Ultrafast Optical Properties Research Articles

Coherent optical response driven by non-equilibrium electron–phonon dynamics in a layered transition-metal dichalcogenide

Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron–hole and electron–phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H–MoTe2 in the presence of an electron–hole (e–h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays (< 1ps) was significantly modified by band-gap renormalization and state filling due to the presence of the e–h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays (< 2ps). The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron–phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.

Ultrafast optical properties of stoichiometric and non-stoichiometric refractory metal nitrides TiNx, ZrNx, and HfNx.

Refractory metal nitrides have recently gained attention in various fields of modern photonics due to their cheap and robust production technology, silicon-technology compatibility, high thermal and mechanical resistance, and competitive optical characteristics in comparison to typical plasmonic materials like gold and silver. In this work, we demonstrate that by varying the stoichiometry of sputtered nitride films, both static and ultrafast optical responses of refractory metal nitrides can efficiently be controlled. We further prove that the spectral changes in ultrafast transient response are directly related to the position of the epsilon-near-zero region. At the same time, the analysis of the temporal dynamics allows us to identify three time components: the "fast" femtosecond one, the "moderate" picosecond one, and the "slow" at the nanosecond time scale. We also find out that the non-stoichiometry does not significantly decrease the recovery time of the reflectance value. Our results show the strong electron-phonon coupling and reveal the importance of both the electron and lattice temperature-induced changes in the permittivity near the ENZ region and the thermal origin of the long tail in the transient optical response of refractory nitrides.

Ultrafast optical properties and applications of anisotropic 2D materials

Abstract Two-dimensional (2D) layered materials exhibit strong light-matter interactions, remarkable excitonic effects, and ultrafast optical response, making them promising for high-speed on-chip nanophotonics. Recently, significant attention has been directed towards anisotropic 2D materials (A2DMs) with low in-plane crystal symmetry. These materials present unique optical properties dependent on polarization and direction, offering additional degrees of freedom absent in conventional isotropic 2D materials. In this review, we discuss recent progress in understanding the fundamental aspects and ultrafast nanophotonic applications of A2DMs. We cover structural characteristics and anisotropic linear/nonlinear optical properties of A2DMs, including well-studied black phosphorus and rhenium dichalcogenides, as well as emerging quasi-one-dimensional materials. Then, we discuss fundamental ultrafast anisotropic phenomena occurring in A2DMs, such as polarization-dependent ultrafast dynamics of charge carriers and excitons, their direction-dependent spatiotemporal diffusion, photo-induced symmetry switching, and anisotropic coherent acoustic phonons. Furthermore, we review state-of-the-art ultrafast nanophotonic applications based on A2DMs, including polarization-driven active all-optical modulations and ultrafast pulse generations. This review concludes by offering perspectives on the challenges and future prospects of A2DMs in ultrafast nanophotonics.

Ultrafast and Nonlinear Optical Properties of Two-Dimensional Mo-Doped Dual Phase Inorganic Lead Halide Perovskite

Ultrafast and Nonlinear Optical Properties of Two-Dimensional Mo-Doped Dual Phase Inorganic Lead Halide Perovskite

Thickness modulation of incline-grown Bi2O2Se for the construction of high-performance phototransistors

Newly emerging two-dimensional (2D) Bi2O2Se has received intense research interest due to its unique band structure and ultrafast optical properties. However, the device performance of Bi2O2Se-based photodetectors is far from the expectation because of the undesirable contact issues of the contaminates from the fabrication process or the high Schottky barrier caused by the large work function mismatch. In this work, highly efficient photodetection based on an “all-Bi2O2Se” device geometry has been demonstrated. By controlling the growth conditions, Bi2O2Se flakes with thicknesses of 8–15 nm (thin) and >40 nm (thick) are obtained. The thin one is a typical n-type semiconductor, while the thick one shows the degenerated n-type behavior with a higher Fermi level. Two thick flakes are adopted as 2D contacts for the absorption layer of thin flake, leading to the upward movement of the thin flake band structures. By tailoring the Schottky barrier frame at the interface junction, the high barriers are eliminated, which boost the transport and collection of photo-generated electrons. The photodetector demonstrates strong photoresponse to visible and near-infrared light. High photoresponsivity and specific detectivity of 3.34 × 104 A/W and 6.61 × 1013 Jones, respectively, are achieved under the 640 nm light illumination.

