Thomas Kämpfe

Depth-minimized, large period half-wave corrugation for linear to radial and azimuthal polarization transformation by grating-mode phase management

The transformation of the polarization distribution of a laser beam from linear to radial and azimuthal by means of a subwavelength binary corrugation etched in a high-index substrate faces fabrication difficulties and an inherent contradiction preventing the achievement of both conditions of 100% transmission and of π phase difference between polarization components. The contradiction is solved by resorting to an easily fabricable high-index corrugation on a low-index substrate where a larger period gives rise to grating-mode reflection/transmission phases that permit the fulfillment of both conditions with a depth-minimized corrugation. From the principle of the solution, a targeted numerical search gives the complete set of the corresponding shallow structures, achieving polarization rotation in a fitting analytical form versus normalized variables.

Azimuthally polarized laser mode generation by multilayer mirror with wideband grating-induced TM leakage in the TE stopband

A new intracavity laser polarization-mode selection scheme relying upon a TE/TM diffractive dichroism principle in a grating multilayer mirror is proposed and demonstrated. The grating diffracts the first orders between the TE and TM band edges of the angular spectra of the laser mirror inducing a leakage of the TM polarization into the mirror substrate through the multilayer stack whereas TE diffraction into the substrate is forbidden. This mechanism is non-resonant, thus relatively wide-band. Applied with a circular-line grating in the 1.0 µm - 1.1 µm wavelength range, this mirror filters out the radially polarization mode and causes the emission of the azimuthally polarized mode. An original amorphous silicon grating technology was developed and the optical function demonstrated in a Nd:YAG laser.

Segmented subwavelength silicon gratings manufactured by high productivity microelectronic technologies for linear to radial/azimuthal polarization conversion

Abstract. A polarization rotation is realized by subwavelength binary gratings, where the round trip phases of the smallest grating modes are fixed to the smallest possible integer numbers of 2π allowing a phase difference of π between TE and TM polarizations and almost 100% transmission. The principle is applied to a polarization transformation in the 1030 to 1064-nm wavelength range, using a segmented polarization rotating element converting a linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kinds of polarization transformers based on assembled birefringent crystals are avoided by using mass-fabrication compatible silicon-on-insulator technology on a wafer scale. It shows the general potential of microelectronic technology, concerning the batch manufacturing of wavelength-scale diffractive, grating-based elements for processing free space waves.

Microstructuring of Mesoporous Titania Films Loaded with Silver Salts to Enhance the Photocatalytic Degradation of Methyl Blue under Visible Light

The microstructuring of the distribution of silver nanoparticles (NPs) in mesoporous titania films loaded with silver salts, using two-beam interference lithography leading to 1 Dimension (1D) grating, induces variations in the photocatalytic efficiency. The influence of the structuration was tested on the degradation of methyl blue (MB) under ultraviolet (UV) and visible illumination, giving rise to a significant improvement of the photocatalytic efficiency. The periodic distribution of the NPs was characterized by transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and scanning electron microscopy (SEM).

Narrow band, large angular width resonant reflection from a periodic high index grid at terahertz frequency

The property of a thin silicon membrane with periodic air slits of definite depth and width to exhibit under normal incidence a close to 100% ultra-narrow band reflection peak is demonstrated experimentally in the terahertz frequency range on a single-crystal silicon grid fabricated by submillimeter microsystem technology. An analysis based on the true modes supported by the grid reveals the nature of such resonances and permits to sort out those exhibiting ultra-narrow band.

Parameter-tolerant binary gratings

A normalized modal analysis of binary gratings under normal TE incidence involving the most condensed set of optogeometrical parameters gives a complete solution to the problem of canceling the 0(th) transmitted order in phase masks of a low-to-high refractive index ratio down to 0.5 with a large tolerance on the corrugation duty cycle or a large spectral bandwidth. The solution is presented in the form of single normalized 3D charts which shed light on the fulfillment of the 0(th)-order cancellation condition: balanced excitation and π-phase difference between two grating modes. Examples of tolerant gratings are given.

