Diffraction-limited system | EPFL Graph Search

In optics, any optical instrument or system a microscope, telescope, or camera has a principal limit to its resolution due to the physics of diffraction. An optical instrument is said to be diffraction-limited if it has reached this limit of resolution performance. Other factors may affect an optical system's performance, such as lens imperfections or aberrations, but these are caused by errors in the manufacture or calculation of a lens, whereas the diffraction limit is the maximum resolution possible for a theoretically perfect, or ideal, optical system. The diffraction-limited angular resolution, in radians, of an instrument is proportional to the wavelength of the light being observed, and inversely proportional to the diameter of its objective's entrance aperture. For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disk. As one decreases the size of the aperture of a telescopic l

Apertureless SNOM

Rubén Esteban Llorente

This thesis studies apertureless Scanning Near Field Optical Microscopy, a technique that uses the apex of a very sharp tip to obtain local optical information with lateral resolution much beyond the diffraction limit. Both theoretical and experimental results are discussed. The theoretical work is a significant advance towards the quantitative convergence of experiments and theoretical predictions, and should be useful in aiding the interpretation of measured images. Extended tips and substrates are used, and the detector is also carefully modeled. A static tip in vacuum serves to study the influence of the tip and illumination geometry on the far fields and on the near fields in the proximity of the tip apex, the volume used to probe the sample. Including a gold substrate and the commonly used demodulation scheme allows to study the discrimination of the components carrying the local information. A very good discrimination is verified for silicon tips and small oscillation amplitudes, as far as the tip interacts closely with the substrate and the oscillation remains highly sinusoidal. The imaging process is studied by including patterned substrates. The obtained signal is mostly sensitive to a few nanometers of depth into the sample, and the influence of the scanning conditions on the level of signal, background suppression and lateral resolution is characterized. Further, a closer look into the behavior of the extended physical detector reveals the influence of the spatial inhomogeneities of the scattered fields and, for interferometric measurements, the large significance of the optical phase. Experimentally, different techniques are first described that can facilitate images with clear local information. A cross polarization scheme is introduced which is very useful for non-perturbative measurements. It is applied to the mapping of the the field distribution surrounding plasmonic structures, for both the phase and the amplitude. Beyond dipolar resonances, I also study coupled dipoles and quadrupole field distributions. When imaging artifacts are avoided, the obtained images closely resemble theoretical expectations.

EPFL2007

Application of Adaptive Optics to Focusing and Imaging

Daniel Iwaniuk

Adaptive optics is used to abate aberrations with a wavefront correction. It is widely used in astronomy and is starting to expand into applications of laser focusing and imaging with high numerical apertures, e.g. microscopy. The physical background of focusing laser light and imaging is the same, so we can use the same methods. In both cases, light is propagating through a lens; either it becomes focused into a small region or light from a small region becomes imaged on a camera. Modulations applied on the lens to implement wavefront corrections behave similarly in both applications. A powerful simulation tool was created to characterize the impact of those modulations. As an example, we validated a design for a Fresnel lens produced on a glass fibre tip to focus its emitting light. We have developed solutions mainly for three different problems. First, a high depth of focus enables keeping a laser beam focused within a larger length or imaging objects from different positions simultaneously. In photography, this can be attained by stopping down the aperture, which introduces a huge loss of light. State-of-the-art for focusing into a line segment also shows an inefficient performance. We present an elegant lens design, which enables highly efficient elongation of the depth of focus. Preliminary studies have shown that it might be a feasible alternative for current intra-ocular lens implants in ophthalmology or for 3D visualization in imaging and microscopy. A high depth of focus also has a large potential in optical lithography and data storage, because the focal position is enlarged and does not have to be adjusted precisely to obtain a useful spot. Second, specimen-induced aberrations can affect even a perfectly adjusted, diffraction limited lens system. A planar refraction index mismatch introduces spherical aberrations, degrading the optical resolution. As a first step to correct them, we were able to characterize simultaneously the refractive index and the thickness of an unknown medium that is placed between the lens and its focal region. This was done by clever manipulation of the beam angles with the same adaptive optics element in the focusing and in the imaging system. Finally, the medium-induced spherical aberrations were corrected based on the characterization results. The point spread function degradation of a focused laser beam was completely removed, which might be useful in optical tweezers or in laser processing of biological samples. While imaging through the planar refractive index mismatch, the bending of the object field was corrected and the diffraction limited performance restored.

EPFL2012