Novel optical absorbers are made using titanium metal and silicon dioxide thin films in specific layer thicknesses that create a low fractional reflectance and transmittance material. The absorbers have high absorption fraction measured from 200 - 1400 nm at normal incidence and specific reflectance band peaks. Layer thicknesses affect the wavelength at which light is reflected or absorbed and opens up tuneability for specific wavelengths within the visible spectrum depending on applications such as for solar cells and light modulation.
Optical eigenmodes describe coherent solutions of Maxwells equations that are orthogonal to each other. These modes form a natural basis set of the electromagnetic Hilbert space that can be used to describe optical scattering interactions in a simple way. Many of the properties defined in quantum mechanics can formally be found in the optical eigenmodes framework. For example, the Hilbert spaces defined by two different scattering operators are separable only if the two operators commute with each other. Here, we expand the optical eigenmode framework to partially coherent light fields. In this case, we remark that the eigenmode decomposition of partially coherent fields leads to a formalism similar to the density matrix formalism used in quantum mechanics.
The use of exotic optical modes is becoming increasingly widespread in microscopy. Particularly, propagation-invariant beams, such as Airy and Bessel beams and optical lattices, have been particularly useful in light-sheet fluorescence microscopy (LSFM) as they enable high-resolution imaging over a large field-of-view (FOV), possess a resistance to the deleterious effects of specimen induced light scattering, and can potentially reduce photo-toxicity.
Although these propagation-invariant beams can resist the effects of light scattering to some degree, and there has been some interest in adaptive-optical methods to correct for beam aberrations when they cannot, scattering and absorption of the illuminating light-sheet limit the penetration of LSFM into tissues and results in non-uniform intensity across the FOV.
A new degree of control over the intensity evolution of propagation-invariant beams can overcome beam losses across the FOV, restoring uniform illumination intensity and therefore image quality. This concept is compatible with all types of propagation-invariant beams and is characterised in the context of light-sheet image quality.
Another property to control is the wavelength of light used. Optical transmission through tissue is greatly improved at longer wavelengths into the near-infrared due to reduced Rayleigh scattering and two-photon excitation has proved beneficial for imaging at greater depth in LSFM. Three-photon excitation has already been demonstrated as a powerful tool to increase tissue penetration in deep brain confocal microscopy, and when combined with beam shaping can also be a powerful illumination strategy for LSFM.
Recent progress in shaping optical fields for LSFM will be presented.
In linear optics, the concept of a mode is well established. Often these modes correspond to a set of fields that are mutually orthogonal with intensity profiles that are invariant as they propagate through an optical medium. More generally, one can define a set of orthogonal modes with respect to an optical measure that is linear in intensity or quadratic/Hermitian in the fields using the method of Optical Eigenmodes (OEi). However, if the intensity of the light is large, the dipole response of an optical medium introduces nonlinear terms to Maxwell’s equations. In this nonlinear regime such terms influence the evolution of the fields and the principle of superposition is no longer valid and consequently, the method of Optical Eigenmodes breaks down. In this work, we define Optical Eigenmodes in the presence of these nonlinear source terms by introducing small perturbation fields onto a nonlinear background interaction and show how this background interaction influences the symmetries associated with the eigenmodes. In particular, by introducing orbital angular momentum (OAM) to the Hilbert space of the perturbation and background fields, we observe conservation laws and symmetries for which we derive associated operators.
When light scatters from an object, it can impart some physical quantity such as momentum or angular momentum. This can act as a measurement on the photon, which collapses on to an eigenstate of the measurement operator. However the corresponding operator is not the same as that describing the total linear or angular momentum in free space. Optical eigenmodes provide a powerful method to describe this interaction by expanding the field as a linear combination of some basis modes and examining the eigenvalues and eigenvectors of the quadratic measure in question. We extend this to the quantum case by writing the quantum operator corresponding to a given measurement such as energy, momentum or angular momentum as a superposition of creation and annihilation operators for each eigenmode. Upon measurement we find that the possible states of a single photon are simply the classical eigenmodes of the measurement. As an application, we examine the force and torque on a general, possibly anisotropic, material. By looking at eigenvalues of the measurement operator we show that the amount of a given quantity transferred in an interaction with matter is not in general the expected amount which a photon carries in free space, even at the single photon level. In particular the difference in linear or angular momentum from before and after is in general not equal to ~k or ~ which are the eigenvalues of these quantities in free space.
We demonstrate the transfer of orbital angular momentum (OAM) to optically levitated microparticles in vacuum. We create two-dimensional (2D) and three-dimensional (3D) optical potentials possessing OAM. In the former case the microparticle is placed within a Laguerre-Gaussian (LG) beam and orbits the annular beam profile with increasing angular velocity as the air drag coefficient is reduced. Our results reveal that there is a fundamental limit to the OAM that may be transferred to a trapped particle, dependent upon the beam parameters and inertial forces present. Whilst a LG beam scales in size with azimuthal index, recently we have created a “perfect vortex” beam whose radial intensity profile and radius are both independent of topological charge. As the Fourier transform of a “perfect vortex” yields a Bessel beam, imaging a “perfect vortex”, with its subsequent propagation thus realises a complex three-dimensional optical field. In this scenario we load individual silica microparticles into this field where the optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle. As a result the trapped microparticle exhibits a complex three-dimensional motion that includes a periodic orbital motion between the Bessel and the “perfect vortex” beam. We are able to determine the three dimensional optical potential in situ by tracking the particle. This first demonstration of trapping microparticles within a complex 3D optical potential in vacuum opens up new possibilities for fundamental studies of many-body dynamics, mesoscopic entanglement, and optical binding.
