Low Pump Fluence Research Articles

Photoinduced Ultrafast Electron Transfer and Charge Transport in a PbI2/C60 Heterojunction

Charge transport and electron transfer in a PbI2/C60 heterojunction were investigated. The charge carrier mobilities and trap densities of PbI2 were extracted from current–voltage characteristics of hole- and electron-only devices. Femtosecond transient absorption spectroscopy was used to elucidate the photophysical processes occurring in the heterojunction. For a pristine PbI2 film, trap-limited bimolecular recombination was the dominant mechanism in the case of low pump fluence. We observed photoinduced ultrafast electron transfer from PbI2 to C60 with a rate constant of 0.45 ps–1. Semiclassical Marcus electron transfer theory was used to estimate the electronic coupling between the conduction band edge of PbI2 and the lowest unoccupied molecular orbital of C60. We also fabricated a solar cell using the PbI2/C60 heterojunction. Electron transfer and charge extract efficiencies in our cell were also deduced. The performance-limiting factors for solar cells based on PbI2 were discussed and a strategy to improve the device performance was developed. Our work is useful for ultraviolet-harvesting transparent solar cells and other photophysical and photochemical applications.

Effect of Laser Irradiation on Graphene Oxide Integrated TE-Pass Waveguide Polarizer

SiO $_{2}$ –HfO $_{2}$ rib waveguides were fabricated using the UV-photolithography method followed by wet chemical etching technique. Graphene oxide (GO) integrated SiO $_{2}$ –HfO $_{2}$ rib waveguide works as an on-chip TE-pass waveguide polarizer and also allows controlled intensity tuning of the selective state of polarization when irradiated using a 532 nm ns laser. The tuning of the TE polarization intensity was confirmed by performing the experiments at three different pump fluences viz., 0.2, 0.3, and 0.6 mJ/cm $^{2}$ , wherein at low pump fluence, TE polarization intensity showed more controlled absorption. The results indicate that GO integrated glass waveguides can act as efficient polarization dependent power splitters for integrated optic applications.

Pump fluence dependence of ultrafast carrier dynamics in InSb measured by optical pump-terahertz probe spectroscopy.

Ultrafast carrier dynamics in intrinsic and n-doped InSb crystals were studied by time-resolved terahertz spectroscopy using an optical pump-terahertz probe setup with pump fluence from 32 μJ/cm2 to 1910 μJ/cm2. With photoexcitation at 800nm, the ultrafast photoinduced absorption and carrier recovery process of intrinsic and n-doped InSb showed strong pump fluence dependence. It was found that the magnitude of photoinduced absorption first increased and then decreased with pump fluence. The carrier recovery process could be well fitted with a single exponential curve at low pump fluence, but could be well fitted with a biexponential curve at high pump fluence when a fast photocarrier relaxation appeared. The magnitude of photoinduced absorption increased gradually at low pump fluence due to the increase of the carrier at the bottom of the conduction band by impact ionization. The magnitude of photoinduced absorption decreased gradually at high pump fluence, possibly due to the efficiency of transient Auger recombination greater than the rate of carriers generated in the impact ionization process. The fast decay process appearing at high pump fluence was thought to be dominated by transient Auger recombination.

Photogenerated metasurfaces at terahertz frequencies induced by a continuous-wave low pump

Active metasurfaces have attracted a great deal of attention due to the unlimited ways of controlling electromagnetic radiation. In this work, we theoretically study the THz electromagnetic properties of photogenerated metasurfaces in an InAs slab. We use a spatially modulated optical pump to modify the permittivity. Those modifications are calculated by solving the ambipolar rate equation for the photocarriers. This work reveals the crucial impact of the photocarrier diffusion for realizing optimal metasurfaces for the control of THz radiation. We demonstrate that InAs is a promising semiconductor that could be used to manufacture fast and efficient on-chip THz components. We also demonstrate that low pump fluence in the continuous regime of only tens of $\text{W}\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$ is sufficient to modulate the absorption of the THz waves up to $67%$ over a broad frequency range from 1 to 3 THz.

