Laser-Printed Plasmonic Arrays Boost IR Emission of Colloidal Quantum Dots

Semiconductor quantum dots (QDs) that emit light in the near- and mid-infrared (IR) spectral ranges have the potential to power a range of optical devices. However, fundamental physical limitations decrease the intensity of IR-emitting QDs. At longer wavelengths, the quantum yield of such QDs decreases as the radiative emission rate drops following Fermi’s golden rule, while nonradiative recombination channels compete with light emission.

Scientists from Far Eastern Federal University (FEFU) and the Far East Branch of the Russian Academy of Sciences (FEB RAS), working with international colleagues, overcame these limitations by applying a special resonant lattice of nanostructures to mercury telluride (HgTe) QDs.

They designed a resonant lattice laser that allowed control of the near- and mid-IR radiation properties of the capping layer of the HgTe QDs. They formed the lattice by ultraprecise direct femtosecond laser printing on the surface of a thin gold film.

“The plasmon lattice we developed consists of millions of nanostructures arranged on the gold film surface. We produced such lattice using advanced direct laser processing. This fabrication technology is inexpensive compared to existing commercial lithography-based methods, easily upscalable, and allows facile fabrication of nanostructures over centimeter-scale areas,” FEFU researcher Aleksander Kuchmizhak said.

The resonant lattice converted the pump radiation into surface plasmons, a special type of electromagnetic wave. These waves, when propagating over the surface of the patterned gold film within the capping layer of the QDs, provided efficient excitation, boosting photoluminescence yield.

With colleagues from China, Hong Kong, and Australia, the scientists manufactured ultracompact bright sources based on IR-emitting HgTe QDs.

“For the visible spectral range, quantum dots have been synthesizing for several decades. Just a few scientific groups in the world, though, are capable of synthesizing QDs for the near and mid-IR range,” FEB RAS researcher Alexander Sergeev said. “Thanks to the plasmon lattice we developed, which consists of plasmon nanostructures arranged in a special way, we are able to control the main light-emitting characteristics of such unique QDs, for example, by repeatedly increasing the intensity and photoluminescence lifetime, reducing the efficiency of nonradiative recombinations, as well as by tailoring and improving emission spectrum.”

a): Artistic representation of the HgTe QD layer coated above the laser-printed gold nanobump array. b): Side-view (view angle of 45°) SEM image showing the gold nanobump array printed at a 1-μm pitch (scale bar corresponds to 1 μm). A close-up SEM image on the top inset demonstrates the difference between the period and the “effective” period of the nanobump array. The bottom inset shows a photograph of two large-scale (3 × 9 mm²) nanobump arrays produced on the glass-supported gold film. c): Typical Fourier transform infrared (FTIR) reflection spectrum of the plasmonic nanobump array printed at a 1-μm pitch (green curve). The contribution of the localized surface plasmon resonance of the isolated nanobumps of a given shape is shown by the orange dashed curve. FLPR denotes the first-order lattice plasmon resonance. The inset provides the distribution of the z-component of the EM field (E_z/E₀) calculated 50 nm above the smooth gold film surface at 1480 nm wavelength. Circles indicate the nanobump positions. d): Side-view (view angle of 70°) SEM image of the cross-section of the nanobump (scale bar is 200 nm). e, f): Calculated EM-field intensity distribution (E²/E₀²) near the isolated nanobump (in the xz plane) and 50 nm above the smooth gold film level (in the xy plane) at an 880 nm pump wavelength (scale bars in e, f are 200, 1000 nm, respectively). Courtesy of FEFU press office.

The researchers believe that these quantum dots are a promising class of luminophores. They can be synthesized using a simple, cost-effective chemical method, are durable, and, unlike organic molecules, they do not experience degradation. “The developed approach [could be applied] to design new optical telecommunication devices, detectors, and emitters, including the first IR-emitting QD-based microlaser,” Kuchmizhak said.

The research was published in Light: Science and Applications (www.doi.org/10.1038/s41377-020-0247-6).