There are many ways to calculate the optical forces acting on scattering particles such as Maxwell's stress tensor, Lorentz forces, gradient and scattering forces, Lorenz-Mie formalism, T-matrix. All these approaches use the electromagnetic field and define the amount of linear momentum transferred to the scattering particles. The resulting momentum transferred is proportional to the intensity of the incident fields, however, the momentum of single photons (hbar k) does not naturally appear in these classical expressions. Here, we discuss an alternative Maxwell's stress tensor based formalism that renders the classical electromagnetic field momentum compatible to the quantum mechanical one. This is achieved through the introduction of the quantum conversion which allows the transformation, including units, of the classical fields to wave-function equivalent fields.
Scattering is one of the simplest light mater interactions possible. For spherical particles, this process can be described using the Lorenz-Mie theory, which makes use of vector spherical harmonic solutions of Maxwell’s equations to represent the fields involved. Using these solutions it is possible to describe the light field scattered from microscopic spherical particles and thus represent the field around a scattering object as a function of the incident fields. These solutions also allow us to determine the optical momentum transfer to the scattering object. This can be calculated using Maxwell stress tensor. Here, we use this approach to calculate the quadratic relationship between the incident field and the optical forces acting on the scattering objects. This relationship defines also a set of orthogonal optical eigenmodes, which delivers a natural basis to describe momentum transfer in light-matter interactions. Using this natural description of the momentum transfer it is possible to define, for each numerical aperture, particle size or geometry the optimal trapping beam in 1, 2 or 3 dimensions. We present a study as a function of the particle parameter and conclude on the maximum achievable trapping stiffness enhancement factor as a function of these parameters.
KEYWORDS: Ultrasonics, Interfaces, Solids, Superposition, Wave propagation, Energy efficiency, Energy transfer, Nondestructive evaluation, Aerospace engineering, Current controlled current source
Non-collinear ultrasonic wave mixing for nonlinear ultrasonics is used for various NDT&E applications to characterize structure/material state. Experiments are frequently conducted using immersion ultrasonic techniques. In this configuration various wave modes are generated at the fluid-solid interface. We have characterised this scenario experimentally and developed a 2D numerical model to analyse the influence of the interface on the possible second-order interactions of the ultrasonic waves. The model is based on the decomposition of the ultrasonic wave into a superposition of monochromatic plane waves. As these waves propagate through the interface and solid, their nonlinear interaction defines a flow of energy between the different modes. Using the Murnaghan's nonlinear interaction potential, we have studied the efficiency of this energy transfer as a function different geometrical parameters.
Widespread use of optical manipulation in combination with advanced imaging techniques will be accelerated by compact, optically simple approaches which are readily integrated into advanced microscopy platforms. For example, optical manipulation has been combined with confocal, multi-photon, and STED microscopes. However, these typically require addition of optical components into the existing beam paths of the microscope, increasing complexity and potentially compromising image quality.
Optical fiber trapping (OFT) offers an ultra-compact and simple solution but compromises on trap quality due to the low numerical aperture (NA) and short manipulation distance of optical fibers. Tapered fibers can be fabricated but this further reduces the manipulation distance and requires access to specialist fabrication facilities.
Here we present a compact, single-beam, high NA OFT probe design based on a graded-index (GRIN) micro-objective lens and single-mode fiber. The OFT probe uses only off-the-shelf components, enables optical trapping at a distance of 200μm from the probe tip, and is compatible with inverted imaging systems.
A challenge with specialist imaging systems is the incompatibility between the specialist imaging modality of the platform and the imaging modality required for trap characterization, resulting in noisy and poor trap characterisation data. To overcome this challenge, we developed an adaptive image filter based on principal component analysis (PCA). The filter separates orthogonal degrees of motion in trap characterisation movies and strong stochastic noise can be removed before tracking, resulting in accurate characterisation.
We demonstrate the use of this PCA image filter for in situ characterisation of the GRIN lens OFT probe.
Light-sheet microscopy (LSM) is an emergent fluorescence microscopy technique showing great promise for biomedical research. LSM enables rapid, high-contrast imaging of large specimens with high spatiotemporal resolution and minimal photo-damage. When imaging large specimens, the intensity of the light-sheet reduces across the field-of-view (FOV) due to absorption. This results in an image with spatially-variant intensity, affecting quantitative measurements, and ultimately limits the penetration depth of the illumination. Some existing approaches to alleviate this issue involve illuminating the sample from multiple directions or rotating the sample. These approaches are not always practical and restrict specimen choice.
