Electromagnetic radiation propagating through any molecular system typically experiences a characteristic change in its polarization state as a result of light-matter interaction. Circularly polarized light is commonly absorbed or scattered to an extent that is sensitive to the incident circularity, when it traverses a medium whose constituents are chiral. This research assesses specific modifications to the properties of circularly polarized light that arise on passage through a system of surface-functionalized spherical nanoparticles, through the influence of chiral molecules on their surfaces. Non-functionalized nanospheres of atomic constitution are usually inherently achiral, but can exhibit local chirality associated with such surface-bound chromophores. The principal result of this investigation is the quantification of functionally conferred nanoparticle chirality, manifest through optical measurements such as circularly polarized emission. The relative position of chiral chromophores fixed to a nanoparticle sphere are first determined by means of spherical coverage co-ordinate analysis. The total electromagnetic field received by a spatially fixed, remote detector is then determined. It is shown that bound chromophores will accommodate both electric and magnetic dipole transition moments, whose scalar product represents the physical and mathematical origin of chiral properties identified in the detected signal. The analysis concludes with discussion of the magnitude of circular differential optical effects, and their potential significance for the characterization of surface-functionalized nanoparticles.
A wide range of mechanisms is available for achieving rapid optical responsivity in material components. Amongst them, some of the most promising for potential device applications are those associated with an ultrafast response and a short cycle time. These twin criteria for photoresponsive action substantially favor optical, over most other, forms of response such as those fundamentally associated with photothermal, photochemical or optomechanical processes. The engagement of nonlinear mechanisms to actively control the characteristics of optical materials is not new. Indeed, it has been known for over fifty years that polarization effects of this nature occur in the optical Kerr effect – although in fluid media the involvement of a molecular reorientation mechanism leads to a significant response time. It has more recently emerged that there are other, less familiar forms of optical nonlinearity that can provide a means for one beam of light to instantly influence another. In particular, major material properties such as absorptivity or emissivity can be subjected to instant and highly localized control by the transmission of light with an off-resonant wavelength. This presentation introduces and compares the key electrodynamic mechanisms, discussing the features that suggest the most attractive possibilities for exploitation. The most significant of such mechanistic features include the off-resonant activation of optical emission, the control of excited-state lifetimes, the access of dark states, the inhibition or re-direction of exciton migration, and a coupling of stimulated emission with coherent scattering. It is shown that these offer a variety of new possibilities for ultrafast optical switching and transistor action, ultimately providing all-optical control with nanoscale precision.
Up-conversion (UC) generally refers to any nonlinear optical process that facilitates the conversion of low energy
radiation into higher energy emission. Typically achieved in materials incorporating rare-earth ions, exploiting their rich
density of available electronic state transitions, non-parametric UC systems are often placed in categories including
excited state absorption, photon avalanche and energy transfer. The latter, energy transfer up-conversion achieves
nonlinear excitation of chromophores as a consequence of non-radiative resonance energy transfer (RET) events, through
coupling with neighboring sensitizer ions. Being susceptible to the local electromagnetic environment, the mechanism
of RET is known to be influenced by surrounding matter, underscoring the importance of similar channels for media
control within UC materials. By developing the principles of two-center UC, a fully quantized representation of local
electronic structure and electrodynamics is extended through the introduction of a mediator species – a vicinal, nonabsorbing
chromophore. This theory underpins a new application of the medium-modified energy transfer theory to
three-center UC. The present report then considers an alternative up-conversion mechanism in which pairs of identical
donors transfer energy to the acceptor species, promoting two-photon excitation and shorter wavelength emission. The
mechanism for this three-center system proves to be significantly influenced by a fourth, essentially passive
chromophore. Investigations of the influence of this mediator, in improving or inhibiting RET, determine parameters
that can be modified to improve the UC efficiency. The results provide insight into factors that might assist the
optimization of laser active media, and the improvement of optical characteristics in designer materials.
Interactions between light and molecular matter featuring photon absorption are commonly associated with excitation of individual chromophores. Subsequent relaxation is achieved through numerous mechanisms, such as scattering and energy transfer to neighbouring chromophores. The efficiency of such processes depends on many factors, including the intensity and wavelength of the optical input, the absorption cross-section of the molecule and the relative orientation of molecular components. New photonic materials are developed on the principle that such factors are controllable, duly tailoring the system to suit new technological applications. The parameters that determine the linear response in multicomponent materials are typically assessed within the theoretical framework of classical optics. However, with improved interest and capability in fabricating nanoscale structures (especially those that exhibit quantum effects), it is becoming increasingly relevant to develop theory that more faithfully represents how individual photons engage with matter. In such constructs, the interaction of materials with optical waves and photons is highly dependent on local molecular environment, therefore surrounding structures and ancillary species exert forms of influence on the photophysical processes inherent within dielectric media. At optical wavelengths where the secondary structure displays little intrinsic optical absorption, the role of such components is often interpreted as modifying the input through a corrective local index of refraction. Although expedient in the discussion of bulk properties of a macroscopic medium, it is reasonably supposed at a local chromophore-photon level that optical mechanisms operate in a different fashion. Using a fully quantized approach to the representation of local molecular electronic structure and electrodynamics, this research develops rigorous theory and a corresponding physical interpretation of how photon absorption, scattering and energy transfer are modified by vicinal, non-absorbing chromophores. The results provide insight into the mechanisms achieved within multi-chromophore systems, highlighting factors that assist in the optimization of optical characteristics in designer materials.
