The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

The present invention is a method and apparatus relating to manufacturing nanostructure patterns and components using molecular science. The method includes overlaying a multilayer organic molecule resist on at least a portion of a parent structure selectively deposited on a substrate, depositing a layer over the parent structure and in contact with at least a portion of the multilayer organic resist, and removing the multilayer organic molecule resist to leave a residual structure.

/pdf/molecular-ruler-for-scaling-down-nanostructures-ctz7lc7kgp.pdf

Molecular ruler for scaling down nanostructures

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

/pdf/optical-gas-sensing-a-review-4uaf589xiq.pdf

Optical gas sensing: a review

The authors review the physics of ultrafast dynamics in semiconductors and their heterostructures, including both the observed experimental phenomena and the theoretical description of the processes. These are probed by ultrafast optical excitation, generating nonequilibrium states that can be monitored by time-resolved spectroscopy. Light pulses create coherent superpositions of states, and the dynamics of the associated phase relationships can be directly investigated by means of many-pulse experiments. The commonly used experimental techniques are briefly reviewed. A variety of different phenomena can be described within a common theoretical framework based on the density-matrix formalism. The important interactions of the carriers included in the theoretical description are the phonon interactions, the interactions with classical and quantum light fields, and the Coulomb interaction among the carriers themselves. These interactions give rise to a strong interplay between phase coherence and relaxation, which strongly affects the non equilibrium dynamics. Based on the general theory, the authors review the physical phenomena in various semiconductor structures including superlattices, quantum wells, quantum wires, and bulk media. Particular results which have played a central role in understanding the microscopic origins of the relaxation processes are discussed in detail.

Theory of ultrafast phenomena in photoexcited semiconductors

Successful building design is becoming an increasingly complex task, due to a growing demand to satisfy more ambitious environmental, societal and economical performance requirements. The application of climate adaptive building shells (CABS) has recently been put forward as a promising alternative within this strive for higher levels of sustainability in the built environment. Compared to conventional facades, CABS offer potential opportunities for energy savings as well improvement of indoor environmental quality. By combining the complementary beneficial aspects of both active and passive building technologies into the building envelope, CABS can draw upon the concepts of adaptability, multi-ability and evolvability. The aim of this paper is to present a comprehensive review of research, design and development efforts in the field of CABS. Based on a structured literature review, a classification of 44 CABS is made to place the variety of concepts in context with each other, and concurrent developments. In doing so, the overall motivations, enabling technologies, and characteristic features that have contributed to the development of CABS are highlighted. Despite the positive perspectives, it was found that the concept of CABS cannot yet be considered mature. Future research needs and further challenges to be resolved are therefore identified as well.

/pdf/climate-adaptive-building-shells-state-of-the-art-and-future-jmf64i6vb8.pdf

Climate adaptive building shells: state-of-the-art and future challenges

Detailed stress investigations of silicon nitride and silicon dioxide defined by PECVD are presented. The spatial variation of the stress of the dielectric material is evaluated by both, laser induced diffraction imaging method (macroscopically averaged stress) and by a large number of laterally distributed MEMS structures on the wafer (microscopically detected stress). By changing the duty cycle of two different plasma excitation frequencies during the deposition, the stress of silicon nitride (deposited at 300 °C) is controlled in a wide range from +850 MPa (compressive) to −300 MPa (tensile). A similar dependence is also observed for silicon nitride deposited at 60 °C. In contrast, silicon dioxide shows in both cases no strong frequency dependence. The microscopically detecting involves a low cost MEMS technology based on photoresist as sacrificial layer. Applying this technology, differently shaped Fabry–Perot filter membranes with various stress values are implemented. The cavity length of the filters and the radii of curvatures of the upper DBR membranes are also varied. Concave, convex and flat membranes with radii of curvature of −0.31 mm, 0.19 mm and −184.71 mm, respectively, are produced.

https://iopscience.iop.org/article/10.1088/0960-1317/14/3/001/pdf

Stress investigation of PECVD dielectric layers for advanced optical MEMS

Optical investigations on the mobility of two-dimensional excitons in GaAs/Ga1-xAlxAs quantum wells.

Optical filters capable of single control parameter-based wide tuning are implemented and studied. The surface micromachined Fabry-Perot filter consists of two InP-air-gap distributed Bragg reflectors and shows a wavelength tuning of more than 140 nm using only a single voltage of up to 3.2 V at currents below 0.2 mA. The membrane-based filter is designed to block all wavelengths in the whole range of 1250-1800 nm apart from its transmission wavelength.

Ultralow biased widely continuously tunable fabry-Perot filter

We propose a freely programmable THz diffraction grating based on an electrostatically actuated, computer controlled array of metallic cantilevers. Switching between different grating patterns enables tailoring spatio-temporal profiles of the THz waves. By characterizing the device with spatially resolved THz time domain spectroscopy, we demonstrate beam steering for a wide frequency band extending from 0.15 THz to 0.9 THz. The steerable range at 0.3 THz exceeds 40°. Focusing is also demonstrated by programming a chirped grating. The proposed approach could be employed to mimic arbitrary diffraction optics, enabling highly integrated and extremely flexible systems indispensable for THz stand-off imaging and communications.

Terahertz beam steering and variable focusing using programmable diffraction gratings.

The authors propose a method capable of providing arbitrarily chirped DFB (distributed feedback) gratings using bent waveguides on homogeneous grating fields and report the first realization as well as preliminary experimental results. >

Hartmut Hillmer

Papers

Stress investigation of PECVD dielectric layers for advanced optical MEMS

Optical investigations on the mobility of two-dimensional excitons in GaAs/Ga1-xAlxAs quantum wells.

Ultralow biased widely continuously tunable fabry-Perot filter

Terahertz beam steering and variable focusing using programmable diffraction gratings.

Chirped gratings for DFB laser diodes using bent waveguides