Excited Transitions Research Articles

Phase transitions of the anisotropic Dicke model

We systematically analyze the various phase transitions of the anisotropic Dicke model that is endowed with both rotating and counter-rotating light-matter couplings. In addition to the ground state quantum phase transition (QPT) from the normal to the super-radiant phase, the anisotropic Dicke model also exhibits other transitions namely the excited state quantum phase transition (ES- QPT), ergodic to non-ergodic transition (ENET) and the temperature dependent phase transition. We show that these phase transitions are profitably studied not only with the standard consecutive level spacing ratio, but also with the aid of various eigenvector quantities such as von Neumann entanglement entropy, the participation ratio, multifractal dimension and mutual information. For ENET, both the statics and dynamics of the participation ratio offer a consistent and useful picture. An exciting finding from our work is that the ESQPT and the ENET are closely related to each other. We show this with the aid of two characteristic energies in the spectrum corresponding to jumps in von Neumann entropy.

Inherent-state melting and the onset of glassy dynamics in two-dimensional supercooled liquids

Below the onset temperature To, the equilibrium relaxation time of most glass-forming liquids exhibits glassy dynamics characterized by a super-Arrhenius temperature dependence. In this supercooled regime, the relaxation dynamics also proceeds through localized elastic excitations corresponding to hopping events between inherent states, i.e., potential-energy-minimizing configurations of the liquid. Despite its importance in distinguishing the supercooled regime from the high-temperature regime, the microscopic origin of To is not yet known. Here, we construct a theory for the onset temperature in two dimensions and find that an inherent-state melting transition, described by the binding-unbinding transition of dipolar elastic excitations, delineates the supercooled regime from the high-temperature regime. The corresponding melting transition temperature is in good agreement with the onset temperature found in various two-dimensional (2D) atomistic models of glass formers and an experimental binary colloidal system confined to a water-air interface. Additionally, we find the predictions for the renormalized elastic moduli to agree with the experimentally observed values for the latter 2D colloidal system. We further discuss the predictions of our theory on the displacement and density correlations at supercooled conditions, which are consistent with observations of the Mermin-Wagner fluctuations in experiments and molecular simulations.

Deep insight into the charge-transfer cocrystals: Decreasing structural overlap induced bathochromically shift emission

Two charge transfer (CT) cocrystals (cocrystal-O and cocrystal-R) were prepared by solvent evaporation method with 9-(3-(pyren-1-yl)phenyl)-9H-carbazole (1PY-mCzB) as donor (D), 1,2,4,5-tetracyanobenzene (TCNB) as acceptor (A). The spectroscopic studies, single-crystal X-ray diffraction structure and theoretical calculations are carried out to explore the relationship between structures and properties of cocrystal system. The results indicate both cocrystals were assembled as the … D-A-D … alternated stacking way, implying strong CT interactions. Cocrystal-O with larger structural overlap and smaller intermolecular distance shows orange fluorescence with λmax = 592 nm, while cocrystal-R with smaller structural overlap and larger intermolecular distance shows red fluorescence with λmax = 614 nm. Actually, the decreasing structural overlap plays a key role in bathochromically shift emission for cocrystal-R, and it make the higher CT excited transition proportions in cocrystal-R (92.74%) than that in cocrystal-O (89.96%). This work not only provides a deep insight into CT cocrystals, but also supplies some guidance for the design and preparation of multicomponent luminescent materials.

Mid-gap trap state-mediated dark current in organic photodiodes

Photodiodes are ubiquitous in industry and consumer electronics. Constantly emerging new applications for photodiodes demand different mechanical and optoelectronic properties from those provided by conventional inorganic-based semiconductor devices. This has stimulated considerable interest in the use of organic semiconductors, which provide a vast palette of available optoelectronic properties, can be incorporated into flexible form factor geometries, and promise low-cost, low-embodied energy manufacturing from earth-abundant materials. The sensitivity of a photodiode depends critically on the dark current. Organic photodiodes (OPDs), however, are characterized by a much higher dark current than expected for thermally excited radiative transitions. Here we show that the dark saturation current in OPDs is fundamentally limited by mid-gap trap states. This new insight is generated by the universal trend observed for the dark saturation current of a large set of OPDs and further substantiated by sensitive external-quantum-efficiency- and temperature-dependent current measurements. Based on this insight, an upper limit for the specific detectivity is established. A detailed understanding of the origins of noise in any detector is fundamental to defining performance limitations and thus is critical to materials and device selection, and design and optimization for all applications. Our work establishes these important principles for OPDs.