Fluorescence Resonance Energy Transfer Properties and Auger Recombination Suppression in Supraparticles Self-Assembled from Colloidal Quantum Dots

Colloid quantum dots (CQDs) are recognized as an ideal material for applications in next-generation optoelectronic devices, owing to their unique structures, outstanding optical properties, and low-cost preparation processes. However, monodisperse CQDs cannot meet the requirements of stability and collective properties for device applications. Therefore, it is urgent to build stable 3D multiparticle systems with collective physical and optical properties, which is still a great challenge for nanoscience. Herein, we developed a modified microemulsion template method to synthesize quantum dot supraparticles (QD-SPs) with regular shapes and a high packing density, which is an excellent research platform for ultrafast optical properties of composite systems. The redshift of the steady-state fluorescence spectra of QD-SPs compared to CQD solutions indicates that fluorescence resonance energy transfer (FRET) occurred between the CQDs. Moreover, we investigated the dynamic processes of energy transfer in QD-SPs by time-resolved ultrafast fluorescence spectroscopy. The dynamic redshift and lifetime changes of the spectra further verified the existence of rapid energy transfer between CQDs with different exciton energies. In addition, compared with CQD solutions, the steady-state fluorescence lifetime of SPs increased and the fluorescence intensity decreased slowly with increasing temperature, which indicates that the SP structure suppressed the Auger recombination of CQDs. Our results provide a practical approach to enhance the coupling and luminescence stability of CQDs, which may enable new physical phenomena and improve the performance of optoelectronic devices.

Detection of Mechanical Deformation Induced by Ultrafast Laser Irradiation Upon a Metallic Cantilever

In this work, we systematically investigated the ultrafast optical properties of aluminum (Al) thin films on silicon cantilevers using a microscopic femtosecond optical pump-probe technique to explore the effect of light irradiation upon cantilevers while considering radiation pressure and photothermal effects. The ultrafast laser pulses used for the study were less than 30 fs in pulse duration and 830 nm in wavelength, and the photon energy (1.49 eV) of the light pulses is close to the interband transition threshold (ITT) of Al. Therefore, the change in ITT due to the strain of the cantilevers induced by the light irradiation is detected through the change in the transient reflectivity, which is dominated by the thermalized (Th) electron signal. We uncovered the position dependency of the transient reflectivity change and the Th electronic signal amplitude of the 100 nm-thick Al films on 160 $\mu$m, 200 $\mu$m and 240 $\mu$m-length cantilevers, and these results are in excellent agreement with two temperature model-based curve fits. Furthermore, to understand the effect of light irradiation, we derived equations for the position-dependent radiation pressure effect and the photothermal effect, and demonstrated that thermal expansion-induced changes in ITT dominate the position dependence of the signal intensity. Our findings offer avenues for exploring strain effects on ultrafast properties and applications for ultrafast scanning probe microscopy.

Ultrafast coherent interlayer phonon dynamics in atomically thin layers of MnBi2Te4

The atomically thin MnBi2Te4 crystal is a novel magnetic topological insulator, exhibiting exotic quantum physics. Here we report a systematic investigation of ultrafast carrier dynamics and coherent interlayer phonons in few-layer MnBi2Te4 as a function of layer number using time-resolved pump-probe reflectivity spectroscopy. Pronounced coherent phonon oscillations from the interlayer breathing mode are directly observed in the time domain. We find that the coherent oscillation frequency, the photocarrier and coherent phonon decay rates all depend sensitively on the sample thickness. The time-resolved measurements are complemented by ultralow-frequency Raman spectroscopy measurements, which both confirm the interlayer breathing mode and additionally enable observation of the interlayer shear mode. The layer dependence of these modes allows us to extract both the out-of-plane and in-plane interlayer force constants. Our studies not only reveal the interlayer van der Waals coupling strengths, but also shed light on the ultrafast optical properties of this novel two-dimensional material.

