The short-wavelength infrared (SWIR) regime between 1 and 3 μm is of high interest especially for surveillance, reconnaissance, and remote sensing applications. The availability of high-power, yet eye-safe SWIR laser sources is an important asset enabling scene illumination and implementation of advanced active imaging concepts like gated viewing (GV) or light detection and ranging (LIDAR). With atmospheric nightglow also a natural, but faint source for scene illumination is available for passive low-light-level imaging in the SWIR region. The most commonly employed material system for realizing SWIR photodetectors is InGaAs with an indium content of 53%. The spectral sensitivity of In0.53Ga0.47As with its cut-off wavelength of 1.7 μm covers a wide part of the nightglow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, for low-light-level passive SWIR imaging a dark-current density around 10-9 A/cm² is considered mandatory. While the international state-of-the-art has already achieved this performance at room-temperature operation, today’s European stateof- the-art is still lagging behind. The development of InGaAs-based SWIR detectors at Fraunhofer IAF aims at pin as well as avalanche photodiodes (APDs) for imaging applications with 640×512 pixels. While InGaAs APDs play to their strength in GV applications with typically rather short integration times, planar processed InGaAs/InP pin photodiodes with lowest possible dark-current and noise characteristics are the detector devices of choice for passive low-light-level detection. Within a few planar-process batches, we approached the European state-of-the-art for the dark-current density of 15-μm-pitch InGaAs pin detectors by a remaining factor of five. The most recent process run yielded further slightly improved dark-current characteristics on test devices. Recently, we have started with in-house characterization of such focal plane detector arrays hybridized with suitable SWIR read-out integrated circuits.
Type-II superlattices (T2SLs) are currently recognized as the sole material system offering comparable performance to HgCdTe, yet providing higher operability, stability over time, spatial uniformity, scalability to larger formats, producibility and affordability. Hence, T2SL technology is very promising for space applications. Fraunhofer IAF played a vital role in the development of III-As/Sb T2SLs right from the beginning. Mono- and bi-spectral focal plane arrays up to 640×512 pixels for the mid- and long-wavelength infrared (IR) have been demonstrated. The growth of T2SL is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. We report on the excellent homogeneity and reproducibility of the growth process, established in the past years at Fraunhofer IAF. After processing this material to detector arrays, the T2SL detectors have been characterized down to low temperatures (below 40K) with promising properties regarding the dark current. For MWIR and LWIR detectors the resolution limit of the measurement setup with a dark current density of 2×10-10 A/cm2 has been reached at 77 K and 36 K, respectively.
Short-wave infrared (SWIR) detection systems are increasingly demanded for surveillance, reconnaissance, and remote sensing applications. Passive SWIR cameras can benefit from an extended spectral range, compared to standard nightvision goggles, and the exploitation of the faint night-glow emission from the night sky. Furthermore, eye-safe SWIR lasers can improve the contrast and range of night-vision systems. High-performance SWIR photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the night-glow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, the low photon flux in night-vision applications demand for high responsivities and very low dark-current characteristics of the InGaAs photodetectors. We report on the current development activities of InGaAs SWIR photodetectors at Fraunhofer IAF. We have implemented a planar process technology of InGaAs/InP pin photodiodes for the fabrication of low-light-level SWIR cameras with 640× 512 pixels with 15 μm pixel pitch. Electro-optical characterization results of early-stage process runs exhibit darkcurrent densities below 10-7 A/cm2 at room temperature for 15-μm pitch detector elements. The current activities focus on further reducing the dark current to reach the international state of the art. Moreover, InGaAs-based avalanche photodiode (APD) have been developed for active SWIR imaging. Gain values of M ≈ 10 on camera level at a reverse bias voltage around 20 V have been achieved by a sophisticated vertical detector design. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch and operated in SWIR laser gated viewing mode.