Benzyl Alcohol Photo-oxidation Based on Molecular Electronic Transitions in Metal Halide Perovskites

Vacancy ordered double perovskites Cs2Te(IV)X6 have been found to exhibit molecule-like electronic behavior when X is Cl– or Br– due to the zero-dimensional (0D) nature of their octahedral units. Electronically isolated building blocks, the [TeBr6]2– ionic octahedron, serve as the fundamental electronic unit of the Cs2TeBr6 solid. Herein, a detailed understanding of the Cs2TeBr6 electronic structure and its photoexcitation is presented with consideration of individual molecular orbitals from these isolated octahedral building blocks. Two optical absorption features correspond to two unique electronic transitions, (1) a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition under 455 nm excitation and (2) mixed transitions including lower HOMO states to LUMO transition and HOMO to higher LUMO states transition under 365 nm excitation. With this in mind, we examined the excitation wavelength-dependent photo-oxidation of benzyl alcohol using Cs2TeBr6 as the photocatalyst. Significant differences in photocatalytic performance were observed, and different forms of activated alcohol radicals were detected under the two excitation wavelengths. As a case study, this work highlights the application of molecule-like halide perovskites in photocatalysis. The highly tunable energy band structures and catalytic centers in perovskites can offer a valuable platform for photocatalytic mechanistic studies and catalyst development in the foreseeable future.

Exploring the excitation spectrum behavior of Dy in CaWO4 for a new excited-state-based ratiometric thermometry

Exploring excited-state transition behavior is of great importance for understanding the optical characteristics of lanthanide ions, which may potentially develop new optical thermometry by the thermally affected excitation processes. In this work, we explore the detailed excitation transition behaviors of Dy3+ and propose a new thermometry strategy based on excited-state absorption intensity ratio (EIR) in Dy3+ doped CaWO4 phosphors. The optimal Dy3+ doping concentration in CaWO4:Dy phosphors was determined and its fine excitation transition lines from low-lying levels of 6HJ (J = 15/2, 13/2, 11/2, and 9/2) to the respective high-lying levels of Dy3+ ions were identified by well-resolving the photoluminescence excitation (PLE) spectra. By taking advantage of the opposite trends of temperature-dependent PLE intensities of Dy3+, a new ratiometric thermometry based on the EIR of Dy3+ was proposed to be independent of the excitation wavelength and only excitation peak intensities required, with the merit of high absolute and relative sensitivities, excellent repeatability and reusability, good excitation wavelength selectivity. These findings demonstrated an effective strategy for developing a Dy3+-based EIR luminescent thermometer, as well as the enormous potential of Dy3+ doped CaWO4 phosphors in temperature sensing.

High-performance broadband photodetector based on plasmonic semiconductor-semiconductor p-n Cu1.8S/AZO interface

Plasmonic semiconductor nanocrystals (NCs) provide intriguing optical properties which offer opportunities to extend the photo-response corresponding to photons with energies less than the semiconductor energy band gap. However, the performance of photodetectors based on individual plasmonic semiconductors is restricted due to very short-living photo-carriers, followed by carrier recombination (∼ 100 ps). Herein, a plasmonic semiconductor-semiconductor p-n interface (Cu1.8S/AZO NCs) was constructed representing 2-fold optical characteristics containing an inter-band electronic transition from the valence to conduction band in response to ultraviolet-visible (UV–vis) excitation and plasmon-induced interfacial charge-transfer transition (PICTT) in response to near infrared (NIR) excitation. We demonstrated that AZO NCs, are appropriate candidates to generate chemical interface damping (CID) of plasmons in Cu1.8S NCs as an acceptor, which is required for PICTT pathway. The device displayed outstanding broadband UV–vis–NIR photoresponsivity (R) and response-time (Trise/Tfall) as high as R = 51.48 A/W, Trise/Tfall= 130/100 ms in 390 nm, R = 37.2 A/W, Trise/Tfall= 50/60 ms in 535 nm, R = 45.6 A/W, Trise/Tfall= 400/10 ms in 750 nm and R = 38.7 A/W and Trise/Tfall= 60/50 ms in 950 nm. This study demonstrates the promising potential of plasmonic semiconductor-semiconductor p-n hybrid systems as high-performance broadband photodetectors (PDs).

