Floquet–Bloch states are known to form in solids under periodic driving of electrons. We visualize their ultrafast build-up with angle-resolved photoemission spectroscopy and subcycle time resolution. We drive the Dirac surface state of a topological insulator with mid-infrared fields in the MV/cm range. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle. Intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics.

In this work, we systematically investigate ultrafast magnetization dynamics and spin wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a function of layer number, temperature, and magnetic field. We find laser-induced (de)magnetization processes can be used to accurately track the distinct magnetic states in different magnetic field regimes, including showing clear odd-even layer number effects. In addition, strongly field-dependent antiferromagnetic magnon modes with tens of gigahertz frequencies are optically generated and directly observed in the time domain. Remarkably, we find that magnetization and magnon dynamics can be observed in not only the time-resolved magneto-optical Kerr effect but also the time resolved reflectivity, indicating strong correlation between the magnetic state and electronic structure. These measurements present the first comprehensive overview of ultrafast spin dynamics in this novel 2D antiferromagnet, paving the way for potential applications in 2D antiferromagnetic spintronics and magnonics as well as further studies of ultrafast control of both magnetization and topological quantum states.

We combine monolayer TMD with chiral lead halide perovskites to achieve enhanced valley polarization via chirality induced spin selective charge transfer and to demonstrate the utility of chiral perovskites as spin filters, preferentially injecting charges with one spin, depending on the handedness of the perovskite material. This elegant, yet simple way to control valley polarization does not require circularly polarized light, and can be realized at room temperature, without a need for cryogenic temperatures, usually implying expensive, bulky, complex setups. Achieving robust valley polarization in TMDs at room temperature is of great importance for practical applications of TMD based valleytronics devices in quantum information science and technology.

A novel approach of characterizing nano-scaled luminescent materials using time-resolved X-ray excited optical luminescence (TR-XEOL) is presented. With the unique possibility to combine nanoscale spatial and pico-second temporal resolution, we investigate perovskite nanowires with exceptional precision. Integrated X-ray fluorescence spectroscopy in combination with TR-XEOL enables correlating carrier dynamics with material composition at the nanoscale. In this study, we focus on CsPbBr3 nanowires, potential X-ray scintillators, studying the influence of degradation effects on the recombination dynamics. This new experimental approach holds promise for advancing optoelectronic and, especially, nano-photonic materials.

In my talk, I will show how the dynamics of coherent superposition of valence electron states generated by < 5 femtosecond long carrier-envelope-phase (CEP) stable laser pulses, can be locally probed with angstrom-scale spatial resolution and 300 attosecond temporal resolution simultaneously, at the single orbital-level with the help of an STM, defying the previously established fundamental space-time limits. Electronic motion in molecules is usually coupled with atomic motion, especially in molecules undergoing photo-induced charge/energy transfer, structural or chemical transformation. In order to understand this coupling, we have recently realized femtosecond broadband coherent anti-Stokes Raman spectroscopy (CARS) in an STM and it has enabled tracking of coherent atomic motions in a single graphene nanoribbon with sub-angstrom scale spatial, meV energy and ~30 fs temporal resolution, simultaneously. Time-resolved CARS implemented in an STM is the key to probing both electronic and atomic motion at the same time.

The probing of coherent lattice vibrations in solids has been conventionally carried out using time resolved transient or reflection spectroscopy where only the relative oscillation amplitude can be obtained. Here we demonstrate that, using standard optical nonlinear spectroscopy, electron-phonon and anharmonic phonon-phonon coupling and their relaxation dynamics can be probed in real time. In combination with optical deformable potential calculated from ab-initio density functional theory, our measurement with absolute amplitude information reveals the maximum, controllable displacement of neighbouring oxygen atoms in alpha-quartz crystal to tens of picometers in real space.

