arXiv subjects

Masaru Kuno

Publications and source records attributed to Masaru Kuno.

11 recordsLinked to original sources

arXiv2026

Deterministic nucleation of nanocrystal superlattices on 2D perovskites for light-funneling heterostructures

Semiconductor heterostructures that combine components with different dimensionality provide an interesting way to manipulate the physical properties of the resulting material. Two-dimensional lead halide perovskites crystallize as flat microcrystals and have efficient in-plane exciton mobility, while perovskite nanocrystals are efficient emitters with a tunable bandgap that can self-assemble into microscopic superlattices. However, combining such intricate architectures into heterostructures has been challenging due to the mismatch in solubility properties and the challenging transfer procedures. Here we realize heterostructures where CsPbBr3 nanocrystal superlattices are deterministically grown along the faces of PEA2PbBr4 two-dimensional layered perovskite microcrystals. The growth can be limited to the lateral faces of the microcrystals and result in core-crown epitaxial heterostructures, or extended to the vertical direction leading to core-shell-like structures. The growth method is simple yet effective and versatile, and promises to be expanded to a large variety of other materials. We demonstrate that these heterostructures can be employed as efficient light-harvesting systems. In fact, energy can be transferred from the two-dimensional microcrystal domain to the superlattices, enabling switching between linear and non-linear carrier recombination regimes by tuning the excitation fluence. Moreover, by exploiting the lifetime shortening of CsPbBr3 nanocrystal emission upon sample cooling, we ensure that energy transfer occurs after the biexcitonic and single-excitonic decays of the nanocrystals, effectively extending the radiative recombination of superlattices.

cond-mat.mtrl-sci↗

arXiv2026

Ionic-instability induced color tuning in lead-based, mixed-halide perovskites

Contrary to conventional wisdom, intermediate photoluminescence energies can be stabilized in mixed-halide lead perovskites during photosegregation. These intermediate energies reside between those of the parent, mixed-halide alloy and fully photosegregated specimens. This demonstrates rudimentary color tuning and has practical implications for potential uses of mixed-halide perovskites in lighting applications. More fundamentally, such color tuning begs the question of how intermediate photosegregation energies arise and how they are kinetically stabilized. What follows is a study of the kinetics of terminal photosegregation energies under pulsed laser excitation. Through concerted continuous wave and pulsed laser photosegregation measurements, we develop a kinetic rationalization for how photosegregations repetition rate or duty cycle and peak fluence dependencies lead to intermediate, terminal photoluminescence energies. The developed model, in turn, explains prior observations of pulsed illumination photosegregation and offers potential insights into other, yet to be explained, phenomena such as spectral blueshifting under high intensity, pulsed illumination.

cond-mat.mtrl-sci↗

arXiv2025

Solid-state Laser Cooling

Since the first proof-of-concept demonstrations of photoluminescence-based optical refrigeration, solid-state laser cooling has developed into a credible competitor to conventional cryogenic technologies. Solid-state laser cooling continues to advance as new materials push cooling limits. These developments have created a need to consolidate progress made to date as well as standardize critical experimental considerations needed for reliable and verifiable cooling measurements. This primer therefore outlines essential concepts and requirements, which underpin solid-state laser cooling. The primer summarizes key milestones achieved with cooling-grade, rare-earth-doped glasses and crystals as well as with semiconductors. It additionally highlights emerging applications of solid-state optical refrigeration. To strengthen the consistency and reproducibility of cooling results going forward, two reporting checklists are introduced. They cover materials, cooling metrics, and thermometry. This primer is intended to serve as both a tutorial and a practical reference for incoming and existing researchers involved in solid-state laser-cooling.

physics.optics↗

arXiv2024

Experimental minimum of condensed-phase optical refrigeration

Since the first demonstration of optical refrigeration in a rare-earth-doped glass nearly 30 years ago, the nascent field of laser cooling solids has progressed significantly. It is now possible to demonstrate payload cooling to ~91 K using laser-excited ytterbium-doped fluoride crystals. Realizing lower temperatures, however, requires achieving ultrahigh purities in existing rare earth-doped crystals or establishing new cooling media. For the latter, semiconductors are an obvious choice given higher cooling power densities and predicted cooling floors as low as 10 K. This has stimulated a race to demonstrate the optical refrigeration of a semiconductor. It is therefore timely to systematize the necessary and sufficient experimental minimum criteria for reporting optical refrigeration results to elevate the reliability and reproducibility of current and future optical refrigeration claims. We distill an optical refrigeration Experimental Minimum (EM) that we propose will standardize the reporting of new cooling results. EM tenets fall into the following categories: demonstrations of explicit heating vs cooling, optical cooling metrics, thermodynamic consistency, and reliable temperature measurements. All optical refrigeration claims should demonstrate the above four criteria to ensure their reliability and verifiability. We further propose that the EM serve as a guide for reviewing literature claims in the field.

