via absorption saturation , and the other beam detects that saturation. This methods is called Doppler-free saturation spectroscopy .
A relatively new field is terahertz spectroscopy , where terahertz radiation (with frequencies e.g. of hundreds of gigahertz to several terahertz) is used instead of light. Although lasers cannot directly emit terahertz radiation, they can be used in different waves for generating such radiation, e.g. by using techniques of electro-optic sampling or nonlinear frequency conversion. Also, ultrashort pulse lasers are useful for the time-resolved detection of terahertz waves. As many optically opaque materials have a substantial transparency for terahertz radiation, terahertz spectroscopy can be used for a wide range of investigations in science and technology. For example, it is now used for fault location in airplane parts and in security screenings.
Some techniques of modern laser spectroscopy employ frequency combs as generated with mode-locked lasers . As such a frequency comb contains some number of exactly equidistant lines in the spectrum, all of its frequency components are known (apart from some noise) if only two parameters are fixed (possibly stabilized with some feedback techniques): the comb spacing, which is related to the pulse repetition rate, and the carrier–envelope offset frequency. Therefore, frequency combs can serve for extremely precise frequency measurements in wide wavelength ranges, if they are produced with a high optical bandwidth (sometimes more than octave-spanning) and with proper frequency stabilization. There are various important applications in frequency metrology (in particular, ultraprecise optical clocks) and other fields.
High Temporal Resolution
Spectroscopy may also involve measurements with an extremely high temporal resolution. In pump–probe spectroscopy one uses an ultrashort pump pulse, followed by a probe pulse with a variable time delay between a few femtoseconds and several nanoseconds. The effect of the pump pulse on the sample and thus on the probe pulse can be measured as a function of the time delay, which is often adjusted simply via a variable optical path length. An alternative is to use to two mode-locked lasers with slightly different pulse repetition rates, so that a range of time delays is scanned continuously.
Lasers for Spectroscopy
Due to the enormously wide range of methods for laser spectroscopy, there is also a wide range of different laser sources which are used for such purposes:
- Small single-frequency laser diodes can be used as cheap and compact wavelength-tunable sources. The emission wavelength is often tuned simply by varying the drive current, which affects the temperature. More refined schemes involve an external laser resonator containing wavelength-selective optical elements. Such external cavity diode lasers allow for higher performance.
- Some broadband tunable solid-state lasers, such as titanium–sapphire lasers, chromium zinc selenide and sulfide lasers (Cr2+:ZnSe, Cr2+:ZnS, see chromium-doped gain media), Cr4+:MgSiO4 (forsterite) lasers and erbium-doped fiber lasers can be quite useful for spectroscopy, as they can cover wavelength ranges of tens to hundreds of nanometers and often also provide a substantial output power and low noise. In some cases, Q-switched operation with the generation of nanosecond pulses is useful; important examples are laser-induced breakdown spectroscopy and Lidar. High pulse energies combined with a narrow linewidth (normally in single-frequency operation) are often of interest.
- Fixed-wavelength solid-state lasers with relatively high power are used in Raman spectroscopy. A narrow linewidth is important.
- Mid-infrared radiation and terahertz radiation can be generated with quantum cascade lasers, often offering better performance and higher convenience than the previously often used lead salt lasers. Very wide wavelength regions are accessible with such sources.
- Mode-locked lasers, usually in the form of solid-state lasers, emitting trains of ultrashort pulses, are used for generating frequency combs (see above), often with highly stabilized optical frequencies.
- Dye lasers can access broad wavelength regions, particularly in the visible spectral range, but also in the near ultraviolet and near infrared. There are continuous-wave, pulsed and mode-locked versions.
- Optical parametric oscillators, pumped with various types of lasers in continuous-wave operation, with nanosecond, picosecond or femtosecond pulses, can often be tuned in very wide wavelength regions, and can access spectral regions which are not directly accessible with lasers. The extension to shorter wavelengths is possible with methods like frequency doubling and sum frequency generation, whereas difference frequency generation is a method for generating long-wavelength (often mid-infrared or even terahertz) radiation.
- Supercontinuum generation can be used as a method to generate broadband light as required for some spectroscopic measurements. This method can offer enormously broad spectra in combination with high spatial coherence and a significant optical power.
This list of laser sources is not complete; even exotic types of lasers and nonlinear sources are sometimes used to meet certain requirements for spectroscopic applications.
A low level of laser noise is often important for precision spectroscopy. Various types of noise, such as intensity noise, phase noise (related to a finite optical bandwidth) or timing jitter, can be relevant. More or less sophisticated schemes for the stabilization of lasers are therefore often employed. These themselves often involve techniques of spectroscopy, as far as frequency stabilization is concerned. In extreme cases, a linewidth below 1 Hz is achieved.
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See also: optical spectrum, intracavity laser absorption spectroscopy, spectrometers, tunable lasers, mid-infrared laser sources, frequency combs, frequency metrology
and other articles in the categories methods, optical metrology
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