Dynamic Control of Nanocavities with Tunable Metal Oxides

Fabry-Pérot metal-insulator-metal (MIM) nanocavities are widely used in nanophotonic applications due to their extraordinary electromagnetic properties and deeply subwavelength dimensions. However, the spectral response of nanocavities is usually controlled by the spatial separation between the two reflecting mirrors and the spacer's refractive index. Here, we demonstrate static and dynamic control of Fabry-Pérot nanocavities by inserting a plasmonic metasurface, as a passive element, and a gallium doped-zinc oxide (Ga:ZnO) layer as a dynamically tunable component within the nanocavities' spacer. Specifically, by changing the design of the silver (Ag) metasurface one can "statically" tailor the nanocavity response, tuning the resonance up to 200 nm. To achieve the dynamic tuning, we utilize the large nonlinear response of the Ga:ZnO layer near the epsilon near zero wavelength to enable effective subpicosecond (<400 fs) optical modulation (80%) at reasonably low pump fluence levels (9 mJ/cm2). We demonstrate a 15 nm red shift of a near-infrared Fabry-Pérot resonance (λ ≅ 1.16 μm) by using a degenerate pump probe technique. We also study the carrier dynamics of Ga:ZnO under intraband photoexcitation via the electronic band structure calculated from first-principles density functional method. This work provides a versatile approach to design metal nanocavities by utilizing both the phase variation with plasmonic metasurfaces and the strong nonlinear response of metal oxides. Tailorable and dynamically controlled nanocavities could pave the way to the development of the next generation of ultrafast nanophotonic devices.

Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces

Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm–2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system.

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm2.

Ultrafast Charge Carrier Dynamics in Extremely Thin Absorber (ETA) Solar Cells Consisting of CdSe-Coated ZnO Nanowires

The extremely thin absorber (ETA) solar cell architecture can enable higher efficiencies than planar cells for absorbers that have low carrier lifetimes or mobilities. Efficient charge separation requires that interfacial electron and hole transfer proceed much faster than the recombination lifetime of photoexcited carriers. In this work, transient absorption spectroscopy was employed to measure these ultrafast photophysical processes in CdSe-coated ZnO nanowire ETA cells and model planar films. At low pump fluences, carrier lifetime was controlled by Shockley–Read–Hall and surface recombination. Annealing the electrodeposited CdSe films increased the lifetime 50-fold, to >1 ns as measured by transient absorption spectroscopy, which correlated to improved ETA cell performance. Interfacial electron transfer from the CdSe coating into the ZnO nanowires occurred 3 orders of magnitude faster, within 1 ps, independent of the presence or absence of an interfacial CdS buffer layer. Interfacial hole transfer to t...

Ultrafast demagnetization of L10 FePt and FePd ordered alloys

The cause of the time scale for ultrafast demagnetization induced by irradiation using a fs pulse laser remains an open issue. Spin-flip mediated by electron–phonon scattering due to spin–orbit interactions is one major theory proposed to explain ultrafast demagnetization. Ultrafast demagnetization in Ni, FePd, and FePt films was investigated in order to systematically study the influence of heavy elements on the demagnetization time. The ultrafast demagnetization in these systems was analyzed using the microscopic three temperature model, which is a theory based on spin-flip mediated by electron–phonon scattering. The spin-flip probability () values for the Ni, FePd, and FePt films were evaluated in the low pump fluence regime. It was found that the value for the Ni film was larger than that for the FePd and FePt films. Thus, there is no correlation between the value and the spin–orbit coupling strength. Fast demagnetization of the FePd film was also observed due to large electron–phonon scattering. In addition, it was found that the values decreased with increasing magnetization quenching for all the films.

Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths

Transparent conducting oxides have recently gained great attention as CMOS-compatible materials for applications in nanophotonics due to their low optical loss, metal-like behavior, versatile/tailorable optical properties, and established fabrication procedures. In particular, aluminum doped zinc oxide (AZO) is very attractive because its dielectric permittivity can be engineered over a broad range in the near infrared and infrared. However, despite all these beneficial features, the slow (> 100 ps) electron-hole recombination time typical of these compounds still represents a fundamental limitation impeding ultrafast optical modulation. Here we report the first epsilon-near-zero AZO thin films which simultaneously exhibit ultra-fast carrier dynamics (excitation and recombination time below 1 ps) and an outstanding reflectance modulation up to 40% for very low pump fluence levels (< 4 mJ/cm2) at the telecom wavelength of 1.3 {\mu}m. The unique properties of the demonstrated AZO thin films are the result of a low temperature fabrication procedure promoting oxygen vacancies and an ultra-high carrier concentration. As a proof-of-concept, an all-optical AZO-based plasmonic modulator achieving 3 dB modulation in 7.5 {\mu}m and operating at THz frequencies is numerically demonstrated. Our results overcome the traditional "modulation depth vs. speed" trade-off by at least an order of magnitude, placing AZO among the most promising compounds for tunable/switchable nanophotonics.