Separately, propagation-invariant light modes have been used to develop high-resolution LSM techniques as they can overcome the natural divergence of a Gaussian beam, producing a thin and uniform light-sheet over long distances. Most notably, Bessel and Airy beam-based LSM techniques have been implemented.
For propagation-invariant beams, there exists a mapping between the transverse coordinate in the pupil plane of a lens, and the axial propagation in the focal plane. Spatially-variant amplitude modulation therefore offers control of the intensity of the beam with propagation.
In this paper, we report that such amplitude modulation in the pupil plane of an Airy LSM can yield a system which counteracts absorption of the light-sheet and gives uniform intensity across the FOV with a single acquisition and without restricting specimen choice. This technique is an alternative to, and may be complimented by, wavefront correction. We demonstrate this technique through numerical simulations and experimental validation in absorbing tissue phantoms.
We synthesize, optically trap, and rotate individual nanovaterite crystals with a
mean particle radius of 423 nm. Rotation rates of up to 4.9 kHz in heavy water are recorded [1].
Laser-induced heating due to residual absorption of the nanovaterite particle results in the
superlinear behavior of the rotation rate as a function of trap power. A finite element method
based on the Navier-Stokes model for the system allows us to determine the residual optical
absorption coefficient for a trapped nanovaterite particle. This is further confirmed by the
theoretical model. Our data reveal that the nanoparticle experiences a different Stokes drag
torque or force depending on whether we consider rotational or translational motion, which is
in a good agreement with the theoretical prediction of the rotational hot Brownian motion [2].
The data allow us to determine the correction factors for the local viscosity for both the
rotational and translational motion of the nanoparticle. The use of nanovaterite particles opens
up new studies for levitated optomechanics in vacuum [3–6] as well as microrheological
properties of cells or biological media [7]. For these latter studies, nanovaterite offers prospects
of microviscosity measurements in ultrasmall volumes and, due to its size, potentially simpler
uptake by cellular media [8].
We demonstrate the transfer of orbital angular momentum to optically levitated
microparticles in vacuum [1]. We prepare two-dimensional and three-dimensional optical
potentials. In the former case the microparticle is placed within a Laguerre-Gaussian beam
and orbits the annular beam profile with increasing angular velocity as the air drag coefficient
is reduced. We explore the particle dynamics as a function of the topological charge
of the levitating beam. Our results reveal that there is a fundamental limit to the orbital angular
momentum that may be transferred to a trapped particle, dependent upon the beam
parameters and inertial forces present. This effect was predicted theoretically [2] and can be
understood considering the underlying dynamics arising from the link between the magnitude
of the azimuthal index and the beam radius [3].
Whilst a Laguerre-Gaussian beam scales in size with azimuthal index `, recently we
have created a “perfect” vortex beam whose radial intensity profile and radius are both
independent of topological charge [4, 5]. As the Fourier transform of a perfect vortex yields
a Bessel beam. Imaging a perfect vortex, with its subsequent propagation thus realises a
complex three dimensional optical field. In this scenario we load individual silica microparticles
into this field and observe their trajectories. The optical gradient and scattering forces
interplay with the inertial and gravitational forces acting on the trapped particle, including
the rotational degrees of freedom. As a result the trapped microparticle exhibits a complex
three dimensional motion that includes a periodic orbital motion between the Bessel and
the perfect vortex beam. We are able to determine the three dimensional optical potential
in situ by tracking the particle. This first demonstration of trapping microparticles within
a complex three dimensional optical potential in vacuum opens up new possibilities for
fundamental studies of many-body dynamics, mesoscopic entanglement [6, 7], and optical
binding [8, 9].
Rotational control over optically trapped particles has gained significant prominence in recent years. The marriage between light fields possessing optical angular momentum and the material properties of microparticles has been useful to controllably spin particles in liquid, air and vacuum. The rotational degree of freedom adds new functionality to optical traps: in addition to allowing fundamental tests of optical angular momentum, the transfer of spin angular momentum in particular can allow measurements of local viscosity and exert local stresses on cellular systems.
We demonstrate optical trapping and controlled rotation of nanovaterite crystals. These particles represent the smallest birefringent crystals ever trapped and set into rotation. Rotation rates of up to 5kHz in water are recorded, representing the fastest rotation to date for dielectric particles in liquid. Laser-induced heating results in the superlinear behaviour of the rotation rate as a function of trap power. We study both the rotational and translational modes of trapped nanovaterite crystals. The particle temperatures derived from those two optomechanical modes are in good agreement, which is supported by a numerical model revealing that the observed heating is dominated by absorption of light by the particles rather than by the surrounding liquid. A comparison is performed with trapped silica particles of similar size.
The use of nanovaterite particles open up new studies for levitated optomechanics in vacuum as well as microrheological properties of cells or biological media. Their size and low heating offers prospects of viscosity measurements in ultra-small volumes and potentially simpler uptake by cellular media.