Type-II superlattices (T2SLs) are considered the III/V alternative to HgCdTe for infrared (IR) detectors and have already reached market maturity. Fraunhofer IAF has demonstrated mono- and bi-spectral T2SL focal plane arrays up to 640×512 pixels for mid- and long-wavelength IR. In order to develop an industry-compatible T2SL technology, we have established the complete chain for detector array fabrication including design and modelling, epitaxial growth, as well as front- and backside processing. The epitaxial growth of T2SLs is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. In this paper, we report on the control of growth rates during epitaxy, uniformity and reproducibility of the growth process, as well as characterization techniques to monitor the quality of the epitaxial layers. For the superlattice period, an average thickness variation far below a single atomic monolayer is required and achieved routinely. The standard deviation of the photoluminescence peak for both colors of bi-spectral IR detectors is around 0.04 μm for consecutive growth runs. With this very stable and reproducible epitaxial growth process in conjunction with our mature front- and backside processing we have been able to set up a pilot line production for bi-spectral T2SL IR detector arrays.
High-performance short-wavelength infrared (SWIR) photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the nightglow spectrum as well as the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, both, active and passive SWIR detection systems are mostly required to provide high responsivities and very low dark currents in order to detect extremely low photon fluxes. This can be achieved with pin photodiodes with high responsivities and very low dark current characteristics or by utilizing the internal signal gain as provided by avalanche photodiodes (APDs). We develop SWIR photodetectors based on InGaAs/InP pin diodes and InGaAs/InAlAs/InP APDs as single-element detectors as well as focal plane arrays (FPAs). The planar processed InGaAs/InP pin photodiodes for low-light-level SWIR cameras exhibit dark current densities of 10-7 A/cm2 at room temperature for 15 μm pitch detector elements. For the APDs, emphasis is put on the vertical detector design. Within three design iterations, the operating voltage for useful gain values M ~ 10 could be reduced from 27 V down to 18 V, which was crucial for the operation with the voltage limitation of the read-out circuit. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch. The avalanche operation on camera level has been demonstrated for both kinds, the standard (passive) as well as gated-viewing operation modes.
Through the choice of appropriate layer thicknesses, the bandgap of InAs/Ga(As)Sb type II superlattices (T2SLs) can be engineered in a wide range covering the mid-wavelength and long-wavelength infrared (MWIR, 3 μm - 5 μm and LWIR, 8 μm - 12 μm) spectral regions. Using this material system, Fraunhofer IAF develops bi-spectral MWIR image sensors based on homojunction photodiodes for missile warning applications and pursues modern heterojunction approaches as well as heteroepitaxial growth of T2SLs on GaAs. We discuss topics arising from efforts to improve the manufacturability of our bi-spectral arrays and report on the progress of the integration with MWIR heterojunction designs that exhibit reduced dark currents.
Photodetectors in the non-visible region of the electromagnetic spectrum are essential for security, defense and space science as well as industrial and scientific applications. The research activities at Fraunhofer IAF cover a broad range in the infrared (IR) regime. Whereas short-wavelength IR (SWIR, <1.7 μm) detectors are realized by InGaAs/InP structures, InAs/GaSb type-II superlattice (T2SL) infrared detectors are developed for the spectral bands from mid- (MWIR, 3-5 μm) to long-wavelength IR (LWIR, 8-12 μm). We report on the extension of the superlattice empirical pseudopotential method (SEPM) to 300 K for the design of LWIR heterostructures for operation near room temperature. Recently, we have also adapted heterostructure concepts to our well established bi-spectral T2SL MWIR detector resulting in a dark current density below 2 × 10-9 A/cm2 for a cut-off wavelength close to 5 μm. Finally, we present first results obtained with a gated viewing system based on our InGaAs/InAlAs/InP avalanche photodiode arrays.
