This PDF file contains the front matter associated with SPIE Proceedings Volume 11278, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Interlayer coupling in van der Waals heterostructures exhibits unique physical properties and presents a new pathway for diverse applications in materials science and photonics. Interestingly, photo-responsivity of graphene with semiconducting transition metal dichalcogenide (TMD) can be enhanced in the UV-visible range by interlayer charge/energy transfer. However, such an effect has been studied only for semiconducting materials. In this work, we will discuss ultrafast hot-electron transfer in atomically thin metallic-layer VSe2/graphene heterostructures investigated by time-resolved pump-probe spectroscopy. Our experimental finding extends the concepts of conventional two-dimensional heterostructures to metallic-layer and can provide a new degree of freedom for designing novel functionalized materials and devices.

Transition metal dichalcogenides (TMDs) have attracted much interest in recent years due to their emerging material properties. In monolayer TMDs, such as MoS₂, extreme quantum confinement is achieved in the monolayer limit. Although monolayer TMDs represent an ideal platform to explore excitonic physics using ultrafast spectroscopy, this exploration is currently limited by confusion regarding the origin of certain spectral features, including the below-bandgap PIA feature observed in pump-probe experiments. In this work, we document an absence of PIA features immediately after photoexcitation, indicating a lack of strong optically-induced biexciton formation. Below-bandgap PIA features are observed to grow in with a time constant of 110 ± 10 fs, indicative of other factors responsible for their origin. These results indicate that optically-induced biexciton formation is most likely not responsible for the previously observed PIA features in MoS₂ monolayers.

In this work, we demonstrated that metamaterials composed of alternating silver and silica layers of subwavelength thickness can act as an effective epsilon-near-zero (ENZ) medium. This ENZ condition is accessible to wide spectral tunability in the entire visible spectrum by adjusting the respective dimensions of constituent metal and dielectric. We also observed a pronounced enhancement of nonlinear response at the ENZ region. The capability to obtain large nonlinearities at pre-assigned optical wavelengths makes this class of metamaterials a flexible platform for nonlinear applications in the visible spectrum.

Dumbbell-shaped metal complexes have received great attention in material science and chemistry because of their high ISC quantum yield and relatively long-lived triplet excited states. However, the working mechanism of the “weight” and “bar” moieties are unclear and vary with structure. In this paper, a novel dumbbell-shaped platinum complex (weight) connecting with TPE (bar) was synthesized and studied by employing time-resolved spectroscopy methods. In this system, the platinum complex, is designed to interfere with the rotation of TPE by Pt•••Pt and π−π stacking interactions, and these interactions also serve as a spin convertor to facilitate the ISC process. A N^C^N Pt^II-TPE decolorized triplet excited state was successfully detected by transient absorption spectroscopy for the first time and a room-temperature phosphorescence with a lifetime around 1404 ns was also observed.

The ultrafast dynamics of non-equilibrium photoexcitations govern the initial energy and charge transport in photovoltaic (PV) materials. Hence, they may strongly influence device operation. Such ultrafast dynamics depend, in turn, on the details of the electronic system and its coupling to the nuclear degrees of freedom. Here we discuss a few examples showing how ultrafast two-dimensional electronic spectroscopy can provide detailed new insight into light-induced charge transfer processes in technologically-relevant PV materials. Specifically, we discuss the role of vibronic couplings and conical intersections for the light-initiated intermolecular dynamics in efficient organic PV thin films, and ultrafast charge carrier relaxation and exciton dynamics in inorganic halide perovskites crystals. Our results suggest potential new strategies to control coherent energy and charge transport in PV materials by tailoring electron-phonon couplings.

Weyl semimetals have been the focus of intense experimental and theoretical investigation, due to their broad appeal in fundamental science and applied technology alike. More recently, several studies have centered on the nonlinear optical properties of these materials, where it is believed that characteristic features of Weyl physics can be observed. To date, these studies have been limited to static or quasi-static measurements, but new and important insights can come about through extending these nonlinear optical spectroscopies into the time domain. To do so, we use terahertz (THz) emission spectroscopy and time-resolved second harmonic generation (TR-SHG) spectroscopy to provide a contact free measure of ultrafast photocurrent dynamics in the transition metal monopnictide family of type-I Weyl semimetals. On the basis of our data, we are able to clearly distinguish between helicity-dependent photocurrents generated within the ab-plane and polarization-independent photocurrents flowing along the non-centrosymmetric c-axis. Such findings are consistent with earlier static photocurrent experiments, and demonstrate on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry. Such generality in the microscopic origin of photocurrent generation in the transition metal monopnictide family of Weyl semimetals makes these materials promising candidates as next generation sources or detectors in the mid-IR and THz frequency ranges.