physics.optics↗

arXiv2024

Resonant multiple-phonon absorption causes efficient anti-Stokes photoluminescence in CsPbBr$_3$ nanocrystals

Lead-halide perovskite nanocrystals such as CsPbBr$_3$, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr$_3$ nanocrystals and attribute it to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL towards condensed phase optical refrigeration.

cond-mat.mes-hall↗

arXiv2023

A thermodynamic band gap model for photoinduced phase segregation in mixed-halide perovskites

Provided is a comprehensive description of a band gap thermodynamic model, which predicts and explains key features of photosegregation in lead-based, mixed-halide perovskites. The model provides a prescription for illustrating halide migration driven by photocarrier energies. Where possible, model predictions are compared to experimental results. Free energy derivations are provided for three assumptions: (1) halide mixing in the dark, (2) a fixed number of photogenerated carriers funneling to and localizing in low band gap inclusions of the alloy, and (3) the statistical occupancy of said inclusions from a bath of thermalized photocarriers in the parent material. Model predictions include: excitation intensity ($I_{\textrm{exc}}$)-dependent terminal halide stoichiometries ($x_{\textrm{terminal}}$), excitation intensity thresholds ($I_{\textrm{exc,threshold}}$) below which photosegregation is suppressed, reduced segregation in nanocrystals as compared to thin films, the possibility to kinetically manipulate photosegregation rates via control of underlying mediators, asymmetries in forward and reverse photosegregation rate constants/activation energies, and a preference for high band gap products to recombine with the parent phase. What emerges is a cohesive framework for understanding ubiquitous photosegregation in mixed-halide perovskites and a rational basis by which to manage the phenomenon.

cond-mat.mtrl-sci↗

arXiv2023

Photoluminescent cooling with incoherent light

Optical refrigeration using anti-Stokes photoluminescence is now well established, especially for rare-earth-doped solids where cooling to cryogenic temperatures has recently been achieved. The cooling efficiency of optical refrigeration is constrained by the requirement that the increase in entropy of the photon field must be greater than the decrease in entropy of the sample. Laser radiation has been used in all demonstrated cases of optical refrigeration with the intention of minimizing the entropy of the absorbed photons. Here, we show that as long as the incident radiation is unidirectional, the loss of coherence does not significantly affect the cooling efficiency. Using a general formulation of radiation entropy as the von Neumann entropy of the photon field, we show how the cooling efficiency depends on the properties of the light source such as wavelength, coherence, and directionality. Our results suggest that the laws of thermodynamics permit optical cooling of materials with incoherent sources such as light emitting diodes and filtered sunlight almost as efficiently as with lasers. Our findings have significant and immediate implications for design of compact, all-solid-state devices cooled via optical refrigeration.

physics.optics↗

arXiv2023

Enhanced robustness and dimensional crossover of superradiance in cuboidal nanocrystal superlattices

Cooperative emission of coherent radiation from multiple emitters (known as superradiance) has been predicted and observed in various physical systems, most recently in CsPbBr$_3$ nanocrystal superlattices. Superradiant emission is coherent and occurs on timescales faster than the emission from isolated nanocrystals. Theory predicts cooperative emission being faster by a factor of up to the number of nanocrystals ($N$). However, superradiance is strongly suppressed due to the presence of energetic disorder, stemming from nanocrystal size variations and thermal decoherence. Here, we analyze superradiance from superlattices of different dimensionalities (one-, two- and three-dimensional) with variable nanocrystal aspect ratios. We predict as much as a 15-fold enhancement in robustness against realistic values of energetic disorder in three-dimensional (3D) superlattices composed of cuboid-shaped, as opposed to cube-shaped, nanocrystals. Superradiance from small $(N\lesssim 10^3)$ two-dimensional (2D) superlattices is up to ten times more robust to static disorder and up to twice as robust to thermal decoherence than 3D superlattices with the same $N$. As the number of $N$ increases, a crossover in the robustness of superradiance occurs from 2D to 3D superlattices. For large $N\ (> 10^3)$, the robustness in 3D superlattices increases with $N$, showing cooperative robustness to disorder. This opens the possibility of observing superradiance even at room temperature in large 3D superlattices, if nanocrystal size fluctuations can be kept small.