This paper reports on advances in the electro-optical characterization of InAs/GaSb short-period superlattice infrared photodetectors with cut-off wavelengths in the mid-wavelength and long-wavelength infrared ranges. To facilitate in-line monitoring of the electro-optical device performance at different processing stages we have integrated a semi-automated cryogenic wafer prober in our process line. The prober is configured for measuring current-voltage characteristics of individual photodiodes at 77 K. We employ it to compile a spatial map of the dark current density of a superlattice sample with a cut-off wavelength around 5 μm patterned into a regular array of 1760 quadratic mesa diodes with a pitch of 370 μm and side lengths varying from 60 to 350 μm. The different perimeter-to-area ratios make it possible to separate bulk current from sidewall current contributions. We find a sidewall contribution to the dark current of 1.2×10-11 A/cm and a corrected bulk dark current density of 1.1×10-7 A/cm2, both at 200 mV reverse bias voltage. An automated data analysis framework can extract bulk and sidewall current contributions for various subsets of the test device grid. With a suitable periodic arrangement of test diode sizes, the spatial distribution of the individual contributions can thus be investigated. We found a relatively homogeneous distribution of both bulk dark current density and sidewall current contribution across the sample. With the help of an improved capacitance-voltage measurement setup developed to complement this technique a residual carrier concentration of 1.3×1015 cm-3 is obtained. The work is motivated by research into high performance superlattice array sensors with demanding processing requirements. A novel long-wavelength infrared imager based on a heterojunction concept is presented as an example for this work. It achieves a noise equivalent temperature difference below 30 mK for realistic operating conditions.
Active and passive short-wave infrared (SWIR) detection systems for surveillance and remote sensing applications are mostly required to detect extremely low photon fluxes. This can be achieved by utilizing the internal signal gain as provided by avalanche photodiodes (APDs). We report on our current development activities of SWIR photodetectors based on InGaAs/InAlAs/InP APDs, covering detector design, epitaxial growth, process technology, and electro-optical characterization results of single-element detectors and fanout hybrids. For the first time, the operation of an InGaAsbased SWIR camera with 640 × 512 pixels utilizing APDs for signal amplification is demonstrated for operating temperatures of 180 K and even 260 K.
For more than two decades, Antimony-based type-II superlattice photodetectors for the infrared spectral range between
3-15 μm are under development at the Fraunhofer Institute for Applied Solid State Physics (IAF). Today, Fraunhofer
IAF is Germany’s only national foundry for InAs/GaSb type-II superlattice detectors and we cover a wide range of
aspects from basic materials research to small series production in this field. We develop single-element photodetectors
for sensing systems as well as two-dimensional detector arrays for high-performance imaging and threat warning
systems in the mid-wavelength and long-wavelength region of the thermal infrared. We continuously enhance our
production capabilities by extending our in-line process control facilities. As a recent example, we present a
semiautomatic wafer probe station that has developed into an important tool for electrooptical characterization. A large
amount of the basic materials research focuses on the reduction of the dark current by the development of bandgap
engineered device designs on the basis of heterojunction concepts. Recently, we have successfully demonstrated
Europe’s first LWIR InAs/GaSb type-II superlattice imager with 640x512 pixels with 15 μm pitch. The demonstrator
camera already delivers a good image quality and achieves a thermal resolution better than 30 mK.
For surveillance and reconnaissance applications in the short-wave infrared (SWIR) spectral range, the imaging systems have to cope with usually very low photon flux densities. Thus, dark-current and noise characteristics of the focal plane array (FPA) are demanding. On the other hand, the challenge of detecting extremely low photocurrents can be mitigated by utilizing an internal gain as provided by avalanche photodiodes (APDs). Fraunhofer IAF has recently started the development of InGaAs-based SWIR detectors. We report on the current development status covering design considerations, epitaxy, process technology and electro-optical characterization. Detector structures based on both, classical InGaAs PIN homojunction diodes as well as InGaAs/InAlAs APDs in separated-absorption-grading-charge-and-multiplication layer heterostructures, have been grown by molecular beam epitaxy on InP. Diodes structures were fabricated with a dry-etch mesa process and a subsequent dielectric passivation of the mesa sidewalls. High-resolution FPAs with 640 x 512 pixels and a 15 μm pixel pitch based on PIN diodes have been assembled to a SWIR camera system in cooperation with AIM Infrarot-Module GmbH. Design variations, in particular for the APDs, were assisted by band-edge-profile simulations. APD test structures as well as fan-out hybrids have been characterized, revealing gain values larger than 300 at room temperature.
