The Prime Suspect: Hot Band Absorption

The hunt was afoot within the laboratory of JILA and NIST Fellow Ralph Jimenez as his team continued to unravel the mystery of entangled two-photon absorption. Entangled photons are pairs of light particles whose quantum states are not independent of each other, so they share aspects of their properties, such as their energies and angular momenta. For many years, these photons have been studied by physicists who are trying to create quantum networks and other technologies. The Jimenez lab has been researching whether entangled photons can excite molecules with greater, even super, efficiency as compared with normal photons.

In a new paper published in the Journal of Physical Chemistry Letters, Jimenez and his team report a new experimental setup to search for the cause of a mysterious fluorescent signal that appears to be from entangled photon excitation. According to Jimenez: “We built a setup where you could use either a classical laser or entangled photons to look for fluorescence. And the reason we built it is to ask: ‘What is it that other people were seeing when they were claiming to see entangled photon-excited fluorescence?’ We saw no signal in our previous work published a year ago, headed by Kristen Parzuchowski. So now, we're wondering, people are seeing something, what could it possibly be? That was the detective work here.” The results of their new experiments suggested that hot-band absorption (HBA) by the subject molecules, could be the potential culprit for this mysterious fluorescent signal, making it the prime suspect.

Case Notes on Hot-Band Absorption

HBA is a classical one-photon absorption process. According to graduate student Kristen Parzuchowski,: “[HBA is] process in which a single photon excites a one-photon transition from thermally populated vibrational levels of the ground electronic state.” Normally, this doesn’t happen for less energetically vibrating molecules, which require two infrared photons to be excited and transition to a higher state.

In order to determine if HBA was the source of the mysterious fluorescence, Jimenez and his team tested two different molecules: Rhodamine 6G (Rh6G) and LDS798 (a fluorophore or fluorescent chemical compound that can re-emit light upon light excitation) dissolved in solvents. “A 1060-nanometer laser was used to directly excite the sample,” Jimenez explained. “The excited molecules emit red light, which is measured by a photomultiplier tube.” To create entangled photons, Jimenez added:  “We used a 532-nanometer laser and focused it into a ppKTP crystal where one in a million photons is turned into an entangled pair of 1064-nanometer photons… which can excite the sample. This way we have a classical and a quantum side of the experiment to compare.”

With their setup, the researchers focused on the “cross section” of the absorption process. “The crosssection tells you how much area a molecule presents for being hit by a photon pair,” Jimenez stated. The cross section sizehas a history of being somewhat controversial, as Parzuchowski explained that: “Right now there is significant controversy about the size of entangled two-photon absorption (E2PA) cross sections.” Jimenez added: “Other groups claimed that the E2PA cross-section was almost as large as that for a single photon, which implies very strong absorption.” The Jimenez group’s previous work showed that other investigators were over-estimating cross-sections by a factor of 10,000 or more. The team was eager to see if their experiment could validate previous observations, or if it would offer something new.

In looking at the cross-sections of the Rh6G and LDS798 molecules, Jimenez pointed out some important parameters. “For regular two-photon absorption, the cross-section is very small. That's why people need to use high-powered lasers with short pulses of light to get a signal,” he stated. “So, the implication was that big cross-sections for entangled two-photon absorption would allow ultralow-power imaging.” But this was where the HBA became important. “If the signal is from hot band absorption, that doesn't allow you to do two-photon imaging, which is 3D.” Jimenez explained that: “For at least one of the molecules we showed here, the signal could be pretty much entirely due to this hot band absorption. The other molecule we looked at did not show this absorption.”

The Evidence Points to Overestimating

In seeing HBA happen in the LDS798 molecule, the researchers realized this small signal may have big implications for the study of entangled photons interacting with molecules. “What we found is that if you calculate the hot band absorption cross-section, it can account for most of the overestimated cross-section that people were reporting for entangled two-photon absorption,” Jimenez said. “We're showing that HBA can mimic what people think is entangled two-photon absorption, so additional tests are needed to verify which process is occurring. We don't know if that's happening with every molecule that others have studied.” Jimenez hopes that others will take this new factor into account when looking at entangled two-photon absorption. In observing the similarities between the entangled two-photon flux and HBA, Parzuchowski noted that: “These two processes share one signature: a linear scaling with excitation photon flux. This is an exciting finding because some researchers who claim to measure E2PA only look for evidence of this one signature. They may have been measuring HBA all along!”

The Case Isn’t Closed

While the researchers found a potential explanation, they still want to understand how to generate a bona fide entangled photon absorption signal. According to Jimenez: “ Now, we know that the real signal is going to be around 10,000 times smaller than what people claimed to see before. But it could still be hundreds or thousands of times stronger than a classical signal under the same conditions.” The team is hoping to tweak their experimental setup to better their chances to find the signal, “There are various ways you can think of doing this experiment,” Jimenez added. “Either by using a different type of quantum light source that provides stronger excitation or by building your experiment in such a way that you get a much stronger interaction between the photons and the sample.” The researchers hope that with their new setup, and results from their previous research they can definitively identify the source of the mystery fluorescent signal, and find a real fluorescence signal from tangled photon excitation. That would be case closed.

Written by Kenna Castleberry, JILA Science Communicator