Ultrafast Carrier Relaxation Research Articles

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Introducing Ag Dopants into CdSe Nanoplatelets (NPLs) Leads to Effective Charge Separation for Better Photodetector Performance.

Solution-processed colloidal cadmium chalcogenide nanoplatelets (NPLs)-based photodetectors (PD) are promising materials for next-generation optoelectronic devices due to their excellent optical properties. Here, we report on ultrafast carrier relaxation dynamics of four monolayer (4 ML) Ag-doped CdSe (Ag: CdSe) NPLs using ultrafast transient absorption spectroscopy and their photodetectors applications. A broad dopant emission is observed at around 650 nm with a large FWHM of ~431 meV and band edge emission at 515 nm. The intragap dopant state acts as a hole acceptor, which leads to better charge separation. The ultrafast transient absorption spectroscopy study shows faster carrier recombination dynamics with a hole transfer time scale of ~10 ps in Ag-doped CdSe NPLs. This supports the excited hole capture phenomenon at the dopant state. Ag-doped CdSe NPLs-based PD performed better than undoped CdSe NPLs with detectivity and responsivity values of 1.3×1010 Jones and 2.4 mA/W, respectively.

Ultrafast Carrier Relaxation Dynamics in a Nodal-Line Semimetal PtSn4.

Topological Dirac nodal-line semimetals host topologically nontrivial electronic structure with nodal-line crossings around the Fermi level, which could affect the photocarrier dynamics and lead to novel relaxation mechanisms. Herein, by using time- and angle-resolved photoemission spectroscopy, we reveal the previously inaccessible linear dispersions of the bulk conduction bands above the Fermi level in a Dirac nodal-line semimetal PtSn4, as well as the momentum and temporal evolution of the gapless nodal lines. A surprisingly ultrafast relaxation dynamics within a few hundred femtoseconds is revealed for photoexcited carriers in the nodal line. Theoretical calculations suggest that such ultrafast carrier relaxation is attributed to the multichannel scatterings among the complex metallic bands of PtSn4 via electron-phonon coupling. In addition, a unique dynamic relaxation mechanism contributed by the highly anisotropic Dirac nodal-line electronic structure is also identified. Our work provides a comprehensive understanding of the ultrafast carrier dynamics in a Dirac nodal-line semimetal.

Momentum-dependent intraband high harmonic generation in a photodoped indirect semiconductor

Nonlinear optical response of solid-state materials exposed to strong non-resonant light fields leads to the generation of harmonic frequencies as a consequence of interband polarization and coherent intraband dynamics of the electrons. The efficient production of a macroscopic wave requires the preservation of the mutual phase between the driving wave and the individual microscopic sources of radiation. Here, we experimentally and theoretically show that the yield of high harmonic generation in a photodoped silicon crystal is enhanced by the nonlinear intraband current whose amplitude depends not only on the volume density of the photogenerated carriers but also on their momentum distributions within the bands. The strongest enhancement is reached when the carrier system is relaxed to the band minima before interacting with the strong nonresonant wave, which drives the high harmonic generation. These results extend the possibilities of high harmonic spectroscopy towards the investigation of ultrafast carrier relaxation in condensed matter.

Research on near-infrared ultrafast excited carrier dynamics of a pyrene-based derivative and its application in femtosecond laser

Femtosecond laser, compared to continuous wave (c.w.) or nanosecond laser, has shown great promise in diversities of frontier applications because of its extremely short pulse width, high peak power and low thermal effect. In order to obtain femtosecond laser output, the design of suitable saturable absorber (SA) materials as a nonlinear modulation device is a key issue for a given laser system. However, the previous SA materials are limited in inorganic semiconductors, which have some constraints such as narrow working wavelength ranges, complex preparation process and high cost. By contrast, organic semiconductors have high molecular design freedom, good flexibility, simple preparation process, and low cost, and would act as an ideal SA material. Here, we report a novel strategy to tune and realize the femtosecond laser out by employing an organic small molecule SA of (Z)-2-(4-(pyrene-1-yl) phenyl)-3-(4-(1,2,2-triphenylvinyl) phenyl) acrylonitrile (Py-TPE). We show that Py-TPE has an ultrafast excited carrier relaxation time less than 20 ps, shorter than most of inorganic semiconductor materials. More importantly, by depositing such Py-TPE on the surface of tapered fiber as an SA device in Er-doped fiber laser system, an ultrafast fiber laser output with a pulse width as narrow as 447 fs and good stability was realized. This is the first report, to the best of our knowledge, to achieve femtosecond laser by taking advantage of the Py-TPE with ultrafast excited carrier relaxation time. These results demonstrate that Py-TPE is a novel organic saturable absorption material with great potential application in ultrafast laser and photonics.

