Ian Galbraith

Third-order non-linearities are important because they allow control over light pulses in ubiquitous high-quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third-order non-linear sheet susceptibility χ⁽³⁾L (where L represents the material thickness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi has a value of χ⁽³⁾L in the THz domain that is higher than that reported for any other material in any wavelength band. The immediate implication of our results is the efficient generation of intense coherent THz light via upconversion (also a χ⁽³⁾ process), and they open the door to exploitation of non-degenerate mixing and optical nonlinearities beyond the perturbative regime.

We provide a set of molecular dynamics simulations employing a force field specifically parameterized for organic π-conjugated materials. The resulting conformation ensemble was coupled to quantum chemistry calculations, and quantities of interest for optoelectronic applications, namely, ground- and excited-state energies, oscillator strengths, and dipole moments were extracted. This combined approach allowed not only exploration of the configurational landscape but also of the resulting electronic properties of each frame within the simulation and thus probe the link between conformation and property. A study was made of the sampling and convergence requirements to yield reliable averages over the ensemble. Typically between 800 and 1000 conformations were sufficient to ensure convergence of properties. However, for some oligomers, more configurations were required to achieve convergence of the oscillator strength and magnitude of the dipole moment.

We study a mean-field model for the dynamics of an interacting Bose-Einstein condensate in two-dimensional pseudorelativistic materials. This model is relatively simple but contains long-lived solutions called oscillons which are absent in simple nonrelativistic condensates. We report on a variety of scenarios including interactions between pairs of oscillons and oscillons propagating across an inhomogeneous material boundary. Hitherto, relativistic oscillons have been studied only in high-energy physics and cosmology, and their relevance has not been highlighted so far in condensed-matter physics.

A wide range of materials like graphene, topological insulators, and transition-metal dichalcogenides (TMDs) share an interesting property: the low-energy excitations behave as Dirac particles. This emergent behavior of Dirac quasiparticles defines a large class of media that are usually called Dirac materials. In this work we build the foundations of a way to study the linear optical properties of these two-dimensional media. Our approach is based on a Dirac-like formulation of the standard semiconductor Bloch equations used in semiconductor physics. We provide an explicit expression of the linear absorbance, which we call the relativistic Elliott formula, and use it to quantify the variation of the continuum absorbance spectrum with the strength of the Coulomb interaction (the Sommerfeld factor). Our calculations also show how the Coulomb enhancements scales with the band gap and vanishes for zero band gap, shedding light on the behavior of graphene for low-light intensities. The results presented are in good quantitative agreement with published experimental results. Our theory can help researchers to explore the nonlinear interactions of intense, ultrashort pulses with TMDs, and the framework is flexible enough to be adapted to different experimental situations, such as cavities, multilayers, heterostructures, and microresonators.

We investigated a damped two-level system interacting with a circularly polarized light as described by an asymmetric non-Hermitian Hamiltonian. This is a simple enough system to be studied analytically while complicated enough to exhibit a rich variety of behaviors. This system exhibits a ring of exceptional points in the parameter space of the real and imaginary dipole couplings where within the ring the energy eigenvalue of the system does not change. This leads to unstable regions inside the exceptional ring, which is shown using a linear stability analysis. These unstable regions are unique to gain-loss systems and have the surprising property that no matter how small the gain/loss ratio, the gain always prevails at long times. We also report on eigenvalue switching, phase rigidity and dynamics of the system around the exceptional points. We highlight that some of these properties are different from those in the widely studied case of symmetric non-Hermitian Hamiltonians.

We describe a general scheme to obtain force-field parameters for classical molecular dynamics simulations of conjugated polymers. We identify a computationally inexpensive methodology for calculation of accurate intermonomer dihedral potentials and partial charges. Our findings indicate that the use of a two-step methodology of geometry optimization and single-point energy calculations using DFT methods produces potentials which compare favorably to high level theory calculation. We also report the effects of varying the conjugated backbone length and alkyl side-chain lengths on the dihedral profiles and partial charge distributions and determine the existence of converged lengths above which convergence is achieved in the force-field parameter sets. We thus determine which calculations are required for accurate parametrization and the scope of a given parameter set for variations to a given molecule. We perform simulations of long oligomers of dioctylfluorene and hexylthiophene in explicit solvent and find peristence lengths and end-length distributions consistent with experimental values.

Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions.

We show that the application of a vertical electric field to the Coulomb interacting system in stacked quantum dots leads to a 90° in-plane switching of charge probability distribution in contrast to a single dot, where no such switching exists. Results are obtained using path integral quantum Monte Carlo with realistic dot geometry, alloy composition, and piezo-electric potential profiles. The origin of the switching lies in the strain interactions between the stacked dots hence the need for more than one layer of dots. The lateral polarization and electric field dependence of the radiative lifetimes of the excitonic switch are also discussed.