Photoexcited Carrier Density Research Articles

Ultrafast Optical Control of Exciton Diffusion in WSe2/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy.

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion in a monolayer WSe2 stacked on graphene. The diffusion dynamics can be optically tuned within a few picoseconds by altering the photoexcited carrier density in graphene. The effective diffusion constant in initial picoseconds in the WSe2/graphene heterostructure is (40.3 ± 4.5) cm2 s-1, representing a substantial improvement over (2.1 ± 0.8) cm2 s-1, typical for an isolated WSe2 monolayer. This enhancement can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe2 by photoexcited charge carriers in graphene. Furthermore, diffusion within WSe2 is affected by interlayer interactions, such as charge transfer, varying with the incident excitation fluence. These findings underscore the dynamical nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide insights for future applications of these systems in ultrafast optoelectronic devices.

Ultrafast charge carrier dynamics of methylammonium lead iodide from first principles

Methylammonium lead iodide (MAPbI3) has been a major focus of photovoltaic research for the last decade. The unique interplay between the structural and electronic properties of this material contributes to its exciting optical properties especially under the action of an ultrafast laser pulse. First-principles methods like real-time time-dependent density functional theory (RT-TDDFT) enable performing corresponding simulations without the aid of empirical parameters: the gained knowledge can be applied to future studies of other complex materials. In this work, we investigate the ultrafast charge-carrier dynamics and the nonlinear optical response of MAPbI3 excited by a resonant pulse above the gap. First, we examine the electronic and optical properties in the static regime. Next, we impinge the system with a femtosecond field of varying intensity and follow the evolution of the photoexcited carrier density. A pronounced intensity-dependent response is observed, manifested by high-harmonic generation and nonlinear trends in the number of excited electrons and excitation energy. Our results provide relevant indications about the behavior of MAPbI3 under strong and coherent radiation and confirm that RT-TDDFT is a viable tool to simulate the photo-induced dynamics of complex materials from first principles.

Raman scattering study of photoexcited plasma in GaAsBi/GaAs heterostructures: Influence of carrier confinement on photoluminescence

GaAsBi alloys are potential candidates for near-infrared optoelectronic applications, such as light-emitting diodes, laser diodes, avalanche photodiodes, and solar cells. In this paper, photoexcited plasma in GaAs1-xBix/GaAs (1.7 ≤ x ≤ 8.6%) heterostructures grown by molecular beam epitaxy was investigated using Raman spectroscopy. The strong LO phonon-plasmon coupled mode (LOPC) was generated by continuous-wave laser excitation at room temperature. The upper branch of the LOPC mode (L+ mode) was clearly observed for Bi contents from x = 3.9–8.6%, which indicates that photoexcited carriers with relatively good carrier mobility were confined in the GaAsBi/GaAs heterostructure. The photoexcited carrier density that was obtained by line shape analysis of the Raman spectra based on a dielectric model increased as the Bi content increased from 1.7 to 6.4%. This indicates that carrier overflow from the GaAsBi layer was suppressed because of the larger conduction band offset at the GaAsBi/GaAs heterointerface. In addition, the optical mobility calculated using the damping constant of the photoexcited L+ mode gradually decreased with increasing Bi content. This may be attributed to an increased defect density with decreasing growth temperature. Our findings provide valuable insight into the development of high-performance optoelectronic devices based on GaAsBi alloys and show the potential of Raman spectroscopy for evaluation of carrier confinement in heterostructures.

Light-induced shear phonon splitting and instability in bilayer graphene

Coherent engineering of landscape potential in crystalline materials is a rapidly evolving research field. Ultrafast optical pulses can manipulate low-frequency shear phonons in van der Waals layered materials through the dynamical dressing of electronic structure and photoexcited carrier density. In this work, we provide a diagrammatic formalism for nonlinear Raman force and implement it to shear phonon dynamics in bilayer graphene. We predict a controllable splitting of double degenerate shear phonon modes due to light-induced phonon mixing and renormalization according to a coherent nonlinear Raman force mechanism. Intriguingly, we obtain a light-induced shear phonon softening that facilitates structural instability at a critical field amplitude for which the shear phonon frequency vanishes. The phonon splitting and instability strongly depend on the laser intensity, frequency, chemical potential, and temperature of photoexcited electrons. This study motivates future experimental investigation of the optical fine tuning and regulation of shear phonons and layer stacking order in layered van der Waals materials.