Citric acid assisted synthesis of luminescent CeF3 and CeF3:Tb3+ nanoparticles: Luminescent &optical properties

Luminescent CeF3 and CeF3:Tb3+ nanoparticles (NPs) were synthesized by citric acid-supported wet chemical method at a lower temperature. X-ray diffraction (XRD) pattern was recorded to examine the crystal structure, particle size, and phase pureness of the nano-products. The shape and size of the as-synthesized nanoproduct were reviewed by transmission electron microscopy (TEM). The development of a hexagonal phase solid solution at the nanoscale by the successful incorporation of Tb3+ ion into the CeF3 crystal lattice was confirmed by the XRD data. The solid solutions of nanocrystalline CeF3:Tb3+ NPs were confirmed by the TEM micrographs. Highly aqueous dispersible, agglomerated, typical porous surface, irregularly spherical shaped, well distributed polycrystalline particles were observed in TEM photographs. The development of a partially transparent colloidal solution with good colloidal stability in the aqueous media was indicated by the examination of optical characteristics. Based on absorption spectral analyses, the occurrence of structural imperfections and their impact on the bandgap energy was explored. The photoluminescence spectrum of the CeF3:Tb NPs was measured to authenticate the successful insertion of Tb3+-ion into the CeF3 crystal matrix. CeF3:Tb NPs exhibit a prominent green color due to a dominant magnetic-dipole (5D4→7F5) emission transition that is visible in the middle of the spectrum at 543 nm on irradiation from a UV light source. The sharp excitation and intense emission transitions astounding luminescence efficiencies indicated that the luminescent NPs are very crystalline. The as-prepared CeF3:Tb NPs have prospective uses in laser-based biological events including fingermark detection, security ink, and bio-tagging agents due to their superior luminescent and optical properties.

Extreme temperature operation for broad bandwidth quantum-dot based superluminescent diodes

The high-temperature resilience of quantum-dot (QD) laser materials is exploited to realize a broad spectral bandwidth emitter in the near infrared. For an InAs/GaAs-based QD-superluminescent light emitting diode (SLEDs), we introduced a 2000 μm long, 5 μm width ridge waveguide that is tilted by 7° and composed of eight multi-sections. With increased temperature operation over 160 °C, the spectral bandwidth is dramatically increased by thermally excited carrier transition in ES1 and ES2. Additionally, the positive net-modal gain is demonstrated at the high operating temperatures, and this is exploited in the QD-SLEDs operating at 180 °C, which exhibit a −3 dB linewidth of 270 nm and a power of 0.34 mW. The simplicity of this approach, utilizing heat alone, is contrasted with other approaches for the extremely broad spectral bandwidth emitter.

Aza-BODIPY with two efficacious fragments for NIR light-driven photothermal therapy by triggering cancer cell apoptosis.

The reasonable structure of aza-BODIPY renders it as an efficient photothermal reagent for photothermal therapy. Herein, we describe the design and synthesis of aza-BODIPY NMeBu with the free rotating tert-butyl group and the dimethylamino-substituted segment to promote the photothermal conversion via the excited state non-radiative transition. NMeBu was found to be the π-π stacking form in the unit cell based on X-ray analysis. NMeBu-NPs by self-assembly possessed a near-infrared absorption (λabs = 772 nm), and once activated by near-infrared light, the photothermal efficiency in aqueous solution can reach 49.3%. NMeBu-NPs can penetrate the cell and trigger cell death via the apoptosis pathway under low concentration and low light power irradiation, thereby avoiding dark toxicity. Aza-BODIPY created using this procedure has excellent photothermal efficiency and could serve as a potential candidate for the treatment of cancer cells and tumors.