Terahertz wave emission with 1.5 μm pump from photoconductive antenna using stacked Er-doped-InAs quantum dot layers with ultrafast carrier relaxation

We fabricated a photoconductive antenna (PCA) with Er-doped InAs quantum dot (QD) layers on a GaAs substrate and demonstrated THz wave generation with 1.5 μm pump. In terahertz time-domain spectroscopy (THz-TDS), PCAs require high mobility and short lifetimes of photocarriers since the carriers need to respond rapidly to optical pulses for broad THz generation and detection with a high time resolution. In this study, to achieve PCA operation at an optical communication wavelength band of 1.5 μm, a substrate for a PCA was fabricated using stacked Er-doped-InAs QD layers. Then, photocurrent measurements were performed to observe optical absorption in the 1.5 μm band. The results indicated the high capability of the PCA for THz wave generation and detection, in fact, which enabled us to generate the THz wave in the 1.5 μm band optical system.

Improved Ultrafast Carrier Relaxation and Charge Transfer Dynamics in CuI Films and Their Heterojunctions via Sn Doping.

CuI is one of the promising hole transport materials for perovskite solar cells. However, its tendency to form defects is currently limiting its use for device applications. Here, we report the successful improvement of CuI through Sn doping and the direct measurement of the carrier relaxation and interfacial charge-transfer processes in Sn-doped CuI films and their heterostructures. Femtosecond-transient absorption (fs-TA) measurements reveal that Sn doping effectively passivates the trap states within the bandgap of CuI. The I-V characteristics of heterostructures demonstrate drastic improvement in transport characteristics upon Sn doping. Fs-TA measurements further confirm that the CuSnI/ZnO heterojunction has a type-II configuration with ultrafast charge transfer (<280 fs). The charge transfer time of a CuI/ZnO heterostructure is ∼2.8 times slower than that of the CuSnI/ZnO heterostructure, indicating that Sn doping suppresses the interfacial states that retard the charge transfer. These results elucidate the effect of Sn doping on the performance of CuI-based heterostructures.

The ultrafast tunable saturable absorption of three azo-Schiff base ligands and related metal complexes

Three azo-Schiff base ligands with large conjugated systems and five related transition metal complexes were synthesized and investigated by femetosecond open-aperture Z-scan and pump-probe system. The ultrafast tunable saturable absorption of the ligands and their metal complexes were observed at 400 nm wavelength. The obtained third-order nonlinear optical (NLO) absorption coefficients of eight compounds are evaluated to be −8.49 ∼ −80.63 cm/GW. Meanwhile, the ultrafast carrier relaxation time (187–592 fs) of all compounds was observed in the pump-probe experiment. The modulated impacts of metal coordination on NLO responses are analyzed by a five-level energy system. We deem that the performance of metal complexes is determined by the number of shell electrons of central metal. All the results indicate that those compounds have potential applications for tunable saturable absorber in NLO field.

