A little over 50 years ago, CU graduate Theodore Maiman (EngrPhys '49) demonstrated the world's first working laser—the ruby laser—at Hughes Research Laboratories in Malibu, California.
Since then, lasers have become an integral part of our lives with applications in consumer electronics, communications, sensors, and medicine. Every compact disc player contains a semiconductor laser, and airplanes rely on laser gyroscopes for navigation, to name a few examples.
"The world just celebrated the 50th birthday of the laser, but there are still many more research opportunities in laser technology," says CU Assistant Professor Juliet Gopinath, who leads a research group focused on lasers and their applications.
Gopinath, who holds joint appointments in electrical, computer, and energy engineering and physics, is pursuing research on short-pulse and high-power lasers, mid-infrared sources, spectroscopy, and microfluidics.
The research is interdisciplinary, spanning several core areas of optics and photonics, solid-state devices, and nanotechnology.
This year, Gopinath received a Young Investigator Award from the Air Force Office of Scientific Research (AFOSR) to study phase and frequency control of laser arrays for pulse synthesis. The prestigious awards program fosters creative basic research and enhances the early career development of outstanding young investigators. The focus of the AFOSR grant is on generating a train of short optical pulses by controlling the optical phase and frequency (color) of different lasers.
Beam combining, in which multiple beams are combined into a single beam with optical elements, will be used to generate the short optical pulses, according to Gopinath. Combining allows increased power from a laser system with the use of multiple lasers, avoiding fundamental physical power limits imposed by nonlinearities from a single device.
"The price you pay for combining is beam quality," she says. "This means that you can't focus the laser array down to as small a spot compared with a single laser, unless you control the frequency and/or the phase of the lasers in the array."
While frequency can be controlled easily through use of an optical prism or grating, which disperses the frequencies as in a wavelength-division multiplexed communication system, controlling the phase is a "multimillion-dollar question."
Gopinath's research group, which includes PhD students Jonathan Pfeiffer in electrical engineering and Robert Niederriter in physics, is conducting experiments to achieve this goal. The result will be an ultrashort optical pulse train, with pulse widths on the order of a femtosecond (10-15 second) to a picosecond (10-12 second), Gopinath says.
Femtosecond lasers locked to optical atomic transitions have been used to generate the world's most accurate clock and resulted in the 2005 Nobel Prize in Physics awarded to CU Professor John Hall.
Short optical pulses are of particular interest for communications, sensing, imaging, and the study of materials. The high peak associated with short optical pulses can be used to study and exploit nonlinearities in materials for optical switching and the generation of new frequencies. A pulse train can be used as an "ultrafast strobe" to study the dynamics of electrons and holes in materials, similar to the high-speed photography pioneered by Harold Edgerton in the first half of the 20th century.
In addition to short optical pulse generation, Gopinath is interested in mid-infrared light sources. The mid-infrared region, ranging from 2 to 50 microns, is well beyond the visible region (0.4 – 0.7 microns) and often referred to as the "chemical fingerprint" region of the spectrum.
Mid-infrared lasers have the potential for a large impact on the world's quality of life with potential applications including sensing, security, and medicine. Mid-infrared lasers can be used in gas sensors to predict volcanic eruptions and monitor greenhouse gases, and in chemical and biological threat detection. Gopinath is working on developing several new materials for mid-infrared lasers and high-resolution sources for sensing and spectroscopy.
She also is interested in developing reconfigurable optical elements, enabling devices that can be modified in the field. The Office of Naval Research is funding a project in this area centering on liquid lenses with variable focal lengths. The technology has the potential to make a large impact in free space communication links, imaging, and medicine, as reconfigurable optical devices can react to atmospheric variations or changes in live cells. Gopinath is leading the research team, which also includes Professor Victor Bright in mechanical engineering and research faculty Carol Cogswell and Robert Cormack of electrical, computer, and energy engineering.
Sustainability projects have attracted Gopinath's attention as well. She is interested in applying ultraviolet lasers and other optical devices to water purification, a large problem for much of the world.
Last year, she advised an undergraduate senior project team that designed and demonstrated a water purification system that could be monitored and operated remotely. The project was sponsored by Manna Energy, which will install the student-designed system in Rwanda.
"While research will have impacts on the quality of life in 10 to 100 years, water purification is a topic with the potential for immediate impact," she says.
It seems clear that we haven't seen the end of what laser technology can do for us—and that CU engineers will continue to be there at the forefront of new developments.