Silicon-based semiconductors offer optically low-loss and high-thermal-conducting lattice for the broad-band terahertz
active media that can be used in the range of 5-7 THz. We report on realization of the terahertz-range stimulated
emission from monocrystalline natural and isotopically enriched silicon crystals doped by group-V donor centers due to
nonlinear frequency conversion. Lasing in the frequency bands of 1.2 - 1.8 THz; 2.5 - 3.4 THz has been achieved from
silicon crystals doped by phosphorus and in the frequency band of 4.6 - 6.4 THz from different donors under optical
pumping by radiation of mid-infrared free electron laser at cryogenic temperatures. Analysis of the data shows that the
emission in high-frequency band corresponds to electronic Stokes-shifted Raman-type lasing. The low-frequency bands
indicate on high-order nonlinear frequency conversion processes similar to four-wave mixing accompanied by highenergy
intervalley g-phonons and f-phonons of host lattice. These lasers supplement terahertz silicon lasers operating on
transitions between donor states.
Detection of concealed threats is a key issue in public security. In short range applications, passive imagers operating at
millimeter wavelengths fulfill this task. However, for larger distances, they will suffer from limited spatial resolution.
We will describe the design and performance of 0.8-THz imaging radar that is capable to detect concealed objects at a
distance of more than 20 meter. The radar highlights the target with the built-in cw transmitter and analyses the returned
signal making use of a heterodyne receiver with a single superconducting hot-electron bolometric mixer. With an
integration time of 0.3 sec, the receiver distinguishes a temperature difference of 2 K at the 20 m distance. Both the
transmitter and the receiver use the same modified Gregorian telescope consisting from two offset elliptic mirrors. The
primary mirror defines limits the lateral resolution of the radar to 2 cm at 20 m distance. At this distance, the field of
view of the radar has the diameter 0.5 m. It is sampled with a high-speed conical scanner that allows for a frame time
less than 5 sec. The transmitter delivers to the target power with a density less than ten microwatt per squared centimeter,
which is harmless for human beings. The radar implements a sensor fusion technique that greatly improves the ability to
identify concealed objects.
Suicide bombers and hidden bombs or explosives have become serious threats especially for mass transportation. Until now there exists no established system which can be used against these threats. Therefore new technologies especially for stand-off detection of threats are required. Terahertz (THz) rays offer an alternative inspection method, which can cope with these new challenges. Major advantages of THz radiation as compared to other spectral regions are the possibility to penetrate through clothes and that THz radiation is not harmful for human health. In this report the design and results of a THz stand-off detection system will be presented. The sensor is based on active illumination of the object and sensitive heterodyne detection of reflected and backscattered radiation. The system operates at about 0.8 THz. A THz laser is used for illumination and a superconducting hot-electron bolometric mixer for detection. The local oscillator required for heterodyne detection is a multiplied microwave source. The optical system is designed to allow for stand-off detection at 20 m with a spatial resolution less than 2 cm.
Based on the Matrix-Operator Method the radiative transfer code
STORM (STOkes vector Radiative transfer Model) is introduced,
which was developed in a joint project of DLR and Institut fuer
Weltraumwissenschaften of the Freie Universitaet Berlin. STORM
calculates the Stokes parameters (I, Q, U, V) in a plane parallel,
multi layered atmosphere in the visible and near infrared spectral
range. The scattering characteristics of aerosols are determined
by Mie theory. The surface represents a Lambertian reflector or a
wind ruffled water surface described by Cox-Munk model. The
results of one calculation are the upward and downward directed
Stokes parameters for one wavelength at a desired number of sun
incident and viewing angles at varying altitudes in the principal
plane and other azimuth angles. STORM is applied for an analysis
in view of designing downward looking Earth observing optical
remote sensing systems and values of the degree of polarization
are presented as generic basis for remote sensing system design
and data processing.
Scattering and radiative models are decisive elements in remote sensing applications. Their accuracy has an essential influence on the accuracy of the retrieved information on the physical and chemical constitution of the earth and its environment. An important scientific goal of the 'Atmospheric Processors' Dept. at the Remote Sensing Technology Institute is the development of sophisticated models to analyze atmospheric light scattering and radiative transfer processes in different spectral ranges. To support the scientific exchange and the usage of these models not only in the various active and passive remote sensing techniques the so-called 'Virtual Scattering and Radiative Transfer Laboratory' was developed. This laboratory is a pilot version of an on-line executable scietnfiic software repository. It is basd on a generic 'Virtual Lab' software infrastructure.
Bidirectional measurements of the downward radiance, degree and plane of polarization were performed with the high resolution spectrometers OVID and HiRES in the spectral range of 650 nm - 830 nm on two different days with meteorologically different conditions. The measurements have shown that the spectral and bidirectional behavior of the degree of polarization is different for these two days, especially in the 02A band. It can be assumed that this behavior is caused either by different aerosol types on these two days, or by different height distributions of the aerosol types, or by a combination of these two effects. To simulate the measured Stokes vector, first of all aerosol optical thickness and its height distribution are varied and different calculations are performed.
In a series of radiative transfer calculations the Stokes vector is derived to simulate radiance and polarization measurements in the visible range for different aerosol types. The used radiative transfer model is based on the matrix operator method. The information content of the calculated multiangle Stokes vectors is extracted by a principal component analysis in order to determine the number of independent parameters that describe the system. A correlation analysis is performed between the input variables of the radiative transfer calculations and the scores of each component to obtain the link between them. The objectives of the analysis are: (1) to examine if there is additional information in polarization measurements; (2) to find out which input parameters are derivable from polarization measurements; and (3) to find the optimal viewing geometry either to use polarization effects for the estimation of aerosol types or to avoid the influence of polarization effects on measurements.