Cubic Halide Double Perovskite Nanocrystals with Anisotropic Free Excitons and Self-Trapped Exciton Photoluminescence

In this work, we first developed Cs2KBiCl6 cubic double perovskite nanocrystals and a series of morphologically isotropic double perovskite nanocrystals. Different contributions of different elements to self-trapped states were revealed by density functional theory. Meanwhile, these double perovskite nanocrystals exhibit the coexistence of free and self-trapped exciton dual-color photoluminescence. Femtosecond transient absorption spectroscopy can confirm that the double perovskite nanocrystals produce a relatively obvious structural deformation in the excited state. We infer that this can lead to a large deviation of the excitation and emission transition dipoles, thus causing large photoluminescence anisotropy. Most importantly, we observe for the first time that both free exciton emission and self-trapped exciton emission are highly anisotropic, which are comparable to or even better than that of lead halide perovskites. This research paves the way for exploring more possibilities and practical applications.

Combined Experimental and Theoretical Investigation into the Photophysical Properties of Halogenated Coelenteramide Analogs.

Marine Coelenterazine is one of the most well-known chemi-/bioluminescent systems, and in which reaction the chemi-/bioluminophore (Coelenteramide) is generated and chemiexcited to singlet excited states (leading to light emission). Recent studies have shown that the bromination of compounds associated with the marine Coelenterazine system can provide them with new properties, such as anticancer activity and enhanced emission. Given this, our objective is to characterize the photophysical properties of a previously reported brominated Coelenteramide analog, by employing a combined experimental and theoretical approach. To better analyze the potential halogen effect, we have also synthesized and characterized, for the first time, two new fluorinated and chlorinated Coelenteramide analogs. These compounds show similar emission spectra in aqueous solution, but with different fluorescence quantum yields, in a trend that can be correlated with the heavy-atom effect (F > Cl > Br). A blue shift in emission in other solvents is also verified with the F−Cl−Br trend. More relevantly, the fluorescence quantum yield of the brominated analog is particularly sensitive to changes in solvent, which indicates that this compound has potential use as a microenvironment fluorescence probe. Theoretical calculations indicate that the observed excited state transitions result from local excitations involving the pyrazine ring. The obtained information should be useful for the further exploration of halogenated Coelenteramides and their luminescent properties.

TDDFT versus GW/BSE Methods for Prediction of Light Absorption and Emission in a TADF Emitter.

Design concepts for organic light emitting diode (OLED) emitters, which exhibit thermally activated delayed fluorescence (TADF) and thereby achieve quantum yields exceeding 25%, depend on singlet-triplet splitting energies of order kT to allow reverse intersystem crossing at ambient temperatures. Simulation methods for these systems must be able to treat relatively large organic molecules, as well as predict their excited state energies, transition energies, singlet-triplet splittings, and absorption and emission cross sections with reasonable accuracy, in order to prove useful in the design process. Here we compare predictions of TDDFT with M06-2X and ωB97X-D exchange-correlation functionals and a GoWo@HF/BSE method for these quantities in the well-studied DPTZ-DBTO2 TADF emitter molecule. Geometry optimization is performed for ground state (GS) and lowest donor-acceptor charge transfer (CT) state for each functional. Optical absorption and emission cross sections and energies are calculated at these geometries. Relaxation energies are on the order of 0.5 eV, and the importance of obtaining excited state equilibrium geometries in predicting delayed fluorescence is demonstrated. There are clear trends in predictions of GoWo@HF/BSE, and TDDFT/ωB97X-D and M06-2X methods in which the former method favors local exciton (LE) states while the latter favors DA CT states and ωB97X-D makes intermediate predictions. GoWo@HF/BSE suffers from triplet instability for LE states but not CT states relevant for TADF. Shifts in HOMO and LUMO levels on adding a conductor-like polarizable continuum model dielectric background are used to estimate changes in excitation energies on going from the gas phase to a solvated molecule.