cond-mat.mes-hall↗

arXiv2020

Solid-state laser refrigeration of nanodiamond quantum sensors

The negatively-charged nitrogen vacancy (NV$^-$) centre in diamond is a remarkable optical quantum sensor for a range of applications including, nanoscale thermometry, magnetometry, single photon generation, quantum computing, and communication. However, to date the performance of these techniques using NV$^-$ centres has been limited by the thermally-induced spectral wandering of NV$^-$ centre photoluminescence due to detrimental photothermal heating. Here we demonstrate that solid-state laser refrigeration can be used to enable rapid (ms) optical temperature control of nitrogen vacancy doped nanodiamond (NV$^-$:ND) quantum sensors in both atmospheric and \textit{in vacuo} conditions. Nanodiamonds are attached to ceramic microcrystals including 10\% ytterbium doped yttrium lithium fluoride (Yb:LiYF$_4$) and sodium yttrium fluoride (Yb:NaYF$_4$) by van der Waals bonding. The fluoride crystals were cooled through the efficient emission of upconverted infrared photons excited by a focused 1020 nm laser beam. Heat transfer to the ceramic microcrystals cooled the adjacent NV$^-$:NDs by 10 and 27 K at atmospheric pressure and $\sim$10$^{-3}$ Torr, respectively. The temperature of the NV$^-$:NDs was measured using both Debye-Waller factor (DWF) thermometry and optically detected magnetic resonance (ODMR), which agree with the temperature of the laser cooled ceramic microcrystal. Stabilization of thermally-induced spectral wandering of the NV$^{-}$ zero-phonon-line (ZPL) is achieved by modulating the 1020 nm laser irradiance. The demonstrated cooling of NV$^-$:NDs using an optically cooled microcrystal opens up new possibilities for rapid feedback-controlled cooling of a wide range of nanoscale quantum materials.

cond-mat.mtrl-sci↗

arXiv2009

Universal emission intermittency in quantum dots, nanorods, and nanowires

Virtually all known fluorophores, including semiconductor nanoparticles, nanorods and nanowires exhibit unexplainable episodes of intermittent emission blinking. A most remarkable feature of the fluorescence intermittency is a universal power law distribution of on- and off-times. For nanoparticles the resulting power law extends over an extraordinarily wide dynamic range: nine orders of magnitude in probability density and five to six orders of magnitude in time. The exponents hover about the ubiquitous value of -3/2. Dark states routinely last for tens of seconds, which are practically forever on quantum mechanical time scales. Despite such infinite states of darkness, the dots miraculously recover and start emitting again. Although the underlying mechanism responsible for this phenomenon remains an enduring mystery and many questions remain, we argue that substantial theoretical progress has been made.

cond-mat.stat-mech↗

arXiv2006

Power Law Blinking Quantum Dots: Stochastic and Physical Models

We quantify nonergodic and aging behaviors of nanocrystals (or quantum dots) based on stochastic model. Ergodicity breaking is characterized based on time average intensity and time average correlation function, which remain random even in the limit of long measurement time. We argue that certain aspects of nonergodicity can be explained based on a modification of Onsager's diffusion model of an ion pair escaping neutralization. We explain how diffusion models generate nonergodic behavior, namely a simple mechanism is responsible for the breakdown of the standard assumption of statistical mechanics. Data analysis shows that distributions of on and off intervals in the nanocrystal blinking are almost identical, $ψ_{\pm}(τ)\propto A_{\pm}τ^{-(1+α_{\pm})}$ with $A_{+}\approx A_{-}$ and $α_{+}\approxα_{-}=α$ and $α\approx0.8$. The latter exponent indicates that a simple diffusion model with $α=0.5$ neglecting the electron-hole Coulomb interaction and/or tunneling, is not sufficient.

cond-mat.stat-mech↗