A gauge-invariant density matrix approach is presented to describe the non-equilibrium dynamics of multiband superconductors after photo-excitation. The derived gauge-invariant Bloch equations extend the Anderson pseudo-spin precession model by fully incorporating the center-of-mass motion of Cooper pairs. We also describe lightwave propagation effects inside a superconducting film by including the self-consistent interaction of the photo-excited superconducting system with the propagating electromagnetic field inside the superconductor using Maxwell's equations.

We use time-resolved THz spectroscopy to study microscopic conductivity and photoinduced carrier dynamics in MBE-grown 100 nm thick (Bi_1-xIn_x)2Se₃ thin films with indium concentration varying from x=0 to x=0.5. Both intrinsic and photoinduced conductivity in Bi2Se3 is significantly higher compared to the films with x=0.25 and x=0.50, with carriers that are not constrained by the twin domain boundaries and exhibit high mobility of 1100 cm²/Vs. We find that introducing indium with concentration of x=0.25 and higher, above the threshold for a topological to trivial transition, suppresses both intrinsic and photoinduced conductivity by over an order of magnitude and reduces the lifetime of photoexcited carriers. These findings demonstrate that controlling indium concentration in (Bi_1-xIn_x)₂Se₃ films provides an avenue to design (Bi_{1- x}In_x)₂Se₃ films with desired properties for high-speed optoelectronic devices.

Germanium sulfide (GeS) is a 2D semiconductor with high carrier mobility and a moderate band gap (~1.5 eV for multilayer crystals), which holds promise for high-speed optoelectronics and energy conversion. Here, we use time resolved THz spectroscopy to investigate how intercalation of Au, Cu, and Sn impacts the photoexcited carrier dynamics and transient photoconductivity of GeS nanoribbons. We find that zero-valent metals affect the photoexcited carrier lifetime and mobility in different ways. Intercalation of GeS with Cu reduces the lifetime of carriers from ~ 120 ps to 60 ps, while Au and Sn intercalation do not. At the same time, intercalation with Cu, Sn and Au significantly enhances the scattering time of photoexcited carriers (~120 fs vs ~65 fs without intercalation), highlighting the potential of zero-valent metal intercalation as a tool for engineering the optoelectronic properties of GeS nanostructures for application in high-speed electronic devices.

High harmonic generation in solids attracted great attentions as a new scheme for frequency conversion. We report observation of an extremely efficient terahertz (THz) third- and fifth-harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal with massless electron dispersion, as it is observable with tabletop THz source at room temperature. Our THz pump-THz probe study with subcycle time resolution elucidates that the intraband current of coherently accelerated Dirac electrons is the main source of the THz harmonics as expected theoretically. The results pave the way toward novel devices for ultrafast THz electronics and photonics based on topological semimetals.

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. These resonant nonlinearities continually attract attention, particularly in newly discovered materials. However, they are frequently not as heightened as currently predicted, limiting their exploitation in nanostructured nonlinear optics. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive, as well as constructive, interference effects in complex systems. Using terahertz quantum-cascade-lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an unexpected interplay of the second-order nonlinear contributions of multiple light and heavy hole states.

In semiconductors and semimetals, strong THz electric fields can induce a controlled coherent motion of the electrons in the conduction band, via ballistic excitation. In the first picoseconds after THz excitation, the nonlinearities induced by this coherent excitation prevail before more incoherent high field effects start dominating the nonlinear response. Disentangling these different nonlinear contributions with 2D THz spectroscopy, we follow the trajectory of the out-of-equilibrium electron population in low-bandgap semiconductor InSb and. We then extract information on the conduction band curvature and evaluate its anharmonicity and its anisotropy, close to the Gamma-point.

Complex oxide thin films, such as La_0.7Sr_0.3MnO₃ (LSMO), are widely studied for a variety of applications. Transient reflectivity (TR) measurements on LSMO indicate enhanced surface recombination of charge carriers in films less than 20 nm in thickness. Oxygen growth pressure variation illustrates that higher oxygen pressures provide more electron dominance in the system, and producing a larger band filling effect which eventually results in higher excitations. Wavelet analysis can distinguish abrupt oscillatory modes with close energy ranges and have been introduced as a method to study sound velocities in ultra-thin films.

