Tunable Lasers Expand for Time-Resolved Spectroscopy

Dr. Mark Little, OPOTEK

In physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds. Within this field, various techniques including transient absorption spectroscopy are used to study the mechanistic and kinetic details of chemical processes that occur within just a few picoseconds to a femtosecond — the equivalent of one millionth of one billionth of a second.

To conduct this research, labs require fast lasers that can create an excited electronic state of a molecule on this time scale. Since all molecules do not absorb the same wavelengths of light, these lasers must be flexible enough to produce wavelengths across a broad spectrum. In the past, it was difficult, if not impossible to conduct spectroscopic research using inexpensive, fixed wavelength lasers. This was extremely inefficient when conducting fundamental research at the academic level on a wide variety of metal-based complexes.

As a result, many research labs are turning to tunable lasers, or optical parametric oscillators (OPOs), for nanosecond time-resolved spectroscopy. OPOs have long been utilized in sophisticated test and measurement applications such as mass spectrometry and photoacoustic imaging. Now, these tunable pulsed lasers are being utilized due to the high resolution and variety of nanosecond wavelength pulses they can produce from visible light to deep UV.

“You don’t want to have to design your chemistry such that it is tailored just to the particular wavelengths you have available in your instrumentation. You want the lasers to be flexible enough to adapt to the chemistry you want to explore,” says Dr. James McCusker, an MSU Foundation Professor in the department of chemistry at Michigan State University who leads a research group of PhD students in the study of cutting-edge techniques in spectroscopy. His group conducts fundamental research on designing and synthesizing molecules to absorb in the visible part of the spectrum. “We only want to be limited by our imaginations, not by what wavelengths we can get out of a particular laser,” says McCusker, who has worked in the field for nearly 30 years.

Ultrafast Spectroscopy of Transition Metals

McCusker’s research team studies a class of compounds known as transition metal complexes. These compounds are based on elements from the so-called transition block of the periodic table. The focus of the group’s research is to understand how a molecule’s structure, composition and absorptive properties relate to their ability to carry out light-induced chemical reactions.

According to McCusker, one of the primary areas of research relates to solar energy conversion. Transition metal complexes are an important class of molecules in this field of research and can be studied using time-resolved spectroscopy for designing molecules whose excited-state properties will enable their use in a wide range of such compounds to enable new kinds of organic transformations of potential interest in the pharmaceutical industry.

“We are pursuing a systematic examination of chemical perturbations to excited-state electronic and geometric structure,” says McCusker. “As a result, we will be able to develop a comprehensive picture of how transition metal chromosphores absorb and dissipate energy.”

Getting on the Right Wavelength

To facilitate this type of research, fast, pulsed lasers are required to selectively excite a molecule to a specific state to study the compound’s excited-state properties. In the early days of laser development, lasers were constructed to operate at very specific wavelengths. Single wavelength Nd:YAG lasers, for example, are inexpensive and simple to use. However, additional hardware is required to modify a 1064-nm laser before it can operate at a different harmonic frequency for testing such as 213, 266, 355, and 532 nm. This adds to the cost of the laser.

There are gaps between the wavelengths, and the jump between 1064 nm to 532 nm is significant, adding that testing each of those harmonics increases the cost. OPO lasers can convert the fundamental wavelength of pulsed mode Nd:YAGs to a selected frequency. This tunability enables OPOs to generate light in a broad range of wavelengths that are amplified within the OPO for a usable output beam.

An OPO laser can be tuned to a very specific wavelength resolution by simply punching in a number like 410, 410.1, or 410.2 nanometers. These pulse-based OPO lasers can deliver concentrated energy in short durations measured in nanoseconds making them ideal for nanosecond time resolved spectroscopy.

McCusker and his research group use the Vibrant 355 II Nd:YAG-pumped OPO laser from OPOTEK, which can quickly provide a tunable wavelength output from 300 to 2400 nm and is used for both time-resolved absorption and time-resolved emission requirements.

These lasers include software applications, which has made them more user-friendly.

Reaching into Deep UV

Much of the research conducted by McCusker and his team involves designing and synthesizing molecules that absorb light in the visible part of the spectrum. However, he notes that for organic carbon-based or aromatic compounds, researchers may want to study UV damage to DNA. Since DNA is composed of organic base pairs that absorb in the ultraviolet spectrum rather than the visible spectrum, OPO lasers are required to access those ultraviolet wavelengths for the study of these compounds.

OPO lasers can be designed to generate wavelengths down to 190 nanometers through multiple stages of optical conversion. Moreover, unlike typical fixed wavelength deep UV lasers, OPO lasers are solid-state and so do not require expensive consumables such as specialized gas or chemical mixtures as the lasing medium.

Future time-resolved spectroscopic research by academic research labs will continue to rely on the flexibility and tunability of OPO lasers to further their knowledge of how materials transmit, absorb, and reflect light.

This article was written by Dr. Mark Little, Technical and Scientific Consultant for OPOTEK, Carlsbad, CA. For more information, call 760-929-0770 or visit here  .

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