Theoretical Nanphototonics and Quantum Optics Group

Members

Dr Marian Florescu, Reader

Dr Chenglong Wan, Research Associate

Mr Richard Spalding, PhD Student

Dr Georgios Gkantzounis, Research Associate

Dr Timothy Amoah, Visiting Scientist

Alumni

Ms Ella Schneider, EPSRC Summer Student

Mr Timothy Eales, EPSRC Summer Student (currently pursuing a PhD in the Photonics and Quantum Sciences group at the University of Surrey)

Mr Richard Splading, EPSRC Summer Student (currently pursuing a PhD in the Theory and Computation group at the University of Surrey)

Mr Usman Waheed, Final Year Project (currently pursuing a PhD in the Department of Materials, Imperial Colege London)

Dr Remi Wache, KTP Research Associate, now with Université de Bretagne Occidentale

Dr Steven Sellers, PhD Student, now with Schroders plc

Dr Ross Maspero, PhD Student, now with Actica Consulting

Dr Zoe Bushell, PhD Student, now a teaching fellow with the Physics Department, University of Bath



Research Interests

Our research interests lie in the fields of nanophotonics, quantum optics, and spintronics. In particular, we focus on the identification of novel phenomena and functionalities in micro and nanostructured photonic materials, in implementations of linear-optical and solid-state quantum information processing in nanostructured materials, and in exploring new effects in the spin dynamics in quantum dots.

Physics and Applications of Microstructured Photonic Materials

The field of photonics has advanced tremendously recently through the development of micro and nanostructured photonic materials. An important class of such materials is represented by photonic band gap (PBG) materials that present frequency ranges over which the electromagnetic light propagation is prohibited for all directions and polarizations. These materials are the optical analogues of semiconductors. Due to their unique ability to mold the flow of light and to control the light-matter interaction, PBG materials lead to a broad new frontier both in basic science and technology. An important part of our research is aimed at developing a theoretical understanding of these materials and at the identification and design of novel functionalities. Directions of research include photonic band gap formation in disordered and quasiperiodic photonic structures, physics and applications of thermal radiation control in photonic crystals and quantum optics and all-information processing in photonic band gap architectures.

Quantum Optics in Structured Photonic Reservoirs

The engineered electromagnetic vacuum associated with microstructured photonic materials is characterized by photonic density of states exhibiting discontinuous changes as a function of frequency and by highly anisotropic electromagnetic field distribution. As a result, the conventional quantum optical formalism can not be applied to describe the light-matter interaction in these materials, and new theories are needed.



Recent Results and Major Research Accomplishments

Hyperuniform Photonic Band Gap Materials

Designer disordered materials with large complete photonic band gaps

Marian Florescu, Salvatore Torquato, and Paul Steinhardt, "Designer disordered materials with large complete photonic band gaps",, Proceedings of the National Academy of Sciences 106, 20658 (2009).

Until recently, the only materials known to have sizeable complete photonic band gaps were photonic crystals, periodic structures, and it was generally assumed that long-range periodic order was instrumental in the PBG formation. We have discovered a new class of materials with large complete band gaps, namely, hyperuniform non-crystallographic  micro-structures. This class of materials characterized by suppressed density fluctuations (hyperuniformity) includes highly-isotropic, translationally-disordered structures. Due to their distinctive optical and structural properties, non-crystallographic PBG materials are expected to facilitate unprecedented capabilities for controlling light, such as waveguiding with arbitrary bending angle and highly-efficient isotropic emission, with great impact for information processing, energy harvesting, sensing, and lighting applications.


High-Q optical cavities in hyperuniform disordered materials

Timothy Amoah and Marian Florescu, "High-Q optical cavities in hyperuniform disordered materials",, Physical Review B, Rapid Communications 91, 020201(R) (2015); editors suggestion.