Fluence dependent dynamics of excitons in monolayer MoSi2Z4 (Z = pnictogen)

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic (Z=N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. We show that on increasing the pump fluence or photo-excited carrier density, the lowest energy exciton first undergoes a redshift followed by a blueshift, due to the renormalized exciton binding energies. The exciton binding energy varies as a Lennard-Jones-like potential as a function of the inter-exciton spacing. This establishes an atom-like attractive and repulsive interaction between excitons depending on the inter-exciton separation. Our study shows that the series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics.

Effects of GaN cap layer thickness on photoexcited carrier density in green luminescent InGaN multiple quantum wells

The effects of GaN cap layers on the optical properties of green luminescent InGaN-based multiple quantum wells were studied by photoluminescence (PL) spectroscopy. The PL peak energy under the selective excitation of the InGaN well layers was lower than that under the band-to-band excitation of the GaN barrier layers. The difference in the PL peak energies between the selective and band-to-band excitations decreased as the cap layer thickness increased, indicating an increase in the nonradiative recombination of photogenerated carriers in the barrier layers. Moreover, the internal quantum efficiency under selective excitation decreased as the cap layer thickness increased because of the increase in the internal electric field strength.

Intrinsic Mobility of Low-Density Electrons in Photoexcited Diamond

The cyclotron-resonance method reveals the drift mobility of carriers in semiconductors, which determines a device's (opto)electronic functionality. However, determining the intrinsic mobility value without interference from other carriers, dislocations, impurities, etc. remains challenging. By minimizing the density of photoexcited carriers in ultrapure diamond, the authors find an extraordinarily narrow cyclotron-resonance curve for electrons in diamond at 3 K. In this manner they obtain a corrected mobility value of 10${}^{8}$ cm${}^{2}$ V${}^{\ensuremath{-}1}$ s${}^{\ensuremath{-}1}$, a 16-fold increase compared to the previous record value for diamond.

Complex energies of the coherent longitudinal optical phonon–plasmon coupled mode according to dynamic mode decomposition analysis

In a dissipative quantum system, we report the dynamic mode decomposition (DMD) analysis of damped oscillation signals. We used a reflection-type pump-probe method to observe time-domain signals, including the coupled modes of long-lived longitudinal optical phonons and quickly damped plasmons (LOPC) at various pump powers. The Fourier transformed spectra of the observed damped oscillation signals show broad and asymmetric modes, making it difficult to evaluate their frequencies and damping rates. We then used DMD to analyze the damped oscillation signals by precisely determining their frequencies and damping rates. We successfully identified the LOPC modes. The obtained frequencies and damping rates were shown to depend on the pump power, which implies photoexcited carrier density. We compared the pump-power dependence of the frequencies and damping rates of the LOPC modes with the carrier density dependence of the complex eigen-energies of the coupled modes by using the non-Hermitian phenomenological effective Hamiltonian. Good agreement was obtained between the observed and calculated dependences, demonstrating that DMD is an effective alternative to Fourier analysis which often fails to estimate effective damping rates.

Dynamic Opening of a Gap in Dirac Surface States of the Thin-Film 3D Topological Insulator Bi2Se3 Driven by the Dynamic Rashba Effect

Optical control of Dirac surface states (SS) in topological insulators (TI) remains one of the most challenging problems governing their potential applications in novel electronic and spintronic devices. Here, using visible-range transient absorption spectroscopy exploiting ∼340 nm (∼3.65 eV) pumping, we provide evidence for dynamic opening of a gap in the Dirac SS of the thin-film 3D TI Bi2Se3, which has been induced by the dynamic Rashba effect occurring in the film bulk with increasing optical pumping power (photoexcited carrier density). The observed effect appears through the transient absorption band associated with inverse-bremsstrahlung-type free carrier absorption in the gapped Dirac SS. We have also recognized experimental signatures of the existence of the higher energy Dirac SS in the 3D TI Bi2Se3 (in addition to those known as SS1 and SS2) with energies of ∼2.7 and ∼3.9 eV (SS3 and SS4). It is evidenced that the dynamic gap opening has the same effect on the Dirac SS occurring at any energy.