Negative electron affinity (NEA) GaAs photocathodes have attracted a wide scope of interest because of their high quantum efficiency and low dark emission. Traditionally, fabrication of GaAs photocathodes has taken two approaches: molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). Understanding the difference between these two methods in terms of device performance can help guide future device development. While past research has indicated that photocathodes grown by MOCVD generally have better spectral response and quantum efficiency, these reports are all based on steady-state analysis and measurement. There has been little prior work comparing the dynamic response of devices fabricated with different technologies. In this presentation, we report a comparative study of the ultrafast response of two gradient-doped GaAs photocathodes fabricated using two different methods, viz. MBE and MOCVD. Our approach is based on femtosecond pump-probe reflectometry (PPR), which measures the transient reflectivity of these devices upon optical excitation by femtosecond pulses. Preliminary PPR result shows that carrier build-up near photocathode surface in the MOCVD device is more efficient compared to the MBE device. A carrier-diffusion model is used to analyze photoelectron transport, accumulation, and decay in the active layer. Experiment-theory comparisons indicate a bi-exponential nature of free-electron population decay near device surface. Excellent agreement between theoretical predictions and measured data not only validates the numerical model but also allows various device parameters to be evaluated quantitatively.

We use k•p perturbation theory as an input for simulations of nonlinear optical properties. A numerical solution of a 30-band k•p-model for bulk gallium arsenide yields both the band structure and complex dipole matrix elements. However, the matrix elements have undesirable features which make them not directly suitable for describing the nonlinear response for far-below resonance excitation frequencies. Besides a random phase originating from numerical diagonalizations, we trace this back to a numerical mixing of near-degenerate bands. As an attempt to resolve this problem we introduce a basis transformation which leads to smooth matrix elements. The material input is used in the semiconductor Bloch equations including the inter- and intraband parts of the light-matter interaction. As an example we evaluate non-degenerate two-photon absorption and compare our results with simpler models.

Multidimensional coherent spectroscopy measures the third-order polarization response of a system to reveal microscopic electronic and many-body phenomena. Applied to semiconductor nanostructures, it can distinguish homogeneous and inhomogeneous broadening due to disorder or strain gradients, resolve coupling between transitions, and optically access transitions that are either non-radiating or outside the bandwidth of the pulses. Two tools often exploited in this versatile technique are (i) the ability to control the polarization of the excitation and emission and thus the optical selection rules, and (ii) the ability to capture the complex spectrum. Here, the polarization of pulses emerging from a multidimensional optical nonlinear spectrometer (MONSTR) and the resulting four-wave mixing emission are controlled automatically using variable retarders, such that multiple spectra are recorded during a single phase-stabilized scan. This improves the acquisition time by ~3x compared to running separate polarization scans. Importantly, only one phase ambiguity exists in the complex spectra across all sets of polarization states measured. This single ambiguity is resolved by comparing the initial spectrally resolved transient absorption to the complex four-wave mixing spectrum for collinear polarization and then applying it to all spectra. Here, the method is applied to a quantum well embedded in a semiconductor microcavity with an adjustable cavity-exciton detuning. The complex 2DCS spectra we report constitute the first measurements of detuning- and polarization-dependent exciton-polariton lineshape across the strong coupling regime.

We demonstrate ultrafast superradiance showing 10-fs decay due to the nonlocal interaction between light waves and exciton waves in CuCl mesoscopic thin films. This time constant is comparable with the thermal dephasing even at room temperature. Actually, we observed photoluminescence signal from one confined mode of translational motion of the exciton surviving at room temperature. In addition, temperature dependence of the photoluminescence spectra with multiple peaks was reproduced well by theoretical calculation. This finding opens a new avenue of “thermal free photonics”, where the exciton coherence is utilized at higher temperature.

We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.

This work further develops a recently proposed time-resolved inline digital holography (TRIDH) [Petrov, N. V. et al. Opt. Lett. 43, 3481 (2018)] for studying degenerate phase modulation induced by an inclined collimated pump beam in the glass substrate with the quantum dots at the surface. Similar to many techniques for measuring nonlinear properties of materials, it is based on a comparison of the prediction obtained by the mathematical model of the phenomenon with experimental data. We have extended the mathematical model for the case of interaction of two femtosecond laser pulses in the double-layered sample. The impact of the ratio between nonlinear refractive indexes of two layers and their thicknesses on induced phase modulation is analyzed.

