171 research outputs found
Localized exciton-polariton modes in dye-doped nanospheres: a quantum approach
We model a dye-doped polymeric nanosphere as an ensemble of quantum emitters
and use it to investigate the localized exciton-polaritons supported by such a
nanosphere. By determining the time evolution of the density matrix of the
collective system, we explore how an incident laser field may cause transient
optical field enhancement close to the surface of such nanoparticles. Our
results provide further evidence that excitonic materials can be used to good
effect in nanophotonics.Comment: 16 pages, 4 figure
Time varying gratings model Hawking radiation
Diffraction gratings synthetically moving at trans-luminal velocities contain
points where wave and grating velocities are equal. We show these points can be
understood as a series of optical event horizons where wave energy can be
trapped and amplified, leading to radiation from the quantum vacuum state. We
calculate the spectrum of this emitted radiation, finding a quasi-thermal
spectrum with features that depend on the grating profile, and an effective
temperature that scales exponentially with the length of the grating, emitting
a measurable flux even for very small grating contrast.Comment: 13 pages, 4 figure
Graph theory approach to exceptional points in wave scattering
In this paper, we use graph theory to solve wave scattering problems in the
discrete dipole approximation. As a key result of this work, in the presence of
active scatterers, we show how one can find arbitrary large-order zero
eigenvalue exceptional points (EPs) in parameter space by solving a set of
non-linear equations. We interpret these equations in a graph theory picture as
vanishing sums of scattering events. We also show how the total field of the
system responds to parameter perturbations at the EP. Finally, we investigate
the sensitivity of the power output to imaginary perturbation in the design
frequency. This perturbation can be employed to trade sensitivity for a
different dissipation balance of the system.Comment: 15 pages, 11 figure
High-dimensional spatial mode sorting and optical circuit design using multi-plane light conversion
Multi-plane light converters (MPLCs) are an emerging class of optical device
capable of converting a set of input spatial light modes to a new target set of
output modes. This operation represents a linear optical transformation - a
much sought after capability in photonics. MPLCs have potential applications in
both the classical and quantum optics domains, in fields ranging from optical
communications, to optical computing and imaging. They consist of a series of
diffractive optical elements (the 'planes'), typically separated by free-space.
The phase delays imparted by each plane are determined by the process of
inverse-design, most often using an adjoint algorithm known as the wavefront
matching method (WMM), which optimises the correlation between the target and
actual MPLC outputs. In this work we investigate high mode capacity MPLCs to
create arbitrary spatial mode sorters and linear optical circuits. We focus on
designs possessing low numbers of phase planes to render these MPLCs
experimentally feasible. To best control light in this scenario, we develop a
new inverse-design algorithm, based on gradient ascent with a specifically
tailored objective function, and show how in the low-plane limit it converges
to MPLC designs with substantially lower modal cross-talk and higher fidelity
than achievable using the WMM. We experimentally demonstrate several prototype
few-plane high-dimensional spatial mode sorters, operating on up to 55 modes,
capable of sorting photons based on their Zernike mode, orbital angular
momentum state, or an arbitrarily randomized spatial mode basis. We discuss the
advantages and drawbacks of these proof-of-principle prototypes, and describe
future improvements. Our work points to a bright future for high-dimensional
MPLC-based technologies
All-optically untangling light propagation through multimode fibres
When light propagates through a complex medium, such as a multimode optical
fibre (MMF), the spatial information it carries is scrambled. In this work we
experimentally demonstrate an all-optical strategy to unscramble this light
again. We first create a digital model capturing the way light has been
scattered, and then use this model to inverse-design and build a complementary
optical system - which we call an optical inverter - that reverses this
scattering process. Our implementation of this concept is based on multi-plane
light conversion, and can also be understood as a diffractive artificial neural
network or a physical matrix pre-conditioner. We present three design
strategies allowing different aspects of device performance to be prioritised.
We experimentally demonstrate a prototype optical inverter capable of
simultaneously unscrambling up to 30 spatial modes that have propagated through
a 1m long MMF, and show how this enables near instantaneous incoherent imaging,
without the need for any beam scanning or computational processing. We also
demonstrate the reconfigurable nature of this prototype, allowing it to adapt
and deliver a new optical transformation if the MMF it is matched to changes
configuration. Our work represents a first step towards a new way to see
through scattering media. Beyond imaging, this concept may also have
applications to the fields of optical communications, optical computing and
quantum photonics.Comment: 18 pages, 11 figure
Classical antennae, quantum emitters, and densities of optical states
We provide a pedagogical introduction to the concept of the local density of
optical states (LDOS), illustrating its application to both the classical and
quantum theory of radiation. We show that the LDOS governs the efficiency of a
macroscopic classical antenna, determining how the antenna's emission depends
on its environment. The LDOS is shown to similarly modify the spontaneous
emission rate of a quantum emitter, such as an excited atom, molecule, ion, or
quantum dot that is embedded in a nanostructured optical environment. The
difference between the number density of optical states, the local density of
optical states, and the partial local density of optical states is elaborated
and examples are provided for each density of states to illustrate where these
are required. We illustrate the universal effect of the LDOS on emission by
comparing systems with emission wavelengths that differ by more than 5 orders
of magnitude, and systems whose decay rates differ by more than 5 orders of
magnitude. To conclude we discuss and resolve an apparent difference between
the classical and quantum expressions for the spontaneous emission rate that
often seems to be overlooked, and discuss the experimental determination of the
LDOS.Comment: 80 pages, 19 figure
Optical simulation of quantum mechanics on the Mobius strip, Klein's bottle and other manifolds, and Talbot effect
We analyse the evolution of the wavefunction of a quantum particle propagating on several compact manifolds, including the Klein bottle, Mobius strip and projective plane. We find analytically the stationary states and the energy spectrum and show that the wavefunction exhibits perfect revivals. Using the orbifold structure of the discussed manifolds, we establish the relation of wave evolution on the manifolds to Fresnel diffraction and consequently to the Talbot effect. This connection provides a novel method of optical simulation of the quantum motion on compact manifolds. We discuss some novel phenomena as well as the effects of topology on the properties of the waves on the manifolds
Quantum-classical correspondence in spin-boson equilibrium states at arbitrary coupling
It is known that the equilibrium properties of nanoscale systems can deviate
significantly from standard thermodynamics due to their coupling to an
environment. For the generalized -angled spin-boson model, here we
derive an explicit form of the classical mean force equilibrium state. Taking
the large spin limit of the quantum spin-boson model, we demonstrate that the
quantum-classical correspondence is maintained at arbitrary coupling strength.
This correspondence gives insight into the conditions for a quantum system to
be well-approximated by its classical counterpart. We further demonstrate that,
counterintuitively, previously identified environment-induced 'coherences' in
the equilibrium state of weakly coupled quantum spins, do not disappear in the
classical case. Finally, we categorise various coupling regimes, from
ultra-weak to ultra-strong, and find that the same value of coupling strength
can either be 'weak' or 'strong', depending on whether the system is quantum or
classical. Our results shed light on the interplay of quantum and mean force
corrections in equilibrium states of the spin-boson model, and will help draw
the quantum to classical boundary in a range of fields, such as magnetism and
exciton dynamics
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