Advisor(s): Mohammed Islam, EECS
Written by: Brian Worthmann
Edited by: Andrew McAllister
Date Published: December 12, 2017
Have you ever wondered how to tell the difference between a cancerous cell and a nearby healthy cell? Or, how vehicle emissions are measured? Or how plastics are sorted at recycling facilities? These are problems in very different fields, but it turns out that the key to solving each of these problems is infrared light.
Infrared light, defined as light with wavelengths between 0.7 microns to 1,000 microns, interacts with the chemical bonds of almost all materials in a unique way. The specific infrared wavelengths that a certain material absorbs defines its infrared ‘fingerprint’. If you had a material of unknown composition, you could measure its fingerprint by sending a variety of infrared light at it and measuring how much absorption was found at each wavelength. Then you could cross-reference the measured fingerprint with the known fingerprints of variousmaterials to determine what your unknown material is, or what it isn’t. Using infrared light to learn about the chemical composition of a material is referred to as infrared spectroscopy.
Infrared spectroscopy is a very useful technique, but finding an appropriate infrared light source is hard because the light needs to contain a continuum of infrared wavelengths, as opposed to the discrete set of wavelengths you’d expect from a typical laser source. Blackbody radiation-based sources, such as halogen bulbs, are the most common infrared light source. However, these bulbs are energy inefficient, and their high operating temperatures necessitate bulky heat management equipment. Additionally, the light from these bulbs is incoherent, and directing this light into a fiber optic cable is difficult and inefficient.
Rather than relying on inefficient blackbody radiation sources like halogen bulbs, Ramon Martinez is working on a new method to produce a continuum source of infrared wavelengths using lasers. An important property for such a laser is its spectral width, or how wide of a range of wavelengths are present in the laser’s output spectrum. Typical spectral widths for narrow-band lasers are a nanometer or less. Broadband lasers typically have spectral widths of around 50 to 100nm. But to accurately measure the material’s infrared ‘fingerprint’, the spectral width needs to be much larger – closer to 1000nm, or a micron. Such lasers are called super continuum, and they are the subject of Ramon’s research.
So how do you create a coherent laser source with such huge spectral widths?
Well, you begin with a standard infrared laser outputting 1.5-micron infrared light into a fiber optic cable. Then, you send that through a series of amplifiers, causing the amplitude of this light to be increased by a factor of a million. At low amplitudes, light obeys the linear Maxwell equations, like those seen in standard E&M texts like Griffiths. But at these much higher amplitudes, the fiber creates nonlinearities in the electromagnetic wave propagation which are ordinarily negligible at lower amplitudes. These nonlinearities mean that an originally narrow band laser will develop lower and higher wavelength components, while still retaining the coherent properties of a laser.
The most important nonlinear interaction that occurs is called the Raman shift. This happens when a photon bumps into one of the atoms in the fiber optic cable. The atom starts moving a little, removing some of the energy from the incident photon. As a result, the outgoing photon is slightly longer in wavelength! This process is a third-order effect, which means it’s only significant at extremely high amplitudes. This Raman shifting process, along with other physical processes, lead to an increasingly wider spectral width. And thus, a super continuum laser is born!
Compared to a halogen bulb, super continuum lasers offer many benefits, but one of the most important is the ability to be highly directional and focusable. This means infrared spectroscopy can be performed in situations where a traditional halogen bulb is insufficient. For example, the subtle differences in the chemical fingerprint of a cancerous cell and a nearby healthy one can be determined spectroscopically, which is particularly important for colon, breast, and skin cancer. Or the gases coming out of the exhaust pipe of a moving car could be probed, and the various chemical compounds and their concentrations could be determined, which is useful for vehicle emissions testing. Monitoring glucose levels in the blood can also be performed spectroscopically, as well as differentiating between plastics in the recycling industry. A super continuum laser can even be used in a 3D printer to print with a wider variety of materials than conventionally available. It seems the possibilities are endless!
About Ramon Martinez:
Ramon is beginning his sixth year of his Applied Physics PhD. He is originally from the Los Angeles area, and received his undergraduate degree in Physics from the University of California at Berkeley. When not found in the lab, he can be found playing board games, practicing improv, swing dancing, or playing Ultimate Frisbee. He is a member of the Central Student Government, co-runs the WISE-GISE physics summer camp, is an advocate for minority education, and was the Vice President of the Applied Physics Student Council in 2016. If you have
any questions for Ramon, contact him at ramartma@umich.edu.