To disentangle the ultrafast dynamics of individual system constituents at the nanoscale we combine photoemission electron microscopy, providing a resolution of ~3 nm, and a 1 MHz NOPA (20 fs pulse duration, tunable from 215 to 970 nm). Specifically, I will focus on a two-pulse experiment in which we detect subtle differences in the local field evolution of individual hot-spots within a plasmonic nanoslit resonator. By using the concept of quasinormal modes we explain these local differences by crosstalk of adjacent resonator modes.[1] Moreover, I will present first results about selecting specific excitation pathways, described by double-sided Feynman diagrams, within single systems using phase-cycled three-pulse sequences, which are provided by a home-built LCD pulse shaper. We aim at adding nanoscale resolution to the well-established method of coherent multidimensional spectroscopy. [1] M. Hensen et al., Nano Lett. 19, 4651-4658 (2019)

This talk will describe the recent progress in plasmon drag effect studies. Recently, we have predicted that illumination of metals by light induces a new force on electrons – which we called the spin force. This force should result in pinning of the net plasmon-induced electromotive force to an atomically thin layer at the metal interface [Durach, Noginova, Phys. Rev. B 96, 195411 (2017)]. Consequently, the plasmonic group at NIST conducted experiments that show that plasmon drag effect (light-induced current) is different in ambient air and in 10-3 Pa vacuum and depends on the molecules adsorbed at the metal interface [Strait et al, arXiv:1812.01673 (2018)], confirming our prediction about the plasmon drag pinning to the atomic layer at the metal interface. Thus our prediction of plasmon drag pinning paves way for a multitude of applications in physics, chemistry and surface science.

We describe the generation of second harmonic light (525 nm) from femtosecond near-infrared (NIR, 1050 nm) pulses from three nanoparticle films: gold nanospheres, hexagonal nanodisks of covellite (CuS), and a bilayer comprising covellite and gold films. Enhanced second-harmonic generation (SHG), fourth order in the NIR pump intensity, arises from coupling of plasmon resonance modes of CuS and Au. Above 6 GW/cm2, the enhanced SHG from the bilayer film is much larger than the incoherent sum of SHG from the Au and CuS films separately, and the SHG efficiency of the bilayer (Au:CuS) films is nine times larger than that of BBO per unit thickness.

Few studies in literature have try to quantitatively compare the 2T model with experimental data at very short time scale and study the dependency and importance of model’s parameters. We have developed a new numerical model and fitting algorithm which combined the 2T model, thermal conduction and 3D FEM EM code to link the spatial distribution of temperature to the optical reflectivity of the sample. Using a state-of-the-art pump-probe setup allowing the acquisition of the full spectro-temporal optical response of the sample, we have compared measurements on thin gold films and array of nanostructures with our model.

We study optical interactions between nano-scaled cavities consist of metal-insulator-metal structures (MIM nano-cavity) with femtosecond surface plasmon polariton (SPP) wavepackets in terms of numerical simulations and time-resolved microscopy methods. Finite-difference time-domain (FDTD) simulations show that when the eigen-frequency of the nanocavity falls within the spectral width of the SPP wave, the incident wavepacket causes resonant excitations of the cavity. The resonance effect of the SPP-cavity interaction is reflected as strong modulations in the spectra of transmitted and reflected wavepackets. Because the MIM nanocavity separates the wavepacket into resonance frequencies as the transmission and others as the reflection, both the envelope and the spectral shapes of the transmitted/reflected components are largely deformed. Particularly, when the spectrum width of the incident wave was equivalent to or narrower than the resonance linewidth of the cavity, the whole intensity of the transmitted wavepacket varied largely. The time-resolved imaging by using a 10 fs laser pulse, of which spectral width is comparable to the linewidth of cavity resonance, shows reasonable agreement with the simulation. In the case the length of the MIM nano-cavity is chosen so that the eigen-modes do not have spectral overlap with that of the 10 fs laser, the intensity of the transmitted wavepackets showed a considerable attenuation. These features can be interpreted in terms of the functionality of the MIM cavity as a Fabry–Pérot etalon.

