Laser Cooling Research Articles

Image-Current Mediated Sympathetic Laser Cooling of a Single Proton in a Penning Trap Down to 170mK Axial Temperature.

We demonstrate a new temperature record for image-current mediated sympathetic cooling of a single proton in a cryogenic Penning trap by laser-cooled ^{9}Be^{+}. An axial mode temperature of 170mK is reached, which is a 15-fold improvement compared to the previous best value. Our cooling technique is applicable to any charged particle, so that the measurements presented here constitute a milestone toward the next generation of high-precision Penning-trap measurements with exotic particles.

Thermal Activation of Anti-Stokes Photoluminescence in CsPbBr3 Perovskite Nanocrystals: The Role of Surface Polaron States.

Optically driven cooling of a material, or optical refrigeration, is possible when optical up-conversion via anti-Stokes photoluminescence (ASPL) is achieved with near-unity quantum yield. The recent demonstration of optical cooling of CsPbBr3 perovskite nanocrystals (NCs) has provided a path forward in the development of semiconductor-based optical refrigeration strategies. However, the mechanism of ASPL in CsPbBr3 NCs is not yet settled, and the prospects for cooling technologies strongly depend on details of the mechanism. By analyzing the Arrhenius behavior of ASPL in CsPbBr3 NCs, we investigated the relationship between the average energy gained per photon during up conversion, ΔE, and the thermal activation energy, Ea. We find that Ea is systematically larger than ΔE, and that Ea increases for larger ΔE. We suggest that the additional energetic cost is due to a rearrangement of the crystal lattice as charge carriers pass from surface localized, structurally distinct sub-gap polaron states to the free exciton state during up-conversion. Our interpretation is further corroborated by quantifying the impact of ligand coverage on the NC surface. These findings help inform the development of CsPbBr3 NCs for applications in optical refrigeration.

Limits of Laser Cooling of Light Alkali Metals in a Polychromatic Light Field

Abstract A model was developed for laser cooling of alkali atoms in a polychromatic field, considering the real structure of atomic levels. The model was tested by the example of the 6Li atom. The minimum achievable temperatures of laser cooling of light alkali atoms were studied for different polarizations of the light field components, and the possibility of cooling below the Doppler limit was shown.

Bose-Einstein Condensation by Polarization Gradient Laser Cooling.

Attempts to create quantum degenerate gases without evaporative cooling have been pursued since the early days of laser cooling, with the consensus that polarization gradient cooling (PGC, also known as "optical molasses") alone cannot reach condensation. In the present work, we report that simple PGC can generate a small Bose-Einstein condensate (BEC) inside a corrugated micrometer-sized optical dipole trap. The experimental parameters enabling BEC creation were found by machine learning, which increased the atom number by a factor of 5 and decreased the temperature by a factor of 2.5, corresponding to almost 2 orders of magnitude gain in phase space density. When the trapping light is slightly misaligned through a microscopic objective lens, a BEC of ∼250 ^{87}Rb atoms is formed inside a local dimple within 40ms of PGC after MOT loading.

Direct Laser Cooling of Rydberg Atoms with an Isolated-Core Transition.

We propose a scheme to directly laser cool Rydberg atoms by laser cooling the residual ion core within the Rydberg-electron orbit. The scheme is detailed for alkaline-earth-metal Rydberg atoms, whose ions can be easily laser cooled. We demonstrate that a closed optical cooling cycle can be found despite the perturbations caused by the Rydberg electron and that this cycle can be driven over more than 100 μs to achieve laser cooling. The cooling dynamics with and without the presence of magnetic fields are discussed in detail.