Coherent acoustic modulation and defect-sensitive ultrafast carrier dynamics of Pr0.5Ca0.5MnO3 thin films investigated by time-resolved terahertz spectroscopy

In transition metal oxides, the potential of competing energetics of interacting fundamental entities is best displayed in perovskite manganites via the formation of a variety of exotic phases; however, there are limitations of extreme sensitivity to extrinsic and intrinsic defects and the slightest of structural modulations. Here, we report the effect of oxygen annealing and epitaxial strain on the ultrafast carrier excitation and relaxation mechanism in charge-ordered (CO) manganite Pr0.5Ca0.5MnO3 (PCMO) thin films of 60 and 150 nm thicknesses, both as-grown and oxygen annealed, as investigated by optical pump–terahertz (THz) probe measurements. Transient THz transmittance is negative for both films. Bi-exponential relaxation behavior accompanied with acoustic modulations was observed that varies along with strain and oxygen content of the films. As fitted by the sum of exponentials, the fast relaxation time constant is found to be fluence independent, while the slow relaxation time constant decreases with pump fluence for both films and is less for the annealed film suggesting that the relaxation in PCMO strongly depends on strain and oxygen content. This study on non-equilibrium carrier dynamics depicting the sensitivity of defects and subtle structural modifications is unprecedented in demonstrating the ultrafast control of CO manganites.

Ultrafast scanning electron microscopy with sub-micrometer optical pump resolution

Ultrafast scanning electron microscopy images carrier dynamics and carrier induced surface voltages using a laser pump electron probe scheme, potentially surpassing all-optical techniques in probe resolution and surface sensitivity. Current implementations have left a four order of magnitude gap between optical pump and electron probe resolution, which particularly hampers spatial resolution in the investigation of carrier induced local surface photovoltages. Here, we present a system capable of focusing the laser using an inverted optical microscope built into an ultrafast scanning electron microscopy setup to enable high numerical aperture pulsed optical excitation in conjunction with ultrafast electron beam probing. We demonstrate an order of magnitude improvement in optical pump resolution, bringing this to sub-micrometer length scales. We further show that temporal laser pump resolution can be maintained inside the scanning electron microscope by pre-compensating dispersion induced by the components required to bring the beam into the vacuum chamber and to a tight focus. We illustrate our approach using molybdenum disulfide, a two-dimensional transition metal dichalcogenide, where we measure ultrafast carrier relaxation rates and induced negative surface potentials between different flakes selected with the scanning electron microscope as well as on defined positions within a single flake.

Coherent surface-to-bulk vibrational coupling in the 2D topologically trivial insulator Bi2Se3 monitored by ultrafast transient absorption spectroscopy

Ultrafast carrier relaxation in the 2D topological insulator (TI) Bi2Se3 [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI Bi2Se3 (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (~ 100 fs) pumping pulses of ~ 340 nm wavelength (~ 3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI Bi2Se3 films with a thickness of 2 and 5 quintuple layers (~ 2 and ~ 5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet–visible spectral region (1.65–3.9 eV). The negative and positive contributions of TA spectra were attributed to absorption bleaching that mostly occurs in the bulk states and to the inverse bremsstrahlung type free carrier absorption in the gapped Dirac SS, respectively. Owing to this direct and selective access to the bulk and surface carrier dynamics, we were able to monitor coherent longitudinal optical (LO) phonon oscillations, which were successively launched in the bulk and surface states by the front of the relaxing electron population within the LO-phonon cascade emission. We have also recognized the coherent surface-to-bulk vibrational coupling that appears through the phase-dependent amplitude variations of coherent LO-phonon oscillations. This unique behavior manifests itself predominantly for the topologically trivial insulator phase of the 2D TI Bi2Se3 (2 nm thick film) in the photon energy range (~ 2.0–2.25 eV) where efficient energy exchange between the bulk and surface states occurs. We also found that the coherent surface-to-bulk vibrational coupling significantly weakens with increasing both the Bi2Se3 film thickness and pumping power.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies.