In this talk, first, we describe chip-scale coherent mode-locking in microresonator frequency combs, verified by interferometric femtosecond timing jitter measurements and phase-resolved ultrafast spectroscopy. Normal dispersion sub-100-fs mode-locking is also observed, supporting by nonlinear modeling and analytics. Second we describe the noise limits in full microcomb stabilization, locking down both repetition rate and one comb line against a reference. Active stabilization improves the long-term stability to an instrument-limited residual instability of 3.6 mHz per root tau and a tooth-to-tooth relative frequency uncertainty down to 50 mHz and 2.7×10−16. Third we describe graphene-silicon nitride hybrid microresonators for tunable frequency modes, variants of soliton mode-locked states and crystals, and controllable Cerenkov radiation. Our studies provide a platform towards precision spectroscopy, frequency metrology, timing clocks, and coherent communications.

With its direct correspondence to the electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a ubiquitous tool for the study of solids. When extended to the temporal domain, time-resolved (TR)-ARPES offers the potential to move beyond equilibrium properties, exploring both the unoccupied electronic structure as well as its dynamical response under ultrafast perturbation. By performing high-harmonic generation inside a femtosecond enhancement cavity (fsEC), we realized a practical source for TR-ARPES that achieves a flux of over 10^11 photons/s delivered to the sample, and operates over a range of 8-40 eV with a repetition rate of 60 MHz. This source enables TR-ARPES studies with a temporal and energy resolution of 190 fs and 22 meV, respectively. To showcase the capabilities of this setup, we develop a novel approach to determine the mode-projected electron-phonon matrix element in graphite, with unprecedented sensitivity, directly in the time domain.

Ultrashort pulse light beam is an important tool in many areas of science and technology. It is used to study ultrafast phenomena to new imaging modalities. Because of its ultra-broad frequency bandwidth, it can be used to control, manipulate, and characterize light–matter interactions. Over the recent years, we have played with non-paraxial beams and short pulses new imaging modalities. However, we also found the ultrashort optical pulse of a fraction or few optical cycles may change many well-known assumptions in optical metrology. Here a few questions: • How the ultrashort pulse affects the diffraction point spread function? • Does ultrashort pulse interferometry to test optical component is possible? • How the far field and near field diffraction are affected by ultrashort pulse duration? • How vectorial beams are affected under short pulse and non-paraxial conditions? In this talk, we will try to introduce how ultrashort pulse may open a new area in optical metrology.

Ultrafast dynamics experiments in physics, chemistry and biology, in condensed matter or gas phase require different types of lasers. We present various OPCPAs as a powerful toolbox to deliver femtosecond pulses from the XUV to THz spectral region. A synchronized pump-probe laser has been realized, generating visible and near-IR pump pulses (350-1300 nm) and UV pulses (250 - 350 nm) operating at 4 MHz. Furthermore, a 1.55 μm wavelength OPCPA has been demonstrated for efficient THz-generation. For XUV generation in the water window, an OPCPA is presented, delivering millijoule-level, femtosecond pulses from 1.7 - 2.2 μm with up to 100 W average power.

We use ultrafast spectroscopy to resolve the interlayer charge scattering processes in heterostructures (HS) of two-dimensional materials. In a WSe2/MoSe2 HS we photogenerate intralayer excitons in MoSe2 and observe hole injection in WSe2 on the sub-picosecond timescale, leading to the formation of interlayer excitons. The temperature dependence of the build-up and decay of interlayer excitons provide insights into the coupling mechanisms. In a graphene/WS2 HS we observe ultrafast charge transfer from graphene to the semiconductor. The data are consistent with hot electron/hole transfer, whereby a tail the hot Fermi-Dirac carrier distribution in graphene tunnels through the Schottky barrier into WS2.

We investigate dynamics of resonantly excited excitons in single-layers of MoSe₂ and WS₂ down to 4.5K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (F M) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS₂ with respect to MoSe₂, which is attributed to the dark character of exciton ground state in the former and the bright in the latter.

Monolayer transition metal dichalcogenides present the opportunity to manipulate quantum states of matter via ultrafast laser pulses, permitting, for example, observation of the phase transition from trivial to topological insulator. However, the nature of ultrafast pulses are far removed from the theoretical assumptions used to model these phenomena. We have used ultrafast pump-probe spectroscopy to observe the influence of an ultrashort laser pulse on the coherent response of monolayers of MoS2 and WS2 to investigate how an ultrashort pulse may cause the system to deviate from the behaviour expected from theoretical predictions.

