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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.

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.

We fabricated terahertz-frequency plasmonic antenna arrays using patterned metallic structures atop a layer of graphene. The antenna design was developed through numerical simulations that were informed by experimentally obtained graphene parameters. Experimental characterization reveals a clear reflection enhancement at 3λ/2, the intensity of which was modified by underlying graphene.

We use terahertz time-domain spectroscopy (THz-TDS) in reflection geometry to investigate the complex optical properties of ultra-thin (i.e., < 10 nm) conductors from 0.3 to 3 THz. We discuss the advantages of this approach, and report complex optical properties of graphene and a 9 nm Ti thin-film.

Electromagnetic wave propagation inside one-dimensional photonic crystal fiber is numerically characterized by computing V-parameter, cladding index and effective mode profiles. Dependence on these parameters on geometrical structure of the fiber reflects the design tolerance, and corresponding modal characteristics. Air-hole spacing and periodicity are varied to observe the effect on V-parameter; along with effective core radius, which is a function of periodicity, is modified to see the effect on cladding index and effective mode profiles. Results obtained from empirical relations method are compared with analytical formula of Nielsen and Mortensen. Periodicity of the structure is regarded as numerically equal to the central wavelength of the desired range throughout the computation. Refractive index of the core is taken as independent of operating wavelength. Simulated parameters have critical importance to estimate the characteristics of photonic crystal fiber which will help the design engineers to study the performance at 870 nm, 1330 nm and 1550 nm. Keywords--V-Parameter Cladding Index, Effective Core Radius, Single Mode Propagation, Empirical Relations Method, Optical Communication

Using time- and angle-resolved photoemission spectroscopy on a microscopic sample of a 2D semiconductor, we visualized directly the excitonic wavefunction in real- and momentum-space.