Laser cooling in Yb:KY3F10: a comparison with Yb:YLF.

Laser cooling by anti-Stokes fluorescence is a technology to realize all-solid-state optical cryocoolers. We grew Yb3+-doped KY3F10 (Yb:KYF) crystals as novel laser cooling media and compare their cooling performance to Yb3+-doped LiYF4 (Yb:YLF) crystals also grown in our institute. We present temperature-dependent absorption and emission cross sections as well as the fluorescence lifetime of Yb:KYF, and calculate its material figure-of-merit for laser cooling. Yb:KYF exhibits a higher figure-of-merit than Yb:YLF at temperatures below 200 K. This is because, in contrast to Yb:YLF, the excitation transition from the second-highest Stark level of the ground state is best-suited for cryogenic cooling in Yb:KYF. Thus, it has the potential to achieve unprecedentedly low temperatures below the boiling point of liquid nitrogen. In this work, we observe the first laser cooling of Yb:KYF, and obtain a background absorption coefficient of ∼10-4 cm-1, which is among the lowest ever reported for Yb3+-doped fluoride crystals. A simple model calculation predicts that our Yb:KYF and Yb:YLF crystals can potentially be cooled down to ≈100 K in a high-power cooling setup. Our Yb:KYF crystals still leave room for further improvement through the optimization of the growth process and the use of purer raw materials.

The weak coupling decay magnetopolaron effect in an asymmetric Gaussian quantum well

The famous Lee–Low–Pines transformation method is employed to receive the ground-state energy (GSE), first excited state energy (FESE), excited energy (EE), transition frequency (TF), and ground state binding energy (GSBE) of the weak coupling magnetopolaron in the asymmetrical Gaussian potential quantum well (AGPQW) under the influence of decay magnetic field (DMF). Results demonstrate that the GSE, FESE, EE, and TF are increasing functions of the depth of the AGPQW and the initial magnetic induction intensity (IMII) and decreasing functions of the range of the AGPQW and the DMF's decay frequency (DF) and decay time (DT) using GaAs crystals for numerical simulations. In contrast, the GSBE can decrease with the depth of the AGPQW and the IMII and increase with the range of the AGPQW and the DF and DT. The obtained research results are theoretically significant for recognizing, understanding, and applying of magnetopolaron effect of GaAs crystals.

Single and double electron excitation transition rates and oscillator strengths calculations in the study of the x-ray absorption spectrum of Neon

We report a new study of the x-ray absorption spectrum of Neon near the K-edge and near the KL-edge. The study is performed by calculating the transition probabilities and oscillator strengths for excitations from the ground state to singly and doubly excited states, the calculations were performed using multi-configurational Dirac-Fock (MCDF) method. Breit interaction between electrons and finite nuclear size effects were taken into account during the calculations, the transition probabilities were convoluted into Lorentzian line shape profiles. The results of the calculations were then compared to previous measurements of the x-ray absorption spectrum of Neon, good agreement was observed and many features in the spectrum were explained.

Study of the Nuclear Structure for Some Nuclei Using Self-Consistent RPA Calculations with Skyrme-Type Interaction

In the present research, some static and dynamic nuclear properties of the closed-shell nuclei; 58Ni, 90Zr, 116Sn, and 144Sm nuclei have been studied using the Random Phase Approximation (RPA) method framework and different Skyrme parameterizations, particularly SyO-, Sk255, SyO+, SLy4, BSk17, and SLy5. In particular, in studies of static properties such as nuclear densities for neutrons, protons, mass, and charge densities with their corresponding rms radii, the single-particle nuclear density distributions All the obtained results agreed well with the relevant experimental data. Concerning the dynamic properties, the excitation energy, transition density, and giant resonance modes for the excitation to the low-lying negative partite excited states 1–, 3–, 5–, and 7– have also been studied. The findings indicate that estimates of RPA with Skyrme-type interactions are a good way to describe the properties of the structure of even-even, closed-shell nuclei.