Effects of spin polarization on absorption saturation and recombination dynamics of carriers in (001) GaAs quantum wells

In this paper, the ultrafast dynamics of spin relaxation and recombination of photoexcited carriers has been studied in (001) GaAs quantum wells using a time-resolved pump-probe absorption spectroscopy under a nearly resonant excitation of heavy-hole excitons. It is found that the spin polarization of carriers influences both absorption saturation of linear polarized light and recombination dynamics of carriers. Pump fluence dependence of the ultrafast dynamics of spin relaxation and recombination of carriers is further studied, which shows that the effect of spin polarization on linearly polarized absorption saturation is reduced with lowering pump fluence. Spin-polarization-dependent absorption saturation effect can be ignored only as the pump fluence is weak. However, spin-polarization dependence of recombination dynamics is presented in turn at low pump fluence. Our analysis shows that such dependence originates from the spin-polarization dependence of the density of excitons formed in the excited carriers because recombination time constants of excitons and free carriers are very different so that the ratio of exciton density to free carrier density can influence the recombination dynamics. The spin-polarization dependence of ultrafast recombination dynamics of photoexcited carriers implies that the recombination time constant in the calculation of spin relaxation time from spin relaxation dynamics should be the recombination time of spin-polarized carriers, rather than the recombination lifetime of non-spin-polarized carriers as done currently. Exciton density is estimated based on 2D mass action law, which agrees very well with our experimental results. The good agreement between theoretical calculation and experimental results reveals that the effect of Coulomb screening on the formation of excitons may be ignored for a lower excited carrier density.

Ultrafast Charge Generation in an Organic Bilayer Film

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.

Simultaneous investigation of ultrafast structural dynamics and transient electric field by sub-picosecond electron pulses

The ultrafast structure dynamics and surface transient electric field, which are concurrently induced by laser excited electrons of an aluminum nanofilm, have been investigated simultaneously by the same transmission electron diffraction patterns. These two processes are found to be significantly different and distinguishable by tracing the time dependent changes of electron diffraction and deflection angles, respectively. This study also provides a practical means to evaluate simultaneously the effect of transient electric field during the study of structural dynamics under low pump fluence by transmission ultrafast electron diffraction.

Effects of Interface States on Photoexcited Carriers in ZnO/Zn2SnO4 Type-II Radial Heterostructure Nanowires

Type-II band alignment of heterostructure contributes to spatially separate electrons and holes leading to an increase in minority carrier lifetime, which has much more advantages in photocatalytic activities and photovoltaic device applications. Here, Zn2SnO4-sheathed ZnO radial heterostructure nanowires were constructed to investigate systematically interfacial charge separation. The lattice mismatch between ZnO and Zn2SnO4 induces interface states to exist at their heterointerface. At low pump fluence, photoexcited charges are localized within the ZnO core rather than separated due to the large interface barrier. Correspondingly, only ZnO-related bandedge ultraviolet (UV) and green emissions are dominated in photoluminescence spectra. At high pump fluence, however, impurities are ionized and electrons trapped in interface states are excited, resulting in a decrease in interface barrier, which makes photogenerated charges efficiently separated at their heterointerface by direct tunneling, and, consequently, an additional blue-violet emission, attributed to the heterointerface recombination of electrons in Zn2SnO4 conduction band (CB) and holes in ZnO valence band. Additionally, the heterointerface can separate effectively photoexcited carriers and form a photovoltaic effect. Our results provide the localization/separation condition of photogenerated charges for the type-II band alignment of core/shell heterostructure, which should be very useful for the realization of underpinned mechanism of the developed optoelectronic devices.

Glass-like recovery of antiferromagnetic spin ordering in a photo-excited manganite Pr0.7Ca0.3MnO3

Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward undertsanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr0.7Ca0.3MnO3 following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature.

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.