We report on the development and optimization of mesa-processed InGaAs/InAlAs avalanche photodiodes (APD)
for short-wave infrared applications with demand for high gain and low breakdown voltage. The APDs were
grown by molecular beam epitaxy. Dark and photo current measurements of fully processed APDs reveal high
dynamic range of 104 and gain larger than 40 for 25 V reverse bias voltage and cooled operation at 140 K. A
maximum gain larger than 300 is demonstrated for room temperature as well as 140 K. Two different approaches
to determine the gain of the APD structures are discussed.
We report on the development and testing of the building blocks of a possible compact heterodyne setup in the mid-infrared,
which becomes particularly relevant for flight instrumentation. The local oscillator is a Quantum Cascade Laser
(QCL) source at 8.6 μm operable at room temperature. The beam combination of the source signal and the local
oscillator will occur by means of integrated optics for the 10 μm range, which was characterized in the lab. In addition
we investigate the use of superlattice detectors in a heterodyne instrument. This work shows that these different new
components can become valuable tools for a compact heterodyne setup.
Fraunhofer IAF can look back on many years of expertise in developing high-performance infrared photodetectors. Since
pioneering the InAs/GaSb type-II superlattice detector development, extensive capabilities of epitaxy, process
technology, and device characterization of single element detectors and camera arrays for the mid- and longwave
infrared (MWIR and LWIR) have been established up to the level of small-scale production. Bispectral MWIR/MWIR
and MWIR/LWIR cameras based on type-II superlattices or HgCdTe are key topics at Fraunhofer IAF. Moreover, the
development of InGaAs-based short-wave infrared (SWIR) photodetectors for low-light-level applications has recently
In this contribution, we report on the status of recent photodetector development activities at IAF, covering detector
design, epitaxial growth, process technology, and most recent electro-optical characterization results of focal plane
arrays as well as single element detectors especially for the SWIR based on InGaAs material system.
We report on materials and technology development for short-wave infrared photodetectors based on InGaAs p-i-n and avalanche photodiodes (APDs). Using molecular beam epitaxy for the growth of thin layers with abrupt interfaces, which are required for optimized APD structures, excellent crystalline quality has been achieved for detector structures grown on 3-inch InP substrates. For the fabrication of focal plane detector arrays, we employed a mesa etching technology in order to compare the results with the commonly utilized planar technology. Camera detector arrays as well as test structures with various sizes and geometries for materials and process characterization are processed using a dry-etch mesa technology. Aspects of the process development are presented along with measured dark-current and photo-current characteristics of the detector devices.
To examine defects in InAs/GaSb type-II superlattices we investigated GaSb substrates and epitaxial InAs/GaSb layers
by synchrotron white beam X-ray topography to characterize the distribution of threading dislocations. Those
measurements are compared with wet chemical etch pit density measurements on GaSb substrates and InAs/GaSb type-II
superlattices epitaxial layer structures. The technique uses a wet chemical etch process to decorate threading dislocations
and an automated optical analyzing system for mapping the defect distribution.
Dark current and noise measurements on processed InAs/GaSb type-II superlattice single element photo diodes reveal a
generation-recombination limited dark current behavior without contributions by surface leakage currents for midwavelength
infrared detectors. In the white noise part of the noise spectrum, the extracted diode noise closely matches
the theoretically expected shot noise behavior.
For diodes with an increased dark current in comparison to the dark current of generation-recombination limited
material, the standard shot-noise model fails to describe the noise experimentally observed in the white part of the
spectrum. Instead, we find that McIntyre’s noise model for avalanche multiplication processes fits the data quite well.
We suggest that within high electric field domains localized around crystallographic defects, electrons initiate avalanche
multiplication processes leading to increased dark current and excess noise.
