Colm Dineen

VECSEL Optimization Using Microscopic Many-Body Physics

Vertical external cavity surface-emitting lasers (VECSELs) are designed and analyzed using an approach based on fully microscopically computed material properties like gain and carrier recombination rates. Very good agreement between theoretical predictions and measured characteristics of the realized devices is demonstrated. The high accuracy of the theoretical models allows one to determine even small deviations between the nominal designs and actual realizations. The models are used to find optimization strategies. It is shown how the external efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the gain-enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be detrimental to the device performance and how this problem can be reduced using optimized distributed Bragg reflectors and metallization layers. A combination of improved metallization and use of such a coating more than doubles the external efficiency and maximum power for a realized VECSEL operating at 1010 nm and the theory indicates that further significant improvements are possible.

Second-order accurate FDTD space and time grid refinement method in three space dimensions

We present an algorithm based on the finite-difference time-domain method for local refinement of a three-dimensional computational grid in space and time. The method has second-order accuracy in space and time as verified in the numerical examples. A number of test cases with material traverse normal to the grid interfaces were used to assess the long integration time stability of the algorithm. Resulting improvements in the computation time are discussed for a photonic crystal microcavity design that exhibits a sensitive dependence of the quality factor on subwavelength geometrical features

Magnetic dipole moments in single and coupled split-ring resonators

We examine the role of magnetic dipoles in single and coupled pairs of metallic split-ring resonators by numerically computing their magnitude and examining their relative contributions to the scattering cross section. We demonstrate that magnetic dipoles can strongly influence the scattering cross section along particular directions. It is also found that the magnetic dipole parallel to the incident magnetic field and/or high-order multipoles may play a significant role in the linear response of coupled split-ring resonators.

Electromagnetic field structure and normal mode coupling in photonic crystal nanocavities.

The electromagnetic field of a high-quality photonic crystal nanocavity is computed using the finite difference time domain method. It is shown that a separatrix occurs in the local energy flux discriminating between predominantly near and far field components. Placing a two-level atom into the cavity leads to characteristic field modifications and normal-mode splitting in the transmission spectra.

Optical Forces on a Quantum Dot in Metallic Bowtie Structures

We present a numerical scheme to calculate the electromagnetic force on a quantum dot in the near-field of subwavelength metallic nano-structures. As an application, we examine the forces on a quantum dot placed in the center of a gold nano-structure. The forces on the quantum dot are calculated using an adaptive mesh refinement finite-difference time-domain code combined with microscopic material equations. Is is shown that the dot may be laterally confined in the proposed geometry.

AMR FDTD solver for nanophotonic and plasmonic applications

Uniform, nonuniform and adaptive mesh refinement FDTD approaches to solving 3D Maxwells equations are compared and contrasted. Specific applications of such schemes to optical memory, nanophotonics and plasmonics problems will be illustrated.

Design and optimisation of VECSELs for the IR and mid-IR

An approach based on fully microscopically computed material properties like gain/absorption, radiative and Auger recombination rates are used to design, analyze and develop optimization strategies for Vertical External Cavity Surface Emitting Lasers for the IR and mid-IR with high quantitative accuracy. The microscopic theory is used to determine active regions that are optimized to have minimal carrier losses and associated heating while maintaining high optical gain. It is shown that in particular for devices in the mid-IR wavelength range the maximum output power can be improved by more than 100% by making rather minor changes to the quantum well design. Combining the sophisticated microscopic models with simple onedimensional macroscopic models for optical modes, heat and carrier diffusion, it is shown how the external efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the gain enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be reduced using optimized metallization layers. This increases the efficiency, reduces heating and strongly improves the maximum power. Applying these concepts to VECSELs operating at 1010nm has already resulted in more than twice as high external efficiencies and maximum powers. The theory indicates that significant further improvements are possible - especially for VECSELs in the mid-IR.

Optical forces on Quantum dots in the near field region of resonant metallic nano-structures

In this work we examine the forces on a single quantum dot in the presence of resonantly enhanced electric fields and steep field gradients confined to a sub-wavelength region in the near-field of the structure. To accurately resolve the sub-wavelength geometrical features of this structure, we employ the AMR-FDTD method with local space and time grid refinement using a 3D grid interface interpolation algorithm that preserves the second-order accuracy of the original FDTD scheme and is stable for long time integration. Using AMR-FDTD gives detailed access to the near field structure of the enhanced fields and consequently the gradient forces experienced by the quantum dot (QD) located near the tips.

Local field enhancement and spectral response of resonant nanostructures

The resonant behavior of metallic nano-structures is simulated using local mesh refinement of the FDTD method in 3 Dimensions. The influence of shape and tip geometry on the resonance structure and intensity of the optical fields is examined.

Electromagnetic interaction between nanoparticles and optical subwavelength devices

A general approach to self-consistently describe the electromagnetic field in a nanophotonic environment is presented. It is applied to dielectric particles and quantum-dots in the optical subwavelength-limit and also yields the radiative force on them.