High‐Efficiency Single‐Photon Upconversion Photoluminescence of Perovskite Quantum Dots via Intragap State Regulation

AbstractLead halide perovskite quantum dots (PQDs) have shown remarkable single‐photon upconversion photoluminescence (SPUC‐PL) bringing them a brilliant perspective as the active media for biological imaging, optiag, and upconversion photovoltaics. However, the mechanism of SPUC in PQDs is still in dispute, and the specific strategy for optimizing PQDs toward a higher SPUC performance has yet to be established, which impedes the application of PQDs‐based SPUC‐PL in practice. In this work it is revealed that the PL quantum yield is declined by surface trap states, while the SPUC efficiency is promoted by Urbach tail states. These two types of intragap states can be chemically manipulated by engineering the surface ligand and the growth kinetics of perovskite nanocrystals, bringing on the improvement of light emission efficiency and the promotion of subgap low‐energy photon absorption, respectively. As a proof of concept, utilizing the synergistic effect of the proposed chemical manipulation method, the SPUC‐PL efficiency of PQDs is boosted by more than 40%, which gives rise to a spectral window of optical refrigeration gain as broad as ≈130 meV.

Design and Control of a Laser Cooling System for Wavelength Stabilization of a Monochromatic Light Source

Design and Control of a Laser Cooling System for Wavelength Stabilization of a Monochromatic Light Source

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr3, 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 CsPbBr3 nanocrystals and attribute them 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 toward condensed phase optical refrigeration.

Atom Chip and Diffraction Grating for the Laser Cooling of Ytterbium Atoms

The possibility of using an atom chip and a diffraction grating to form a compact magneto-optical trap for ytterbium atoms, which can be used to develop compact atomic interferometers and optical clocks based on ultracold atoms, has been studied. An experiment on the laser cooling of the 171Yb and 174Yb isotopes in a first‑stage magneto-optical trap has been carried out to determine initial requirements for the mentioned elements. The design of the atom chip forming a magnetic field gradient up to 60 G/cm has been calculated. The optimal configurations of the diffraction grating that allow forming both the first- and second-stage magneto-optical traps have been evaluated.

Method for Fast Measuring Cold Atom Density by Isotropic Laser Cooling

Method for Fast Measuring Cold Atom Density by Isotropic Laser Cooling

Design and Simulation of Optical Field in Quasi-Two-Dimensional Isotropic Laser Cooling

Design and Simulation of Optical Field in Quasi-Two-Dimensional Isotropic Laser Cooling

Analysis of the Principle and State-of-art Applications for Laser Cooling and Trapping

As a matter of fact, the laser cooling and trapping has already been widely used in various fields in advanced atoms controlling. With this in mind, this study gives a systematical analysis of the principle of laser cooling and trapping, and the development of them in the recent years. To be specific, this study will discuss the applications that has invented in the recent year based on the laser colling and trapping (especially for the state-of-art facilities), and makes a comparison between the concepts of laser cooling and laser trapping accordingly. Then, based on the analysis and evaluations, the limitations of laser cooling and laser trapping have been shown, and the future outlooks are also claimed and demonstrated as well. To sum up, the study concludes by discussing the limitations of these techniques, and gives some methods to solve these issues. Overall, these results shed light on guiding further exploration of laser cooling and trapping.

Sharp Focusing of an Atomic Beam with the Doppler and Sub-Doppler Laser Cooling Mechanisms in a Two-Dimensional Magneto-Optical Trap

The focusing of an atomic beam with the use of a two-dimensional magneto-optical trap in order to increase the number of atoms in the region of their laser cooling and localization near an atom chip is discussed. Two regimes of the interaction of atoms with a focusing laser field are considered: (i) the Doppler interaction regime, which occurs at small detunings of the laser field from the atomic resonance, and (ii) the sub-Doppler interaction regime, which occurs at large detunings of the laser field from the atomic resonance. The efficiency of focusing in the first case is low because of the momentum diffusion. It has been shown that the momentum diffusion in the sub-Doppler cooling mechanism is insignificant and, as a result, the broadening of the transverse velocity distribution of atoms is small. The sharp focusing of the atomic beam is possible in this interaction regime.

Uncovering the mechanisms of efficient upconversion in two-dimensional perovskites with anti-Stokes shift up to 220 meV.