Over the past few decades, coherent broadband spectroscopy has been widely used to improve our understanding of ultrafast processes (e.g., photoinduced electron transfer, proton transfer, and proton-coupled electron transfer reactions) at femtosecond resolution. The advances in femtosecond laser technology along with the development of nonlinear multidimensional spectroscopy enabled further insights into ultrafast energy transfer and carrier relaxation processes in complex biological and material systems. New discoveries and interpretations have led to improved design principles for optimizing the photophysical properties of various artificial systems. In this review, we first provide a detailed theoretical framework of both coherent broadband and two-dimensional electronic spectroscopy (2DES). We then discuss a selection of experimental approaches and considerations of 2DES along with best practices for data processing and analysis. Finally, we review several examples where coherent broadband and 2DES were employed to reveal mechanisms of photoinitiated ultrafast processes in molecular, biological, and material systems. We end the review with a brief perspective on the future of the experimental techniques themselves and their potential to answer an even greater range of scientific questions.

Importance of surface oxygen vacancies for ultrafast hot carrier relaxation and transport in Cu2 O

${\mathrm{Cu}}_{2}\mathrm{O}$ has appealing properties as an electrode for photoelectrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (${V}_{\mathrm{O}}$) in the experimentally observed ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ reconstruction of the dominant (111) surface. Our results show that these ${V}_{\mathrm{O}}$ are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged ${V}_{\mathrm{O}}$ plays a crucial role in the nonradiative electron capture process with a capture coefficient of about ${10}^{\ensuremath{-}9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{3}$/s and a lifetime of 0.04 ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface ${V}_{\mathrm{O}}$ chemistry will be a crucial step in optimizing ${\mathrm{Cu}}_{2}\mathrm{O}$ for photoelectrode applications.

Phonon Bottleneck in Temperature-Dependent Hot Carrier Relaxation in Graphene Oxide

We report temperature-dependent ultrafast hot carrier relaxation in graphene oxide (GO). At all temperatures in the range from ∼6 to 400 K, the time evolution of the photoexcited relaxation reveals...

Stimulating polarization switching dynamics in mid-infrared quantum cascade lasers

A unique feature of quantum cascade lasers relies on the ultrafast carrier relaxation lifetime that occurs on a picosecond time scale. Furthermore, the very sharp electronic transitions among the conduction-band states (subbands) lead to specific selection rules, which in theory exclude the TE-polarized light emission. Under cross-polarization reinjection, we stimulate the polarization switching dynamics in such an intersubband device. The mid-infrared modulation signal is phase-shifted between the TM mode and the TE mode, with a typical microsecond time scale different from pure time-delay dynamics. We also prove that both the frequency and the duty cycle of the modulated pattern can be slightly tuned by varying the characteristics of the reinjected light. These results reveal the possibility of favoring the TE polarization in a quantum cascade laser and generating a square-wave modulation with cross-polarization reinjection.

Ultrafast electron–hole relaxation dynamics in CdS nanocrystals

Surface traps significantly influence the charge carrier dynamics within semiconductor nanocrystals, introducing non-radiative exciton recombination channels which are detrimental for their applications. Understanding the nature of these trap states and modulating them synthetically bears immense potential in designing defect-free colloidal semiconductor nanocrystals for efficient optoelectronic devices. Thus, systems devoid of surface traps can be used to study the relaxation pathways of excitons generated within these nanocrystals. In this work, we study the ultrafast charge carrier relaxation dynamics upon near-edge resonance excitation and above-resonance excitation in CdS nanocrystals using ultrafast transient absorption spectroscopy, in order to understand intraband cooling and mid-gap trap states. The time-resolved studies reveal that the above bandgap excitation results in a three-step process, including instantaneous growth followed by a fast sub-picosecond decay and a long-lived (>1 ns) excited state or a trap state recombination. The large percentage of long-lived excitons in CdS nanocrystals elucidates the defect-free nature of the system arising from the absence of surface states.

High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics

AbstractThe Dirac semimetal cadmium arsenide (Cd3As2), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high‐performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power‐efficiency and switching speed. Here, all‐optical, low‐power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd3As2 are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd3As2 with high terahertz electron mobility of 7200 cm2 (Vs)−1. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power‐efficient, high speed, compact, tunable electronic, and photonic devices.

