Do chiral molecules and their mirror images vibrate at exactly the same frequencies? Or, as theorists have suggested, does the electroweak interaction’s known nonconservation of parity introduce a slight difference? A team of researchers at the Laser Physics Laboratory in Paris (LPL, affiliated with the University of Paris 13 and the CNRS) is engaged in a long-term project to answer that question.1
Molecular vibrations fall in the mid-IR, a spectral region best accessed with quantum cascade lasers (QCLs; see the article by Federico Capasso, Claire Gmachl, Deborah Sivco, and Alfred Cho, Physics Today, May 2002, page 34). But QCLs exhibit short-term frequency fluctuations of tens to thousands of kilohertz whereas parity violation is expected to create frequency shifts on the order of 1 Hz or less. Furthermore, the tools that have been developed to stabilize the frequencies of visible and near-IR lasers don’t work as well in the mid-IR.
Now Anne Amy-Klein, her postdoc Bérengère Argence (pictured in figure 1), and their LPL colleagues have stabilized a mid-IR QCL by locking it to an ultrastable near-IR laser.2 The near-IR light was imported via a fiber-optic link from the metrology lab LNE-SYRTE (associated with the Paris Observatory, the CNRS, and Pierre and Marie Curie University) across town, where another team of researchers, led by Yann Le Coq, compared it with primary frequency standards. As a result, the stabilized QCL’s absolute frequency had an uncertainty of 10−14, or less than 1 Hz—nearly two orders of magnitude better than previous QCL stabilization efforts.
Figure 1. At the Laser Physics Laboratory in Paris, Bérengère Argence adjusts the sum-frequency-generation crystal used to combine a quantum cascade laser’s mid-IR output with a near-IR frequency comb.
Figure 1. At the Laser Physics Laboratory in Paris, Bérengère Argence adjusts the sum-frequency-generation crystal used to combine a quantum cascade laser’s mid-IR output with a near-IR frequency comb.
Ultrastability
For precision spectroscopy in the UV, visible, and near-IR, experimenters have many tools to choose from. Lasers in those regimes can be stabilized by locking to a mode of a high-finesse, ultrastable Fabry–Perot cavity. Optical-frequency-comb (OFC) lasers, which produce dense forests of equally spaced frequency peaks, provide a way to measure absolute frequencies with high precision (see Physics Today, December 2005, page 19). Atomic resonances also provide useful reference frequencies.
In the mid-IR region of molecular vibrations—about 3–15 µm—the situation changes, mostly because of materials challenges. Ultrastable cavities and OFC lasers both rely on highly reflective partial mirrors made with good- quality optical coatings, and the best available mirrors for the mid-IR just don’t perform as well as those designed for higher frequencies. Certain molecular vibrations can be used as reference frequencies, but in general, they’re much harder to control than atomic resonances.
The Paris researchers’ setup, shown schematically in figure 2, uses near-IR techniques to stabilize a mid-IR QCL. The 1.54-µm near-IR reference laser at LNE-SYRTE is locked to an ultrastable cavity. To prevent long-term frequency drift, the LNE-SYRTE researchers constantly check the laser against the metrology lab’s primary frequency standards, including a hydrogen maser and a cesium-fountain atomic clock, and adjust its frequency every 100 s.
Figure 2. An ultrastable near-IR laser, referenced to primary frequency standards at the Paris metrology lab LNE-SYRTE, is transferred 43 km to the Laser Physics Laboratory (LPL), where it’s phase locked to an optical frequency comb (OFC) that’s used to stabilize a mid-IR quantum cascade laser (QCL). The QCL output is combined with the OFC’s 1.82-µm component in a silver gallium selenite (AgGaSe2) sum-frequency-generation crystal. The sum frequency is compared with the 1.55-µm OFC component and the error signal fed back into the QCL. (Adapted from ref. 2.)