A photon can carry orbital angular momentum equal to an integer number of the reduced Planck’s constant. This principle expresses itself in geometrical quantization of optical vortex beams, which thus can propagate only in the form of fields having a helically wavefront characterized by an integer valued topological charge. However, one can create an optical vortex beam of an effective fractional charge by combining multiple integer vortices. Here, we investigate this apparent violation of the geometrical quantization of orbital angular momentum of light. Our approach relies on observation of the light-induced motion of a microscopic particle, which thus acts as an optomechanical probe for the optical vortex beam. A fractional topological charge corresponds to an abrupt jump in the helical phase front of the beam. This singularity expresses itself as an off-axis disturbance in the intensity profile, and thus complicates the optomechanical probing. We overcome this problem by distributing the disturbance along the vortex ring, so that a microparticle can continuously orbit due to the orbital angular momentum transfer. We demonstrate theoretically that whatever effort is put into smoothing the fractional vortex ring (as long as the net topological charge is fixed), the particle’s orbital motion cannot be as uniform as in the case of an integer vortex beam. We support this prediction by experimental proof. The experimental technique benefits from the recently introduced “perfect” vortex beams which allow an optically trapped particle to orbit along a constant trajectory irrespective of any topological charge.
There are many ways to calculate the optical forces acting on scattering particles such as Maxwell’s stress tensor, Lorentz forces, gradient and scattering forces, Lorenz-Mie formalism, T-matrix. All these approaches use the electromagnetic field and define the amount of linear momentum transferred to the scattering particles. The resulting momentum transferred is proportional to the intensity of the incident fields, however, the single photon momentum (hbar k) does not naturally appear in these classical expressions. In this paper, we discuss an alternative Maxwell’s stress tensor based formalism that renders the classical electromagnetic field momentum compatible to the quantum mechanical one.
Vortex beams with different topological charge usually have different profiles and radii of peak intensity. This introduces a degree of complexity the fair study of the nature of optical OAM (orbital angular momentum). To avoid this, we introduced a new approach by creating a perfect vortex beam using an annular illuminating beam with a fixed intensity profile on an SLM that imposes a chosen topological charge. The radial intensity profile of such an experimentally created perfect vortex beam is independent to any given integer value of its topological charge. The well-defined OAM density in such a perfect vortex beam is probed by trapping microscope particles. The rotation rate of a trapped necklace of particles is measured for both integer and non-integer topological charge. Experimental results agree with the theoretical prediction. With the flexibility of our approach, local OAM density can be corrected in situ to overcome the problem of trapping the particle in the intensity hotspots. The correction of local OAM density in the perfect vortex beam therefore enables a single trapped particle to move along the vortex ring at a constant angular velocity that is independent of the azimuthal position. Due to its particular nature, the perfect vortex beam may be applied to other studies in optical trapping of particles, atoms or quantum gases.
We developed a SERS biosensor based on gold fishnets fabricated by using e-beam lithography. This device is used for glycerophosphoinositol (GroPIns) molecule sensing. GroPIns is an abundant component of cell cytosol and high GroPIns levels have been reported in several tumour cells. We demonstrate that our SERS sensor is able to accurately and quantitatively determine the concentration of GroPIns. These results indicate that SERS may provide a novel platform technology to identify GroPIns profiles in disease pathogenesis.
For any optical system, optical eigenmodes describe solutions of Maxwells equations that are orthogonal to each other. In their simplest free space form, these modes correspond, for example, to Bessel, Laguerre-Gaussian or Hermite-Gaussian beams. However, the orthogonality property is not limited to the intensity of the optical field but more generally the optical eigenmode decomposition can be applied to the linear and angular momentum arising from complex coherent beams. These modes can be seen as describing the independent degrees of freedom of the optical system and are characterized by the mode, their density and coupling efficiency. It is interesting to study the effect of different optical systems on the density of the optical degrees of freedom propagating through them. Here, we look at systems containing different elements such as: dielectric, meta-material and random lenses. Using the optical eigenmode decomposition, we determine their density in these different cases and discuss the origin of the variations observed. Further, we study the overall number of optical degrees of freedom accessible including linear and angular momentum of optical beams.
It is known that a properly arranged distribution of nanoholes on a metallic slab is able to produce, in far field conditions, light confinement at sub-diffraction and even sub-wavelength scale. The same effect can also be implemented by the use of Optical Eigenmode (OEi) technique. In this case, a spatial light modulator (SLM) encodes phase and amplitudes of N probe beams whose interference is able to lead to sub-wavelength confinement of light focused by an objective. The OEi technique has been already used in a wide range of applications, such as photoporation, confocal imaging, and coherent control of plasmonic nanoantennas. Here, we describe the application of OEi technique to a single valve of a marine diatom. Diatoms are ubiquitous monocellular algae provided with an external cell wall, the frustule, made of hydrated porous silica which play an active role in efficient light collection and confinement for photosynthesis. Every frustule is made of two valves interconnected by a lateral girdle band. We show that, applying OEi illumination to a single diatom valve, we can achieve unprecedented sub-diffractive focusing for the transmitted light.