We introduce designs for high-Q photonic cavities in slab architectures in hyperuniform disordered solids displaying isotropic band gaps. Despite their disordered character, hyperuniform disordered structures have the ability to tightly confine the transverse electric-polarized radiation in slab configurations that are readily fabricable. The architectures are based on carefully designed local modifications of otherwise unperturbed hyperuniform dielectric structures. We identify a wide range of confined cavity modes, which can be classified according to their approximate symmetry (monopole, dipole, quadrupole, etc.) of the confined electromagnetic wave pattern. We demonstrate that quality factors Q>10^9 can be achieved for purely two-dimensional structures, and that for three-dimensional finite-height photonic slabs, quality factors Q>20,000 can be maintained.


Isotropic band gaps and freeform waveguides observed in hyperuniform disordered photonic solids

Weining Man, Marian Florescu, Eric Paul Williamson, Yingquan He, Seyed Reza Hashemizad, Brian YC Leung, Devin Robert Liner, Salvatore Torquato, Paul M Chaikin, Paul J Steinhardt, "Isotropic band gaps and freeform waveguides observed in hyperuniform disordered photonic solids",, Proceedings of the National Academy of Sciences 110, 15886 (2013).

Recently, disordered photonic media and random textured surfaces have attracted increasing attention as strong light diffusers with broadband and wide-angle properties. We report the experimental realization of an isotropic complete photonic band gap (PBG) in a 2D disordered dielectric structure. This structure is designed by a constrained optimization method, which combines advantages of both isotropy due to disorder and controlled scattering properties due to low-density fluctuations (hyperuniformity) and uniform local topology. Our experiments use a modular design composed of Al2O3 walls and cylinders arranged in a hyperuniform disordered network. We observe a complete PBG in the microwave region, in good agreement with theoretical simulations, and show that the intrinsic isotropy of this unique class of PBG materials enables remarkable design freedom, including the realization of waveguides with arbitrary bending angles impossible in photonic crystals. This experimental verification of a complete PBG and realization of functional defects in this unique class of materials demonstrate their potential as building blocks for precise manipulation of photons in planar optical microcircuits and has implications for disordered acoustic and electronic band gap materials.


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Photonic Band Gap Materials: Physics and Applications

Fast Assembly of Gold Nanoparticles in Large-Area 2D Nanogrids Using a One-Step, Near-Infrared Radiation-Assisted Evaporation Process

André Utgenannt, Ross Maspero, Andrea Fortini, Rebecca Turner†, Marian Florescu, Christopher Jeynes, Antonios G. Kanaras, Otto L. Muskens, Richard P. Sear, and Joseph L. Keddie, "Fast Assembly of Gold Nanoparticles in Large-Area 2D Nanogrids Using a One-Step, Near-Infrared Radiation-Assisted Evaporation Process", ACS Nano, 10 (2), 2232 (2016).

When fabricating photonic crystals from suspensions in volatile liquids using the horizontal deposition method, the conventional approach is to evaporate slowly to increase the time for particles to settle in an ordered, periodic close-packed structure. Here, we show that the greatest ordering of 10 nm aqueous gold nanoparticles (AuNPs) in a template of larger spherical polymer particles (mean diameter of 338 nm) is achieved with very fast water evaporation rates obtained with near-infrared radiative heating. Fabrication of arrays over areas of a few cm2 takes only 7 min. The assembly process requires that the evaporation rate is fast relative to the particles’ Brownian diffusion. Then a two-dimensional colloidal crystal forms at the falling surface, which acts as a sieve through which the AuNPs pass, according to our Langevin dynamics computer simulations. With sufficiently fast evaporation rates, we create a hybrid structure consisting of a two-dimensional AuNP nanoarray (or “nanogrid”) on top of a three-dimensional polymer opal. The process is simple, fast, and one-step. The interplay between the optical response of the plasmonic Au nanoarray and the microstructuring of the photonic opal results in unusual optical spectra with two extinction peaks, which are analyzed via finite-difference time-domain method simulations. Comparison between experimental and modeling results reveals a strong interplay of plasmonic modes and collective photonic effects, including the formation of a high-order stopband and slow-light-enhanced plasmonic absorption. The structures, and hence their optical signatures, are tuned by adjusting the evaporation rate via the infrared power density.


Thermal radiation from finite photonic crystals

Christian Schuler, Christian Wolff, Kurt Busch, and Marian Florescu, "Thermal radiation from finite photonic crystals", Applied Physics Letters 95, 241103 (2009).