Observation of intra- and inter-band transitions in the transient optical response of graphene

The transient optical conductivity of freely suspended graphene was examined by femtosecond time-resolved spectroscopy using pump excitation at 400 nm and probe radiation at 800 nm. The optical conductivity (or, equivalently, absorption) changes abruptly upon excitation and subsequently relaxes to its initial value on the time scale of 1 ps. The form of the induced change in the optical conductivity varies strongly with excitation conditions, exhibiting a crossover from enhanced to decreased optical conductivity with increasing pump fluence. We describe the graphene response in terms of transient heating of the electrons, with the characteristic relaxation time of the transient conductivity reflecting the cooling of the electron system and the strongly coupled optical phonons through emission of lower energy phonons. The change in the optical conductivity is attributed to a combination of induced absorption from intra-band transitions of the photo-generated carriers and bleaching of the inter-band transitions by Pauli blocking. The former effect, which corresponds to the high-frequency wing of the Drude response, dominates at low pump fluence. In this regime of a limited rise in the electron temperature, an increase in the optical conductivity is observed. At high pump fluence, elevated electron temperatures are achieved. The decrease in the inter-band bleaching then dominates the transient response, the intra-band contribution being overwhelmed despite an increase in the Drude scattering rate with temperature. The temporal evolution of the optical conductivity in all the regimes can be described within a model including the intra- and inter-band contributions with a time-varying electronic temperature. An increased Drude scattering rate is inferred for high electron temperature and mechanisms for this enhancement are considered. The calculated scattering rate for interactions of the carriers with zone-center and zone-edge optical phonons agrees well with the rates obtained from experiment.

Absorption saturation in optically excited graphene

We investigate the saturation of the optical absorption in graphene induced by ultrafast optical pulses. Within a microscopic theory, we study the momentum-, angle-, and time-resolved interplay of anisotropic excitation, carrier-carrier, and carrier-phonon scattering, and its influence on the saturation of absorption and transmission. In agreement with performed experiments, we observe a linear regime for the intensity-dependence of the transmission at low pump fluences and a nonlinear saturation in the high excitation regime. Applying 10 fs-pulses, we obtain a saturation fluence of approximately 0.65 mJ/cm2. We demonstrate how the interplay of Pauli-blocking and intensity-dependent relaxation determines the saturation behavior.

Impact of Auger processes on carrier dynamics in graphene

Auger processes are of great importance for both fundamental research and technological applications, since they change the number of charge carriers in a system. Here, we present a microscopic study on the influence of Auger relaxation channels on the carrier dynamics in graphene. The presented time-, momentum-, and angle-resolved calculations reveal the importance of the impact excitation giving rise to a significant multiplication of optically excited carriers and a remarkable Coulomb-induced carrier cooling effect. We propose low pump fluence, high excitation energy, and low ambient temperature as optimal conditions for an efficient carrier multiplication reaching values of up to $2.5$ even in the presence of phonons. The optimal regime is confirmed by an analytic expression for the carrier multiplication, which gives further insights into the role of Auger processes for a Dirac-like carrier system. Our results can help to guide future experiments to demonstrate the carrier multiplication in graphene and related structures.

Precessional switching by ultrashort pulse laser: Beyond room temperature ferromagnetic resonance limit

Conventional pure magnetic recording schemes have a serious and unavoidable problem known as the ferromagnetic resonance (FMR) limit. Here we demonstrate a fast precessional switching via interplay of ultrafast heating by ultrashort pulse laser and large temperature dependency of magnetic resonance. In ferrimagnetic GdFeCo, when the temperature approaches the angular momentum compensation point TA, both frequency of FMR mode and the Gilbert damping parameter increase significantly. High-speed and strongly damped precessional switching was triggered with ultrafast heating of a Gd24.5Fe66.1Co9.4 across its magnetization compensation point TM up to TA, under a static applied magnetic field. To initiate and investigate the magnetization reversal, we have used an all-optical pump-probe technique employing an amplified Ti:sapphire laser system with 90 fs pulses. In particular, following the laser excitation with low pump fluence (0.9 mJ/cm2) the magnetic system relaxes back toward initial state while the rather high pump fluence (3.3 mJ/cm2) excitation induces a metastable opposite magnetic state. After the sudden heating which causes just 30% reduction of magnetization at first breakdown, consequently magnetization was started to rotate across Mz = 0 within 6 ps during first precession and finished the high damped precessional motion within few cycles into opposite direction.