Giant nonlinear optical responses from photon-avalanching nanoparticles

Avalanche phenomena use steeply nonlinear dynamics to generate disproportionately large responses from small perturbations, and are found in a multitude of events and materials¹. Photon avalanching enables technologies such as optical phase-conjugate imaging², infrared quantum counting³ and efficient upconverted lasing^4-6. However, the photon-avalanching mechanism underlying these optical applications has been observed only in bulk materials and aggregates^6,7, limiting its utility and impact. Here we report the realization of photon avalanching at room temperature in single nanostructures-small, Tm³⁺-doped upconverting nanocrystals-and demonstrate their use in super-resolution imaging in near-infrared spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave lasers, and exhibit all of the defining features of photon avalanching, including clear excitation-power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is more than 10,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26th power of the pump intensity, owing to induced positive optical feedback in each nanocrystal. This enables the experimental realization of photon-avalanche single-beam super-resolution imaging⁷ with sub-70-nanometre spatial resolution, achieved by using only simple scanning confocal microscopy and without any computational analysis. Pairing their steep nonlinearity with existing super-resolution techniques and computational methods^8-10, ANPs enable imaging with higher resolution and at excitation intensities about 100 times lower than other probes. The low photon-avalanching threshold and excellent photostability of ANPs also suggest their utility in a diverse array of applications, including sub-wavelength imaging^7,11,12 and optical and environmental sensing^13-15.

Controlled Assembly of Upconverting Nanoparticles for Low-Threshold Microlasers and Their Imaging in Scattering Media

Micron-sized lasers fabricated from upconverting nanoparticles (UCNP) coupled to whispering gallery mode (WGM) microresonators can exhibit continuous-wave anti-Stokes lasing useful for tracking cells, environmental sensing, and coherent stimulation of biological activity. The integration of these microlasers into organisms and microelectronics requires even smaller diameters, however, which raises threshold pump powers beyond practical limits for biological applications. To meet the need for low lasing thresholds and high fidelity fabrication methods, we use correlative optical and electron microscopy to uncover the nanoparticle assembly process and structural factors that determine efficient upconverted lasing. We show that 5 μm microspheres with controlled submonolayer UCNP coatings exhibit, on average, 25-fold lower laser thresholds (1.7 ± 0.7 kW/cm²) compared to the mean values of the lowest threshold UCNP lasers, and variability is reduced 30-fold. WGMs are observed in the upconversion spectra for TiO₂-coated microspheres as small as 3 μm, a size at which optical losses had previously prevented such observations. Finally, we demonstrate that the WGM signatures of these upconverting microlasers can be imaged and distinguished through tissue-mimicking phantoms. These advances will enable the fabrication of more efficient upconverting lasers for imaging, sensing, and actuation in optically complex environments.

Room-temperature continuous-wave upconverting micro- and nanolasing for bio-optofluidics

Nanolasers that operate under the continuous-wave pump and are robust in diverse environments will make possible compact optoelectronic devices, biomedical imaging, and large-scale quantum photonics. However, current nanolasers require low temperatures or pulsed excitation because their small mode volumes severely limit gain relative to cavity loss. Here, I will present continuous-wave upconverting micro- and nanolasing at room temperature with record-low thresholds and high photostability. I will explore the future implications of such a low-threshold laser for optofluidics.

Ultralow-threshold, continuous-wave upconverting lasing from subwavelength plasmons

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones^1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions^4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales^6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires^10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation^6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb³⁺/Er³⁺-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W cm^-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light.

Continuous-wave upconverting nanoparticle microlasers

Reducing the size of lasers to microscale dimensions enables new technologies¹ that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors² to live brain tissue³. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers^4-7 that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs)⁸ as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm³⁺-doped UCNPs⁹ to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm^-2. Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions¹⁰. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.

Femtosecond Laser Ablation Synthesis of Aryl Functional Group Substituted Gold Nanoparticles

Femtosecond laser ablation synthesis of gold-aryl nanoparticles in solution was explored. Laser irradiation of the yellow solution of diazonium tetrachloroaurate(III) salt [C8F17-4-C6H4N≡N]AuCl4 in acetonitrile formed ruby red color of gold-aryl nanoparticles without the need for external chemical reducing agent. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and nanodrop UV-Vis spectroscopy were used in the nanoparticles characterization. XPS showed the presence of the core–shell and the formation of gold(0) oxidation state only. The nanoparticles size distribution estimated by TEM is dependent on the duration of laser irradiation. Longer irradiation time resulted in decreasing the nanoparticles size. UV-Vis studies in acetonitrile showed that the absorption of gold(III) at 310 nm vanished with a concomitant formation of a plasmon absorbance at 532 nm due to the formation of “embryonic” gold-aryl nanoparticles. The novelty of this work is the in situ conjugation of core–shell structure without the need for adjusting the conjugate/gold ratio, chemicals-free synthesis from reducing agents and surfactants, synthesis of nanoparticles using gold salts unlike the common ablation of flat metal surfaces, and the use of reactive [AuCl4]− counter-ion that permits the co-deposition of gold and conjugates. Released solvated electrons and hydrogen radicals are believed to induce the reduction reaction of the gold salts. Isolation of pure nanoparticles is important for further biomedical applications including cellular uptake.

Energy-Looping Nanoparticles: Harnessing Excited-State Absorption for Deep-Tissue Imaging

Near infrared (NIR) microscopy enables noninvasive imaging in tissue, particularly in the NIR-II spectral range (1000-1400 nm) where attenuation due to tissue scattering and absorption is minimized. Lanthanide-doped upconverting nanocrystals are promising deep-tissue imaging probes due to their photostable emission in the visible and NIR, but these materials are not efficiently excited at NIR-II wavelengths due to the dearth of lanthanide ground-state absorption transitions in this window. Here, we develop a class of lanthanide-doped imaging probes that harness an energy-looping mechanism that facilitates excitation at NIR-II wavelengths, such as 1064 nm, that are resonant with excited-state absorption transitions but not ground-state absorption. Using computational methods and combinatorial screening, we have identified Tm(3+)-doped NaYF4 nanoparticles as efficient looping systems that emit at 800 nm under continuous-wave excitation at 1064 nm. Using this benign excitation with standard confocal microscopy, energy-looping nanoparticles (ELNPs) are imaged in cultured mammalian cells and through brain tissue without autofluorescence. The 1 mm imaging depths and 2 μm feature sizes are comparable to those demonstrated by state-of-the-art multiphoton techniques, illustrating that ELNPs are a promising class of NIR probes for high-fidelity visualization in cells and tissue.