Ta2NiSe5 nanosheets as a novel broadband saturable absorber for solid-state pulse laser generation

Two-dimensional (2D) ternary chalcogenides have attracted great attentions because of their novel chemical and physical properties arising from the synergistic effect and stoichiometric variation with the additional third element compared with their binary counterparts. Here, high-quality 2D tantalum nickel selenide (Ta2NiSe5) nanosheets are successfully fabricated by a liquid-phase exfoliation (LPE) method. The ultrafast excited carrier relaxation time and nonlinear optical absorption response are investigated and reveal that the prepared 2D Ta2NiSe5 nanosheets have excellent broadband saturable absorption properties, which are further illustrated by three passively Q-switched (PQS) all-solid-state lasers operating at 1.0, 2.0 and 2.8 µm with the Ta2NiSe5 nanosheet-based saturable absorber (SA). Furthermore, mode-locked laser operation with the pulse width as short as 356 fs is also realized at 1.0 µm. This work not only demonstrates the excellent nonlinear optical proprieties and optical modulation performance of Ta2NiSe5, but also paves the way for exploring the photonic and optoelectronic proprieties of ternary chalcogenide materials.

Mobility and activation energy of lateral photocurrent of InAs quantum dot layers with ultrafast carrier relaxation

We determined the lateral mobility of photocarriers and activation energy of the photocurrent (PC) in stacked InAs quantum dot (QD) layers with ultrafast carrier relaxation times and 1.5-μm band absorption. Lateral mobility is one of the important parameters in the application of QD layers to photoconductive antennas operating with 1.5-μm gating light for terahertz (THz) wave detection. The activation energy obtained from temperature dependence of PC spectroscopy is important for elucidating the mechanism, and it is an indicator of the temperature range for antenna operation. The dependence of the mobility on the photocarrier density was evaluated from the dependence on the pump power. The mobility sharply increased from 42 cm 2 /V･s in the dark to 416 cm 2 /V･s upon laser illumination and then decreased with the increase in the number of generated photocarriers. The broad PC peak from InAs QD layers at around 1400 nm was continuously monitored in the temperature range of 4–292 K. The activation energy of the PC from the InAs QD layers was evaluated in the temperature range of 159–240 K where the PC significantly increased. In the broad PC peak in the wavelength range of 1200–1600 nm, the activation energy increased from 48 to 62 meV with the wavelength. The PC observed at lower temperatures was not derived considering the thermal escape of electrons, but it was determined through the Auger process. The InAs QD layer with ultrafast carrier relaxation has considerable potential in photoconductive antenna applications for THz wave detection. • Lateral mobility increased: 42 cm 2 /V･s in darkness to 416 cm 2 /V･s with illumination. • Thermally stable and broad peak of InAs QD layers occurred during PC spectroscopy. • From 159 to 240 K, PC increased and activation energy of the PC depended on QD sizes. • At cryogenic temperatures, PC was derived using the Auger process.

Ultrafast carrier relaxation in SnSe (x=1, 2) thin films observed using femtosecond time-resolved transient absorption spectroscopy

Abstract In this study, SnSex (x = 1, 2) films are prepared using pulsed laser deposition (PLD) and characterized using X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The crystal structure and valence state of SnSex films can be observed using XRD patterns and XPS spectra. The carrier dynamics are investigated using femtosecond time-resolved TA spectroscopy. The carrier relaxation process is strongly influenced by thin film stoichiometry. The TA spectra observed in the SnSe and SnSe2 films associated with photoinduced absorption, free carrier absorption, and state filling effects. The carrier ultrafast dynamics of two films are well fitted using a tri-exponential decay model with fast (several picoseconds), medium (tens of picoseconds), and slow (several nanoseconds) components. These components are associated with carrier scattering, Auger recombination, and diffusion, respectively. The component with medium lifetime (Auger recombination) in the SnSe2 film is faster than that of SnSe film because defect state trapping. The slow relaxation component in SnSe2 film, on the other hand, is much longer than that of SnSe film. This is also due to trapping by defect states in SnSe2, which affects the diffusion of the photo-induced carriers. We investigated the carrier relaxation processes in SnSex (x = 1, 2) films and offered an insight in the ultrafast dynamic mechanism in the two tin selenides.