An ultrafast spectroscopy system for studying dynamic properties of superconductors under high pressure and low temperature conditions

An ultrafast pump-probe spectroscopy system combined with a cryogenic diamond anvil cell(DAC) instrument is developed to investigate the photo-excitation dynamic properties of condensed materials under low temperature and high pressure(LTHP) conditions.The ultrafast dynamics study is performed on Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi-2212) thin film under LTHP conditions.The superconducting(SC) phase transition has been observed by analyzing the ultrafast dynamics of Bi-2212 as a function of pressure and temperature.Our results suggest that the pump-probe spectroscopy system combined with a cryogenic DAC instrument is an effective method to study the physical mechanism of condensed matter physics at extreme conditions,especially for the SC phase transition.

Review of ultrafast spectroscopy studies of valley carrier dynamics in two-dimensional semiconducting transition metal dichalcogenides

The two-dimensional layered transition metal dichalcogenides provide new opportunities in future valley-based in- formation processing and also provide an ideal platform to study excitonic effects. At the center of various device physics toward their possible electronic and optoelectronic applications is understanding the dynamical evolution of various many- particle electronic states, especially exciton which dominates the optoelectronic response of TMDs, under the novel con- text of valley degree of freedom. Here, we provide a brief review of experimental advances in using helicity-resolved ultrafast spectroscopy, especially ultrafast pump-probe spectroscopy, to study the dynamical evolution of valley-related many-particle electronic states in semiconducting monolayer transitional metal dichalcogenides.

Attosecond spectroscopy for filming the ultrafast movies of atoms,molecules and solids

Three decades ago,a highly nonlinear nonpertubative phenomenon,now well-known as the high harmonic generation(HHG),was discovered when intense laser irradiates gaseous atoms.As the HHG produces broadband coherent radiation,it becomes the most promising source to obtain attosecond pulses.The door to the attosecond science was opened ever since.In this review,we will revisit the incredible adventure to the attoworld.Firstly,the progress of attosecond pulse generation is outlined.Then,we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions,utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.

Pressure manipulation of ultrafast carrier dynamics in monolayer WS_(2)

Two-dimensional transition metal dichalcogenides(TMDs)have intriguing physic properties and offer an exciting platform to explore many features that are important for future devices.In this work,we synthesized monolayer WS_(2)as an example to study the optical response with hydrostatic pressure.The Raman results show a continuous tuning of the lattice vibrations that is induced by hydrostatic pressure.We further demonstrate an efficient pressure-induced change of the band structure and carrier dynamics via transient absorption measurements.We found that two time constants can be attributed to the capture process of two kinds of defect states,with the pressure increasing from 0.55 GPa to 2.91 GPa,both of capture processes were accelerated,and there is an inflection point within the pressure range of 1.56 GPa to 1.89 GPa.Our findings provide valuable information for the design of future optoelectronic devices.

Photophysics of metal-organic frameworks:A brief overview

Metal-organic frameworks(MOFs),which are self-assembled porous coordination materials,have garnered considerable attention in the fields of optoelectronics,photovoltaic,photochemistry,and photocatalysis due to their diverse structures and excellent tunability.However,the performance of MOF-based optoelectronic applications currently falls short of the industry benchmark.To enhance the performance of MOF materials,it is imperative to undertake comprehensive investigations aimed at gaining a deeper understanding of photophysics and sequentially optimizing properties related to photocarrier transport,recombination,interaction,and transfer.By utilizing femtosecond laser pulses to excite MOFs,time-resolved optical spectroscopy offers a means to observe and characterize these ultrafast microscopic processes.This approach adds the time coordinate as a novel dimension for comprehending the interaction between light and MOFs.Accordingly,this review provides a comprehensive overview of the recent advancements in the photophysics of MOFs and additionally outlines potential avenues for exploring the time domain in the investigation of MOFs.

Progress and realization platforms of dynamic topological photonics

Dynamic topological photonics is a novel research field, combining the time-domain optics and topological physics.In this review, the recent progress and realization platforms of dynamic topological photonics have been well introduced.The definition, measurement methods and the evolution process of the dynamic topological photonics are demonstrated to better understand the physical diagram. This review is meant to bring the readers a different perspective on topological photonics, grasp the advanced progress of dynamic topology, and inspire ideas about future prospects.

