Molecular Fingerprinting and Control

The Jun Ye group demonstrated the first direct frequency comb spectroscopy with a femtosecond optical frequency comb. The group is now expanding its research on coherent spectroscopy by extending the precise control of femtosecond combs into the mid-infrared (mid-IR) and vacuum ultraviolet (VUV) regions of the electromagnetic spectrum. Ye and his colleagues are investigating frequency comb-based spectroscopy across wider regions of the spectrum with the goal of achieving high-resolution quantum control of atomic and molecular states. Their coherent spectroscopic techniques also promise important applications in homeland security, atmospheric research (for the monitoring of greenhouse gases), and medical analysis.

The frequency comb-based spectroscopy at the heart of molecular fingerprinting combines a broad bandwidth and high spectral resolution frequency comb with ultrahigh detection sensitivity from an optical enhancement cavity. This combination makes it possible to readily identify many individual kinds of molecules in a mixture such as a human breath. Because fundamental molecular vibration frequencies are located in the mid-IR, molecular fingerprinting works most efficiently at those wavelengths. In 2009, the Ye group developed a mid-IR frequency comb that produces broadband radiation with wavelengths tunable between 3 and 5 µm and power levels greater than 1 W. In 2010, the group will be working to reach wavelengths of 5–10 µm. It will also be using the 3–5 µm comb for interesting applications.

Unlike in the visible and near-IR, where frequency combs can be generated directly from a laser, the production of a mid-IR comb requires an optical parametric oscillator. In this device, pump photons from a near-IR frequency comb laser are down-converted to pairs of lower energy photons that maintain the comb structure. One challenge in designing the new cavity-enhanced comb-spectroscopy system has been getting low-loss mirrors that are highly reflective to mid-IR light. The group has spent many hours working with mirror-coating specialists at local Boulder companies to perfect the new mirrors.

While enhancing molecular fingerprinting technology, the Ye group also continues working on its many important applications. First, the group is extending its studies of molecular-fingerprinting-based breath analysis by collaborating with several departments at the CU Health Sciences Center and the CU Cancer Center. The medical school departments are providing breath samples from patients or research animals with severe lung diseases such as cancer or emphysema. By comparing the spectra of breath samples from very sick patients or lab animals with those of healthy student volunteers, the group plans to demonstrate that its new technology can discriminate between people who are healthy or sick via a simple, noninvasive test. Once it succeeds, its medical collaborators plan to launch double-blind studies of the new technology.

Second, the group is working with a local semiconductor manufacturer on the detection of contaminants (e.g., water or carbon dioxide) in arsine gas used to make laser diodes for laser pointers, CD-ROM drives, and DVDs. Even at very low levels, these contaminants can greatly influence the performance of the diodes. The researchers have already used comb spectroscopy to detect water contamination in the gas. They were able to scan a broad region of the spectrum and zoom in on single water absorption lines. Their job was complicated by the fact that the arsine gas absorbed throughout the region under study, making it very difficult to find the water absorptions and impossible to "see" absorption from carbon dioxide. The results of this study reinforced the group’s goal of developing a mid-IR comb, which should make it much easier to resolve the spectra of individual molecules.

Finally, the group recently demonstrated the ultrahigh resolution possible with a comb spectroscopy system by probing cold molecules. The experiment used an erbium-doped fiber laser comb to characterize a supersonic jet expansion of acetylene in argon gas inside an optical enhancement cavity, as shown in the figure. In this high-resolution broadband system, it is possible to detect all of the acetylene absorption lines at once while also looking closely at how the shape of each line changes. By moving the nozzle in two spatial dimensions, the system can extract different information about the molecules under study and perform 3D mapping of the rotational temperature (basically, how fast the molecules are spinning) and density of the supersonic jet. The system has not only made it easier to perform high-resolution spectroscopy, but also much faster since the comb lines act as thousands of independent detection channels. The independent detection channels allow for the processing of multiple molecular absorption lines in an instant.

Their early results were so encouraging that the researchers believe this technology could allow them to "see" and measure short-lived intermediates in chemical reactions. The group also envisions the application of comb-based spectroscopy to the study of cold chemistry and cold collisions, the investigation of greenhouse gases in the atmosphere, and the identification of explosives, toxins, or industrial contaminants.

In an exciting collaboration with Eric Cornell, the Ye group is a applying its expertise in comb-based spectroscopy to the search for the elusive electron electric dipole moment (eEDM). The two groups plan to use comb-based spectroscopy to scan for two key electronic transitions predicted by Cornell and Bohn to exist in hafnium fluoride ions (HfF⁺), which are under investigation in the Cornell lab. Once the collaborating team has precisely identified these transitions (predicted to be in the vicinity of 800 nm but with uncertainty up to 200 nm), it plans to develop new technology to precisely measure them. The goal is to develop a comb-based system that uses hundreds of detectors to find and record the absorption features of many rotational and vibrational transitions of the ions, while ignoring absorptions from neutral species that would complicate the spectrum.