In this paper we present theory and simulations of an optical spring mirror with emphasis on the incident laser
beam configuration and the associated optical trapping forces. We elucidate the physical mechanisms underlying the
optical trapping using the example of an incident Gaussian beam and demonstrate that guided-wave trapping shows
particular promise for stable trapping in both the translational and rotational degrees of freedom.
Binding between optically co-trapped micro-particles occurs when the scattered optical fields are sufficient to com pete with the trapping forces. Such optical binding is seen as being pertinent to large-scale micro-manipulation due to the significant forces present within an optical trapping system comprising multiple micro-particles. One aspect of optical binding involves an inter-particle force relationship that is strongly dependent on optical fre quency. In our study we theoretically show that a broadened spectral content associated with frequency doubled broadband ultrashort pulses can result in spatial overlap of contained frequencies. The binding force oscillations are averaged out when sufficient spectral content is present within the pulse.
We present an experimental technique allowing size-based all-optical sorting of gold nanoparticles. The technique is based on the red-shift of plasmon resonance, due to retardation effects, with increasing particle size. As a result, smaller gold nanoparticles are influenced strongly by shorter wavelengths whereas larger gold nanoparticles are influenced more strongly by longer wavelengths. We utilise this retardation effect and realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.
The optical eigenmode technique offers a global optimisation method delivering the electromagnetic field
profile enhancing any linear light-matter interaction. Here, we use this approach to study the optimal beams
for trapping, pushing and pulling (tractor beam) a mesoscopic micro-object at resonance with the incident
light field.
KEYWORDS: Raman spectroscopy, Luminescence, Tissues, Modulation, Signal to noise ratio, Bone, Tissue optics, Principal component analysis, Spectroscopy, Chemical analysis
In the field of biomedical optics, Raman spectroscopy is a powerful tool for probing the chemical composition of biological samples. In particular, fiber Raman probes play a crucial role for in vivo and ex vivo tissue analysis. However, the high-fluorescence background typically contributed by the auto fluorescence from both a tissue sample and the fiber-probe interferes strongly with the relatively weak Raman signal. Here we demonstrate the implementation of wavelength-modulated Raman spectroscopy (WMRS) to suppress the fluorescence background while analyzing tissues using fiber Raman probes. We have observed a significant signal-to-noise ratio enhancement in the Raman bands of bone tissue, which have a relatively high fluorescence background. Implementation of WMRS in fiber-probe-based bone tissue study yielded usable Raman spectra in a relatively short acquisition time ( ∼ 30 s), notably without any special sample preparation stage. Finally, we have validated its capability to suppress fluorescence on other tissue samples such as adipose tissue derived from four different species.
We decompose the light field in the focal plane of an imaging system into a set of optical eigenmodes. Subsequently,
the superposition of these eigenmodes is identified, that optimizes certain aspects of the imaging process.
In practice, the optical eigenmodes modes are implemented using a liquid crystal spatial light modulator. The
optical eigenmodes of a system can be determined fully experimentally, taking aberrations into account. Alternatively,
theoretically determined modes can be encoded on an aberration corrected spatial light modulator. Both
methods are shown to be feasible for applications. To achieve subdiffractive light focussing, optical eigenmodes
are superimposed to minimize the width of the focal spot within a small region of interest. In conjunction with
a confocal-like detection process, these spots can be utilized for laser scanning imaging. With optical eigenmode
engineered spots we demonstrate enhanced two-point resolution compared to the diffraction limited focus and a
Bessel beam. Furthermore, using a first order ghost imaging technique, optical eigenmodes can be used for phase
sensitive indirect imaging. Numerically we show the phase sensitivity by projecting optical eigenmodes onto a
Laguerre-Gaussian target with a unit vortex charge. Experimentally the method is verified by indirect imaging
of a transmissive sample.
Raman spectroscopy is a non-invasive technique offering great potential in the biomedical field for label-free
discrimination between normal and tumor cells based on their biochemical composition. First, this contribution describes
Raman spectra of lymphocytes after drying, in laser tweezers, and trapped in a microfluidic environment. Second,
spectral differences between lymphocytes and acute myeloid leukemia cells (OCI-AML3) are compared for these three
experimental conditions. Significant similarities of difference spectra are consistent with the biological relevance of the
spectral features. Third, modulated wavelength Raman spectroscopy has been applied to this model system to
demonstrate background suppression. Here, the laser excitation wavelength of 785 nm was modulated with a frequency
of 40 mHz by 0.6 nm. 40 spectra were accumulated with an exposure time of 5 seconds each. These data were subjected
to principal component analysis to calculate modulated Raman signatures. The loading of the principal component shows
characteristics of first derivatives with derivative like band shapes. The derivative of this loading corresponds to a
pseudo-second derivative spectrum and enables to determine band positions.