Ultracompact Deep Neural Network for Ultrafast Optical Property Extraction in Spatial Frequency Domain Imaging (SFDI)

Spatial frequency domain imaging (SFDI) is a powerful, label-free imaging technique capable of the wide-field quantitative mapping of tissue optical properties and, subsequently, chromophore concentrations. While SFDI hardware acquisition methods have advanced towards video-rate, the inverse problem (i.e., the mapping of acquired diffuse reflectance to optical properties) has remained a bottleneck for real-time data processing and visualization. Deep learning methods are adept at fitting nonlinear patterns, and may be ideal for rapidly solving the SFDI inverse problem. While current deep neural networks (DNN) are growing increasingly larger and more complex (e.g., with millions of parameters or more), our study shows that it can also be beneficial to move in the other direction, i.e., make DNNs that are smaller and simpler. Here, we propose an ultracompact, two-layer, fully connected DNN structure (each layer with four and two neurons, respectively) for ultrafast optical property extractions, which is 30×–600× faster than current methods with a similar or improved accuracy, allowing for an inversion time of 5.5 ms for 696 × 520 pixels. We further demonstrated the proposed inverse model in numerical simulations, and comprehensive phantom characterization, as well as offering in vivo measurements of dynamic physiological processes. We further demonstrated that the computation time could achieve another 200× improvement with a GPU device. This deep learning structure will help to enable fast and accurate real-time SFDI measurements, which are crucial for pre-clinical, clinical, and industrial applications.

Enhanced ultrafast nonlinear absorption and optical limiting of indolium squaraine for laser protection

The ultrafast optical nonlinear properties of an indolium squarine 1,3-bis[(3,3-dimethylindolin-2-ylidene)methyl]squaraine (ISQ) are systematically investigated via multiple pulse-width Z-scan technique and femtosecond transient absorption spectrum. The results indicate that the sample exhibits superior excited-state absorption which mainly originates from the singlet excited-state. The relatively long lifetime measured in ISQ provides efficient excitons accumulation on excited-state, making it suitable for optical limiting (OL) under various pulse duration. The nonlinear absorption coefficient of ISQ ranges from ∼10−8 m/W with nanosecond (ns) pulse to ∼10−12 m/W with femtosecond (fs) pulse. ISQ film is also prepared via doping ISQ into polymethyl methacrylate, the OL thresholds of ISQ solution and ISQ film are measured with 532 nm ns pulse. The OL threshold of ISQ/toluene is 25.5 mJ/cm2, and that of the film is 30.6 mJ/cm2, which basically retain the OL performance of solution. Our results indicate that indolium squaraine could be a potential candidate for future application in laser protection.

Deciphering Photoinduced Charge Transfer Dynamics in a Cross-Linked Graphene-Dye Nanohybrid.

The search for synthetic materials that mimic natural photosynthesis by converting solar energy into other more useful forms of energy is an ever-growing research endeavor. Graphene-based materials, with their exceptional electronic and optical properties, are exemplary candidates for high-efficiency solar energy harvesting devices. High photoactivity can be conveniently achieved by functionalizing graphene with small molecule organic semiconductors whose band-gaps can be tuned by structural modification, leading to interactions between the π-conjugated electronic systems in both the semiconductor and graphene. Here we investigate the ultrafast transient optical properties of a cross-linked graphene–dye (diphenyl-dithiophenediketopyrrolopyrrole) nanohybrid material, in which oligomers of the organic semiconductor dye are covalently bound to a random network of few-layer graphene flakes, and compare the results to those obtained for the reference dye monomer. Using a combination of ultrafast transient absorption and two-dimensional electronic spectroscopy, we provide substantial evidence for photoinduced charge transfer that occurs within 18 ps in the nanohybrid system. Notably, subpicosecond photoinduced torsional relaxation observed in the constituent dye monomer is absent in the cross-linked nanohybrid system. Through density functional theory calculations, we compare the competing effects of covalent bonding, increasing conjugation length, and the presence of multiple graphene flakes. We find evidence that the observed ultrafast charge transfer process occurs through a superexchange mechanism in which the oligomeric dye bridge provides virtual states enabling charge transfer between graphene–dye covalent bond sites.

