This PDF file contains the front matter associated with SPIE Proceedings Volume 12884, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Single or entangled photon states are vital for quantum communication. They can be generated on-demand by solid-state quantum emitters. Achieving high-fidelity photons depends on the excitation process. In addition to more established schemes, we present a radically different excitation scheme using two red-detuned laser pulses through the swing-up (SUPER) effect. The SUPER scheme is compared to other schemes like Rabi rotations, phonon-assisted preparation and adiabatic rapid passage. Experimental demonstrations of the different schemes are pointed out. With this, we highlight advancements in photon generation from solid-state quantum emitters.

The dynamics of semiconductor quantum wires and wells that are coupled to a single-mode quantum field are analyzed. Within a two-band tight-binding model the Coulomb interaction between electrons and holes is included on a microscopic basis and the light-matter interaction is quantized. The dynamics of the system is described by equations of motion for the relevant set of expectation values of the coupled electronic-photonic system. Starting from the initial condition of a single photon occupying the field mode, we study the dynamics of the mean photon number. To analyze effects arising from the many-body Coulomb interaction, we use an exact truncation of the electronic hierarchy problem by employing the fact that N photons cannot excite more than N electron-hole pairs. We compare Rabi oscillations with and without Coulomb interaction for different excitation conditions. When the quantum field mode is initially occupied by two photons, two interacting electron-hole pairs, i.e., biexcitons, can be generated which characteristically modify the dynamics. Within a consistent and fully-quantized approach we show consequences of biexcitonic many-body correlations that are coupled to a quantum field and discuss the obtained dynamics.

Controlling time-reversal symmetry on very short time scales may enable novel non-equilibrium phases of matter with broken, as well as ultrafast spintronics for energy-efficient information processing. Strongly driven phonons directly influence the interatomic distances, orbital symmetry, and exchange interactions that are closely related to magnetism in materials. Chiral phonons, where atoms rotation unidirectionally around the equilibrium position inside crystalline lattice, break time reversal symmetry and are expected to directly couple with magnetization. We excited chiral phonons using circular-polarized terahertz pulses and observed a prominent transient magneto-optic Kerr effect in cerium trifluoride, which was known to have unusually large spin-phonon coupling. The effective magnetic field needed to polarize the paramagnetic spins is on the order of 1 tesla under very moderate absorbed fluence of 0.2 mJ/cm². The temperature dependence of the spin dynamics indicates the magnetic field indeed is generated by the phonons, as opposed to a pure electromagnetic inverse Faraday effect. Our result may open a new route to investigate spin-phonon interaction in ultrafast magnetism.

Highly ordered nanocrystal (NC) assemblies, namely superlattices (SLs), have been investigated as a building block of novel bright (quantum) light sources because of their unique collective emission process, so-called superfluorescence. Thus far, the primary preparation method for perovskite NC SLs has been drying-mediated self-assembly, in which the NCs spontaneously assemble into SLs while the solvent evaporates. However, this method does not give control over the position and size of assemblies. Hence, it has been challenging to place NC assemblies in photonic device structures. Here, we demonstrate template-assisted self-assembly of CsPbBr₃ NCs to achieve precise control of the geometrical features of NC assemblies. A solution of NCs is drop-casted on a substrate with hollow, lithographically-defined template structures made from SiO2. We allow slow solvent evaporation and remove excess NCs from the substrate surface afterward. Thus, NCs only remain in the templates, and the position and size of these NC assemblies can be controlled by changing the design of the hollow structures. We performed time-resolved photoluminescence measurements on these NC assemblies and observed signatures of collective photon emission. Our results provide an important step forward for the development of optical devices that harness embedded perovskite NC assemblies.

The decay dynamics of excited carriers in graphene have attracted wide scientific attention, as its gapless Dirac electronic band structure opens up relaxation channels that are not allowed in conventional bulk materials. We report bright mid-infrared emission from laser-pumped graphene, originating from a previously unobserved decay channel: hot plasmon emission. The observed Fermi-level dependence of radiation rules out all possible mid-infrared Planckian light emission mechanisms. Calculations for our experimental conditions indicate that conditions for plasmon gain exist on the sub-100 fs timescale due to the net positive emission processes. As the population inversion of excited carriers depletes at longer times, spontaneous plasmon emission dominates stimulated plasmon emission, and the cumulative observed emission per excitation pulse is dominated by spontaneous plasmon emission. The calculated time-integrated plasmon-assisted spontaneous emission spectra corroborate with the experimentally measured Fermi-level-dependent emission spectra from a planar graphene. The plasmon emission processes produce non-Planckian behavior. The spectral flux of spontaneously generated plasmons is found to be several orders of magnitude higher than that of spontaneous photon emission. Evidence for bright hot plasmon emission is further supported by a large emission enhancement detected from graphene decorated with gold NDs, which promote localized plasmon excitation and serve as outcoupling scatterers. These observations pave a new avenue for the exploration of ultrafast and ultrabright mid-infrared stimulated and spontaneous emission processes and bright infrared light sources.

