By Brooke Kuei
Users and staff at the Molecular Foundry have combined the properties of two tiny materials to produce ultra-stable lasers that boost the energy of light in spaces smaller than the light waves themselves.
These lasers, which convert infrared light into visible red light, operate at the lowest powers recorded for so-called upconverting lasers. Unlike other lasers that utilize similar phenomena, these new lasers run continuously at room temperature and have a narrow, directional beam that will make it easier to use these devices for biomedical imaging and three-dimensional mapping.
The study, published in Nature Materials, leverages two types of nanometer-sized particles, (a nanometer is a millionth of a meter): ceramic nanoparticles that upconvert light, and arrays of metal nanopillars that enhance the intensity of light and produce lasing.
The upconverting nanoparticles, developed by co-authors and Molecular Foundry staff scientists Bruce Cohen and Emory Chan, use rare earth elements known as lanthanides to absorb several low energy photons (particles of light) and then emit single photon with a higher energy. “It’s like exchanging five pennies for a nickel,” explained Chan. “The total value is the same, but that nickel is more useful for paying a parking meter.”
Last year, the Foundry team collaborated with co-author Jim Schuck, Associate Professor of Mechanical Engineering at Columbia University, to develop the world’s most efficient and stable upconverting lasers using nanoparticles embedded with lanthanides. But because the upconversion process is inherently inefficient, these lasers still required a large amount of power to operate.
For the current study, Chan, Cohen, and Shuck joined forces with co-author Teri Odom, Professor and Chair of Chemistry at Northwestern University, to leverage her laboratory’s expertise using metal nanoparticle arrays and dyes to produce nanometer-sized lasers. The arrays consist of tiny silver pillars spaced about 450 nanometers apart on a glass surface. “When light hits the silver nanopillars at specific frequencies, the electrons inside of the metal collectively slosh back in forth in sync,” said Chan. This oscillation, known as surface plasmon resonance, creates a strong electromagnetic hot spot close to the pillars that can increase the intensity of the light emitted by dyes in those hot spots by millions of times.
But most dyes and nanoparticles degrade after a few minutes under the intense light needed to produce lasing in these plasmonic devices. To combat this, researchers either lower the temperature of their lasers with liquid nitrogen or helium, or operate them in intermittently in short pulses, neither of which is convenient in real-world applications.
By covering the nanopillar arrays with upconverting nanoparticles, which the Foundry team previously demonstrated do not degrade under intense light, the research team created a new kind of nanolaser: now, the nanoparticles that sit in the hotspots can emit light orders of magnitude more effectively than before. “Our tiny lasers operate at powers that are orders of magnitude smaller than observed in any existing upconverting or plasmonic lasers,” Schuck said.
Angel Fernandez-Bravo, a Molecular Foundry postdoc with Chan and Schuck, demonstrated that these nanolasers can emit a continuous stream of photons, rather than pulses, and they last for over six hours operating at room temperature. Simulations by Danqing Wang, a graduate student in Odom’s group, showed that the hot spots that produce the lasers are smaller than the wavelength of light itself, which normally limits the size of optical components to several hundred nanometers. This ability to confine lasers to tiny, sub-wavelength spaces gives them the potential to sense disease biomarkers in tissue or treat neurological disorders.
“Our nanolaser is transparent but can generate visible photons when optically pumped with light our eyes cannot see,” said Odom. “The continuous wave, low-power characteristics will open numerous new applications, especially in biological imaging.”
The technology could also be useful for quantum-optical technologies, and lab-on-a-chip photonic devices.
“When we realized our nanoparticles had this unusual property of perfect photostability, we knew that would open up a lot of applications. Stable, tiny lasers weren’t the first thing that came to mind, but this turns out to be a really exciting area,” Cohen adds.
Moving forward, the researchers hope to explore the different emission colors that are possible with different lanthanides and also possibly by coupling those nanoparticles with different types of nanostructure arrays.
“This is a great example of combining user capabilities with those at the Foundry, and it shows how such synergies can open up new fields of research,” said Chan.