We report the control of spontaneous emission from CdSe/ZnS core-shell quantum dots coupled to novel open-access optical microcavities. The cavities are fabricated by focused ion beam milling and provide mode volumes less than a cubic micrometre. The quantum dot emission spectrum, spatial modes and lifetime are all modified substantially by the presence of the cavity, and can be tuned by actively varying the cavity length. An increase in emission rate of 75% is achieved at room temperature, attributed to the Purcell effect in the 'bad emitter' regime. We demonstrate a high degree of control over the emission from the dots, including near single-mode operation and the ability to detect strong emission from individual nanocrystals.
The effect of a thin dielectric film on the plasmonic behaviour of metal nanoparticles (MNPs) above a high refractive index substrate is explored. Using finite-difference time domain simulations, the optical properties of Ag nanoparticles are investigated as a function of film thickness, refractive index, and particle position within the film. We demonstrate that the addition of a film around a MNP at the air interface of a high-index substrate, where n air < n f ilm < n substrate , will always increase the fraction of light coupled to the substrate (F subs ). It is found that placement within a layer that does not conform to n air < n f ilm < n substrate can lead to reduced enhancements in F subs . The principal application for this work is for light-trapping in thin-film solar cells. We show that the inclusion of a thin film can increase the fraction of radiation coupled into the substrate by up to 30% for solar wavelengths. Additional potential benefits of the film structure, such as greater tunability of scattering resonances, an increase in path length of light in the substrate, and some control over the emission pattern are demonstrated. MNPs in a film are found to produce a more finely structured emission pattern than particles at a simple interface, showing potential for this research to be applied to optical nanoantennae. V
Adaptive optics (AO) methods are widely used in microscopes to improve image quality through correction of phase aberrations. A range of wavefront-sensorless AO schemes exist, such as modal, pupil segmentation zonal, and pixelated piston-based methods. Each of these has a different physical implementation that makes direct comparisons difficult. Here, we propose a framework that fits in all sensorless AO methods and facilitates systematic comparisons among them. We introduce a general model for the aberration representation that encompasses many existing methods. Through modeling and experimental verification in a two-photon microscope, we compared sensorless AO schemes with a range of aberration representations to correct both simulated and sample induced aberrations. The results show that different representations can provide a better basis for correction in different experimental scenarios, which can inform the choice of sensorless AO schemes for a particular application.
We present a scheme for active compensation of complex extrinsic polarization perturbations introduced into an optical system. Imaging polarimeter is used to measure the polarization state across a beam profile and a liquid crystal spatial light modulator controls the polarization of the input beam. A sequence of measurements permits determination of the birefringence properties of a perturbing specimen. The necessary correction is calculated and fed back to the polarization modulator to compensate for the polarization perturbation. The system capabilities are demonstrated on a range of birefringent specimens.where Jmirror represents the Jones matrix of the mirror, same for the other components; δ1 is the retardance introduced by the first pass of the SLM, δ2 is the retardance from the second pass and
Adaptive optics is being applied widely to a range of microscopies in order to improve imaging quality in the presence of specimen‐induced aberrations. We present here the first implementation of wavefront‐sensorless adaptive optics for a laser‐free, aperture correlation, spinning disk microscope. This widefield method provides confocal‐like optical sectioning through use of a patterned disk in the illumination and detection paths. Like other high‐resolution microscopes, its operation is compromised by aberrations due to refractive index mismatch and variations within the specimen. Correction of such aberrations shows improved signal level, contrast and resolution.
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