Due to its distinct optical characteristics, such as electrical adjustability and robust spatial confinement, graphene surface plasmons (GSPs) possess significant potential for controlling light within a compressed two-dimensional (2D) domain at extremely small sub-wavelength scales. While the interaction of light with materials in three-dimensional (3D) space is well-established, our comprehension of 2D plasmons is still nascent, primarily explored in limited scenarios, such as the interaction of GSPs with structured graphene like nano-gaps. This investigation expands our insights by exploring how GSPs interact with metallic nano-plates, specifically focusing on GSP reflection phase shift. By developing a rigorous model, we show that highly confined GSPs, with significantly larger momentum than free space photons, undergo nearly complete internal reflection associated with an anomalous phase shift of 0.885π in the reflection. Our findings contribute a comprehensive understanding of manipulating GSPs using a straightforward nanostructure. This knowledge is essential for advancing nanostructure-integrated low-dimensional devices and 2D nanophotonics.

Light fields can sculpt the optical and electronic properties of solids by injecting charge carriers from the valence to the conduction band. The photoinjection process typically unfolds on time scales shorter than the period of the driving radiation, thus being - for visible and near-infrared light - below one femtosecond. Despite being pervasive, only a few experiments investigated the photoinduced charge excitation produced by an intense, ultrashort laser pulse with attosecond temporal resolution. Here we used attosecond transient reflection spectroscopy in combination with a two-fold, advanced theoretical approach to demonstrate that there is not a single photoexcitation mechanism responsible for charge injection in germanium. Instead, it stems from multi-photon absorption and band dressing, while intra-band motion has a hindering effect.

Use of nanomaterials for photocatalysis faces challenges such as complex synthesis, high cost, low scalability, and dependance on UV radiation for initiating the photocatalytic activity. We recently demonstrated scalable, one-pot syntheses of one-dimensional (1D) lepidocrocite-based nanofilaments (NFs), 1DL NFs, that have the potential to overcome some of the challenges. 1DL NFs are exceptionally stable in water, have a large surface to volume ratio, and sub-square-nanometer cross sections. Initial reports show the semiconducting nature of this material, with an indirect band gap energy of 4.0 eV, one of the highest ever reported for a titania material. In this work, we present a study of the electronic and optical properties of these newly discovered 1DL NFs using ultrafast transient optical absorption. We show that despite the large band gap of this material, sub-gap states can be accessed with visible light illumination only, and photoexcited species reveal decay times in the nanosecond scale. Long lived photoexcitations in the visible range, without assistance by UV illumination, pave the way for possible application in photocatalysis.

The optical and electronic properties of two-dimensional (2D) materials make them attractive for a variety of applications in quantum information processing, solar-energy harvesting, and catalysis. However, the dynamics of electrons, holes, and excitons formed after photo-excitation are often complex and involve many states that are optically dark, making their characterization with optical spectroscopy alone difficult. We present our recent work imaging the quantum states of 2D materials and their heterostructures in momentum space using time- and angle-resolved photoemission. A unique combination of tunable ultrashort extreme ultraviolet (XUV) pulses with 61 MHz repetition rate and time-of-flight momentum microscopy (ToF k-mic) enables the parallel recording of electron dynamics in all states across the full Brillouin zone after perturbative excitation. We will present results on pseudospin dynamics in graphene, valley circular dichroism in monolayer WS₂, and hybridized excitons in MoSe₂/WS₂ bilayers.

Liquid phase exfoliation (LPE) is a frequently employed technique for the scalable production of dispersible graphene and graphite nanosheets for electronic, optoelectronic, and photonic applications. Fundamental information on how the liquid exfoliation process and resulting size-distribution of these nanoflakes affects their electrical properties is lacking. To address this gap, terahertz time-domain spectroscopy (THz-TDS) is employed as a non-contact optical approach for determining the AC conductivity behavior and charge transport dynamics of these materials. Cascade centrifugation is employed to separate exfoliated flakes into a range of sizes as confirmed by profilometry measurements. Correlations between the flake area and carrier concentration/scattering time are observed and related to flake morphology through changes to carrier mean free paths and edge defects. A lack of control to the flake chemistry leaves open questions regarding the influence of doping levels on the charge carrier dynamics, which will be the focus of future work.

Colloidal quantum dots (QDs) are heavily investigated for their applications in light emission such as light emitting diodes and, more challenging, lasers. This is due to their appealing processing conditions, compared to e.g. epitaxy, resulting in lowering cost. They can also be patterned and their optical properties can be tuned. Using quantum confined Cd-based QDs, several groups have shown light amplification and ensuing lasing action in the red part of the spectrum. Although impressive milestones were achieved, there is to date no single material that can provide the demanding combination of gain metrics to be truly competitive with existing epitaxial growth approaches. In this talk, we take a look at material properties of CdS/Se nanocrystals in the regime of vanishing quantum confinement, so-called ‘bulk nanocrystals’. We show that these unique materials display disruptive optical gain metrics in the green optical region. Indeed, while showing similar gain thresholds compared to state-of-the-art QD materials, the gain window (440-600 nm, 640-750 nm), amplitude (up to 50.000/cm) and gain lifetime (up to 3 ns) vastly outpace other solution processible materials. These results, while very impressive, are also puzzling. In the solution processible community a material system without quantum confinement does not exactly inspire confidence to have good emission metrics. We attempt to explain the physics behind these huge gain coefficients, by using a bulk semiconductor model which includes a strong band-gap renormalization effect, and argue why going to a bulk semiconductor can be advantageous compared to confined systems for making integrated lasers.

