This work is dedicated to present a versatile, metamaterial-based sensing platform for biomarker detection in the THz frequency regime. The sensor can be classified as frequency selective surface and exhibits a characteristic dual resonance feature in the transmission frequency response which has been proven to perform well in biosensing applications. The platform is able to selectively detect proteins such as transcription factor EGR2 (early growth response protein 2) from a complex sample matrix with more than 7 · 103 other protein species, to sensitively detect reverse-transcribed MIA (melanoma inhibitory activity) DNA from a complex sample matrix and to respond to extracellular vesicular structures as well.
Diamond offers unique opportunities for applications in photonics, essentially because its very large band-gap leads to a wide transparency window and to the availability of numerous color centers. However, it is still challenging to obtain scalable and reproducible optoelectronics based on these defects. An important aspect in this respect is represented by the feasibility of electroluminescence devices, which combine diamond electronics with the photo-physics of color centers. Here, we demonstrate electrically-driven light emission from color centers based on a phosphorous-doped diamond Schottky diodes. Compared to conventional p-i-n systems, our approach simplifies the fabrication process, and it promotes the implementation of novel light sources based on diamond.
Compact, highly accurate and fast optical systems for three-dimensional image acquisition are becoming increasingly important in various future relevant applications. A special optoelectronic detector that can be used for distance measurement is the Photonic Mixer Device (PMD). We developed a graphene-based PMD, which benefits from the advantages of graphene (high charge carrier mobility and broadband absorption spectrum) and results in a broadband PMD with enhanced depth resolution. Our graphene PMD has a mirror-symmetrical design and combines two photodetectors in such a way that they have a common source contact made of palladium and two separate drain contacts made of titanium. The charge carrier swing, which is the main feature of a standard PMD, is realized via two separate buried gate contacts. Two versions of the graphene PMD, one silicon-substrate based and the other glass-substrate based, were fabricated. The fabrication steps as well as measurements of the electrical behavior and the photocurrent of the manufactured graphene PMDs are presented.
KEYWORDS: Terahertz radiation, Signal to noise ratio, Data modeling, Signal processing, Imaging systems, Image processing, Super resolution, Image resolution, Diffraction
THz imaging and sensing has demonstrated a wide-ranging application potential. However, the transfer of such basic applicability observations to real-world application scenarios is severely obstructed by fundamental limitations imposed by the comparatively long wavelength of this analytic technique. In this presentation, an overview of recent signal processing developments for the enhancement of the analytic performance of THz imaging and sensing systems is presented. The first part of the presentation introduces advanced signal processing techniques to enhance the spectroscopic investigation capability of THz systems. Experiments are performed at particularly difficult application situations, including inter alia very thin material systems or measurements with strongly absorptive features beyond the signal to noise limitations of spectroscopic instrumentation. Model- based numeric procedures for spectroscopic investigation with pulsed THz systems are derived, which enhance the analytic material data quality by two orders of magnitude in comparison to established numeric procedures. Furthermore, computer vision based blind-deconvolution superresolution approaches are introduced, which allow the unassisted increase of imaging resolution beyond the diffraction limit. Experiments performed with a FMCW- based THz imaging system operating from 514 - 640 GHz demonstrate a resolution increase by a factor of 2.3 beyond the diffraction limit, without requiring any prior knowledge on the point-spread function size or shape of the imaging system, but based on a direct analysis of the imaging data of an unknown target sample.
Multiple-input multiple-output (MIMO) imaging systems in the terahertz frequency range have a high potential in the field of non-destructive testing (NDT). With such systems it is possible to detect defects in composite materials, for example cracks or delaminations in fiber composites. To investigate mass-produced products it is necessary to study the objects in close to real-time on a conveyor without affecting the production cycle time. In this work we present the conception and realization of a 3D MIMO imaging system for in-line investigation of composite materials and structures. To achieve a lateral resolution of 1 mm, in order to detect such small defects in composite materials with a moderate number of elements, precise sensor design is crucial. In our approach we use the effective aperture concept. The designed sparse array consists of 32 transmitters and 30 receivers based on planar semiconductor components. High range resolution is achieved by an operating frequency between 220 GHz and 260 GHz in a stepped frequency continuous wave (SFCW) setup. A matched filter approach is used to simulate the reconstructed 3D image through the array. This allows the evaluation of the designed array geometry in regard of resolution and side lobe level. In contrast to earlier demonstrations, in which synthetic reconstruction is only performed in a 2D plane, an optics-free full 3D recon- struction has been implemented in our concept. Based on this simulation we designed an array geometry that enables to resolve objects with a resolution smaller than 1mm and moderate side lobe level.
We report on the development of an active stand-off imaging system operating in the 80 GHz - 110 GHz frequency
range. 3D real-time imaging is enabled by a combination of a mechanically scanned one-dimensional conventional
imaging projection with a rotating metallic reflector and a two-dimensional synthetic imaging reconstruction with a
linear array of transmitter (Tx) and receiver (Rx) elements. The system is conceived, in order to allow a resolution better
than 1cm both in lateral, as well as in range directions by using a multi-view imaging geometry with an aperture larger
than 2 m x 2 m. The operation distance is 8.5 - 9 m. The 2D synthetically reconstructed imaging planes are derived from
the correlation of 20 sources and 24 coherent detectors. Range information is obtained by operating in a frequency
modulated continuous wave (FMCW) mode. Real-time imaging is enabled by implementing the synthetic image
reconstruction algorithms on a general purpose graphics processing unit (GPGPU) system. A multi-view imaging
geometry is implemented, in order to enhance the imaging resolution and to reduce the influence of specular reflections.