Phonon-assisted photon upconversion holds great potential for numerous applications, e.g., optical refrigeration. However, traditional semiconductors face energy gain limitations due to thermal energy, typically achieving only ~25 milli-electron volts at room temperature. Here, we demonstrate that quasi-two-dimensional perovskites, with a soft hybrid organic-inorganic lattice, can efficiently upconvert photons with an anti-Stokes shift exceeding 200 milli-electron volts. By using microscopic transient absorption measurements and density functional theory calculations, we explicate that the giant energy gain stems from strong lattice fluctuation leading to a picosecond timescale transient band energy renormalization with a large energy variation of around ±180 milli-electron volts at room temperature. The motion of organic molecules drives the deformation of inorganic framework, providing energy and local states necessary for efficient upconversion within a time constant of around 1 ps. These results establish a deep understanding of perovskite-based photon upconversion and offer previously unknown insights into the development of various upconversion applications.

Laser Cooling of Traveling-Wave Phonons in an Optical Fiber

Laser Cooling of Traveling-Wave Phonons in an Optical Fiber

Stimulated Laser Cooling in a Compact Geometry Using Microfabricated Atomic Beam Collimators

Stimulated Laser Cooling in a Compact Geometry Using Microfabricated Atomic Beam Collimators

(Invited) Photoluminescence up-Conversion in CsPbBr3 Nanocrystals

Photoluminescence up-conversion is a near ubiquitous process in colloidal semiconductor nanocrystals. Observed efficient up-conversion in high emission quantum yield CsPbBr3 nanocrystals suggest their possible use in optical refrigeration as well as light harvesting. We discuss efforts to understand the mechanism behind efficient up-conversion as well as studies into the identity of intermediate states responsible for the phenomenon. Here, photothermal measurements suggest real intermediate states deep in the gap that are responsible for efficient up-conversion.

Energy Levels and Transition Rates for Laser Cooling Os− and a General Approach to Produce Cold Atoms and Molecules

High-resolution photoelectron energy spectra of osmium anions are obtained using the slow-electron velocity-map imaging method. The energy levels of excited states 4 F 7/2, 4 F 5/2 and 4 F 3/2 of Os− are determined to be 148.730(13), 155.69(15), and 176.76(13) THz [or 4961.09(41), 5193.4(49), and 5896.1(42) cm−1], respectively. The lifetime of the opposite-parity excited state is determined to be 201(10) μs using a cold ion trap, about 15 times shorter than the previous result 3(1) ms. Our high-level multi-configuration Dirac–Hartree–Fock calculations yield a theoretical lifetime 527 μs. Our work shows that the laser cooling rate of Os− is as fast as that of Th−. The advantages of Os− are its near-IR range cooling transition and simple electronic structure, which make Os− a promising candidate for laser cooling of negative ions. We propose a general approach to produce cold atoms and molecules based on the sympathetic cooling of negative ions in combination with a threshold photodetachment.

Phonon-assisted upconversion photoluminescence of monolayer MoS2 at elevated temperatures.

Upconversion photoluminescence (UPL) lies at the heart of optical refrigeration and energy harvesting. Monolayer transition metal dichalcogenides (TMDCs) have been identified as an excellent platform with robust phonon-exciton coupling for studying the phonon-assisted UPL process. Herein, we investigate the multiphonon-assisted UPL emission in monolayer MoS2 at elevated temperatures and the temperature-dependent phonon contributions in the UPL process. When temperature goes up from 295 K to 460 K, the enhancement of the integrated UPL intensity is demonstrated due to the increased phonon population and the reduced phonon numbers involved in the UPL process. Our findings reveal the underlying mechanism of phonon-assisted UPL at high temperatures, and pave the way for the applications of photon upconversion in display, nanoscale thermometry, anti-Stokes energy harvesting, and optical refrigeration.

Laser Cooling of Lithium-6 Atoms in a Bichromatic Light Field

Laser Cooling of Lithium-6 Atoms in a Bichromatic Light Field