Optical eigenmode imaging

We present an indirect imaging method that measures both amplitude and phase information from a transmissive target. Our method is based on an optical eigenmode decomposition of the light intensity and the first-order cross correlation between a target field and these eigenmodes. We demonstrate that such optical eigenmode imaging does not need any a priori knowledge of the imaging system and corresponds to a compressive full-field sampling leading to high image extraction efficiencies. Finally, we discuss the implications with respect to second-order correlation imaging

Optical eigenmodes for illumination & imaging

This thesis exploits so called “Optical Eigenmodes” (OEi) in the focal plane of an optical system. The concept of OEi is introduced and the OEi operator approach is outlined, for which quadratic measures of the light field are expressed as real eigenvalues of an Hermitian operator. As an example, the latter is employed to locally minimise the width of a focal spot. The limitations of implementing these spots with state of the art spatial beam shaping technique are explored and a selected spot with a by 40 % decreased core width is used to confocally scan an in focus pair of holes, delivering a two-point resolution enhanced by a factor of 1.3. As a second application, OEi are utilised for fullfield imaging. Therefore they are projected onto an object and for each mode a complex coupling coefficient describing the light-sample interaction is determined. The superposition of the OEi weighted with these coefficients delivers an image of the object. Compared to a point-by-point scan of the sample with the same number of probes, i.e. scanning points, the OEi image features higher spatial resolution and localisation of object features, rendering OEi imaging a compressive imaging modality. With respect to a raster scan a compression by a factor four is achieved. Compared to ghost imaging as another fullfield imaging method, 2-3 orders of magnitude less probes are required to obtain similar images. The application of OEi for imaging in transmission as well as for fluorescence and (surface enhanced) Raman spectroscopy is demonstrated. Finally, the applicability of the OEi concept for the coherent control of nanostructures is shown. For this, OEi are generated with respect to elements on a nanostructure, such as nanoantennas or nanopads. The OEi can be superimposed in order to generate an illumination of choice, for example to address one or multiple nanoelements with a defined intensity. It is shown that, compared to addressing such elements just with a focussed beam, the OEi concept reduces illumination crosstalk in addressing individual nanoelements by up to 70 %. Furthermore, a fullfield aberration correction is inherent to experimentally determined OEi, hence enabling addressing of nanoelements through turbid media

Coherent control of plasmonic nanoantennas using optical eigenmodes

The last decade has seen subwavelength focusing of the electromagnetic field in the proximity of nanoplasmonic structures with various designs. However, a shared issue is the spatial confinement of the field, which is mostly inflexible and limited to fixed locations determined by the geometry of the nanostructures, which hampers many applications. Here, we coherently address numerically and experimentally single and multiple plasmonic nanostructures chosen from a given array, resorting to the principle of optical eigenmodes. By decomposing the light field into optical eigenmodes, specifically tailored to the nanostructure, we create a subwavelength, selective and dynamic control of the incident light. The coherent control of plasmonic nanoantennas using this approach shows an almost zero crosstalk. This approach is applicable even in the presence of large transmission aberrations, such as present in holographic diffusers and multimode fibres. The method presents a paradigm shift for the addressing of plasmonic nanostructures by light.Publisher PDFPeer reviewe

Nonredundant Raman imaging using optical eigenmodes

We thank funding from the UK Engineering and Physical Sciences Research Council namely grant EP/J01771X/1 and EU FP7 grant FAMOS.Various forms of imaging schemes have emerged over the last decade that are based on correlating variations in incident illuminating light fields to the outputs of single “bucket” detectors. However, to date, the role of the orthogonality of the illumination fields has largely been overlooked, and, furthermore, the field has not progressed beyond bright field imaging. By exploiting the concept of orthogonal illuminating fields, we demonstrate the application of optical eigenmodes (OEis) to wide-field, scan-free spontaneous Raman imaging, which is notoriously slow in wide-field mode. The OEi approach enables a form of indirect imaging that exploits both phase and amplitude in image reconstruction. The use of orthogonality enables us to nonredundantly illuminate the sample and, in particular, use a subset of illuminating modes to obtain the majority of information from the sample, thus minimizing any photobleaching or damage of the sample. The crucial incorporation of phase, in addition to amplitude, in the imaging process significantly reduces background noise and results in an improved signal-to-noise ratio for the image while reducing the number of illuminations. As an example we can reconstruct images of a surface-enhanced Raman spectroscopy sample with approximately an order of magnitude fewer acquisitions. This generic approach may readily be applied to other imaging modalities such as fluorescence microscopy or nonlinear vibrational microscopy.PostprintPeer reviewe

Optical eigenmodes for imaging applications

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.</p

Far field subwavelength focusing using optical eigenmodes

We report the focusing of light to generate a subdiffractive, subwavelength focal spot of full width half maximum 222 nm at an operating wavelength of 633 nm using an optical eigenmode approach. Crucially, the spot is created in the focal plane of a microscope objective thus yielding a practical working distance for applications. The optical eigenmode approach is implemented using an optimal superposition of Bessel beams on a spatial light modulator. The effects of partial coherence are also discussed. This far field method is a key advance toward the generation of subdiffractive optical features for imaging and lithographic purposes