3rd generation IR modules - dual-color (DC), dual-band (DB), and large format two-dimensional arrays - require
sophisticated production technologies such as molecular beam epitaxy (MBE) as well as new array processing
techniques, which can satisfy the rising demand for increasingly complex device structures and low cost detectors. AIM
will extend its future portfolio by high performance devices which make use of these techniques. The DC MW / MW
detectors are based on antimonide type-II superlattices (produced by MBE at Fraunhofer IAF, Freiburg) in the 384x288
format with a 40 μm pitch. For AIM, the technology of choice for MW / LW DB FPAs is MCT MBE on CdZnTe
substrates, which has been developed in cooperation with IAF, Freiburg. 640x512, 20 μm pitch Focal Plane Arrays
(FPAs) have been processed at AIM. The growth of MW MCT MBE layers on alternate substrates is challenging, but
essential for competitive fabrication of large two-dimensional arrays such as megapixel (MW 1280x1024, 15 μm pitch)
FPAs. This paper will present the development status and latest results of the above-mentioned 3rd Gen FPAs and
Integrated Detector Cooler Assemblies (IDCAs).
InAs/GaSb-based type-II superlattice photodiodes have considerably gained interest as high-performance infrared
detectors. Beside the excellent properties of InAs/GaSb superlattices, like the relatively high effective electron mass
suppressing tunneling currents, the low Auger recombination rate, and a high quantum efficiency, the bandgap can be
widely adjusted within the infrared spectral range from 3 - 30 μm depending on the layer thickness rather than on
composition. Superlattice growth and process technology have shown tremendous progress during the last years. Fully
integrated superlattice cameras have been demonstrated by several groups worldwide.
Within very few years, the InAs/GaSb superlattice technology has proven its suitability for high-performance infrared
imaging detector arrays. At Fraunhofer IAF and AIM, the efforts have been focused on developing a mature fabrication
technology for bispectral InAs/GaSb superlattice focal plane arrays for a simultaneous, co-located detection at 3-4 μm
and 4-5 μm in the mid-wavelength infrared atmospheric transmission window. A very low number of pixel outages and
cluster defects is mandatory for dual-color detector arrays. Sources for pixel outages are manifold and might be caused
by dislocations in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. Process refinements, intense root cause analysis and specific test methodologies employed at various stages
during the process have proven to be the key for yield enhancements.
InAs/GaSb short-period superlattices (SL) have proven their large potential for high performance focal plane array
infrared detectors. Lots of interest is focused on the development of short-period InAs/GaSb SLs for mono- and bispectral
infrared detectors between 3 - 30 μm. InAs/GaSb short-period superlattices can be fabricated with up to 1000
periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of InAs/GaSb SL
camera systems with very high responsivity, comparable to state of the art CdHgTe and InSb detectors. The material
system is also well suited for the fabrication of dual-color mid-wavelength infrared InAs/GaSb SL camera systems.
These systems exhibit high quantum efficiency and offer simultaneous and spatially coincident detection in both spectral
An essential point for the performance of two-dimensional focal plane infrared detectors in camera systems is the
number of defective pixel on the matrix detector. Sources for pixel outages are manifold and might be caused by the
dislocation in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. The goal is to grow defect-free epitaxial layers on a dislocation free large area GaSb substrate. Permanent
improvement of the substrate quality and the development of techniques to monitor the substrate quality are of particular
importance. To examine the crystalline quality of 3" and 4" GaSb substrates, synchrotron white beam X-ray topography
(SWBXRT) was employed. In a comparative defect study of different 3" GaSb and 4" GaSb substrates, a significant
reduction of the dislocation density caused by improvements in bulk crystal growth has been obtained. Optical
characterization techniques for defect characterization after MBE growth are employed to correlate epitaxially grown
defects with the detector performance after hybridization with the read-out integrated circuit.
In the past years, the development of the type-II InAs/GaSb superlattice technology at the Fraunhofer-Institute for
Applied Solid State Physics (IAF) has been focused on achieving series-production readiness for third generation dualcolor
superlattice detector arrays for the mid-wavelength infrared spectral range. The technology is ideally suited for
airborne missile threat warning systems, due to its ability of low false alarm remote imaging of hot carbon dioxide
signatures on a millisecond time scale. In a multi-wafer molecular beam epitaxy based process eleven 288×384 dualcolor
detector arrays are fabricated on 3" GaSb substrates. Very homogeneous detector arrays with an excellent noise
equivalent temperature difference have been realized. The current article presents the type-II superlattice dual-color
technology developed at IAF and delivers insights into a range of test methodologies employed at various stages during
the fabrication process, which ensure that the basic requirements for achieving high detector performance are met.