We have developed a microscopic theory of thermal emission from finite-sized photonic crystals and show that the directional spectral emissivity and related quantities can be evaluated via standard bandstructure computations without any approximation. We then identify the physical mechanisms through which interfaces modify the potentially super-Planckian radiation flow inside infinite photonic crystals, such that thermal emission from finite-sized samples is consistent with the fundamental limits set by Planck's law. As an application, we further demonstrate that a judicious choice of a photonic crystal's surface termination facilitates considerable control over both the spectral and angular thermal emission properties.

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All-optical information processing in Photonic Band Gap Materials

Resonance fluorescence in photonic band gap waveguide architectures: Engineering the vacuum for all-optical switching

Marian Florescu and Sajeev John, "Resonance fluorescence in photonic band gap waveguide architectures: Engineering the vacuum for all-optical switching", Physical Review A 69, 053810 (2004).

We have described the spectral characteristics of the radiation scattered by two-level atoms (quantum dots) driven by a strong external field, and coupled to a photonic crystal radiation reservoir. In the presence of strong variations with the frequency of the photonic reservoir density of states, the atomic, Mollow, sideband components of the scattered intensity can be strongly modified. Consequently, a weak optical probe field experiences a substantial differential gain in response to slight variations in the intensity of an optical driving field. We have suggested that these effects may be of relevance to all-optical transistor action in photonic crystals. Using a specific photonic crystal heterostructure, we suggest that an all-optical microtransistor based on photonic crystals may operate at less than 100 nW switching threshold power.

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Linear Optical Quantum Information Processing in Photonic Nanostructures

Single photons on demand from 3D photonic band-gap structures

Marian Florescu, Stefan Scheel, Hartmut Haeffner, Hwang Lee, Dmitry V.Strekalov, Peter L. Knight, Jonathan P. Dowling, "Single photons on demand from 3D photonic band-gap structures", Europhysics Letters 69 (6), 945 (2005).

We describe a practical implementation of a photon gun based on stimulated Raman adiabatic passage pumping and the strong enhancement of the photonic density of states in a photonic band-gap material. This device allows deterministic and unidirectional production of single photons with a high repetition rate of the order of 100 kHz. We also discuss specific 3D photonic micro-structure architectures in which our model can be realized and the feasibility of implementing such a device using Er ions that produce single photons at the telecom wavelength of 1.55 μm.

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Quantum Optics in Structured Photonic Reservoirs

Single atom switching in photonic band gap materials

Marian Florescu and Sajeev John, "Single-atom switching in photonic crystals", Physical Review A 64, 033801 (2001).

We have investigated the role of the first non-Markovian corrections to the resonance fluorescence in photonic crystals, using a perturbative expansion of the Heisenberg equations of motion in powers of the atom-field coupling strength. Our method recaptures the physics of the photon-atom bound state in the presence of a full photonic band gap (PBG). For the anisotropic three-dimensional PBG, it predicts fundamentally new features in the resonance fluorescence, such as atomic population inversion and switching behaviour in a two-level atom for moderate values of the applied field. The magnitude of the switching depends sensitively on the external laser field intensity and the detuning of its frequency with respect to atomic resonant frequency. The robustness of these effects against non-radiative decay and dephasing mechanisms of the atomic system is also investigated.

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Lateral Quantum Dots

Spin relaxation in lateral quantum dots

Marian Florescu and Pawel Hawrylak, "Spin relaxation in lateral quantum dots: Effects of spin-orbit interaction", Physical Review B 73, 045304 (2006).

Using exact diagonalization techniques, we investigate the influence of the spin-orbit interaction on the energy levels of a two-electron droplet and we show that the spin-orbit interaction strongly affects the expectation values of the total and z-projection spins of the two-electron system. We then evaluate the energy relaxation rates for the two-electron droplet through the emission of longitudinal acoustic phonons Our study shows that the spin-orbit interaction provides an effective coupling between the spin-polarized triplet states and the singlet state, and the calculated scattering rates reveal a microsecond time scale. The relaxation mechanism presents a built-in magnetic field asymmetry, in qualitative agreement with experimental findings.

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