Ultrafast Optical Properties of Cavity‐Enhanced Superfluorescence

AbstractCavity‐enhanced superfluorescence is an ultrafast and intense lasing produced by strong interaction between superfluorescence and optical cavity. However, due to the quite difficulty of particle cooperative emission in solid‐state systems, and the low coupling efficiency between superfluorescence and optical cavity, research on ultrafast optical properties of cavity‐enhanced superfluorescence is thus far in its infancy. Fortunately, perovskites being made into quantum dot superlattice microcavity is an excellent candidate as such light source. This work reveals the time‐dependent redshift process of cavity‐enhanced superfluorescence under high‐power injection, and interprets the physical mechanism of lasing broadening with increasing power from the dynamic perspective. At the same time, the multi‐mode lasing dynamic photoluminescence image output at different times is also tracked. The short‐wavelength resonance mode has a lower stimulated threshold, but its radiation lags behind the long‐wavelength resonance mode by 4 ps in high excited density. These ultrafast optical property researches may promote the application of quantum dot superlattices in information technology, optical communications, ultrafast lasers, and other fields.

Effects of many-body interactions on the transient optical properties of lead halide perovskites

Lead halide perovskites (LHPs) are emerging as promising candidates for use in various high-performance optoelectronic applications, yet their photophysics remains a topic of debate. Here, we theoretically investigated how the ultrafast optical properties of a few prototype LHPs are affected by many-body interactions, including the bandgap renormalization (BGR) effect, the band-filling (BF) effect, the free-carrier absorption effect, and the exciton effect, at carrier densities ranging from 1016 to 1019 cm−3. The results show that the exciton absorption becomes more obvious near the bandgap with increasing exciton energy (as the halogen component of the LHP is varied from I to Cl). Transient reflectivity results indicate that the signal has one peak below the bandgap as a result of the BGR effect at low carrier densities and one valley above the bandgap originating from the BF effect at high carrier densities. In addition, the absorbance decreases with increasing the carrier density as a result of the BF effect because the filled energy levels are observed at 2 meV above the bottom of the conduction band. The results of the present work are expected to promote the application of LHPs in solar cells, light-emitting diodes, and other photoelectric devices.

Ultrafast carrier dynamics in SnSe thin film studied by femtosecond transient absorption technique

The femtosecond transient absorption (TA) technique is used to study the ultrafast carrier dynamics of SnSe thin film prepared by pulsed laser deposition (PLD), which is basically characterized by X-ray diffraction (XRD). The XRD pattern shows pure single-phase of SnSe in term of the diffraction peak of crystalline direction (400), and the indirect bandgap can be estimated by Kubelka-Munk treatment in transmittance spectrum. The TA spectra of SnSe thin film are observed in picosecond timescale, corresponding to the process of carrier scattering, carrier recombination and lattice thermal effects. A triple exponential decay model, including fast (~2 ps), medium (~100 ps), and slow (ns) components, is well fitted to the TA dynamics of SnSe film. There are different dominant processes in the fast, medium and slow components, which are related to the carrier process of scattering, recombination and lattice diffusion, respectively. And these responses are modulated by single pulse energy, the carrier concentration and temperature can affect the microscopic relaxation process. The thickness of SnSe thin films also affect the ultrafast carrier dynamics. And we find there are differences in different probe wavelengths. Our results can improve the comprehension of the relaxation mechanism in SnSe thin film, and provide the important view about the ultrafast optical properties of this layered IV-VI chalcogenide.

Ultrafast and nonlinear optical properties of two-dimensional CdSe nanostructures prepared using MoS2 nanosheets as template

Two-dimensional (2D) materials have attracted tremendous research interest because of their unique optical, electronic, and mechanical properties which make them suitable building blocks for various electronic and optoelectronic applications. The heterostructures formed by 2D semiconductors can play an important role in modern semiconductor industry. Here, we have grown 2D-CdSe structures over MoS2 nanosheets via successive ion layer adsorption and reaction method. The dynamics of photoinduced charge carriers in 2D-CdSe nanostructures have been investigated using femtosecond transient absorption spectroscopy. Our photophysical studies infer that charge carrier recombination is monomolecular at low and bimolecular at high excitation densities. Nonetheless, femtosecond Z-scan technique was employed to demonstrate optical limiting behavior of 2D-CdSe films. This study will open up a new avenue of designing 2D nanomaterials for optoelectronic applications.

