For Ultrafast Photonics, New Applications Emerge

HANK HOGAN, CONTRIBUTING EDITOR, hank.hogan@photonics.com

Ultrafast lasers can do a lot, including create a room-temperature superconductor. That type of material transformation is just one emerging area of ultrafast photonics. Today, new ultrafast photonics applications found in lasers and sensors enable systems to see deep into tissue and improve radar performance. Tomorrow, the technology could combat climate change, via more energy-efficient magnetic polarity switching, by transforming ceramics into room-temperature superconductors and more.

An example of a new use of ultrafast photonics improves the performance of radio-frequency (RF) signal processing. This is critical to building better radar and communication systems that must operate at frequencies in the hundreds of gigahertz. The enhanced performance arises because the system makes optimum use of photonic and acoustic components, according to Aleem Siddiqui, a research and development engineer at Sandia National Laboratories in Albuquerque, N.M.

Transduction of a radio-frequency (RF) signal via photons and acoustic waves improves RF filtering performance. Depicted above is the transduction process (a) with devices showing the paths of optical and acoustic signals (b) and the typical response (c). Courtesy of Sandia National Laboratories.

“You have the best of everything. You’re using the best properties of light. You can shuttle it around long distances with low losses. The nice thing about the acoustic domain is that the frequencies are low compared to optical frequencies. To get certain filtering bandwidths, it’s a lot simpler because of the lower relative frequencies,” said Siddiqui.

He was lead author of a Sandia group’s April 2017 SPIE Ultrafast Bandgap Photonics II conference paper, “Toward High Fidelity Spectral Sensing and RF Signal Processing in Silicon Photonic and Nano-Optomechanical Platforms.” The researchers used a chip-scale nano-optomechanical system to make high-performance bandpass filters. This technology could create multichannel filters with narrower channels and less loss than possible with current methods, the group stated in its paper.

System implementation showing multichannel spectral sensing. Courtesy of Sandia National Laboratories.

Converting RF signals

This is done with a cascaded transduction process, which begins by converting an RF signal into an optical one. The interaction of these photons with a silicon nitride membrane creates an acoustic signal, downshifting the frequency from terahertz to gigahertz in the process. Filtering is done using phononic crystals, which have bandgap for acoustic waves engineered with sharp resonances. The filtered result is then changed back by a reverse process of interaction with an optical waveguide into light. This is captured by a fast photo-diode sensor and finally converted back into an RF signal.

The lower frequency in the acoustic domain eases the burden of filtering substantially. The quality, or Q factor, of the filter needed is thousands of times less because the transduction moves the frequency down by about three orders of magnitude. This relaxes requirements on the stability of the laser frequency and device temperature, making fabrication significantly easier, according to Siddiqui.

Optical setup used to image through volumetric scattering using all photons imaging. Courtesy of Guy Satat and other researchers, MIT.

For the future, the researchers are looking to further characterize the cascaded transduction process. They also plan to scale up the prototype to more complex systems, Siddiqui said.

While this approach used a continuous wave laser for its light source, other researchers combine pulsed lasers with ultrafast time-resolved sensors. Here, the ability of a sensor to capture a brief burst of light, which may only last a trillionth of a second or less, is critical.

At the Massachusetts Institute of Technology in Cambridge, Mass., Guy Satat, a Media Lab graduate student, is part of a Camera Culture group that works on time-of-flight or femto-photography projects. They include investigating seeing around corners, imaging without a lens, and peering deeper into biological tissue. The latter is accomplished by making use of all photons passing through the material, as described in a 2016 Nature Scientific Reports paper, “All Photons Imaging Through Volumetric Scattering.”

Reconstruction results demonstrating seeing through 1.5-cm-thick tissue phantom with all photons imaging. Courtesy of Guy Satat, and other researchers MIT.

Basically, some of the light travels straight through while part of it is scattered. By collecting everything, what is deep within tissue emerges.

“Having a time-resolved measurement helps an algorithm to basically correct for optical scattering and recover the actual underlying signal,” Satat said.

Ongoing research is bringing closer the day when such techniques can emerge from the lab and move into practical applications. One advance that will make this possible is a simplification of the setup. In initial demonstrations, scientists may use a streak camera, a relatively complicated piece of equipment that is difficult for nonexperts to employ.

