Dynamic gain and frequency comb formation in exceptional-point lasers

Exceptional points (EPs)--singularities in the parameter space of non-Hermitian systems where two nearby eigenmodes coalesce--feature unique properties with applications for microcavity lasers such as sensitivity enhancement and chiral emission. Present EP lasers operate with static populations in the gain medium. Here, we show theoretically that a laser operating sufficiently close to an EP will spontaneously induce a multi-spectral multi-modal instability that creates an oscillating population inversion and generates a frequency comb. The comb formation is enhanced by the non-orthogonality of modes via the Petermann factor. Such an "EP comb" features an ultra-compact size and a widely tunable repetition rate, without requiring external modulators or a continuous-wave pump. We develop an exact ab initio dynamic solution of the space-dependent Maxwell-Bloch equations, describing all steady-state properties of the EP comb. We illustrate this phenomenon in a realistic parity-time-symmetric 5-{\mu}m-long AlGaAs cavity and validate our prediction with finite-difference time-domain simulations. This work reveals the rich physics that connect non-Hermitian degeneracies and the nonlinear dynamics of gain media to fundamentally alter the laser behavior

A global picture of the S-1/S-0 conical intersection seam of benzene

A global picture of the S1/S0intersection seam of benzene is presented. Eleven new conical intersection critical points were located at the CASSCF level, the connectivity was mapped and the energies refined with CASPT 2. There are two seam branches related with pairs of degenerate A1g/B2uand Egstates at D6hsymmetry, respectively, and the two branches are connected by a seam segment of Cssymmetry. The global energy minimum of the seam is the half-boat shaped intersection that leads to a pre-fulvenic intermediate [I.J. Palmer, I.N. Ragazos, F. Bernardi, M. Olivucci, M.A. Robb, J. Am. Chem. Soc. 115 (1993) 673]. Several other intersections that can lead to the same intermediate or vibrationally hot benzene lie in a range of 3.7 eV above the global seam minimum. There is a recurrent connectivity pattern where permutationally isomeric seam segments are connected by intersections of a higher symmetry point groupThis work has been supported by Grant CTQ2008-06696 from the Spanish Ministerio de Ciencia e Innovación (MICINN

Products liability.

The goal of this Products Liability IQP is to learn the basic concepts of products liability law. First, to gain this fundamental understanding, we watched nine videos and read two books pertaining to products liability. Using this solid foundation, we analyzed two liability cases and performed one accident reconstruction. This experience expanded our knowledge of safety, ethics, and good engineering practice

Implementation and evaluation of speaker testing techniques

This project completed in conjunction with Bruel & Kjaer of Naerum, Denmark evaluates speaker testing techniques for measuring frequency and phase response in reverberant environments. The techniques investigated were maximum length sequence, gating, dual channel fast Fourier transform and complex cepstrum. The project team developed an application in C++ integrated with Bruel & Kjaer's PULSE platform. The application emulates the anechoic environment and successfully measure the frequency and phase response of a speaker using the four techniques