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Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of the momentum forbidden dark excitons, and study their properties, including their near-degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominate the excited state distribution - a surprising finding that highlights their importance in atomically thin semiconductors., Comment: 34 pages

We use a table-top time-resolved ARPES based on a MHz XUV source to directly observe direct and momentum-forbidden excitons in the full first Brillouin zone of WSe2 monolayer and measure their ultrafast dynamics.

Two different types of freebase porphyrins with variation in the central core, namely H2TPP (meso-5,10,15,20- tetrakis phenyl porphyrin) and HSTPP (meso-tetrakis 5,10,15,20-tetra-phenyl-23-H-21-thiaporphyrin) were introduced into the borate glass matrix and developed the thin films by melt quenching technique. Due to core modification and extended non-planarity in the glass matrix, the absorption spectrum of HSTPP shows a strong redshift in the soret and Q bands. Emission spectra of HSTPP show an intense emission at 739 nm (S1 → S0) and ~ 505 nm due to coordination of boron atom at the central core and freezing of N-BO3 vibrations furnishes slow relaxation from higher energy states. Comparison studies are made by treating with BF3O(C2H5)2 to mimic structure in the borate glass matrix due to boron coordination at the central core. The excited state lifetimes are shortened due to BF3 interaction in the solution as well as in the borate glass matrix endowing its potential to develop the optical limiting devices. Nonlinear optical studies reveal reverse saturable absorption (RSA) behaviour in the picosecond (ps) region and show enhanced two photon absorption (TPA) by core modification. Optical power limiting measurements on HSTPP in glass matrix delivers a better limiting threshold value of 15.2 mJ/cm2.

We use a table-top time-resolved ARPES based on a MHz XUV source to directly observe direct and momentum-forbidden excitons in the full first Brillouin zone of WSe2 monolayer and measure their ultrafast dynamics.

Covalent organic frameworks (COFs) have aroused immense scientific interest as an exhilarating class of porous materials due to their structure tunability and diverse properties. However, understanding of their response towards laser induced nonlinear optical (NLO) applications is in its infancy and demands prompt attention. Herein, we report three novel regioregular porphyrin based porous COFs, Por‐COF‐HH and its dual metalated congeners (Por‐COF‐ZnCu and Por‐COF‐ZnNi) with excellent NLO properties. Notably, intensity dependent NLO switching behavior was observed for these Por‐COFs, which is highly desirable for optical switching and optical limiting devices. Moreover, the efficient π‐conjugation and charge transfer transition in ZnCu‐Por‐COF enable a high nonlinear absorption coefficient (β=4470 cm/GW) and figure of merit (FOM = σ1/σo, 3565) values compared to other state‐of‐art materials including molecular porphyrins (β=~100‐400 cm/GW), metal‐organic frameworks (MOFs; β=~0.3‐0.5 cm/GW) and graphene (β=900 cm/GW).

Graphene, in its pristine form and analogs, is revolutionizing the architecture and designing of materials for high-performance optoelectronic devices. The functionalization of graphene is very attractive and emerging field of interest for nonlinear optical devices. In this work, we have synthesized Graphene oxide (GO) decorated with TiO2 semiconducting nanoparticles for nonlinear photonic devices. The nonlinear optical (NLO) measurements of GO-TiO2 composite were studied with a popular Z-scan technique for optical power limiting applications. Functionalized graphene oxide with TiO2 nanoparticles exhibits a strong enhancement in optical-response with nanosecond laser pulses. The composite show fingerprints of reverse saturable absorption (RSA) dominated by two-photon absorption (TPA) along with enhanced nonlinear absorption due to transfer of electrons/energy between TiO2 nanoparticles and graphene oxide. The systematic spectroscopic studies with TEM, XRD, FT-IR, micro-Raman, steady-state and time-resolved photoluminescence reveals that successful conjugation of GO and TiO2 nanoparticles and probes the structural and optical properties. The nonlinear optical measurements demonstrate that GO-TiO2 composite exhibits promising optical power limiting features than individual counterparts implicating the potential contingent towards broadband optical limiting applications.

We measure the opto-electronic properties of femtosecond-laser-ablated GaAs and demonstrate its utility towards THz devices. In particular, we show that laser-ablated THz antennas are 65% more efficient than non-ablated antennas at high powers. Our results demonstrate the possibility of using femtosecond-laser-ablation as a cost-effective technique to engineer material properties for THz devices.

© 2018 Author(s). We report ultrafast optical measurements of the Dirac line-node semimetal ZrSiS and the Weyl semimetal NbAs, using mid-infrared pump photons from 86 meV to 500 meV to directly excite Dirac and Weyl fermions within the linearly dispersing bands. In NbAs, the photoexcited Weyl fermions initially form a non-thermal distribution, signified by a brief spike in the differential reflectivity whose sign is controlled by the relative energy of the pump and probe photons. In ZrSiS, electron-electron scattering rapidly thermalizes the electrons, and the spike is not observed. Subsequently, hot carriers in both materials cool within a few picoseconds. This cooling, as seen in the two materials' differential reflectivity, differs in sign, shape, and timescale. Nonetheless, we find that it may be described in a simple model of thermal electrons, without free parameters. The electronic cooling in ZrSiS is particularly fast, which may make the material useful for optoelectronic applications.