Figure 2. An ultrastable near-IR laser, referenced to primary frequency standards at the Paris metrology lab LNE-SYRTE, is transferred 43 km to the Laser Physics Laboratory (LPL), where it’s phase locked to an optical frequency comb (OFC) that’s used to stabilize a mid-IR quantum cascade laser (QCL). The QCL output is combined with the OFC’s 1.82-µm component in a silver gallium selenite (AgGaSe2) sum-frequency-generation crystal. The sum frequency is compared with the 1.55-µm OFC component and the error signal fed back into the QCL. (Adapted from ref. 2.)
The near-IR reference is transmitted 43 km to the LPL, where it’s used to stabilize an OFC. The OFC has two outputs: one centered at 1.55 µm and one, centered at 1.82 µm, generated by feeding part of the 1.55-µm output through a nonlinear optical fiber. The frequency difference between the two components corresponds to the QCL wavelength—in this case, 10.3 µm. So when the QCL output is combined with the 1.82-µm comb in a sum-frequency-generating nonlinear crystal, the output should coincide with the 1.55-µm comb. Any discrepancy must be due to fluctuations in the QCL, so by feeding the phase difference back into the QCL, the researchers can negate the fluctuations and stabilize the laser.
All the ingredients of that setup have been demonstrated before. For example, in 2007 Paolo De Natale of Italy’s National Institute of Optics in Florence, Livio Gianfrani of the Second University of Naples, and their colleagues used sum-frequency generation to measure (but not stabilize) a QCL’s frequency against a near-IR OFC.3 In 2013 De Natale and colleagues phase locked a QCL to an OFC to stabilize its frequency to better than 1 kHz.4 The two Paris teams have long been using their fiber-optic link to transport frequency references from LNE-SYRTE to the LPL.5 And in 2013 they used the link to stabilize a carbon dioxide laser—a powerful and useful mid-IR source, but one whose output, limited to a few vibrational resonances of CO2, leaves most of the mid-IR range inaccessible.6
In the new Paris result, all the key pieces—the QCL, the OFC, sum- frequency generation, and an ultrastable frequency reference—were put together for the first time. “The choice of the reference is a key point in the setup,” explains Amy-Klein, “since the stabilized laser will never be more stable than the reference. We used one of the best references available.” As a result, the QCL’s linewidth was narrowed to just 0.2 Hz, and its fluctuation was reduced to less than 0.06 Hz.
For ease of comparison with their previous results, the researchers chose a QCL tunable over a 60-nm range—10.28 µm to 10.34 µm—that encompassed one of the CO2 laser’s outputs. But QCLs are available in all parts of the mid-IR. With their current setup, the researchers can access any part of the 9- to 11-µm range just by swapping out the QCL. Outside that range, they’d also need to replace the nonlinear fiber and sum-frequency-generation crystal.
Good vibrations
The LPL researchers haven’t yet used their stabilized laser in their search for parity violation. Among other things, they’re still searching for the right molecule to put to the test. They need a molecule that has a vibrational frequency with a large predicted parity splitting and that can be synthesized in quantities of 1 g or more and purified into its two mirror-image forms. Furthermore, they need to get the molecule into the gas phase—not an easy task when most of the candidates are heavy, nonvolatile organometallic compounds—and suppress its Doppler broadening and other effects that obscure the parity-violation signal. Still, the QCL has already opened up new possibilities. “Previously we were limited to molecules with absorption lines in the narrow range accessible to the CO2 laser,” says Amy-Klein. “Now we can look at absorption lines anywhere in the mid-IR.”
And of course, ultrastable mid-IR lasers have plenty of other applications. From precision molecular vibrational spectra one can derive extremely accurate measurements of fundamental quantities such as the fine-structure constant and the proton-to-electron mass ratio—and perhaps observe those ostensible constants drifting over time.7 Sensors that detect trace components of a mixture based on their vibrational spectra can be made even more sensitive and specific—with applications to atmospheric, planetary, medical, and industrial sciences. Teams at LNE-SYRTE and the LPL are working on expanding their 43-km fiber-optic link into a continent-wide network so researchers across Europe can benefit from LNE-SYRTE’s frequency standards.
