Tunable light sources gain considerable interest as key components in various spectroscopic systems, e.g. for gas or chemicals detection. A novel approach to employ new active materials for such sources is aimed by the TAGGED consortium and relies on the integration of electrically biased graphene membrane with electrostatically actuated membrane which will allow tuning of source wavelength in visible or IR range. In this work, preliminary results of optical characterization of electrically biased graphene chips are presented. Graphene was grown on a copper foil using chemical vapor deposition technique. After the growth it was transferred over the trenches of silicon chips. Fabricated chips contain several test structures for suspending the graphene membranes, and electrical contacts to the membranes for biasing. Presented results pave the way to fully integrated miniaturized opto-electric-mechanical tunable light sources.
As part of a future optical platform on-chip, we present a waveguide integrated tunable Fabry-Pérot Interferometer (FPI) for the long infrared wavelength range. The FPI consists of two parallel Bragg reflectors that are located at the ends of two waveguides facing each other. The waveguides are made of silicon and are suspended in air. The reflectors are realized as an alternating stack of silicon and air layers with high (H) and low (L) refractive index. The filter transmittance is evaluated by analytic calculations and electromagnetic finite difference time domain simulations. Filters with (HL)² layer stack show a full width half maximum of 270 nm and a peak transmittance of more than 25% at a wavelength of 9.4 μm at the first interference order in the simulation. It is evaluated by measurements. A MEMS actuator is used to tune the filter wavelength by changing the distance between both reflectors. A digital electrostatic actuator concept with a linear drive characteristic, designed for a large travel range up to 4 μm with a driving voltage of less than 30 V, is presented and evaluated together with the filter. The MEMS fabrication process for the structures is based on bonding and deep reactive ion etching (DRIE). The DRIE etch process was optimized, hereafter achieving a reduced roughness of less than 3 nm of the waveguide sidewalls. For transmission measurements the silicon waveguides are coupled to a laser source and a detector using optical fibers together with optical couplers on the chip. The filter performance was characterized in the range from 9μm to 9.4 μm.
Colloidal quantum dots (QDs) have attracted considerable interest because of their unique properties, like high quantum efficiencies, size dependent emission wavelength, high color purity and a low-cost solution processability. These properties make them one of the most promising emitting materials for thin film light-emitting diodes (LEDs) of the future. Although QD-LEDs have been in the focus of science for quite some time, many topics are still neglected, like the dynamics, that show the speed of switching for these LEDs. However, this knowledge could be decisive for the use of such LEDs in communication or sensor technology. This work is intended to provide an overview of the influence of the organic hole transport layer (HTL) on the dynamics of QD LEDs, since the organic semiconductors used represent a limiting factor due to their low mobility. The substances 4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP), N,N-Bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) and N,N,N,N-Tetrakis(3-methylphenyl)-3,3-dimethylbenzidine (HMTPD) are used as HTLs. The examined LEDs are constructed in the same way and differ only in the material used for the HTL as well as their thicknesses. The dynamics of these samples will be investigated by electro-optical measurements. To get a better insight, hole only devices were also produced with the HTLs to compare their properties. We assume that these results mark the progress in the development and realization of faster QD LEDs
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