Fabrication and characterization of erbium-doped toroidal microcavity lasers

Erbium-doped SiO2 toroidal microcavity lasers are fabricated on a Si substrate using a combination of optical lithography, etching, Er ion implantation, and CO2 laser reflow. Erbium is either preimplanted in the SiO2 base material or postimplanted into a fully fabricated microtoroid. Three-dimensional infrared confocal photoluminescence spectroscopy imaging is used to determine the spatial distribution of optically active Er ions in the two types of microtoroids, and distinct differences are found. Microprobe Rutherford backscattering spectrometry indicates that no macroscopic Er diffusion occurs during the laser reflow for preimplanted microtoroids. From the measured Er doping profiles and calculated optical mode distributions the overlap factor between the Er distribution and mode profile is calculated: Gamma=0.066 and Gamma=0.02 for postimplanted and preimplanted microtoroids, respectively. Single and multimode lasing around 1.5 µm is observed for both types of microtoroids, with the lowest lasing threshold (4.5 µW) observed for the preimplanted microtoroids, which possess the smallest mode volume. When excited in the proper geometry, a clear mode spectrum is observed superimposed on the Er spontaneous emission spectrum. This result indicates the coupling of Er ions to cavity modes

Implicit Density Functional Theory

A fermion ground state energy functional is set up in terms of particle density, relative pair density, and kinetic energy tensor density. It satisfies a minimum principle if constrained by a complete set of compatibility conditions. A partial set, which thereby results in a lower bound energy under minimization, is obtained from the solution of model systems, as well as a small number of exact sum rules. Prototypical application is made to several one-dimensional spinless non-interacting models. The effectiveness of "atomic" constraints on model "molecules" is observed, as well as the structure of systems with only finitely many bound states.Comment: 9 pages, 4 figure

Visualization of defect-induced excitonic properties of the edges and grain boundaries in synthesized monolayer molybdenum disulfide

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attractive materials for next generation nanoscale optoelectronic applications. Understanding nanoscale optical behavior of the edges and grain boundaries of synthetically grown TMDCs is vital for optimizing their optoelectronic properties. Elucidating the nanoscale optical properties of 2D materials through far-field optical microscopy requires a diffraction-limited optical beam diameter sub-micron in size. Here we present our experimental work on spatial photoluminescence (PL) scanning of large size ( $\geq 50$ microns) monolayer MoS$_2$ grown by chemical vapor deposition (CVD) using a diffraction limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small as 200 nm allowing us to probe nanoscale excitonic phenomena which was not observed before. We have found several important features: (i) there exists a sub-micron width strip ($\sim 500$ nm) along the edges that fluoresces $\sim 1000 \%$ brighter than the region far inside; (ii) there is another brighter wide region consisting of parallel fluorescing lines ending at the corners of the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic peak, as large as $\sim 120$ meV, in the PL spectra from the edges. Using density functional theory calculations, we attribute this giant blue shift to the adsorption of oxygen dimers at the edges, which reduces the excitonic binding energy. Our results not only shed light on defect-induced excitonic properties, but also offer an attractive route to tailor optical properties at the TMDC edges through defect engineering.Comment: 10 pages, 4 figures in Journal of Physical Chemistry C, 201

Beating the PNS attack in practical quantum cryptography

In practical quantum key distribution, weak coherent state is often used and the channel transmittance can be very small therefore the protocol could be totally insecure under the photon-number-splitting attack. We propose an efficient method to verify the upper bound of the fraction of counts caused by multi-photon pluses transmitted from Alice to Bob, given whatever type of Eve's action. The protocol simply uses two coherent states for the signal pulses and vacuum for decoy pulse. Our verified upper bound is sufficiently tight for QKD with very lossy channel, in both asymptotic case and non-asymptotic case. The coherent states with mean photon number from 0.2 to 0.5 can be used in practical quantum cryptography. We show that so far our protocol is the $only$ decoy-state protocol that really works for currently existing set-ups.Comment: So far this is the unique decoy-state protocol which really works efficiently in practice. Prior art results are commented in both main context and the Appendi

A decoy-state protocol for quantum cryptography with 4 intensities of coherent states

In order to beat any type of photon-number-splitting attack, we propose a protocol for quantum key distributoin (QKD) using 4 different intensities of pulses. They are vacuum and coherent states with mean photon number $\mu,\mu'$ and $\mu_s$. $\mu_s$ is around 0.55 and this class of pulses are used as the main signal states. The other two classes of coherent states ($\mu,\mu'$) are also used signal states but their counting rates should be studied jointly with the vacuum. We have shown that, given the typical set-up in practice, the key rate from the main signal pulses is quite close to the theoretically allowed maximal rate in the case given the small overall transmittance of $10^{-4}$

An effective method of calculating the non-Markovianity N\mathcal{N} for single channel open systems

We propose an effective method which can simplify the optimization of the increase of the trace distance over all pairs of initial states in calculating the non-Markovianity $\mathcal{N}$ for single channel open systems. For the amplitude damping channel, we can unify the results of Breuer $et$ $al$. [Phys. Rev. Lett. \bf 103\rm, 210401 (2009)] in the large-detuning case and the results of Xu $et$ $al$. [Phys. Rev. A \bf 81\rm, 044105 (2010)] in the resonant case; furthermore, for the general off-resonant cases we can obtain a very tight lower bound of $\mathcal{N}$. As another application of our method, we also discuss $\mathcal{N}$ for the non-Markovian depolarizing channel.Comment: 7 pages, 3 figures,to be published in Phys. Rev.

Absolute calibration of GafChromic film for very high flux laser driven ion beams.

We report on the calibration of GafChromic HD-v2 radiochromic film in the extremely high dose regime up to 100 kGy together with very high dose rates up to 7 × 1011 Gy/s. The absolute calibration was done with nanosecond ion bunches at the Neutralized Drift Compression Experiment II particle accelerator at Lawrence Berkeley National Laboratory (LBNL) and covers a broad dose dynamic range over three orders of magnitude. We then applied the resulting calibration curve to calibrate a laser driven ion experiment performed on the BELLA petawatt laser facility at LBNL. Here, we reconstructed the spatial and energy resolved distributions of the laser-accelerated proton beams. The resulting proton distribution is in fair agreement with the spectrum that was measured with a Thomson spectrometer in combination with a microchannel plate detector

Geometric Phases for Mixed States during Cyclic Evolutions

The geometric phases of cyclic evolutions for mixed states are discussed in the framework of unitary evolution. A canonical one-form is defined whose line integral gives the geometric phase which is gauge invariant. It reduces to the Aharonov and Anandan phase in the pure state case. Our definition is consistent with the phase shift in the proposed experiment [Phys. Rev. Lett. \textbf{85}, 2845 (2000)] for a cyclic evolution if the unitary transformation satisfies the parallel transport condition. A comprehensive geometric interpretation is also given. It shows that the geometric phases for mixed states share the same geometric sense with the pure states.Comment: 9 pages, 1 figur