Sum-Frequency Generation (SFG) is an optical phenomenon, wherein two photons impinge on a sample surface and generate a third photon at the sum of their frequency.
SFG is a second-order nonlinear optical process – it occurs when the fields caused by incident photons are so intense that the response of local media is no longer linear. It can happen at any medium where there is no spatial inversion symmetry; meaning that it can only occur in materials that have a nonlinear crystal structure, or at the interface between two different media.
Most commonly encountered materials will possess some degree of inversion symmetry, which means that in most cases, SFG is interface specific. Consequently, it can be used to probe buried interfaces, where the only limiting factor is whether the interface can be accessed by light.
The SFG process requires a great deal of energy, so pulsed ultrafast laser sources are usually used to supply the required peak power. By manipulating the frequency of one of these laser sources, it is possible to generate a spectrum of molecular vibrational resonances at the interface between two media.
Traditionally, in Sum-Frequency Spectroscopy, picosecond (10-12 seconds) laser pulses are used. One of the input beams is swept through a region of infrared frequencies (typically using optical parametric amplification). The other beam is held at a constant, higher frequency to shift the infrared beam into a higher frequency (upconversion), which can then be detected with a photomultiplier tube-based detector.
A spectrum is subsequently built in a piece-wise fashion, which will be comparable to a traditional infrared spectrum but will contain exclusively molecular vibrations that occur at the interface where the two input beams meet.
Broadband Sum-Frequency Spectroscopy
More recently, femtosecond (10-15 seconds) sources have been used, where each pulse inherently contains a broader spread of frequencies, as a consequence of the uncertainty principle.
Mode-locked Ti:Sapphire (titanium-doped sapphire) laser sources are often used, which have naturally broad lasing bandwidths. A beam of ultrashort pulses is split into two paths, one will become the static beam as before (after spectral narrowing by a grating or an étalon), and the other will be frequency shifted into the IR region, by an optical parametric amplifier.
The static beam and the broad beam are directed towards a sample surface as before, and spectrally broad SFG is produced. The SFG beam is then directed towards a spectrometer system, which will typically consist of a monochromator, a microchannel plate, and a CCD detector, which spatially disperses the light based on its frequency, and records an entire spectrum at once.
This provides a few key advantages; the acquisition times of SFG spectra are now much quicker (spectra that would traditionally take several hours to produce can be acquired in a few minutes), and spectra are not sensitive to fluctuations in power of either of the beams.
Applying Broadband Sum-Frequency Spectroscopy
Due to the insensitivity of SFG to bulk phases, it can be applied to any interface that is accessible by light, in a way that traditional vibrational spectroscopies cannot. This includes the air-solid interface, the liquid-solid interface, and the liquid-liquid interface.
With the improved acquisition times achieved with Broadband SFG, it is possible to observe local changes in interfacial chemistry with very high temporal resolution. As an example of this, it is possible to monitor the self-assembly of a monolayer of surfactants on a sample surface, at the solution-sample interface.
Due to the strict symmetry requirements of SFG, it is also possible to assess the local geometry of this interface. Specific vibrational modes are sensitive to the polarisation of the input beams, and the SFG output they produce will also have an inherent polarisation. These are all highly dependent on the molecular orientation of the interfacial layer.
As a result, if spectra are recorded with different polarisations of light, using BB-SFG, it is even possible to ascertain the molecular conformation and orientation at a buried interface, while chemical processes are taking place.
BB-SFG has recently been used to probe the interfacial structure of water, to investigate structural properties of electronic devices like solar cells, and to study biological processes in situ. The versatility of the technique has resulted in much interest from a wide variety of fields, and improvements are continually being made.
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