InAs/GaSb short-period superlattices (SL) based on GaSb, InAs and AlSb have proven their great potential for high
performance infrared detectors. Lots of interest is currently focused on the development of short-period InAs/GaSb SLs
for advanced 2nd and 3rd generation infrared detectors between 3 - 30 μm. For the fabrication of mono- and bispectral
thermal imaging systems in the mid-wavelength infrared region (MWIR) a manufacturable technology for high
responsivity thermal imaging systems has been developed. InAs/GaSb short-period superlattices can be fabricated with
up to 1000 periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of
InAs/GaSb SL camera systems with high responsivity comparable to state of the art CdHgTe and InSb detectors. The
material system is also ideally suited for the fabrication of dual-color MWIR/MWIR InAs/GaSb SL camera systems with
high quantum efficiency for missile approach warning systems with simultaneous and spatially coincident detection in
both spectral channels.
A mature production technology for Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs) and
InAs/GaSb superlattice (SL) FPAs has been developed. Dual-band and dual-color QWIP- and SL-imagers are
demonstrated for the 3-5 μm and 8-12 μm atmospheric windows in the infrared. The simultaneous, co-located detection
of both spectral channels resolves the temporal and spatial registration problems common to existing bispectral IRimagers.
The ability for a reliable remote detection of hot CO2 signatures makes tailored dual-color superlattice imagers
ideally suited for missile warning systems for airborne platforms.
InAs/GaSb type-II short-period superlattice (SL) photodiodes have been shown to be very promising for 2nd and 3rd
generation thermal imaging systems with excellent detector performance. A multi-wafer molecular beam epitaxy (MBE)
growth process on 3"-GaSb substrates, which allows simultaneous growth on five substrates with excellent homogeneity
has been developed. A reliable III/V-process technology for badge processing of single-color and dual-color FPAs has
been set up to facilitate fabrication of mono- and bi-spectral InAs/GaSb SL detector arrays for the mid-IR spectral range.
Mono- and bispectral SL camera systems with different pitch and number of pixels have been fabricated. Those imaging
systems show excellent electro-optical performance data with a noise equivalent temperature difference (NETD) around
In Germany, InAs/GaSb superlattice detector technology for the mid-wavelength infrared spectral range has been
intensively developed in recent years. Mid-IR InAs/GaSb superlattice photodiodes achieve a very high quantum
efficiency. The world's first high-performance infrared imagers based on InAs/GaSb superlattices were realized offering
high spatial and excellent thermal resolution at short integration times. Additionally, the technology for dual-color
superlattice detectors featuring simultaneous, pixel-registered detection of two separate spectral regimes in the mid-IR
has been developed. Due to the ability to detect signatures of hot carbon dioxide, dual-color superlattice detectors are
ideally suited for missile alerting sensors. The capability for small volume production of InAs/GaSb superlattice
detectors has been established.
InAs/GaSb short-period superlattices (SL) for the fabrication of mono- and bispectral thermal imaging systems in the
mid-wavelength infrared region (MWIR) have been optimized in order to increase the spectral response of the imaging
systems. The responsivity in monospectral InAs/GaSb short-period superlattices increases with the number of periods in
the intrinsic region of the diode and does not show a diffusion limited behavior for detector structures with up to 1000
periods. This allows the fabrication of InAs/GaSb SL camera systems with high responsivity. Dual-color MWIR/MWIR
InAs/GaSb SL camera systems with high quantum efficiency for missile approach warning systems with simultaneous
and spatially coincident detection in both spectral channels have been realized.
The worldwide production volume of polymers is still rising exponentially and the number of applications for plastic
components steadily increases. Yet, many branches within the polymer industry are hardly supported by non-destructive
testing techniques. We demonstrate that terahertz (THz) spectroscopy could be the method of choice to ensure high-quality
polymer products. Applications range from the in-line monitoring of extrusion processes and the quality control
of commodities in a mass production up to a total inspection of high-tech safety relevant products. Furthermore, we
present an extension to THz time-domain spectroscopy in the form of a new data extraction algorithm, which derives the
absorption coefficient, the refractive index and the thickness of a sample with very high precision in a single pass.
