Spectroscopy

Laser spectroscopy, a broad field of studies of interaction

between matter and laser as a function of laser light

wavelength, is used to probe properties of matter

on all scales from fundamental particles to stars.

 

IPG lasers for spectroscopy feature:

• wavelengths from UV to Mid-IR

• CW lasers, ns, ps and fs pulses

• average powers from mW to >kW

• fixed frequency or tunable sources

• single frequency to broadband

• excellent pointing stability

• excellent beam mode

 

Raman scattering is an inelastic, light scattering process, and affords a nondestructive technique whereby the vibrational “fingerprint” of an analyte molecule is measured following its photoexcitation and a subsequent change in its molecular polarization.  After its initial discovery in 1928, it has since become famous for its scientific versatility, with applications spanning art, archaeology, biosciences, analytical chemistry, solid state physics, liquids and liquid interactions, nanomaterials, phase transitions, pharmaceutical studies, and forensic science.  For example, Raman spectroscopy has found extensive usage as a tool in the biopharmaceutical industry, where the identification of pharmaceutical ingredients may be determined, in the semiconductor industry where the purity of wafers may be investigated, and in forensics sciences where the detection of explosives may be monitored.

In Raman spectroscopy, lasers of different wavelength may be employed to initiate the excitation process of the molecule of interest.  Although the vast majority of these excited molecules scatter the light as elastic Rayleigh scattering of the same energy, a select few will experience a change in their vibrational state during relaxation to the ground electronic state, resulting in a shift of the energy of the scattered light that is characterized by the energy in that vibrational mode.  This is the Raman effect.  IPG Photonics offers a range of continuous wave lasers spanning UV-visible-IR wavelengths for traditional Raman spectroscopy studies, and ultrafast pulsed lasers including the Mid-IR CLPF for state-of-the-art femtosecond stimulated Raman.  Such laser systems may also be used in the excitation of plasmonic substrates, as is the case in surface enhanced Raman spectroscopy (SERS).

Raman spectroscopy has found extensive use in a wide host of industries, spanning: semiconductors and superconductors, pharmaceutical, medical, optical communications, and academic research.

 

Within the framework of laboratory research, conventional laser spectroscopic studies on the ultrafast time scale employ the concept of pump-probe spectroscopy. In this general setup, two separate femto- or picosecond optical pulses are required: one to excite (“pump”) the sample of interest, and another to interrogate (“probe”) the deexcitation of the sample, both of which are required to overlap both spatially and temporally. An optical delay line can effectively lengthen the path the probe pulse takes, thereby temporally delaying it with respect to the pump pulse. As the pump pulse excites the molecule, the increasingly delayed probe pulse monitors the decay of the excited electrons. From this, dynamic, time-resolved data for the sample of interest can be obtained and analyzed. Pump-probe spectroscopy is used, most typically, to monitor the recovery of saturable absorbers after photoinduced excitation, to measure the temporal signatures of chemical reactions, or the transfer of energy from one molecule to another.  This information may then guide further design, synthesis, and implementation of studied materials for a wide variety of applications, including but not limited to: photocatalysis, photoelectrochemistry, and photovoltaics. The main industries utilizing pump-probe spectroscopic technology are academia, aerospace, metallurgical, biophotonics, microscopy and medicine. The efficiency of solar cell materials, as an example, may be determined by such pump-probe techniques through monitoring crucial excitation recombination dynamics, or the charge carrier recombination efficiency of water hydrolysis materials to generate clean-burning hydrogen fuel.

 

The study of the dynamics utilizing pump-probe spectroscopy allows for a more profound and fundamental insight into the properties of the material of interest. As a dynamic measurement, it affords information that supplements that of steady-state measurements.  For these considerations, IPG offers pulsed fiber lasers with wavelengths spanning 355 nm to 1.5 μm, all with femtosecond (or picosecond) pulse widths and pulse energies appropriate to explore maximally the aforementioned applications at temporal resolutions in the femtosecond regime.

  Cavity ring-down spectroscopy is an optical technique whereby the measurement of the optical extinction of materials that both scatter and absorb light may be determined.  It has found widespread applications in the field of gas-phase research, where gaseous samples may be quantified to the parts per trillion level.  In such an experiment, a laser is used to illuminate an optical cavity; when in resonance with the cavity mode, laser intensity builds up due to constructive interference.  Once laser is turned off, the exponentially decaying light intensity is measured. This application is particularly beneficial for environmental monitoring, emissions monitoring, and biopharmaceutical processes due to the inherent high sensitivity of the technique.  The measurement of greenhouse gases, as a specific example, has helped ensure the construction of increasingly “green” technologies, ranging from automotive engines to chemical processing plants.

Because cavity ring-down spectroscopy is predicated upon the absorption of light by gaseous materials, and because different gases absorb at different wavelengths, varying laser wavelengths are necessary to successfully complete such an experiment.  As such, IPG offers a broad range of CW lasers capable of handling application requirements for gas absorption.  Most typical gases have unique absorption spectra at mid-infrared wavelengths, such as IPG's Mid-IR hybrid lasers

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