Volume 3, Issue 5-6 - Green Photonics / Guest Editors: Andreas Tünnermann, Kimio Tatsuno

It has been proven that the only realistic path to practical ultra-high efficiency solar cells is the monolithic multi-junction approach, i.e., to stack pn-junctions made of different semiconductor materials on top of each other. Each sub pn-junction, i.e., sub solar cell, converts a specific part of the sun’s spectrum. In this way, the energy of the sunlight photons is converted with low thermalization losses. However, large-area multi-junction solar cells are still far too expensive if applied in standard PV modules. A viable solution to solve the cost issue is to use tiny solar cells in combination with optical concentrating technology, in particular, high concentrating photovoltaics (HCPV), in which the light is concentrated over the solar cells more than 500 times. The combination of ultra-high efficient solar cells and optical concentration lead to low cost on system level and eventually to low levelized cost of electricity, today, well below 8 €cent/kWh and, in the near future, below 5 €cent/kWh. A wide variety of approaches exists for III-V multi-junction solar cells and HCPV systems. This article is intended to provide an overview about the different routes being followed.

Femtosecond laser interactions inside transparent dielectric films of refractive index, n film , with tight focusing presents strong nonlinear interactions that can be preferentially confined at the fringe maxima as formed by Fabry-Perot interference, to generate thin nanoscale plasma disks separated on half-wavelength, λ /2 n film . The nano-thin disk explosions can be controlled inside the film to cleave open subwavelength internal cavities at single or multiple periodic depths at low laser exposure, while higher exposure will eject a quantised number of film segments with segment thickness defined by the laser wavelength. This new method enables high-resolution film patterning for ejecting nanodisks at quantised film depth for colouring and three-dimensional (3D) surface structuring, as well as for fabrication of free-standing nanofilms.

A novel integrated technology chain of laser-microstructured multilayer foils for fast, flexible, and low-cost manufacturing of lab-on-a-chip devices especially for complex cell and tissue culture applications, which provides pulsatile fluid flow within physiological ranges at low media-to-cells ratio, was developed and established. Initially the microfluidic system is constructively divided into individual layers, which are formed by separate foils or plates. Based on the functional boundary conditions and the necessary properties of each layer, their corresponding foils and plates are chosen. In the third step, the foils and plates are laser microstructured and functionalized from both sides. In the fourth and last manufacturing step, the multiple plates and foils are joined using different bonding techniques like adhesive bonding, welding, etc. This multilayer technology together with pneumatically driven micropumps and valves permits the manufacturing of fluidic structures and perfusion systems, which spread out above multiple planes. Based on the established lab-on-a-chip platform for perfused cell-based assays, a multilayer microfluidic system with two parallel connected cell culture chambers was successfully implemented.

For many lighting applications, light-emitting diodes (LEDs) are replacing traditional light sources providing the possibility for smart and efficient systems as well as a reduction in the product weight. A next step in this development is the integration of laser-based light sources to increase luminance and to further scale down the optics possibly leading to a reduction of necessary resources. This article reviews the possibilities and challenges arising from the use of laser diodes especially compared to current high-power LED systems in terms of efficiency, color-rendering properties, and thermal management.

This paper describes wired and wireless seamless networks consisting of radiowave and optical fiber links. Digital coherent technology developed for high-speed optical fiber transmission can mitigate signal deformation in radiowave links in the air as well as in optical fibers. Radio-over-fiber (RoF) technique, which transmits radio waveforms on intensity envelops of optical signals, can provide direct waveform transfer between optical and radio signals by using optical-to-electric or electric-to-optical conversion devices. Combination of RoF in millimeter-wave bands and digital coherent with high-performance digital signal processing (DSP) can provide wired and wireless seamless links where bit rate of wireless links would be close to 100 Gb/s. Millimeter-wave transmission distance would be shorter than a few kilometers due to large atmospheric attenuation, so that many moderate distance wireless links, which are seamlessly connected to optical fiber networks should be required to provide high-speed mobile-capable networks. In such systems, reduction of power consumption at media converters connecting wired and wireless links would be very important to pursue both low-power consumption and large capacity.

Minimizing mechanical losses and friction in vehicle engines would have a great impact on reducing fuel consumption and exhaust emissions, to the benefit of environmental protection. With this scope, laser surface texturing (LST) with femtosecond pulses is an emerging technology, which consists of creating, by laser ablation, an array of high-density microdimples on the surface of a mechanical device. The microtexture decreases the effective contact area and, in case of lubricated contact, acts as oil reservoir and trap for wear debris, leading to an overall friction reduction. Depending on the lubrication regime and on the texture geometry, several mechanisms may concur to modify friction such as the local reduction of the shear stress, the generation of a hydrodynamic lift between the surfaces or the formation of eddy-like flows at the bottom of the dimple cavities. All these effects have been investigated by fabricating and characterizing several LST surfaces by femtosecond laser ablation with different features: partial/full texture, circular/elliptical dimples, variable diameters, and depths but equivalent areal density. More than 85% of friction reduction has been obtained from the circular dimple geometry, but the elliptical texture allows adjusting the friction coefficient by changing its orientation with respect to the sliding direction.

Hydrogen peroxide (H 2 O 2 ) detection was demonstrated with multi-pass absorption spectroscopy using a commercial 76-m astigmatic multi-pass absorption cell. An ∼7.73-μm continuous wave, distributed feedback quantum cascade laser (CW DFB-QCL) was employed for targeting a strong H 2 O 2 line (1296.2 cm -1 ) in the fundamental absorption band. Wavelength modulation spectroscopy combined with a second harmonic detection technique was utilized to increase the signal-to-noise ratio. By optimizing the pressure inside the multi-pass cell and the wavelength modulation depth, a minimum detection limit (1σ) of 13.4 ppbv was achieved for H 2 O 2 with a 2-s sampling time. From an Allan-Werle deviation plot, the detection limit could be improved to 1.5 ppbv with an averaging time of 200 s. Interference effects of atmospheric air components are also discussed.