Materials having tailor-made electronic and phononic properties are vital for energy conversion, thermal management and novel sub-THz applications. To this end, we have demonstrated phonon cavities designed to operate at 96GHz with parabolic, quartic and linear 1D phonon potentials using GaAs/AlAs superlattices. Ultrafast vibrational spectroscopy shows long lived coherent acoustic phonon modes confined by the cavity at room temperature. Moreover, time resolved photoluminescence characterization showed a slow carrier energy loss rate compared to a DBR sample fabricated to exhibit linear phonon dispersion without confinement. These results can help develop new device architectures for applications where phonons either need to be confined or efficiently guided.

Excitations in spatially indirect transitions feature such as excitons in type-II heterostructures or charge-transfer excitations in molecular crystals feature a permanent dipole moment which influences the transition dipole moments commonly probed by optical spectroscopy. Epitaxially grown III-V quantum heterostructures featuring suitable band alignments are ideal model systems to study the interplay between the two. Selected topics discussed include the AC Stark effect and signatures of spatially indirect coherent biexciton states as well as the potential for coherent optical current injection dynamics.

In this study, we place a strong emphasis on understanding the ultrafast dynamics of carrier recombination pathways in p-type ZnO, especially in the midgap region. Synthesizing and controlling the properties of p-type ZnO remains a pivotal yet challenging task for numerous optoelectronic and spintronic applications due to intrinsic midgap (defect) states. Through an advanced sol-gel process, we have successfully produced ZnO quantum dots (QD), eliminating unreacted molecules that decrease the excitonic emission. This refined method supports the generation of ZnO with p-type characteristics, primarily attributed to zinc vacancies in oxygen-rich scenarios. Notably, our analysis across timescales from femtoseconds to microseconds unveiled carrier lifetimes at room temperature, and associated long-lasting carriers with zinc vacancy defects, corroborating the p-type nature of our synthesized ZnO QDs.

The study of strong field-driven nonlinear responses from high harmonic generation (HHG) in two dimension (2D) materials has been recognized as a valuable tool for probing ultrafast electron dynamics in the condensed matter systems. The laser polarization dependence of HHG in solids provides significant scientific insights into the crystal and band structure of the generation medium. In this work we experimentally investigate the impact of HHG in MoS2 monolayer by changing the laser polarization from linear to circular for different orientations relative to the symmetry axes of the crystal. The polarization analysis of harmonic emission reveals ellipticity and helicity dependent enhancement of even order harmonics which is strongly asymmetric near the symmetry directions of the crystal.

Van der Waals (vdW) materials hold immense promise for new nanotechnologies. However, more insight into their behavior under non-equilibrium conditions on femtosecond time scales and their support of exotic nonlinear processes is needed. In this talk, we will first discuss our recent studies of ultrafast lattice and spin dynamics in vdW magnets. Here, we uncover unusually strong transient spin-phonon coupling, manifesting as pump helicity-dependencies and dynamic magnetostriction-driven stiffening of coherent phonon modes. In both these cases, these phenomena originate from the pronounced modulation of exchange interactions through the distortion of the lattice. We will then turn to nonlinear optical processes, namely harmonic conversion and optical parametric amplification driven by twisted light. Here, we show both the efficient harmonic scaling of orbital angular momentum and the free tuning of wavelength and topological charge of vortex beams, both supported by atomically thin semiconductors. Our work points to new ways to exploit femtosecond and structured illumination to extend the versatility of vdW materials in the pursuit of new spintronic and optoelectronic applications.

In this study, we present the control over second-harmonic generation (SHG) in MoS2 with single-cell bacteria. SHG signal changes in response to bacteria deposited on monolayer MoS2, resulting in anisotropic polarization responses as a function of bacteria orientation relative to MoS2 crystal axis; the anisotropic enhancement originated from bacteria tensile stress. By monitoring the UV-induced changes in SHG signals, we determine the rupture and ablation times for individual bacteria. The rupture time differs noticeably between species, which enables us to distinguish individual bacteria sharing the same habitat, such as E. coli and L. casei.