Metal-Organic Hybrid Metamaterials for Spectral-Band Selective Active Terahertz Modulators

Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An active layer of an organic thin film (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, also called PCBM) was deposited on the metamaterial-silicon structure for modulating the transmission of incident THz radiation. The metal-organic hybrid metamaterials enabled active control of spectral bands present in the transmission spectra of THz waves. In addition, the changes in the photo-excited carrier density due to the transfer of charges between the layers were quantitatively analyzed by simulation results.

Investigation of THz Radiation from Longitudinal Optical Phonon- Plasmon Coupling in p-i-n Diodes

Background: Terahertz radiation (THz) in infrared domain at room temperature has many applications in science and technology, especially in the technology of analyzing and processing image. This paper introduces a relatively simple method to investigate the Terahertz radiation from the coupling of Longitudinal Optical (LO) phonon and coherent plasmon in p-i-n diode structure. The frequency spectra are found from the Fast Fourier Transform (FFT) of the voltage between two neighboring points in the insulating region of this diode. Numerical calculations have been applied for GaAs semiconductor device with the photo-excited carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3 and the insulating layer size of 500 nm. Methods: In order to study of the coupling of LO phonon in p-i-n diode based on the EMC simulations, the equation of density vibration is solved simultaneously with the simulation of carrier dynamics, this is performed with spatial resolution of 1A° and time resolution of 0.20 fs . We solve the Poisson’s equation to derive the potentials along the x-axis in both cases with and without taking into account the coupling. The frequency spectrum derived from the FFT of the voltage of two layers separated by a distance 10 nm in insulating layer (i). Results: The frequency spectrum derived from the Fourier transform of the voltage between two neighboring points with and without LO phonon–plasmon coupling is shown. We can easily observe the existence of the modes which are close to the frequency values of bulk semiconductor. It should be noted that, our calculations are reasonable agreement with experiments measured by the Ibanez et al in Phys. Rev. B 69 (7), 075314 (2004). Conclusion: In this paper, we present a relatively simple approach to investigate the THz radiation from the coupling of LO phonon-plasmon in p-i-n diode structure by taking the FFT of the voltage of two neighboring points in insulating semiconductor region. The voltage is calculated through the electric potentials which relate to the charge density via Poisson’s equation. Numerical calculations have been performed for GaAs semiconductor device with carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3. Our simulations calculations show that with appropriate photoexcited carrier density, two strong coupling LO phonon-plasmon coherent modes are appear.

Observing dynamic and static Rashba effects in a thin layer of 3D hybrid perovskite nanocrystals using transient absorption spectroscopy

The dynamic and static Rashba effects in hybrid methylammonium (MA) lead halide perovskites have recently been theoretically predicted. However, only the static effect was experimentally confirmed so far. Here, we report on the dynamic (sub-picosecond/picosecond timescale) and static (nanosecond/microsecond timescale) Rashba effects observed in a fully encapsulated layer with various thicknesses (ranging from ∼40 nm to ∼100 nm) of ∼20-nm-sized 3D MAPbBr3 nanocrystals (NCs) using transient absorption (TA) spectroscopy. The effect appears as a splitting of the corresponding peaks in TA spectra. We argue that the physical reason for the Rashba effect to be observed is fundamentally determined by configurational entropy loss in NCs possessing a strong spin asymmetry. Specifically, owing to an enhanced flexibility of the NC lattice, a built-in electric field initially induced by an ultrashort (100 fs) pumping pulse through the photo-Dember effect and subsequently developed due to dynamic charge separation throughout NCs is able to initiate the order–disorder transition associated with the MA cation reorientations, the process that efficiently breaks structural inversion symmetry and hence induces the Rashba spin–orbit interaction. The dynamic Rashba effect is found to be strongly dependent on photoexcited carrier density (pumping power), whereas it weakens sharply upon increasing the NC layer thickness up to ∼80 nm due to the NC stacking effect. The integrated intensities of the corresponding spin-split subbands demonstrate a photon-helicity-dependent asymmetry, thus proving the Rashba-type spin-splitting. The magnitudes of the Rashba and Fröhlich polaron effects and the methods of controlling the dynamic Rashba effect are discussed.