Periodic TiO2 Nanostructures with Improved Aspect and Line/Space Ratio Realized by Colloidal Photolithography Technique

This paper presents substantial improvements of the colloidal photolithography technique (also called microsphere lithography) with the goal of better controlling the geometry of the fabricated nano-scale structures—in this case, hexagonally arranged nanopillars—printed in a layer of directly photopatternable sol-gel TiO2. Firstly, to increase the achievable structure height the photosensitive layer underneath the microspheres is deposited on a reflective layer instead of the usual transparent substrate. Secondly, an increased width of the pillars is achieved by tilting the incident wave and using multiple exposures or substrate rotation, additionally allowing to better control the shape of the pillar’s cross section. The theoretical analysis is carried out by rigorous modelling of the photonics nanojet underneath the microspheres and by optimizing the experimental conditions. Aspect ratios (structure height/lateral structure size) greater than 2 are predicted and demonstrated experimentally for structure dimensions in the sub micrometer range, as well as line/space ratios (lateral pillar size/distance between pillars) greater than 1. These nanostructures could lead for example to materials exhibiting efficient light trapping in the visible and near-infrared range, as well as improved hydrophobic or photocatalytic properties for numerous applications in environmental and photovoltaic systems.

High efficiency, high selectivity ultra-thin resonant diffractive elements

Resonant diffractive elements as the association of a surface corrugation with a surface wave exhibit boosted diffraction efficiency and high selectivity properties under the effect of ultra-shallow subwavelength surface reliefs. This is demonstrated by four examples of resonant functional structures made of very different material systems over the optical spectrum. All four structures are fabricated by slow wet etching as the inherent lateral broadening in corrugations of very small aspect ratio can be neglected.

Cost effective LED-based multi-spot projection system for stereo 3D reconstruction

This paper presents a multi spot projection unit, used in a 3D volume measurement system employed in a heart failure monitoring device, observing the volume of a patient’s feet for symptoms of heart problems (peripheral edema - swelling of the extremities). The stereoscopic image acquisition requires a surface with enough optically detectable texture, usually not present on human skin, which can be resolved by projecting an infrared, static multi-spot optical pattern. The focus of this paper is on creating a very cost-effective, energy efficient, eye-safe projection system, realizing a strongly divergent (up to ±60°) spot pattern, using infrared LEDs and mass fabricable micro-optical elements. Two different setup were tested: a) an LED array combined with a microlens array, and b) a combination of a single LED with a microlens array and a computer generated hologram (CGH) that adds a pseudo-random spot multiplication. For approach a) the microlens array was optimized by ray-tracing. The CGH function for approach b) was found using a wave optical design algorithm (iterative Fourier-Transform Algorithm – IFTA). The micro-lens array master was fabricated by diamondturning, whereas electron-beam lithography was employed for the CGH-master. Both masters were replicated using hotembossing of PMMA. Installed in a prototype of the medical measurement device, the influence on the 3D reconstruction was measured. The proposed solutions allow installing a competitively priced product for automatic peripheral edema monitoring in chronically ill patient homes, which is of great interest for improving their quality of life and the efficiency of their treatment.This paper presents a multi spot projection unit, used in a 3D volume measurement system employed in a heart failure monitoring device, observing the volume of a patient’s feet for symptoms of heart problems (peripheral edema - swelling of the extremities). The stereoscopic image acquisition requires a surface with enough optically detectable texture, usually not present on human skin, which can be resolved by projecting an infrared, static multi-spot optical pattern. The focus of this paper is on creating a very cost-effective, energy efficient, eye-safe projection system, realizing a strongly divergent (up to ±60°) spot pattern, using infrared LEDs and mass fabricable micro-optical elements. Two different setup were tested: a) an LED array combined with a microlens array, and b) a combination of a single LED with a microlens array and a computer generated hologram (CGH) that adds a pseudo-random spot multiplication. For approach a) the microlens array was optimized by ray-tracing. The CGH function for approach b) was found using a wave optical design algorithm (iterative Fourier-Transform Algorithm – IFTA). The micro-lens array master was fabricated by diamondturning, whereas electron-beam lithography was employed for the CGH-master. Both masters were replicated using hotembossing of PMMA. Installed in a prototype of the medical measurement device, the influence on the 3D reconstruction was measured. The proposed solutions allow installing a competitively priced product for automatic peripheral edema monitoring in chronically ill patient homes, which is of great interest for improving their quality of life and the efficiency of their treatment.

Plasmonic resonances in metal covered 2D hexagonal gratings fabricated by interference lithography

We present both modeling and experimental results devoted to design, fabrication and characterization of metal covered hexagonal diffraction gratings. Variation of exposition and development time allow to modify the shape of the elementary cell, leaving the depth and periodicity unchanged. The fabrication process was modeled using real parameters of the lithography bench and the photoresist, substantially improving experimental results. The high quality of metal covered gratings is confirmed by excitation of plasmonic resonances, which are in a good agreement with theoretical predictions. The described approach allows to better understand plasmonic effects in 2D periodic structures and leads to an optimized design of plasmonic sensors.