Photothermal spectroscopy is a group of high sensitivity
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the thermodynamic properties of the sample or of a suitable material adjacent to it. Measurement of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements.
As with
photoacoustic spectroscopy
Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell sho ...
, photothermal spectroscopy is an indirect method for measuring
optical absorption
In physics, absorption of electromagnetic radiation is how matter (typically electrons bound in atoms) takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber (for example, thermal energy). ...
, because it is not based on the direct measure of the light which is involved in the absorption. In another sense, however, photothermal (and photoacoustic) methods measure ''directly'' the absorption, rather than e.g. calculate it from the transmission, as is the case of more usual (transmission) spectroscopic techniques. And it is this fact that gives the technique its high sensitivity, because in transmission techniques the
absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
is calculated as the difference between total light impinging on the sample and the
transmitted (plus
reflected, plus
scattered) light, with the usual problems of accuracy when one deals with small differences between large numbers, if the absorption is small. In photothermal spectroscopies, instead, the signal is essentially proportional to the absorption, and is zero when there is zero ''true'' absorption, even in the presence of reflection or scattering.
There are several methods and techniques used in photothermal spectroscopy. Each of these has a name indicating the specific physical effect measured.
* Photothermal lens spectroscopy (PTS or TLS) measures the thermal blooming that occurs when a beam of light heats a transparent sample. It is typically applied for measuring minute quantities of substances in homogeneous gas and liquid solutions.
* Photothermal deflection spectroscopy (PDS), also called the
mirage
A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...
effect, measures the bending of light due to optical absorption. This technique is particularly useful for measuring surface absorption and for profiling thermal properties in layered materials.
* Photothermal diffraction, a type of
four wave mixing Four-wave mixing (FWM) is an intermodulation phenomenon in nonlinear optics, whereby interactions between two or three wavelengths produce two or one new wavelengths. It is similar to the third-order intercept point in electrical systems. Four-wave ...
, monitors the effect of transient
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
s "written" into the sample with coherent lasers. It is a form of real-time
holography
Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional images, but it also has a wide range of other applications. In principle, i ...
.
* Photothermal emission measures an increase in sample
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
radiance
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiatio ...
occurring as a consequence of absorption. Sample emission follows
Stefan's law of thermal emission. This methods is used to measure the thermal properties of solids and layered materials.
*
Photothermal single particle microscopy. This technique allows the detection of single absorbing nanoparticles via the creation of a spherically symmetric thermal lens for imaging and correlation spectroscopy.
Photothermal deflection spectroscopy
Photothermal deflection spectroscopy is a kind of
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
that measures the change in refractive index due to heating of a medium by light. It works via a sort of "
mirage
A mirage is a naturally-occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the Latin ''mirari'', meanin ...
effect" where a refractive index gradient exists adjacent to the test sample surface. A probe laser beam is refracted or bent in a manner proportional to the temperature gradient of the transparent medium near the surface. From this deflection, a measure of the absorbed excitation radiation can be determined. The technique is useful when studying optically thin samples, because sensitive measurements can be obtained of whether absorption is occurring. It is of value in situations where "pass through" or transmission spectroscopy can't be used.
There are two main forms of PDS: Collinear and Transverse. Collinear PDS was introduced in a 1980 paper by A.C. Boccara, D. Fournier, et al.
In collinear, two beams pass through and intersect in a medium. The pump beam heats the material and the probe beam is deflected. This technique only works for transparent media. In transverse, the pump beam heats come in normal to the surface, and the probe beam passes parallel. In a variation on this, the probe beam may reflect off the surface, and measure buckling due to heating. Transverse PDS can be done in Nitrogen, but better performance is gained in a liquid cell: usually an inert, non-absorbing material such as a
perfluorocarbon
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
is used.
In both collinear and transverse PDS, the surface is heated using a periodically modulated light source, such as an optical beam passing through a mechanical chopper or regulated with a function generator. A lock-in amplifier is then used to measure deflections found at the modulation frequency. Another scheme uses a pulsed laser as the excitation source. In that case, a boxcar average can be used to measure the temporal deflection of the probe beam to the excitation radiation. The signal falls off exponentially as a function of frequency, so frequencies around 1-10 hertz are frequently used. A full theoretical analysis of the PDS system was published by Jackson, Amer, et al. in 1981.
The same paper also discussed the use of PDS as a form of microscopy, called "Photothermal Deflection Microscopy", which can yield information about impurities and the surface topology of materials.
PDS analysis of thin films can also be performed using a patterned substrate that supports optical resonances, such as guided-mode resonance and whispering-gallery modes. The probe beam is coupled into a resonant mode and the coupling efficiency is highly sensitive to the incidence angle. Due to the photoheating effect, the coupling efficiency is changed and characterized to indicate the thin film absorption.
See also
*
Photothermal effect Photothermal effect is a phenomenon associated with electromagnetic radiation. It is produced by the photoexcitation of material, resulting in the production of thermal energy (heat).
It is sometimes used during treatment of blood vessel lesions, l ...
*
Photothermal microspectroscopy
*
Photothermal optical microscopy Photothermal optical microscopy / "photothermal single particle microscopy" is a technique that is based on detection of non-fluorescent labels. It relies on absorption properties of labels ( gold nanoparticles, semiconductor nanocrystals, etc.), a ...
*
Urbach energy The Urbach Energy, or Urbach Edge, is a parameter typically denoted E_, with dimensions of energy, used to quantify energetic disorder in the band edges of a semiconductor. It is evaluated by fitting the absorption coefficient as a function of energ ...
References
* J. A. Sell ''Photothermal Investigations of Solids and Fluids'' Academic Press, New York 1989
* D. P. Almond and P. M. Patel ''Photothermal Science and Techniques'' Chapman and Hall, London 1996
* S. E. Bialkowski ''Photothermal Spectroscopy Methods for Chemical Analysis'' John Wiley, New York 1996
External links
Quantities, terminology, and symbols in photothermal and related spectroscopies (IUPAC Recommendations 2004)
{{Branches of spectroscopy
Spectroscopy