Decelerated Hot Carrier Cooling in Graphene via Nondissipative Carrier Injection from MoS2.

One key to improve the performance of advanced optoelectronic devices and energy harvesting in graphene is to understand the predominant carrier scattering via optical phonons. Nevertheless, low light absorbance in graphene yields a limited photoexcited carrier density, hampering the hot carrier effect, which is strongly correlated to the hot optical phonon bottleneck effect as the energy-loss channel. Here, by integrating graphene with monolayer MoS2 possessing stronger light absorbance, we demonstrate an efficient interfacial hot carrier transfer between graphene and MoS2 in their heterostructure with a prolonged relaxation time using broadband transient differential transmittance spectroscopy. We observe that the carrier relaxation time of graphene in the heterostructure is 4 times slower than that of bare graphene. This is explained by nondissipative interlayer transfer from MoS2 to graphene, which is attributed to the enhanced hot optical phonon bottleneck effect of graphene in the heterostructure by an increased photoexcited carrier population. A significant reduction of both amplitude and relaxation time in A- and B-excitons is another evidence of the interlayer transfer from MoS2 to graphene. The nondissipative interlayer charge transfer from MoS2 to graphene is confirmed by density functional calculations. This provides a different platform to further study the photoinduced hot carrier effect in graphene heterostructures for photothermoelectric detectors or hot carrier solar cells.

Zero-field spin precession dynamics of high-mobility two-dimensional electron gas in persistent spin helix regime

We investigated the spin-orbit (SO) effective magnetic-field-induced spin precession of a high-mobility two-dimensional electron gas (2DEG) in a modulation-doped (001) GaAs/AlGaAs quantum well close to the persistent spin helix (PSH) state. The oscillating spin signal induced by the SO fields in zero external magnetic fields was clearly observed. When the photoexcited carrier density is sufficiently small compared with the 2DEG density, the spin precession length in the space domain was obtained by analyzing the precession frequency in the PSH regime. The excitation density dependence of the spin dynamics was reproduced well using the scattering time-dependent spin dynamics simulation. We revealed that the increase in the photoexcited carriers results in a transition from ballistic motion to diffusive motion, leading to a decrease in the spin-diffusion coefficient.

Photocarrier-Induced Active Control of Second-Order Optical Nonlinearity in Monolayer MoS2.

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light-TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second-harmonic generation (SHG) from a monolayer MoS2 crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low-dimensional materials for all-optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second-order dielectric susceptibility χ(2) and the density of photoexcited carriers in MoS2 . Indeed, the depopulation of the conduction band electrons, at the vicinity of the high-symmetry K/K' points of MoS2 , suppresses the contribution of interband electronic transitions in the effective χ(2) of the monolayer crystal, enabling the all-optical modulation of the SHG signal. The strong dependence of the second-order optical response on the density of photocarriers reveals the promise of time-resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.

Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence

Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes. Localized photodoping in mixed-cation perovskites is shown to modify charge-carrier recombination and thus offer a route for more efficient light emission.

Comparative analysis of nonlinear optical properties of single-layer graphene and few-layer graphene nanosheets

Nonlinear absorption of suspensions of graphene nanosheets with the number of layers from one to three was studied using the Z-scan method with femtosecond excitation at 1030 nm wavelength. A large modulation depth and lower saturation intensity of the suspensions as compared with nonlinear absorption of single-layer graphene were shown. Dynamics of photoexcited carriers for different duration of excitation pulse was considered. The values of the absorption cross section and the density of photoexcited carriers in single-layer graphene were estimated. The presence of two-photon absorption (TPA) in suspensions of graphene nanosheets and the absence of noticeable TPA in single-layer graphene were shown.

