Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009. Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480−1250 GHz with SIS mixers and the 1410−1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s −1 . Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.
Excitons dominate
the light absorption and re-emission spectra
of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations
of the excitonic response in TMD almost invariably extract information
from the radiative recombination step, which only constitutes one
part of the picture. Here, by exploiting imaging spectroscopic ellipsometry
(ISE), we investigate the spatial dependence of the dielectric function
of chemical vapor deposition (CVD)-grown WS2 flakes with
a microscopic lateral resolution, thus providing information about
the spatially varying, exciton-induced light absorption in the monolayer
WS2. Comparing the ISE results with imaging photoluminescence
spectroscopy data, the presence of several correlated features was
observed, along with the unexpected existence of a few uncorrelated
characteristics. The latter demonstrates that the exciton-induced
absorption and emission features are not always proportional at the
microscopic scale. Microstructural modulations across the flakes,
having a different influence on the absorption and re-emission of
light, are deemed responsible for the effect.
Optimizing the processing of organic photovoltaic devices by laser radiation requires a fundamental understanding of the excitation processes during laser radiation−matter interaction. Spatially and temporally resolved pump−probe ellipsometry on poly(methyl methacrylate) (PMMA), excited by single-pulsed ultrafast mid-IR laser radiation, reveals electronic and vibrational excitation as competing excitation processes. Mid-IR laser radiation in the femtosecond regime induces mainly wavelength independent nonlinear excitation of electrons, as predicted by a theoretical model of the induced free charge carrier density. In contrast, mid-IR laser radiation in the picosecond regime enables linear vibrational excitation, provided the laser radiation frequency corresponds to a resonance frequency of PMMA. Thereby, a wavelength selective processing of organic materials may enable faster processing of multilayer systems depending on the polymer specific vibrational modes.
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