However, much simpler single-photon avalanche photodiodes have improved in performance over the years. Therefore, after completing a proof of principle, researchers may see if this type of easier-to-use sensor works. By reworking algorithms, revisiting theory, revising setups and taking advantage of new technology, that can turn out to be the case, according to Satat.

There are other ways that ultrafast photonics may have new applications. Photons can change material properties, either permanently or temporarily. In the case of bandgap materials and ultrafast lasers, the results could save significant amounts of energy.

Switching polarity

One example of what may be possible comes from research done over the last 15 years by Theo Rasing, a physics professor at Radboud University in Nijmegen, the Netherlands. Rasing studies the interaction of femtosecond laser pulses with ferromagnetic and antiferromagnetic material. The first exhibits bulk magnetism and is found in everyday magnets and data recording media. The second is magnetic on an individual layer basis but overall is not magnetic because the layers are ordered in an antiparallel fashion and so counteract each other.

All-optical switching of magnetic media, done by moving a laser over a magnetic film. Bits are written with single, 40-fs laser pulses. Courtesy of Theo Rasing, Radboud University.

No matter the type of material, hitting it with a 50 fs-laser pulse can switch the magnetic polarity from north to south and vice versa. Thus, data can be written into and stored by the material, something that traditionally has been done via a read/write head. Using light offers advantages, Rasing noted.

“It’s about 1000 times faster,” he said.

Perhaps more importantly, the pulsed light approach can make such a switch in antiferromagnetic materials. This is something that cannot readily be done with traditional methods, and the new recording medium offers energy savings.

“It is already orders of magnitude more efficient,” Rasing said in comparing the two approaches.

That could be significant in cutting energy consumption. Large data centers are a growing class of energy users. They already gobble up several percent of total American electricity, according to a 2016 report from the U.S. Department of Energy. So, making their data storage more energy efficient would improve their long-term sustainability.

However, researchers need to figure out how best to distribute pulses and demonstrate that the technique can be scaled beyond a laboratory. Rasing hopes to have a working proof of principle of the basic concept ready in a few years. For antiferromagnetic materials, new sensors are needed, as current technology detects bulk magnetism.

While changes in magnetic polarity are permanent, the final example of ultrafast photonics and material interaction involves transitory transformation. In a 2014 Nature article, “Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa₂Cu₃O_6.5,” researchers at Germany’s Max Planck Institute for the Structure and Dynamics of Matter and elsewhere showed that ultrashort infrared pulses could briefly transform an yttrium barium copper oxide ceramic into a room-temperature superconductor.

Later, the investigators determined that the change was due to a small shift in the location of the copper dioxide layers within the ceramic crystalline matrix, with this movement driven by the laser pulse. That relocation was only “two picometers, or one hundredth of an atomic diameter,” according to the researchers. But, it was enough to increase the quantum coupling between copper dioxide layers and thereby make the ceramic a room-temperature superconductor. ?However, the time that the crystal was in this excited state of lossless conductivity was also tiny. Relaxation back into a normal state and the disappearance of superconductivity took only a few picoseconds.

This discovery gave scientists insight into how to construct a room-temperature superconductor. It also opened the door to the creation of metastable materials, ones that temporarily became a superconductor, or magnetic or transparent, or otherwise exhibit altered properties. These can be studied to uncover information about the basis for such properties.

What’s more, it may be possible to extend the time in the metastable state long enough to be useful. A covering, for instance, that switched from transparent to opaque would be beneficial for defense applications that seek to probe the world for information and targets while, in turn, being effectively invisible. A material that switched from opaque to transparent, even if only briefly and for specified wavelengths, and back again could help accomplish that.

Room-temperature superconductivity, if it can be achieved, would be very valuable. The challenge is on several fronts. For one thing, a metastable state that lasts only picoseconds implies that maintaining it will require a laser pulse rate in the hundreds of gigahertz, far more than possible with current technology. Beyond that, the initial transformation happens largely without heat, according to researchers. Pumping in more and more laser pulses, though, would change that because each pulse would deposit some energy in the material. After enough pulses, this heat would swamp everything. So, investigators are searching for the right material and setup that can overcome these and other issues.

Success in this quest to transform material properties could have a big payoff in sustainability. That is one driving reason behind the research done by Radboud University’s Rasing.

As he said, “Scientifically it’s challenging, but the potential impact in terms of energy efficiency, I found right now, personally, extremely challenging as well, and it’s one of the strong motivations for the coming years.”