Giant nonlinear optical responses from photon avalanching nanoparticles

Abstract: Avalanche phenomena leverage steeply nonlinear dynamics to generate disproportionately high responses from small perturbations and are found in a multitude of events and materials, enabling technologies including optical phase-conjugate imaging, infrared quantum counting, and efficient upconverted lasing. However, the photon avalanching (PA) mechanism underlying these optical innovations has been observed only in bulk materials and aggregates, and typically at cryogenic temperatures, limiting its utility and impact. Here, we report the realization of PA at room temperature in single nanostructures--small, Tm-doped upconverting nanocrystals--and demonstrate their use in superresolution imaging at near-infrared (NIR) wavelengths within spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave or pulsed lasers and exhibit all of the defining features of PA. These hallmarks include excitation power thresholds, long rise time at threshold, and a dominant excited-state absorption that is >13,000x larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26th power of pump intensity. This enables the realization of photon-avalanche single-beam superresolution imaging (PASSI), achieving sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational analysis. Pairing their steep nonlinearity with existing superresolution techniques and computational methods, ANPs allow for imaging with higher resolution and at ca. 100-fold lower excitation intensities than is possible with other probes. The low PA threshold and exceptional photostability of ANPs also suggest their utility in a diverse array of applications including sub-wavelength bioimaging, IR detection, temperature and pressure transduction, neuromorphic computing, and quantum optics.

Overview of Photon Avalanching Nanoparticles

The paper "Giant nonlinear optical responses from photon avalanching nanoparticles" presents the realization of photon avalanching (PA) in Tm(^3+)-doped upconverting nanocrystals at room temperature, marking a significant advancement in the field of nonlinear optics. This study addresses a longstanding challenge of achieving PA outside of bulk materials and aggregates, traditionally constrained to cryogenic conditions.

Key Findings and Numerical Results

1. Room Temperature Photon Avalanching: The researchers successfully achieved PA in individual nanoparticles at room temperature, which previously was observed predominantly at cryogenic temperatures in bulk materials. The demonstration utilized Tm(^3+)-doped β-NaYF(_4) nanocrystals with core/shell architectures.

2. Exceptional Nonlinearity: The nanoparticles exhibited a strong nonlinear response whereby the emission intensity scaled with the (26^{th}) power of excitation intensity. This behavior is attributed to induced optical feedback mechanisms within the nanocrystals.

3. Low Threshold and High Sensitivity: The PA occurred at a low threshold pump intensity of about 20 kW/cm(^2), with a remarkable sensitivity reflected in an excited-state absorption to ground-state absorption rate ratio exceeding 13,000.

4. Sub-70 nm Imaging Resolution: The availability of PA in these nanoparticles has led to the development of photon-avalanche single-beam superresolution imaging (PASSI), achieving imaging resolutions below 70 nm without computational enhancements.

5. Wide Excitation Wavelength Range: PA was observed for wavelengths between 1400 nm to 1470 nm, with optimal performance at 1450 nm.

Implications and Future Directions

The findings have profound implications for several domains. In bioimaging, the ability to achieve superresolution with reduced photodamage is particularly advantageous for imaging in living systems. Additionally, the low PA threshold and robust photostability strengthen the utility of these nanoparticles across various applications, such as infrared detection, temperature, and pressure transduction.

From a theoretical standpoint, the successful implementation of PA in nanocrystals invites exploration into the underlying dynamics of lanthanide ion interactions at nanoparticle surfaces, offering new avenues in sensor technology where changes in environmental conditions can be translated into luminescent signals. Furthermore, the extreme nonlinearity of these materials could foster advances in quantum optics and neuromorphic systems.

Conclusion

This paper underscores the transformative potential of photon avalanching nanoparticles in optical technology, bridging the gap between bulk and nanomaterial applications under ambient conditions. As research progresses, emphasis should be placed on refining the design of such nanoparticles to enhance their efficiency and further reduce the excitation thresholds, paving the way for integration into various optical and electronic systems. The innovative application of photon avalanching at room temperature challenges traditional paradigms and prompts further investigation into novel material architectures and their quantum mechanical properties.