We describe a high power CW solid-state dye laser setup. With perylene orange in PMMA as gain medium an output
power up to 800 mW at 576 nm and a tuning range between 565 and 595 nm is reached. The laser output shows good
long time power stability. The durability can be adjusted by variation of the pump power. A feedback loop controls the
laser output. At a setpoint of e.g. 100 mW, the laser output can be provided for more than eight hours with a low noise
level (RMS < 10%). The spectral width of the laser emission is less than 3 GHz and can be tuned over more than 30 nm.
A circular mode-profile is achieved with M2 < 1.4 [1].
Via intra-cavity second harmonic generation more than 1 mW of 290 nm UV-radiation is achieved. As nonlinear element
a 7 mm BBO (Beta-Barium Borate) crystal is used. The UV laser radiation can be tuned over 10 nm. The theoretical
limit of UV output is estimated to 3.5 mW. To our knowledge we present the first tunable CW polymer UV laser.
While the output stability at the fundamental wavelength is reasonably good, in the UV region a significant enhancement
of the noise level is observed. In addition to this the long time stability is reduced to few minutes. The limitation is
mainly given by the photo-decomposition of the organic dye molecules.
Terahertz biosensors are used for sensing chemical and biochemical material. In order to sense small material quantities,
such as DNA strands, sensors with a high sensitivity are needed. Our recent approach applies asymmetric double-split
ring resonators (aDSR) in a two-dimensional array. Interaction of resonances within the structure results in a steep flank
in the frequency response which is sensitively shifted by small amounts of biomolecules loaded on the sensor surface.
Additionally a high E-field concentration connected to the split ring resonances is used to maximize the frequency shift
induced by biomaterial covering only a small fraction of the sensor area. Minutes amounts of biomaterial can therefore
be detected. In this presentation, the approach is analyzed with numerical simulation. We demonstrate the functionality
and optimization of the aDSR array structures, and the capability to detect submicrometer layers of dielectric material
with a spatially selective deposition on the resonant structures. A measurement of the complementary structure is
presented as proof of principle.
The phoenix project aims to develop all-optical switches based on
the combination of inorganic and organic materials in hybrid
devices. We present first results in developing low-loss ring
resonators fabricated in silicon-on-insulator (SOI) technology,
with Q-factors as high as 125.000, and losses of α≈3.5dB/cm in the ring.
Because of its fast reversible phase change between a crystalline and an amorphous phase and a corresponding change in optical properties, Ge-Sb-Te alloys are well known as materials for phase change optical data storage [1]. Especially the stoichiometric Ge2Sb2Te5 of the GeTe-Sb2Te3 pseudobinary line is suited for this purpose and already commercially used [2]. Nevertheless, the physical principles of this technique are not yet completely understood. In the presented paper a composition-spread approach was used to deposit Ge-Sb-Te films with compositions around the ternary phase Ge2Sb2Te5. The deposition took place in a UHV sputtering chamber using three magnetron cathodes equipped with pure Ge-, Sb- and Te-targets, respectively, for film deposition. Films were deposited on Si-wafers as well as on Si-Al-5i02 stacks. The resulting composition-spread was analyzed by EPMA-mappings and GI-XRD with respect to composition and structure. The velocity of the phase change was determined using a static tester. The correlation between film constitution and kinetics of the phase change revealed that the change from the initialized crystalline phase to the amorphous phase could be achieved in about 20 ns for optimized compositions. Even slight deviations from this composition resulted in a strong decrease of the phase change velocity. Structural analysis proved the existence of two crystalline phases with cubic and hexagonal structure in the initialized films.
Time-resolved THz imaging for the incidence-angle dependent 3D tomographic characterization of layered structures is presented. We illustrate the capabilities of the developed system on multi-layer ceramic samples used for solid oxide fuel cells. Diverse methods for determining unknown refractive indices are discussed. The significant influence of the angle of incidence of a THz imaging system on the measured signal is demonstrated, which can be exploited especially in Brewster-angle configurations to enhance the capabilities of any THz tomography system.
State-of-the-art coherent THz radiation sources are reviewed and inversion-less amplification mechanisms are presented, which are applicable to a wide variety of optically impulsively excited THz emitters. The amplification schemes will be experimentally demonstrated and fundamental limitations and prerequisites discussed, stressing analogies and differences to standard amplification by stimulated emission.
The measurements of the spatial coherence of a macroscopic ensemble of carriers excited coherently by femtosecond laser pulses is presented. The spatial coherence of the excited ensemble is derived from time- and spatially resolved measurements of the far-field THz-emission pattern. The analysis concentrates on surface field emitters, which are widespread broadband sources of coherent THz-radiation. We find that these emitters are fully spatially coherent for emission frequencies up to 1.6 THz, above which frequency the spatial coherence starts to decrease. For frequencies above 2.5 THz the spatial coherence of the emitter is limited to one THz wavelength.
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