Electroreflectance spectroscopy of few-layer MoS2 : Issues related to A1s exciton subspecies, exciton binding energy, and inter-layer exciton

Optical spectra of few-layer transition metal dichalcogenide semiconductors reveal several transitions whose character and origins continue to be debated. We have studied hBN encapsulated few-layer MoS2 films using electroreflectance (ER) spectroscopy. Two strong features are seen in the reflectance spectrum of trilayer MoS2 around the ground state A–exciton transition. In ER, the corresponding features show opposite phase response to the applied voltage. Evidence from first principles ER line shape simulation and photoluminescence spectroscopy suggests that these two features are likely to be A1s exciton subspecies proposed earlier, whose energy depends on which layer the electron–hole pair is located in. The first excited state A2s exciton transition is also identifiable in ER. Through the two-dimensional hydrogenic exciton model, it enables an approximate estimation of the exciton binding energy Eb. The increase in Eb with decreasing film thickness, which originates from reduced dielectric screening, is phenomenologically analyzed through a film thickness and capping material dependent effective dielectric constant. Extending this idea, we show that the A1s exciton is mostly confined to a single S–Mo–S layer, as predicted by theory. An inter-layer (IL) exciton is expected in bilayer and thicker 2H-MoS2 films. However, we show that there can be bilayer films where the IL exciton is absent, which may be related to increased carrier concentration.

Atmospheric Optical Characteristics in the Area of 30–400 km

Extremely weak lidar reflections in the thermosphere, which correlate with ionosonde data, were detected in 2008 and 2017 over Kamchatka during seasons of low aerosol filling of the atmosphere at solar activity minima. Here, these reflections are considered in comparison with mesospheric and stratospheric lidar signals that makes it possible to determine favorable conditions for thermospheric lidar observations. In 2014, it was shown that lines of transitions between the excited states of atomic nitrogen ions fall within the 532 nm lidar signal emission band, and in 2017, lidar reflections in the thermosphere were simultaneously obtained at 561 and 532 nm excited transitions of atomic oxygen and nitrogen ions, thereby the resonant nature of thermospheric lidar reflections was established and confirmed. Here, using lidar signals at wavelengths of 561 and 532 nm in the altitude range of 30–400 km, by solving the inverse problem, we restore the light scattering coefficients corresponding to these wavelengths that makes it possible to compare the optical characteristics of the thermosphere, mesosphere, and upper stratosphere and to determine the relationship between resonant, Rayleigh, and aerosol light scattering at different heights of the atmosphere. In conclusion, using the scattering coefficients in the thermosphere, we find the cross-sections of light scattering at the 561 and 532 nm transitions of atomic oxygen and nitrogen ions and explain why the scattering coefficients for O+, 561 nm are less than for N+, 532 nm, while the concentration of O+ is two orders of magnitude higher than N+. The results obtained here are of interest for understanding the ionization effect of solar activity on the optical characteristics of the atmosphere that determine weather and climate changes.

ALMA explores the inner wind of evolved O-rich stars with two widespread vibrationally excited transitions of water

AbstractALMA observations with angular resolution in the range ∼20–200 mas demonstrate that emission at 268.149 and 262.898 GHz in the (0,2,0) and (0,1,0) vibrationally excited states of water are widespread in the inner envelope of O-rich AGB stars and red supergiants. These transitions are either quasi-thermally excited, in which case they can be used to estimate the molecular column density, or show signs of maser emission with a brightness temperature of ∼103–107 K in a few stars. The highest spatial resolution observations probe the inner few stellar radii environment, up to ∼10–12 R* in general, while the mid resolution data probe more thermally excited gas at larger extents. In several stars, high velocity components are observed at 268.149 GHz which may be caused by the kinematic perturbations induced by a companion. Radiative transfer models of water are revisited to specify the physical conditions leading to 268.149 and 262.898 GHz maser excitation.