Apart from that, we discuss the ability of THz systems for quality control of polymeric compounds. Here, it is essential
to monitor the additive content as well as additive inhomogeneities within the mixture. Recently, we built a fiber-coupled
THz spectrometer for in-line monitoring of compounding processes. Additionally, we demonstrate the potential of THz
systems for the non-destructive and contactless testing of structural components. THz imaging is capable of analyzing
material thicknesses, superstructures, the quality of plastic weld joints, and of detecting flaws in components.
Plastics and THz form a very fruitful symbiosis. In return, plastics industry can provide THz systems with custom-tailored
components, which have very attractive properties and extremely low costs. Examples of this development are
photonic crystals or polymeric Bragg filters, which have recently been demonstrated.
To evaluate the potential of THz imaging systems for mail and luggage inspection we study a set of letters containing different hazardous items. The samples are investigated with three different THz systems available in our group: A microwave based system working around 100 GHz, a THz time-domain system and a THz gas laser. We provide a comparative discussion on our results and the advantages and disadvantages of each system.
Dielectric mirrors are widely used in optical setups for spectral regions such as UV, visible, as well as IR. Yet, for the rapidly growing field of terahertz spectroscopy dielectric multilayer optics are sparsely utilized. But with low-loss materials high quality THz optics can be obtained. We present two approaches for the realization of highly effective dielectric THz mirrors. First, four thin slices of high-resistivity silicon and five common polypropylene (PP) foils were alternately stacked together to obtain a broad reflection band. This stop-band blueshifts with increasing angles of incidence. But due to the high index step between Si and PP a band from 0.32 to 0.375 THz always remains the stopband for all incidence angles and both the s- and p-polarization. The measurement data obtained in reflection and transmission geometry are reproduced well by numerical simulations. With a minor change of the layer sequence a microresonator is obtained which reveals a sharp transmission peak at around 0.3 THz within the reflection band. The second material system consists of ceramic laminates of alumina (A) and alumina-zirconia (AZ). Measurements on 12.5 pairs of A/AZ layers yield a strong stop-band from 0.3 to 0.37 THz at normal incidence, which again match numerical simulations. The big advantage of the ceramic mirror is the rugged, quasimonolithic design of the sintered multilayer structure.
Polymers are often mixed with other additives or fillers to yield compounds with modified physical properties. In most cases a homogeneous mixture is desired. Yet, it often remains difficult to verify the degree of homogeneity of the resulting compound especially for nano-scaled fillers. We present initial experiments to evaluate the potential of terahertz (THz) spectroscopy for the quality control of polymeric compounds. We study low-density polyethylene (LDPE) samples which contain titanium dioxide nanospheres with a typical diameter of 270 nm which themselves are coated with even smaller silver nanoparticles with a typical diameter of 20 nm. Images obtained with a standard terahertz time-domain spectrometer show significant inhomogeneities in the compound on a millimeter scale. The imaging results indicate imperfectly mixed material regions. On the other hand, we show that also fluctuations in the sample thickness can lead to inhomogeneous terahertz images. A final conclusion, if the inhomogeneities observed in our LDPE/Ag-TiO2 samples result from variations in the compound composition or from thickness fluctuations, cannot be drawn at this point.
We investigate small peptides using standard terahertz (THz) time-domain spectroscopy. As a test set we examine the tripeptides glutathione, gly-gly-gly and enalapril maleate at room temperature. While earlier investigations of short-chain polypeptides with a conventional FTIR spectrometer were performed at higher THz frequencies, we present first measurements between 150 GHz and 2 THz and compare our measurements to density functional theory (DFT) calculations in order to assign the measured resonances to distinct molecular motions. DFT calculations obtained for a single molecule (glutathione), dimers (gly-gly-gly) and ion pairs (enalapril maleate) coupled via H-bonds, reproduce correctly the number of resonances observed in the experiment.