Standard Raman spectroscopy (SRS) is a noninvasive technique that is used in the biomedical field to discriminate between normal and cancer cells. However, the presence of a strong fluorescence background detracts from the use of SRS in real-time clinical applications. Recently, we have reported a novel modulated Raman spectroscopy (MRS) technique to extract the Raman spectra from the background. In this paper, we present the first application of MRS to the identification of human urothelial cells (SV-HUC-1) and bladder cancer cells (MGH) in urine samples. These results are compared to those obtained by SRS. Classification using the principal component analysis clearly shows that MRS allows discrimination between Raman spectra of SV-HUC-1 and MGH cells with high sensitivity (98%) and specificity (95%). MRS is also used to distinguish between SV-HUC-1 and MGH cells after exposure to urine for up to 6 h. We observe a marked change in the MRS of SV-HUC-1 and MGH cells with time in urine, indicating that the conditions of sample collection will be important for the application of this methodology to clinical urine samples.
In this paper, we explore the propagation of light through disordered material and ask whether we can create an optimal
focus in such a scenario. We use the complex modulation of the input light (i.e. modulation in both phase and amplitude)
for these studies, implemented by use a spatial light modulator (SLM) and show trapping and manipulation through a
static turbid medium. We then extend the system to create a tandem SLM system with an acousto-optic deflector. This
has further advantages as we can now not only project light fields into turbid media but can also create interference-free
mode superpositions of light fields such as Laguerre-Gaussian (LG) and Bessel modes. This is illustrated by controlled
rotation of trapped particles in weighted, interference-free superpositions of LG beams of opposite order but equal
magnitude.
We discuss a powerful technique for removing optical aberrations from optical systems with a spatial light modulator.
In optical trapping systems this technique enables compensating for wavefront and amplitude deviations
directly at the sample chamber thus bringing significant enhancement of optical trapping performance.
We have used the Maxwell stress tensor method to calculate the optical forces acting upon a dielectric nanosphere
in the proximity of gold nanoantenna structure optically excited by a plane wave. We have explored the dependence
of optical forces for the full range of excitation angles with the conclusion that the maximum force occurs
for the excitation at critical angle. The large force at this angle is, however, at the expense of greatly increased
intensity in the volume of the particle from which we conclude that the important measure for the trapping
efficiency in the case of plasmonic nanostructures is not the incident intensity of the plane wave, but rather
the local intensity averaged over the volume of the particle. Our calculations further show multiple trapping
sites with similar trapping properties, which leads to uncertainty in the trapping position. Furthermore, our
calculations show that the heating effects might play a significant role in the experimentally observed trapping.
KEYWORDS: Raman spectroscopy, Luminescence, Modulation, Signal to noise ratio, Polymers, Molecules, Molecular spectroscopy, Biological research, Optical filters, In vivo imaging
Raman spectroscopy is a label-free and non-invasive method that measures the inelastic scattered light
from a sample giving insight into the vibration eigenmodes of the excited molecules. Raman spectroscopy
provides a detailed chemical composition of the sample, constituting a sort of its chemical fingerprint.
Although Raman spectroscopy is a useful technique to identify and quantify species in a given matrix, it
has been severely limited in its applicability by fluorescence. Spectrally, this fluorescence occurs at the
same wavelength as the Raman signal and is often several orders of magnitude more intense that the weak
chemical transitions probed by Raman spectroscopy. Often, this fluorescence background and its natural
variability make biochemical analysis using Raman spectroscopy impractical. In this work, we present the
theory and the implementation of an innovative modulated Raman spectroscopy technique to filter out
the Raman spectra from the fluorescence background by modulating of the excitation wavelength. The
method is based on the continuous wavelength shift of the Raman peaks with the modulation of the laser
wavelength while the fluorescence background remains static. Exploiting this physical property allows us
to clearly distinguish between the Raman signal and the fluorescence background. Our method is related
to wavelength shifting Raman spectroscopy but incorporates two key novel elements: (i) the use of more
than two excitation wavelengths and (ii) multi-channel lock-in detection of the Raman signal for suppression
of the fluorescence background. Our results establish a direct and practical approach for fluorescence
background suppression in 'real-time' Raman spectroscopy for in-vivo biomedical applications.
The use of Raman spectroscopy for biomedical applications requires overcoming the obstacle of the broad
fluorescence background that is generally generated in biological samples. Recently, we have developed a
new modulation method for separating the weak Raman peaks from the strong fluorescence background.
The novel method is based on the periodical modulation of the excitation wavelength and uses the
principle of multi-channel lock-in detection. By continuously modulating the excitation wavelength it
is possible to shift the Raman peaks while the fluorescence background remains essentially constant.
The powerful capabilities of this novel method are demonstrated by acquiring spectra from different
location (nucleus, cytoplasm and membrane) inside a CHO cell. In fact, we show that our modulated
Raman spectroscopy provides, with higher efficiency than the standard one, Raman spectra of different
locations within a single cell, suggesting that this minimally invasive optical technology could be applied
for bio-medical diagnosis and imaging.