(Invited) Element-Specific Measurement of Hole Transport in a Photoanode By Transient Extreme Ultraviolet Spectroscopy

A passivating oxide layer is critical for the performance and stability of solar-fuel photoelectrodes. While the semiconductor surface can be passivated by a few nanometer oxide film, the best performance often correlates with a thicker and defect-rich amorphous TiO2 layer. The defect states are suggested to facilitate hole transport between the semiconductor and metal catalyst. In this presentation, transient extreme ultraviolet (XUV) absorption spectroscopy is used to measure the electron and hole transport between each element of a photoexcited Ni-TiO2-Si junction. The transient changes in the Ni M2,3 edge, Si L2,3­ edge, and Ti M2,3 edge are quantified using a broad-band, high harmonic XUV continuum that spans 30 to 120 eV. Hole tunneling from the p-type Si to the Ni metal is measured immediately following 800 nm optical excitation. The measured transport time and electric field indicate a ballistic tunneling through the TiO2 that originates from a Si hole with a mobility of ~400 cm2/Vs. An appreciable transfer of photoexcited electrons is not measured. The quantum yield of the hole tunneling is calculated to be ~50% percent by using the photoexcited carrier density in Si and the measured shift in the Ni M2,3 edge Fermi energy. On a picoseconds time scale, the holes are measured to back-transfer from the Ni to the Si through the mid-gap states in the TiO2. The extracted kinetics suggest a hole diffusion constant of ~1 cm2/s. The surface recombination velocity of electrons and holes at the Si-TiO2 interface is also quantified as ~200 cm/s. These results suggest that transient XUV spectroscopy can be used to quantitatively measure electron and hole transport in solar-fuel relevant nanoscale junctions.

Ultrafast asymmetric Rosen-Zener-like coherent phonon responses observed in silicon

We investigate the spectral profiles of time signals attributed to coherent phonon generation in an undoped Si crystal. Here, the retarded longitudinal-optical (LO) phonon Green function relevant to the temporal variance of induced charge density of ionic cores is calculated by employing the polaronic quasiparticle model developed by the authors [Y. Watanabe et al., Phys. Rev. B 95, 014301 (2017); ibid., 96, 125204 (2017)]. The spectral asymmetry is revealed in the frequency domain of the signals under the condition that an LO phonon mode stays almost energetically resonant with a plasmon mode in the early time region; this lasts for approximately 100 fs immediately after the irradiation of an ultrashort pump-laser pulse. It is understood that based on the adiabatic picture in time, this asymmetry is caused by the Rosen-Zener coupling between both modes. The associated experimental results are obtained by measuring time-dependent electro-optic reflectivity signals, and it is proved that these are in harmony with the calculated ones. The spectra become more symmetric, as the photoexcited carrier density further changes from that meeting the above condition to higher and lower sides of carrier densities. Moreover, the effect of optical nutation of carrier density on the CP signals is addressed, and the present results are compared with the asymmetry caused by transient Fano resonance, and the spectral profiles observed in a GaAs crystal in the text.

(Invited) Element-Specific Measurement of Hole Transport and Kinetics in a Ni-TiO2-Si Photolectrode Using Transient Extreme Ultraviolet Spectroscopy

A passivating oxide layer is critical for the stability and the performance of solar-fuel photoelectrodes. While the semiconductor surface can be passivated by a few nanometer oxide film, the best performance often correlates with a thicker and defect-rich amorphous TiO2 layer. The defect states are suggested to facilitate hole transport between the semiconductor and metal catalyst. In this presentation, transient extreme ultraviolet (XUV) absorption spectroscopy measures the electron and hole transport between each element of a photoexcited Ni-TiO2-Si photoelectrode. The transient changes in the Ni M2,3 edge, Si L2,3­ edge, and Ti M2,3 edge are quantified using a broad-band, high harmonic XUV continuum that spans 30 to 120 eV. Photoexcitation with a 30 fs, 800 nm optical pulse leads to hole tunneling from the p-type Si to the Ni metal. The transport time and electric field indicate a ballistic tunneling through the TiO2 that originates from a Si hole with mobility of ~400 cm2/Vs. An appreciable transfer of photoexcited electrons is not measured. The quantum yield of the hole tunneling is calculated to be ~50% percent by using the photoexcited carrier density in Si and the measured shift in the Ni M2.3 edge Fermi energy. The holes are measured to back-transfer from the Ni to the Si through the mid-gap states in the TiO2 on a picoseconds time scale. The measured kinetics suggest a hole diffusion constant of ~1 cm2/s. The surface recombination velocity of electrons and holes at the Si-TiO2 interface is also quantified as ~200 cm/s. These results suggest that transient XUV spectroscopy can be used to quantitatively measure electron and hole transport in solar-fuel relevant nanoscale junctions.