The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for
extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve
mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can
fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are
synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing
the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of
producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not
involving the limitations associated with using photochemical change, nor the high intensities required to produce and
manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can
be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the
generation of continuous rotary motion, the ultimate aim of molecular motor design.
In the optical excitation of many nanoscale systems, the primary result of photon absorption is an electronic excitation
that is typically followed by ultrafast relaxation processes. The losses associated with such relaxation generally produce
a partial degradation of the optical energy acquired, before any ensuing photon emission occurs. Recent work has shown
that the intensity and directional character of such emission may be significantly influenced through engagement with a
completely off-resonant probe laser beam of sufficient intensity: the mechanism for this optical coupling is a secondorder
nonlinearity. It is anticipated that the facility to actively control fluorescent emission in this way may lead to new
opportunities in a variety of applications where molecular chromophores or quantum dots are used. In the latter
connection it should prove possible to exploit the particle size dependence of the nonlinear optical dispersion, as well as
that of the emission wavelength. Specific characteristics of the effect are calculated, and suitable experimental
implementations of the mechanism are proposed. We anticipate that this all-optical control device may introduce
significant new perspectives to fluorescence imaging techniques and other analytical applications.
The accessibility of tunable, ultrafast laser sources has spurred the development and wide application of specialized
microscopy techniques based on chromophore fluorescence following
two- and three-photon absorption. The attendant
advantages of such methods, which have led to a host of important applications including three-dimensional biological
imaging, include some features that have as yet received relatively little attention. In the investigation of cellular or subcellular
processes, it is possible to discern not only on the location, concentration, and lifetime of molecular species, but
also the orientations of key fluorophores. Detailed information can be secured on the degree of orientational order in
specific cellular domains, or the lifetimes associated with the rotational motions of individual fluorophores; both are
accessible from polarization-resolved measurements. This paper reports the equations that are required for any such
investigation, determined by robust quantum electrodynamical derivation. The general analysis, addressing a system of
chromophores oriented in three dimensions, determines the fluorescence signal produced by the nonlinear polarizations
that are induced by multiphoton absorption, allowing for any rotational relaxation. The results indicate that multiphoton
imaging can be further developed as a diagnostic tool, either to selectively discriminate micro-domains in vivo, or to
monitor dynamical changes in intracellular fluorophore orientation.
Multiphoton fluorescence microscopy is now a well-established technique, currently attracting much interest across all
fields of biophysics - especially with regard to enhanced focal resolution. The fundamental mechanism behind the
technique, identified and understood through the application of quantum theory, reveals new optical polarization features
that can be exploited to increase the information content of images from biological samples. In another development,
based on a newly discovered, fundamentally related mechanism, it emerges the passage of off-resonant probe laser pulses
may characteristically modify the intensity of single-photon fluorescence, and its associated optical polarization
behavior. Here, the probe essentially confers optical nonlinearity on the decay transition, affording a means of optical
control over the fluorescent emission. Compared to a catalogue of other laser-based techniques widely used in the life
sciences, most suffer limitations reflecting the exploitation of specifically lifetime-associated features; the new optical
control mechanism promises to be more generally applicable for the determination of kinetic data. Again, there is a
prospect of improving spatial resolution, non-intrusively. It is anticipated that tight directionality can be imposed on
single-photon fluorescence emission, expediting the development of new imaging applications. In addition, varying the
optical frequency of the probe beam can add another dimension to the experimental parameter space. This affords a
means of differentiating between molecular species with strongly overlapping fluorescence spectra, on the basis of their
differential nonlinear optical properties. Such techniques significantly extend the scope and the precision of spatial and
temporal information accessible from fluorescence studies.
With appropriately selected optical frequencies, pulses of radiation propagating through a system of chemically distinct
and organized components can produce areas of spatially selective excitation. This paper focuses on a system in which
there are two absorptive components, each one represented by surface adsorbates arrayed on a pair of juxtaposed
interfaces. The adsorbates are chosen to be chemically distinct from the material of the underlying surface. On
promotion of any adsorbate molecule to an electronic excited state, its local electronic environment is duly modified, and
its London interaction with nearest neighbor molecules becomes accommodated to the new potential energy landscape.
If the absorbed energy then transfers to a neighboring adsorbate of another species, so that the latter acquires the
excitation, the local electronic environment changes and compensating motion can be expected to occur. Physically, this
is achieved through a mechanism of photon absorption and emission by molecular pairs, and by the engagement of
resonance transfer of energy between them. This paper presents a detailed analysis of the possibility of optically
effecting such modifications to the London force between neutral adsorbates, based on quantum electrodynamics (QED).
Thus, a precise link is established between the transfer of excitation and ensuing mechanical effects.
On the propagation of radiation with a suitably resonant optical frequency through a dense chromophoric system - a
doped solid for example - photon capture is commonly followed by one or more near-field transfers of the resulting
optical excitation, usually to closely neighboring chromophores. Since the process results in a change to the local
electronic environment, it can be expected to also shift the electromagnetic interactions between the participant optical
units, producing modified inter-particle forces. Significantly, it emerges that energy transfer, when it occurs between
chromophores or particles with electronically dissimilar properties (such as differing polarizabilities), engenders hitherto
unreported changes in the local potential energy landscape. This paper reports the results of quantum electrodynamical
calculations which cast a new light on the physical link between these features. The theory also elucidates a significant
relationship with Casimir-Polder forces; it transpires that there are clear and fundamental links between dispersion forces
and resonance energy transfer. Based on the results, we highlight specific effects that can be anticipated when laser light
propagates through an interface between two absorbing media. Both steady-state and pulsed excitation conditions are
modeled and the consequences for interface forces are subjected to detailed analysis.