Atomically thin transition metal dichalcogenides (TMDs) and halide perovskites have rapidly grown as promising materials for efficient optoelectronic devices. In both material classes, the dynamics of optical excitations and their interactions on ultrafast timescales are still debated. Using a newly developed theory, we discuss the interaction of TMD excitons and unbound charge carriers with optical phonons in the dielectric environment of the 2d layer. We find a significant reduction of exciton binding energies as well as linewidth broadening due to the dynamical coupling to environmental phonons. Moreover, we investigate near-band-edge optical transitions in CsPbBr3 single crystals at room temperature by combining ultrafast two-dimensional electronic spectroscopy and semiconductor Bloch equation calculations. An exciton binding energy of ~30 meV and remarkably short <30-fs carrier relaxation rates are extracted.

Hybrid organic-inorganic perovskites have been a focus of study due to the impressive performance of methylam- monium lead iodide (MAPI) in photovoltaic (PV) applications. These materials family also provide a new versatile platform for other optoelectronic applications such lasers. In this work we studied exciton dynamics that leads to lasing in various perovskite structures using time resolved absorption, THz conductivity and time resolved photoluminescence experiments. Hybrid perovskite thin films with different morphologies processed to exhibit polycrystalline morphologies with multiphasic nature. These studies reveal the exciton transfer dynamics into lower energy domains, and formation and recombination kinetics of the population inverted state.

La0.7Sr0.3MnO3 is a strongly correlated complex oxide. There is limited information about La0.7Sr0.3MnO3 thin films’ transient reflectivity (TR), where differences in ultra-fast dynamics due to surface and interface effects are expected. By decreasing the film thickness, additional energy states emerge, providing extra relaxation channels. Due to the reduced absorption in thin films, observing the effects of these extra states in TR signal is challenging, especially in a hole doped system such as La0.7Sr0.3MnO3. Moreover, in lower thicknesses, sinusoidal behavior superimposed on the TR signal is not analyzable by Fourier transforms. Wavelet transforms are perfect tools to analyze these fast-vanishing oscillations.

Solution-processed metal-halide perovskite solar cells (PSCs) have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency (PCE), which has surpassed 24% in a relative short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a pre-requisite to bring the efficiency closer to the Shockley-Queisser limit. Ultrafast transient absorption spectroscopy is a vital tool to study such a processes and specifically interfacial recombination can accessed through these measurements, and further in-sights into losses associated with the open circuit voltage Voc are gained. Transient spectroscopy techniques will be used to unravel the dynamics of processes limiting the photoluminescence quantum efficiency and thus the Voc. Employing both transient photoluminescence and transient absorption techniques, enables differentiation be-tween various recombination processes. Here we study the impact of the different hole transport layers, namely, PDPP-3T, NiO and PTAA hole transport layers and reveal the charge carrier recombination. We report the direct observation of hole extraction and carrier recombination dynamics of mixed-cation lead mixed-halide perovskite layers interfacing with a polymeric hole transport layer: PDPP-3T. The dynamics of the ground state bleach of the polymer, which directly reveals the hole extraction and re-combination at the perovskite/polymer interface. The perovskite hole mobility was found to be 3.08 cm2 V-1 s-1. To gain further insight into the hole extraction dynamics, we vary the thickness of the perovskite film. We observe that the hole extraction time is slower with increasing the perovskite thickness following optical excitation from the perovskite side. Mimicking the device architecture via introducing an electron transport layer to the perovskite/PDPP-3T stack resulted in slower carrier recombination dynamics due to decreased charge carrier recombination in the perovskite.

In this work, time-resolved microscopic photoluminescence (PL) spectra of self-assembled CdSe quantum dots (QDs) grown by Molecular Beam Epitaxy were investigated under excitation of femto-second laser. Interface-state-phonon (ISP) assisted carrier relaxation in self-assembled CdSe quantum dots are investigated by ultrafast time-resolved photoluminescence (PL). Electrons excited in barriers are found to relax into quantum dots and then have radiative recombination with holes by the mean of ISP assisted relaxation. Temperature dependence of rise time and decay time of time-resolved PL spectra are measured in the temperature range from 77 K to 286 K. The rise time decreases from 76 ps to 32 ps while the decay time first decreases then increases accordingly. The rise time shows exponential decay with the increasing of temperature. The thermal activation temperature for ISP process is deduced to be 184.9 K, corresponding to a thermal activation energy of 16 meV.