Selective excitation of molecular mode in a mixture by femtosecond resonance-enhanced coherent anti-Stokes Raman scattering spectroscopy

Femtosecond time-resolved coherent anti-Stokes Raman scattering （CARS） spectroscopy is used to investigate gaseous molecular dynamics. Due to the spectrally broad laser pulses, usually poorly resolved spectra result from this broad spectroscopy. However, it can be demonstrated that by the electronic resonance enhancement optimization control a selective excitation of specific vibrational mode is possible. Using an electronically resonance-enhanced effect, iodine molecule specific CARS spectroscopy can be obtained from a mixture of iodine-air at room temperature and a pressure of 1 atm （corresponding to a saturation iodine vapour as low as about 35 Pa）. The dynamics on either the electronically excited state or the ground state of iodine molecules obtained is consistent with previous studies （vacuum, heated and pure iodine） in the femtoseeond time resolved CARS spectroscopy, showing that an effective method of suppressing the non-resonant CARS background and other interferences is demonstrated.

Ultrafast solvation dynamics at internal sites of staphylococcal nuclease investigated by site-directed mutagenesis

Internal solvation of protein was studied by site-directed mutagenesis, with which an intrinsically fluorescent probe,tryptophan, is inserted into the desired position inside a protein molecule for ultrafast spectroscopic study. Here we review this unique method for protein dynamics research. We first introduce the frontiers of protein solvation, site-directed mutagenesis, protein stability and characteristics, and the spectroscopic methods. Then we present time-resolved spectroscopic dynamics of solvation dynamics inside cavities of active sites. The studies are carried out on a globular protein, staphylococcal nuclease. The solvation at sites inside the protein molecule＇s cavities clearly reveals characteristics of the local environment. These solvation behaviors are directly correlated to enzyme activity.

Monitoring the reaction between AlCl_3 and o-xylene by using terahertz spectroscopy

Terahertz time-domain spectroscopy （THz-TDS） is used to study the interaction between AlCl3 and o-xylene in a temperature range from 300 K to 368 K. For comparison, the three isomers of o-, m-, and p-xylene are measured by using THz-TDS. The o-xylene carries out isomerization reaction in the presence of catalyst AlCl3. The absorption coefficient of the mixed reaction solution is extracted and analyzed in the frequency range from 0.2 THz to 1.4 THz. The temperature dependence of the absorption coefficient, which is influenced by both the dissolution of AlCl3 and the production of the two other isomer resultants, is obtained, and it can indicate the process of the isomerization reaction. The results suggest that THz spectroscopy can be used to monitor the isomerization reaction and other reactions in chemical synthesis, petrochemical and biomedical fields.

Photo-induced absorption in the pump probe spectroscopy of single-walled carbon nanotubes

Femtosecond pump probe spectroscopy is employed to study the photo-induced absorption feature in the single-walled carbon nanotube transient spectrum. The two advantages of the experiment, a chirality enriched sample and tuning the pump wavelength to the resonance of a specific nanotube species, greatly facilitate the identification of the photo-induced absorption signal of one tube species. It is found that a photo-induced absorption feature is located at one radial breathing mode to the blue side of the E11 state. This finding prompts a new explanation for the origin of the photo-induced absorption： the transition from the ground state to a phonon coupled state near the E ii state. The explanation suggests a superposition mechanism of the photo-bleach and photo-induced absorption signals, which may serve as a key to the interpretation of the complex pump probe transient spectrum of carbon nanotubes. The finding sheds some light on the understanding of the complex non-radiative relaxation process and the electronic structure of single-walled carbon nanotubes.

Ultrafast optical spectroscopy evidence of pseudogap and electronphonon coupling in an iron-based superconductor KCa_(2)Fe_(4)As_(4)F_(2)

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa_(2)Fe_(4)A_(s4)F_(2)with T_(c)= 33.5 K. Our results reveal a possible pseudogap(△_(PG)=(2.4 ± 0.1) me V) below T*≈ 50 K but prior to the opening of a superconducting gap(△SC(0) ≈(4.3 ± 0.1) me V). Measurements under high pump fluence reveal two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency A1 g(2) mode corresponds to the c-axis polarized vibrations of Fe As planes with a nominal electron-phonon coupling constant λA1 g(2)= 0.194 ± 0.02.Our findings suggest that the pseudogap is likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa_(2)Fe_(4)A_(s4)F_(2).

Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

In this work,the fundamental mechanism of ultrabright fluorescence from surface-modified colloidal silicon quantum dots is investigated in depth using ultrafast spectroscopy.The underlying energy band structure corresponding to such highly efficient direct bandgap-like emissions in our surface-modified silicon quantum dots is unraveled by analyzing the transient optical spectrum,which demonstrates the significant effect of surface molecular engineering.It is observed that special surface modification,which creates novel surface states,is responsible for the different emission wavelengths and the significant improvement in the photoluminescence quantum yields.Following this essential understanding,surface-modified silicon quantum dots with deep blue to orange emission are successfully prepared without changing their sizes.

Photocatalytic N2 Fixation by Plasmonic Mo-Doped TiO_(2) Semiconductor

Photocatalytic N_(2)xation has attracted substantial attention in recent years,as it represents a green and sustainable devel-opment route toward effciently convert-ing N_(2)to NH_(3)for industrial applications.How to rationally design catalysts in this regard remains a challenge.Here we pro-pose a strategy that uses plasmonic hot electrons in the highly doped TiO_(2)to ac-tivate the inert N_(2)molecules.The synthesized semiconductor catalyst Mo-doped TiO_(2)shows a NH_(3)production effciency as high as 134μmol·g^(-1)·h^(-1)under ambient conditions,which is comparable to that achieved by the conventional plasmonic gold metal.By means of ultra-fast spectroscopy we reveal that the plasmonic hot electrons in the system are responsible for the activation of N_(2)molecules,enabling improvement the catalytic activity of TiO_(2).This work opens a new avenue toward semiconductor plasmon-based photocatalytic N_(2)xation.

Scanning the optical characteristics of lead-free cesium titanium bromide double perovskite nanocrystals

Cs_(2)TiBr_(6) nanocrystals(NCs)are a type of promising optoelectronic materials,owing to their high photoelectric properties and non-toxicity.Here,we synthesize the colloidal Cs_(2)TiBr_(6)NCs using a hot-injection approach.The temperaturedependent absorption data shows that its energy band changes about 30 me V with temperature,reflecting that its energy band structure is much stable.The excitation intensity-dependent transient absorption data confirms its linear absorption cross-sections and carrier recombination rate constants,involving monomolecular and bimolecular recombination,which are all superior to those of conventional perovskite bromide counterparts.In addition,its nonlinear absorption cross-sections are also measured based on femtosecond Z-scan.Our results suggest that Cs_(2)TiBr_(6)NCs can be extensively applied in the field of optoelectronics,owing to its excellent carrier dynamics and nonlinear optical properties.

Steady and transient behavior of perylene under high pressure

Pressure can reduce the distances among atoms, thereby modifying the overall optical characteristics of molecules.In this article, the excited state behavior of perylene is carefully observed under isotropic pressure and non-complexing condition. In a steady state, absorption peak shows red shift and spectral width are broadened with pressure increasing,which is ascribed to the π-electron delocalization between molecules. In a transient state, the transition dynamics presents a wavelike tendency with pressure increasing because the shift of self-tapping exciton state is contrary to that of Y-state with pressure increasing. The results conduce to understanding the influence of inter-molecule interaction on excited state behavior with inter-molecule distance decreasing, which contributes to studying the materials under extreme condition.

Optical study on topological superconductor candidate Sr-doped Bi_(2)Se_(3)

Utilizing infrared spectroscopy,we study the charge dynamics of the topological superconductor candidate Sr_(x)Bi_(2)Se_(3).The frequency-dependent reflectivity R(ω)demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing.The plasma edge shows a slight blue shift upon cooling,similar to the behavior of Cu_(x)Bi_(2)Se_(3).As the carrier concentration n obtained by Hall resistivity increases slightly with the decreasing temperature,the effective mass is proved to increase as well,which is in contrast with that of Cu_(x)Bi_(2)Se_(3).We also perform the ultrafast pump-probe study on the Sr_(0.2)Bi_(2)Se_(3)compounds.Resembling its parent compound Bi_(2)Se_(3),three distinct relaxation processes are found to contribute to the transient reflectivity.However,the deduced relaxation times are quite different.In addition,the electron-optical-phonon coupling constant is identified to beλ=0.88.