The Maxwell stress tensor method is used to calculate the optical forces acting upon a glass nanosphere in the proximity of optically excited gold nanoantenna structures. The dependence of optical forces over a full range of excitation angles is explored: the total internal reflection excitation does not bring any particular advantage to trapping efficiency when compared to the normal incidence excitation. Our calculations show multiple trapping sites with similar trapping properties for the normal and the total internal reflection cases, respectively; furthermore, the convective heating probably dominates over any optical forces in such systems.
Optical micromanipulation offers a unique insight into light-matter interaction at the mesoscopic level within
the classical framework. The optical forces used in this process originate from the conservation of energy and
momentum during an optical interaction. They can be determined through the use of the electromagnetic
energy-momentum tensor which describes the flux of momentum and energy. However, can we use this direct
opto-mechanical interaction to further our understanding of the classical nature of photons? In this paper,
I generalize the momentum energy tensor and determine its eigenmodes for a given scattering object. These
eigenmodes decompose the classical electromagnetic field into modes that have formally similar properties to
those associated with photons i.e. modes with uniquely defined energy, momentum and polarization. This
classical interpretation of photons is put forward and discussed in the context of small scattering particles.
When a single photon is scattered from an object, transferring its momentum, is its final state one of the
momentum energy eigenmodes? Potential experiments are discussed that could confirm this assumption in
the case of single photon optical trapping and verify the "classical" nature of the photon.
Airy beams are of great interest as a result of their unusual characteristics, they are non-diffracting and also
propagate along a parabolic path due to the presence of a transversal acceleration component. In this paper
the generation of a white light Airy beam is presented, an investigation is also carried out to determine
how the properties of an Airy beam change with the wavelength and spatial coherence of the source. A
supercontinuum source is used in conjunction with a spatial light modulator to produce the Airy beams.
The wavelength dependence study of the Airy beam parameters was carried out by inserting interference
filters into the supercontinuum beam path to select each wavelength. The parameters investigated are
the deflection coefficient of the Airy beam, b0, this quantifies the parabolic path traveled by the beam;
the characteristic length, x0, which is related to the lobe spacing, and lastly the aperture coefficient, a0.
The deflection coefficient and the characteristic length were both found to be wavelength dependent. The
aperture coefficient did not alter as a result of wavelength, however it was found to be dependent on the
spatial coherence, and therefore on the M2 value, of the beam. The other parameters, b0 and x0, are
unaffected by the spatial coherence of the source.
Non-diffracting beams, such as Bessel and Mathieu beams, offer a wide range of potential applications in
the fields of bio-photonics, micromanipulation and spectroscopy. One of the main features of these beams
is their self-healing behavior where the beams reconstruct after an obstacle. Higher order versions of these
beams incorporate non-diffracting optical singularities or vortices propagating together with the beams in a
straight line. Vortices are ubiquitous in many parts of physics and their dynamics, especially their creation
and annihilation processes are very important in fundamental physics. Newly demonstrated Airy beams
represent a different class of non-diffracting beams that do not propagate in a straight line but exhibit
a constant transversal acceleration. The self-healing properties of these Airy beams together with their
transversal acceleration can be used to optically clear entire regions of microparticles. These Airy beams
are created using a spatial light modulator that encodes a cubic phase front on an incident Gaussian beam.
Using the same method and suitable computer generated holograms we are able to generate Airy like beams
that include optical vortices. In this paper, we study the creation and evolution of Airy beam accelerating
vortices from the theoretical and experimental perspective.
Optical chromatography is a powerful technique, capable of separating micron-sized particles within a fluid flow, based
on their intrinsic properties, including size, shape and refractive index. Briefly, particles in a fluid flow are subject to two
forces, the Stokes drag force due to the fluid and then an introduced optical force as supplied by a laser beam, acting in
opposite but collinear directions. According to the particle's intrinsic hydrodynamic and optical properties, equilibrium
positions may form where the two forces balance, which is highly dependent on the properties of the particle and as a
result provides a means for spatial separation in a sample mixture. Optical chromatography is a passive sorting
technique, where pre-tagging of the particles of interest is not required, allowing for non-discrete distributions to be
evaluated and/or separated. Firstly we review the current stage of optical chromatography. We present a new advance in
optical chromatography potentially enabling the unique beam delivery properties of photonic crystal fiber (PCF) to be
employed and integrated into microfluidic chips. Also, for the first time a finite element method is applied to the optical
field in the theoretical analysis of optical chromatography, which is found to be in excellent agreement with the current
ray optics model, even for particles much smaller than the optical wavelength. This will pave the way for the technique
to be extended into the nanoparticle regime.
The year 2007 witnessed the experimental realization of extraordinary laser beams termed Airy and parabolic
beams. Surprisingly, these beams are immune to diffraction and in addition exhibit transverse acceleration while
propagating. This peculiar property of both Airy and parabolic beams facilitates the clearance of both microparticles
and cells from a region in a sample chamber through particle/cell transport along curved trajectories. We
term this concept "Optically mediated particle clearing" (OMPC) and, alternatively, "Optical redistribution"
(OR) in the presence of a microfluidic environment, where particles and cells are propelled over micrometersized
walls. Intuitively, Airy and parabolic beams act as a form of micrometer-sized "snowblower" attracting
microparticles or cells at the bottom of a sample chamber to blow them in an arc to another region of the sample.