III-V semiconductors, such as Aluminium Gallium Arsenide (AlGaAs), are known for their high refractive index and strong non-resonant second-order optical nonlinearity making them useful for building active nonlinear photonic devices. AlGaAs-based nanophotonic structures supporting anapole resonances can significantly boost the internal electric fields resulting in enhanced nonlinear optical response. However, low-quality factors of these anapolar resonances can result in poor conversion efficiency for the nonlinear optical process. Here we report enhanced sum frequency generation (SFG) from vertically stacked three-disk AlGaAs-based nanoresonators supporting anapole type resonance that exhibit an order of magnitude increase in field enhancement compared to a single-disk resonator system at the SFG wavelength. The vertically stacked resonators consist of three individual AlGaAs layers separated vertically by an under-cut Gallium Arsenide (GaAs) stem. Finite difference time domain (FDTD) simulations of the scattering cross sections were performed to optimize the dimensions of AlGaAs resonator. The optimal structure consists of three vertically stacked AlGaAs nanodisks (550 nm diameter, 50 nm height) separated by a 100 nm GaAs stem. This design ensures anapole type resonance overlap at SFG wavelength (600-670 nm). Multi-spectral SFG images were acquired by varying the input signal wavelength in the range of 1400-1800 nm, keeping the pump wavelength fixed at 1040 nm. The experimental results show a maximum SFG enhancement of approximately 50-times near the resonance wavelength of 645 nm in comparison to unpatterned multilayer samples. Optical nanostructures based on stacked AlGaAs resonators provide a very exciting platform to tailor the light-matter interactions for linear and non-linear optical applications.

We developed a THz bandwidth quantum memory using a photon echo technique in InAs quantum dot ensemble with a large inhomogeneous broadning. To improve photon echo generation efficiency, we applied Adiabatic Rapid Passage (ARP) technique to quantum manipulation of quantum memory. In addition, we developed pulse pumped up conversion single photon detector (UCSPD) which enables us to achieve the femtosecond scale temporal resolution and we successfully demonstrated femtosecond time bin photon echo signals.

Nanoparticles of cuprous oxide with and without Pd-coatings are explored for charge carrier dynamics that can influence efficiency in applications of solar photocatalysis. Transient absorption/reflection spectroscopy is used to determine the excited-state dynamics, which depends on size and shape. Nanoparticles of a few tens of nanometers exhibit simple resonances toward the blue end of the visible spectrum, while particles over 100 nm exhibit various Mie resonances that enhance their performance in applications and lead to more complicated dynamics. Global and inversion analysis are used to determine the rate dynamics of the photocarriers. Small spheres exhibit Auger scattering in the high photocarrier injection regime, whereas the low carrier injection regime is characterized by Shockley-Read-Hall dynamics. Adding a near uniform, 2-nm thick Pd coating results in a heterojunction between Cu₂O and Pd that modifies the relaxation dynamics. The reduction of slope value from 3, characterizing Auger scattering, to the lower values on Pd coated samples shows the admixture of possibly two or more recombination mechanisms. Also, the intermediate injection regime exhibits radiative recombination/trap-assisted scattering in the Pd-coated Cu₂O nanoparticles. Power-dependent analysis, by solving the rate equation at the higher injection regime, validates the suppression of Auger scattering in Pd-coated samples. This result indicates the possibility of charge separation across the heterojunction, either from metal to the semiconductor or vice-versa, lowering the effective photocarrier overlap, reducing Auger scattering, and potentially improving photocatalytic activity.

We use a spatially non-local model of semiconductor Bloch equations to simulate ultrashort laser-induced plasma dynamics in a direct gap semiconductor nanowire. After laser exposure, the generated electron-hole plasma briefly undergoes plasma oscillations even in the near absence of Coulomb screening in 1D material. The cause of these oscillations is shown to be the dynamics of the propagating laser field coupled with the spatially non-local material model, resulting in momentum transfer from the laser field to the carriers during absorption and the appearance of non-zero off-diagonal density matrix elements. We further show how this plasma responds to probing THz radiation and examine the role of Coulomb and phonon scattering during the plasma oscillations.

In earlier work, we suggested using a curved waveguide as a quantum simulator to simulate the tunnel ionization process. Here we implemented for the first time such a simulator and tested our results against the Keldysh and bending loss ionization rates. We observed also straight rays which are reminiscent of above-threshold-ionization (ATI) electrons.

Laser induced x-ray fluorescence were observed against laser polarization ellipticity. While emission from krypton peaks at linear polarization, a signature of recollision, emission from neon shows opposite trend. We attribute it to two competing processes.