Transient absorption (TA) spectroscopy gives dynamical information about excited states and spectral information about their excitations. If the pump pulses are strong, then multiple excitations can be produced, and the signal has contributions from single excitations mixed with those from multiple excitations. I will describe a new technique in which TA spectra are acquired at several pump intensities, enabling extraction of high signal-to-noise TA spectra and systematically separated high-order spectra [1,2]. I will show the spectral and dynamical information in high-order spectra. The higher orders contain information both about multiply excited states and singly excited states that are usually dark. I will give an intuitive taxonomy of the response pathways that characterize these high-order signals, extending the standard TA pathways -- stimulated emission, excited-state absorption, and ground-state bleach -- to higher orders. I will show examples from several molecular and solid-state systems. [1] Malý, Lüttig, Rose, Turkin, Lambert, Krich, and Brixner, Nature 616 280 (2023) [2] Lüttig, Rose, Malý, Turkin, Bühler, Lambert, Krich, and Brixner, J Chem Phys 158 234201 (2023)

A 15-laboratory round-robin inter-comparison of ultrafast Terahertz and Microwave time-domain measurements will be presented. Detailed time-to-frequency domain spectral analyses to extract photo-generated carrier conductivity/mobility of high-mobility Perovskite film samples were examined. Experiments comparing non-contact Terahertz to Hall conductivity measurements conducted at NIST for bulk semiconductors (Float Zone Si, n/p-doped Si, GaAs, ZnTe, GaP, etc.) will be briefly shown. An overview of ultrafast terahertz studies of conducting polymers, 2D monolayer and exciton “multiplication” in multilayer transition metal dichalcogenide films (MoTe2, MoS2) will be given.

In this study, with the use of in-situ ultrafast optical detection and sub-picosecond laser systems, we resolve the influence of the various energetic species in an atmospheric plasma on the resulting electronic and career response of semiconductors in real time. Through the development of new plasma diagnostics with sub-picosecond to microsecond temporal resolution, we measure the optical response of material surfaces subjected to various types, intensities, and temporal profiles of atmospheric pressure plasma excitations.

The epsilon-near-zero (ENZ) materials have been shown to be as one of the most promising optical materials in recent years. This talk will review our recent studies on the active linear and nonlinear properties of conducting oxide epsilon-near-zero materials in planar and optical fiber platforms. I will discuss the observation of abnormal nonlinear temporal dynamics of hot electrons and enhanced optical nonlinearity in AZO and ITO ENZ thin films under different pump fluences and excitation angles using a degenerate pump-probe spectroscopy technique. I will also discuss the enhanced nonlinear absorption mechanism of ITO near the ENZ region, where the nonlinear absorption is dominated by (i) saturable absorption caused by the conduction band electrons and (ii) three- photon absorption caused by valance band electrons. These studies enrich the fundamental understanding of nonlinear properties on ENZ thin films and will open the path to revolutionary ultra-compact nonlinear optical devices.

THz coherent acoustic phonons on a scale of sub-picoseconds in temporal-periodicities and nanometers in wavelengths are promising in various fields such as nanometrology/nanoimaging, THz devices and heat management. By merging advanced ultrafast laser spectroscopy and sophisticated nanotechnologies, the excitation and detection of bulk coherent acoustic phonons in the THz regime have been accomplished. Nevertheless, coherent surface acoustic waves (SAWs) realized by metallic gratings deposited on a substrate, are still below 100 GHz. In this report, we take advantage of the cleaved superlattice (SL) surface with immediately reachable nanometer periodicity and atomic-level interface quality to monitor SAWs by femtosecond lasers. Rayleigh SAWs above 100 GHz and bulk surface skimming acoustic waves up to 1THz, with deeply sub-optical-wavelength periodicities, are observed on cleaved Al_0.3Ga_0.7As/GaAs SLs and cleaved In_0.2Ga_0.8N/GaN SLs, respectively. The observations open a path towards THz opto-acoustic/acousto-optic transducers.