Optically induced abnormal terahertz absorption in black silicon

The absorption responses of blank silicon and black silicon（silicon with micro/nano-conical surface structures） wafers to an 808-nm continuous-wave（CW） laser are investigated at room temperature by terahertz time-domain spectroscopy. The transmission of the blank silicon shows an appreciable change, from ground state to the pump state, with amplitude varying up to 50%, while that of the black silicon（BS） with different cone sizes is observed to be more stable. Furthermore,the terahertz transmission through BS is observed to be strongly dependent on the size of the conical structure geometry.The conductivities of blank silicon and BS are extracted from the experimental data with and without pumping. The non-photo-excited conductivities increase with increasing frequency and agree well with the Lorentz model, whereas the photo-excited conductivities decrease with increasing frequency and fit well with the Drude–Smith model. Indeed, for BS, the conductivity, electron density and mobility are found to correlate closely with the size of the conical structure.This is attributed to the influence of space confinement on the carrier excitation, that is, the carriers excited at the BS conical structure surface have a stronger localization effect with a backscattering behavior in small-sized microstructures and a higher recombination rate due to increased electron interaction and collision with electrons, interfaces and grain boundaries.

Hot-exciton effects on exciton diffusion and circular polarization dynamics in a single PbI_(2)nanoflake

As one of the emerging two-dimensional lead halide materials,lead iodide(PbI_(2))nanosheets have proven to possess strong application potential in the fields of high-energy radiation detection and highly efficient perovskite solar cells.However,the underlying photophysical properties such as hot-exciton-related carrier dynamics remain unclear for PbI_(2)nanosheets.Here,we report the exciton dynamics of a single PbI_(2)nanoflake prepared by an aqueous solution method.Through a three-dimensional(3D)diffusion model,we obtain the exciton annihilation radius and diffusion coefficient of a single PbI_(2)nanoflake under nonresonant and resonant excitation conditions of band-edge exciton state.As initial exciton densities increase,we find the carrier recombination mechanism for a single PbI_(2)nanoflake gradually changes from exciton-exciton annihilation to free-carrier recombination.Finally,we reveal the room-temperature circular polarization of a single PbI_(2)nanoflake is due to free-carrier recombination with a band-edge exciton dissociation time of~120 fs under the resonant excitation condition.

Role of the interlayer interactions in ultrafast terahertz thermal dynamics of bilayer graphene

Bilayer graphene,which is highly promising for electronic and optoelectronic applications because of its strong coupling of the Dirac–Fermions,has been studied extensively for the emergent correlated phenomena with magic-angle manipulation.Due to the low energy linear type band gap dispersion relationship,graphene has drawn an amount of optoelectronic devices applications in the terahertz region.However,the strong interlayer interactions modulated electron-electron and electron-phonon coupling,and their dynamics in bilayer graphene have been rarely studied by terahertz spectroscopy.In this study,the interlayer interaction influence on the electron-electron and the electron-phonon coupling has been assigned with the interaction between the two graphene layers.In the ultrafast cooling process in bilayer graphene,the interlayer interaction could boost the electron-phonon coupling process and oppositely reduce the electron-electron coupling process,which led to the less efficient thermalization process.Furthermore,the electron-electron coupling process is shown to be related with the electron momentum scattering time,which increased vividly in bilayer graphene.Our work could provide new insights into the ultrafast dynamics in bilayer graphene,which is of crucial importance for designing multi-layer graphene-based optoelectronic devices.

Hybridized electronic states between CdSe nanoparticles and conjugated organic ligands： A theoretical and ultrafast photo-excited carrier dynamics study

Formation of densely packed thin films of semiconductor nanocrystals is advantageous for the exploitation of their unique optoelectronic properties for real-world applications. Here we investigate the fundamental role of the structure of the bridging ligand on the optoelectronic properties of the resulting hybrid film. In particular, we considered hybrid films formed using the same CdSe nanocrystals and two organic ligands that have the same bidentate dithiocarbamate bInding moiety, but differ in their bridging structures, one bridged by ethylene, the other by phenylene that exhibits conjugation. Based on the results of photo- excited carrier dynamics experiments combined with theoretical calculations on the electronic states of bridged CdSe layers, we show that only the phenylene- based ligand presents a strong hybridization of the molecular HOMO state with CdSe layers, that is a marker of formation of an effective bridge. We argue that this hybridization spread favors the hopping of photo-excited carriers between nanocrystals, which may explain the reported larger photo-currents in phenylene-based hybrid films than those observed in ethylene-based ones.