In this work, we discuss the performance and limitations of OMPC and OR which are currently based on a single
Airy beam optionally fed by a single parabolic beam. A possible strategy to massively enhance the performance
of OMPC and OR is based on large arrays of Airy beams. We demonstrate the first experimental realization of
such arrays.
In their pioneering work, Burns et al. [Phys. Rev. Lett. 63, 1233 (1989)] discovered a laser-induced optical
interaction between dielectric microparticles dispersed in water. This interaction occurred in the plane transversal
to the laser beam and, interestingly, induced bound pairs of particles. Accordingly, the observed phenomenon
was termed "transverse optical binding" (TOB). Burns et al. argued that TOB arises from coherently induced
electric dipoles in the microspheres. Indeed, this explanation verified the experimental observation that the
spatial periodicity of the TOB interaction matched the laser wavelength in water. However, relatively little
experimental evidence has been provided, to date, for both the strength and functional dependence of this effect
on the particle distance. In our study, we used an experimental method which allowed us to directly measure
the TOB interaction. As a result, we found that this interaction is surprisingly long-ranged.
Optical trapping is an established field for movement of micron-size objects and cells. However, trapping of metal nanoparticles, nanowires, nanorods and molecules has received little attention. Nanoparticles are more challenging to optically trap and they offer ample new phenomena to explore, for example the plasmon resonance. Resonance and size effects have an impact upon trapping forces that causes nanoparticle trapping to differ from micromanipulation of larger micron-sized objects. There are numerous theoretical approaches to calculate optical forces exerted on trapped nanoparticles. Their combination and comparison gives the reader deeper understanding of the physical processes in an optical trap. A close look into the key experiments to date demonstrates the feasibility of trapping and provides a grasp of the enormous possibilities that remain to be explored. When constructing a single-beam optical trap, particular emphasis has to be placed on the choice of imaging for the trapping and confinement of nanoparticles.
We demonstrate stable three-dimensional optical trapping of 780nm silica particles using a dispersion-compensated
12.9fs infrared pulsed laser and a trapping microscope system with 1.40NA objective. To achieve these pulse
durations we use the Multiphoton Intrapulse Inteference Phase Scan (MIIPS) method to compensate for the
significant temporal dispersion introduced by the trapping system. We demonstrate orders of magnitude reduction
in pulse duration at the sample, and a dramatic increase in the efficiency of multiphoton excitation at the
sample. The use of dispersion-compensated ultrashort pulses will therefore be a valuable tool for enhancing non-linear
processes in optically trapped particles. In addition, ultrashort pulses can allow the use of pulse shaping
to control these nonlinear processes, yielding the possibility of advanced applications using coherent control of
trapped particles.
Early detection of malignant tumours, or their precursor lesions, can dramatically improve patient outcome. High risk
human Papillomavirus (HPV), particularly HPV16, infection can lead to the initiation and development of uterine
cervical neoplasia. Bearing this in mind the identification of the effects of HPV infection may have clinical value. In this
manuscript we investigate the application of Raman microspectroscopy to detect the presence of HPV in cultured cells
when compared with normal cells. We also investigate the effect of sample fixation, which is a common clinical practice,
on the ability of Raman spectroscopy to detect the presence of HPV. Raman spectra were acquired from Primary Human
Keratinocytes (PHK), PHK expressing the E7 gene of HPV 16 (PHK E7) and CaSki cells, an HPV16 containing cervical
carcinoma derived cell line. The average Raman spectra display variations, mostly in peaks relating to DNA and
proteins, consistent with HPV gene expression and the onset of neoplasia in both live and fixed samples. Principle
component analysis was used to objectively discriminate between the cells types giving sensitivities up to 100% for the
comparison between PHK and CaSki. These results show that Raman spectroscopy can discriminate between cell lines
representing different stages of cervical neoplasia. Furthermore Raman spectroscopy was able to identify cells expressing
the HPV 16 E7 gene suggesting the approach may be of value in clinical practice. Finally this technique was also able to
detect the effects of the virus in fixed samples demonstrating the compatibility of this technique with current cervical
screening methods. However if Raman spectroscopy is to make a significant impact in clinical practice the long
acquisition times must be addressed. In this report we examine the potential for beam shaping and advanced to improve
the signal to noise ration hence subsequently facilitating a reduction in acquisition time.
We present a study into the small particle size and resolution limits of Light Induced Dielectrophoresis (LIDEP). Here the illumination of a photoconductive layer creates virtual electrodes whose associated electric field gradients cause the dielectrophoretic response of the particles. In this way a potential energy landscape can be created that is optically controlled giving reconfigurable control over a large area [1]. In this paper we discuss the interlinked limits of size of particle it is possible to manipulate and the resolution these particles can be manipulated with. We compare traditional dielectrophoresis (DEP) experiments with LIDEP experiments, and discuss the mechanisms behind the physical limits comparing the effects of carrier diffusion verses the spreading of the electric fields in the medium.
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