First-principles study of ultrafast and nonlinear optical properties of graphite thin films

We theoretically investigate ultrafast and nonlinear optical properties of graphite thin films based on first-principles time-dependent density functional theory. We first calculate electron dynamics in a unit cell of graphite under a strong pulsed electric field and explore the transient optical properties of graphite. The optical response of graphite shows a sudden change from the conducting to the insulating phase at a certain intensity range of the applied electric field. It also appears to be a saturable absorption (SA) in the energy transfer from the electric field to electrons. We next investigate a light propagation in graphite thin films by solving the coupled dynamics of the electrons and the electromagnetic fields simultaneously. It is observed that the SA manifests in the propagation with small attenuation in the spatial region where the electric field amplitude is about $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{4.pt}{0ex}}\text{to}\phantom{\rule{4.pt}{0ex}}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\AA{}$.

Hot Electrons Modulation of Third-Harmonic Generation in Graphene

Hot electrons dominate the ultrafast ($\sim$fs-ps) optical and electronic properties of metals and semiconductors and they are exploited in a variety of applications including photovoltaics and photodetection. We perform power-dependent third harmonic generation measurements on gated single-layer graphene and detect a significant deviation from the cubic power-law expected for a third harmonic generation process. We assign this to the presence of hot electrons. Our results indicate that the performance of nonlinear photonics devices based on graphene, such as optical modulators and frequency converters, can be affected by changes in the electronic temperature, which might occur due to increase of absorbed optical power or Joule heating.

Slowing down photocarrier relaxation in Dirac semimetal Cd3As2 via Mn doping.

In this Letter, we successfully introduce a long-lived non-radiative photocarrier decay component in a Dirac semimetal Cd3As2 thin film via Mn doping. The long-lived decay component is found to vary between 200ps and 2.8ns with different Mn concentrations and probing wavelengths. Most remarkably, the elongated transients persist over the important mid-infrared wavelengths (observed up to 4μm). Saturable absorption measurement reveals stronger modulation effects for long-width pulses (∼80 ps) from the Mn-doped samples. Our results provide new insights into the effect of transition-metal doping on the ultrafast optical properties of Dirac semimetal Cd3As2 and establish Cd3As2 as a highly amendable material for mid-infrared photonic applications.

Few-layer bismuthene for robust ultrafast photonics in C-Band optical communications

The ultrafast photonics of different conventional two-dimensional (2D) materials have been studied intensively. Few-layer structure bismuthene has been reported as a new type of 2D material with high efficient electronics, strong mechanics and outstanding photonics properties. In this paper, a robust ultrafast pulse generation in communications-Band (C-Band) based on few-layer bismuthene has been reported. The characteristics and the ultrafast optical nonlinear properties of few-layer bismuthene have been investigated experimentally. The optical induced deposition method is employed to fabricate the saturable absorber based on bismuthene (BiSA). Most importantly, we also utilize BiSA for the ultrafast photonics, which demonstrates that a high-splitting-threshold robust ultrafast fiber laser with 1.3-ps pulse duration at 1531 nm has been obtained in the experiments. Even though we increase the pump power from the self-starting threshold (i.e. 86 mW) to 314 mW, the soliton pulse does not split. Moreover, the high-splitting-threshold laser operation can be achieved stably even if the lasers are exposed in air for at least half a year. It is demonstrated that the proposed bismuthene nonlinear components can be potentially applied to the optical communications with C-Band (i.e. 1530–1565 nm wavelength) to broaden the communications window.

Deep learning model for ultrafast multifrequency optical property extractions for spatial frequency domain imaging

Spatial frequency domain imaging (SFDI) is emerging as an important new method in biomedical imaging due to its ability to provide label-free, wide-field tissue optical property maps. Most prior SFDI studies have utilized two spatial frequencies (2-fx) for optical property extractions. The use of more than two frequencies (multi-fx) can vastly improve the accuracy and reduce uncertainties in optical property estimates for some tissue types, but it has been limited in practice due to the slow speed of available inversion algorithms. We present a deep learning solution that eliminates this bottleneck by solving the multi-fx inverse problem 300× to 100,000× faster, with equivalent or improved accuracy compared to competing methods. The proposed deep learning inverse model will help to enable real-time and highly accurate tissue measurements with SFDI.