Columbia Engineers are the first to miniaturize dual-frequency combs by putting two frequency comb generators on a single millimeter-sized silicon-based chip; could lead to low-cost, portable sensing and spectroscopy in the field in real-time.
In a new paper published today in Science Advances, researchers under the direction of Columbia Engineering Professors Michal Lipson and Alexander Gaeta have miniaturized dual-frequency combs by putting two frequency comb generators on a single millimeter-sized chip.
“This is the first time a dual comb has been generated on a single chip using a single laser,” says Lipson, Higgins Professor of Electrical Engineering.
A frequency comb is a special kind of light beam with many different frequencies, or “colors," all spaced from each other in an extremely precise way. When this many-color light is sent through a chemical specimen, some colors are absorbed by the specimen's molecules. By looking at which colors have been absorbed, one can uniquely identify the molecules in the specimen with high precision. This technique, known as frequency-comb spectroscopy, enables molecular fingerprinting and can be used to detect toxic chemicals in industrial areas, to implement occupational safety controls, or to monitor the environment.
“Dual-comb spectroscopy is this technique put on steroids,” says Avik Dutt, former student in Lipson’s group (now a postdoctoral scholar at Stanford) and lead author of the paper. “By mixing two frequency combs instead of a single comb, we can increase the speed at which measurement are made by thousandfolds or more.”
The work also demonstrated the broadest frequency span of any on-chip dual comb—i.e., the difference between the colors on the low-frequency end and the high-frequency end is the largest. This span enables a larger variety of chemicals to be detected with the same device, and also makes it easier to uniquely identify the molecules: the broader the range of colors in the comb, the broader the diversity of molecules that can see the colors.
Conventional dual-comb spectrometers, which have been introduced over the last decade, are bulky tabletop instruments, and not portable due to their size, cost, and complexity. In contrast, the Columbia Engineering chip-scale dual comb can easily be carried around and used for sensing and spectroscopy in field environments in real time.
“There is now a path for trying to integrate the entire device into a phone or a wearable device,” says Gaeta, Rickey Professor of Applied Physics and of Materials Science.
In a later May 2018 article, New Study First to Demonstrate a Chip-Scale Broadband Optical System that Can Sense Molecules in the Mid-Infrared, Columbia University provides additional detail on how the technology is being applied to develop spectroscopy lab-on-a-chip for real-time sensing in the microseconds.
“Our results show the broadest optical bandwidth demonstrated for dual-comb spectroscopy on an integrated platform,” said Alexander Gaeta, David M. Rickey Professor of Applied Physics and of Materials Science and senior author of the study, published May 14 in Nature Communications.
Creating a spectroscopic sensing device on a chip that can realize real-time, high-throughput detection of trace molecules has been challenging. A few months ago, teams led by Gaeta and Michal Lipson, Higgins Professor of Electrical Engineering, were the first to miniaturize dual-frequency combs by putting two frequency comb generators on a single millimeter-sized chip. They have been working on broadening the frequency span of the dual combs, and on increasing the resolution of the spectrometer by tuning the lines of the comb.
In this current study, the researchers focused on the mid-infrared (mid-IR) range, which, because its strong molecular absorption is typically 10 to 1,000 times greater than those in the visible or near-infrared, is ideal for detecting trace molecules. The mid-IR range effectively covers the “fingerprint” of many molecules.
The team performed mid-IR dual-comb spectroscopy using two silicon nanophotonic devices as microresonators. Their integrated devices enabled the direct generation of broadband mid-infrared light and fast acquisition speeds for characterizing molecular absorption.