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Here, we model a thin layer of graphene located above a metal surface by means of a surface boundary condition in two different stencils, namely, a simple cubic grid (SC-Grid) and a body centered cubic grid (BCC-Grid). We extend the methodology described in the literature by taking into account the interband contribution of the graphene’s conductivity in addition to the intraband contribution. The mathematical description of the presented developments is explained. Besides this, the metal in contact with the graphene sheet is considered as a dispersive medium; therefore, we deal with the problem by using the auxiliary differential equation finite-difference time-domain (ADE-FDTD) method. In this context, we compared the two stencils and demonstrated that BCC-Grid does not present discontinuities in the normal components of the electric and magnetic fields located on the graphene surface; in this respect, BCC-Grid is also more appropriate than the traditional Yee’s cell for these type of applications.

A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH(+)-He and CH(+)-He(4) complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm(−1). The spectrum of CH(+)-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm(−1), and thus confirms it to have a T-shaped structure. For CH(+)-He(4), the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

In this paper, a scalar wave is solved in the fully explicit finite difference time domain scheme for different stencils based on the cubic crystal system. In particular, we study four systems: the simple cubic, the body-centered cubic, the face-centered cubic and the compact packing cubic. In many papers that are focused on the artificial anisotropy induced by these grids in the propagated wave, one candidate is often better than all the others. In this manuscript, we study the stability, the physical phase velocity error and the anisotropy under two views. Firstly, we consider the same burdens or density of nodes per cubic wavelength and secondly we look at the asymptotic case. We also investigate the computational complexity based on several considerations: burdens, asymptotic time and implementation difficulties. Therefore, we pointed out how each problem or application, due to its different characteristics, has an appropriate grid in order to be treated properly.

Finite-difference time-domain discrete plane wave technique (FDTD-DPW) is a numerical method which by using total the field/scattered field scheme allows one to propagate a plane wave quasi...

In this paper, a novel optically-activated cascode structure is proposed to be used with a normally-on gallium nitride (GaN) field-effect transistor (FET) device to achieve an overall normally-off configuration. Using this novel configuration, cost-effective infrared (IR) lasers can be utilized instead of expensive ultraviolet (UV) lasers to activate this structure which includes a wide-bandgap-material device (GaN FET). Furthermore, the effect of parasitic inductance available in the package and connections of this proposed configuration is evaluated using Silvaco TCAD simulations. In practice, one high-power normally-on FET device is connected in series with a low-power optical switch (OS) to make the proposed overall normally-off cascode structure. The capability of being optically-activated for the proposed structure has many advantages over the conventional electrically-activated cascode structures including but not limited to: more immunity to electromagnetic interference (EMI), using only one main bias as the power source, using cost-effective long-wavelength laser instead of expensive short-wavelength lasers to trigger the GaN devices, reduced current and voltage ringing during switching transitions, etc. Comprehensive device modeling and parasitic inductance analysis for this new proposed optical cascode GaN FET device are provided in this work.

The cross-correlation sensitivity of two identical balanced photodiode heterodyne receivers is characterized. Both the balanced photodiodes receive the same weak signal split up equally, a situation equivalent to an astronomical spatial interferometer. A common local oscillator is also split up equally and its phase difference between both the receivers is stabilized. We show by a semi-classical photon deletion theory that the post-detection laser shot noise contributions on both the receivers must be completely uncorrelated in this case of passing three power splitters. We measured the auto- and cross-correlation outputs as a function of the weak signal power (system noise temperature measurement) and obtained a cross-correlation system noise temperature up to 20 times lower than for the auto-correlation system noise temperature of each receiver separately. This is supported by Allan plot measurements showing cross-correlation standard deviations 30 times lower than in auto-correlation. Careful calibration of the source power shows that the auto-correlation (regular) noise temperature of the single balanced receivers is already very near to the quantum limit as expected, which suggests a cross-correlation system noise temperature below the quantum limit. If validated further, this experimentally clear finding will not only be relevant for astronomical instrumentation but also for other fields, such as telecommunications and medical imaging.

For the telescope number and size of the PFI proposal (20 x 8m), the Q-band (around 25 μm) was shown to be an optimal complement to submm- and optical interferometry, to detect substructures in proto-planetary disks. Here, broadband medium-resolution heterodyne would be an appropriate option, considering recent experimental findings on a quantum-limit heterodyne detection (HD) scheme. Under high thermal background and the very small beam filling factors in interferometry, HD was shown to be more sensitive than direct detection for the same bandwidth for wavelengths larger 3-8 micron, depending on the conditions. A new class of heterodyne mixers based on graphene could extend the instantaneous IF-bandwidth up to hundred GHz, for which photonic correlators may be developed since a resolution of R=30000 would be sufficient at 10μm. Such large IF-bandwidths could then even be multiplied in “dispersed heterodyne receivers” using optical frequency combs and photonic techniques derived from the DWDM technology.

Recent results for the cross-correlation signal of a newly proposed balanced correlation receiver at 1.5 μm pointed towards a possible bypassing of the standard quantum limit for the receiver noise-temperature hν⁄k in cross-correlation by a factor of 4-6. The only radiation source strong enough for a clear hot-cold measurement was a heavily attenuated fiber-coupled superluminant LED (SLED), because a multi-mode fiber-coupled thermal halogen lamp was difficult to control in polarization due to its weakness when coupled to a single-mode fiber. This peculiarity left some doubts regarding a possible “strange” quantum-mechanical behavior of the signal light from the SLED. Here we want to present the concept for more convincing measurements using a true thermal signal source.

The current work presents a fiber coupling tip-tilt controller developed for a three-telescope experimental prototype of an Astronomical Fiber-Based Near-Infrared Heterodyne Interferometer. It is based on a commercial magneto-mechanical compact-disk laser-beam actuator on which the fiber-ferrule is mounted. The actuator is driven by a two-axis controller electronics board which was developed by us based on digital processing in a dsPIC33EP device with analog periphery, which reads the quad-photodiode signals amplified by 109, and drives the actuator with two high-current outputs. While this realizes the very fine and relatively fast (up to 100 Hz) fiber-position control in the telescope focus, as a basis to this, a relatively coarse and slow auto-guiding is given by an amateur guiding camera. During first optical bench testing we obtained an average coupled power increase of up to 50% under certain perturbations.

In the context of a numerical study about mid-infrared receivers based on a metal–insulator–metal junction, we study how a top multilayer of graphene is capable to remove heat from the thin metal layer in which the laser impinges on. Due to its extremely high thermal conductivity, the graphene film (made of several layers) extracts the heat and injects it inside bulky lateral metal paddings of the receiver. Therefore, the lateral metal contacts not only detect the electromagnetic field transforming it into a current through this quantum diode, but they can also help to drain the heat. A metal–insulator–metal junction, which combines Kretschmann illumination and a distributed light on the junction (or are illuminated through a grating), allows introducing the electromagnetic field into the junction, biasing in this manner the insulator all along its entire length. The main challenge is the relationship between the insulator thickness and the electromagnetic wavelength. In the mid-infrared region, the insulator thickness of a few nanometers is several orders of magnitude under the diffraction limit, and the electromagnetic field cannot penetrate directly into the structure. In this way, the described technique delivers the infrared radiation into the junction by means of surface-plasmon-polariton traveling wave (SPP-TW). Therefore, a right illumination improves the diode responsivity considerably. However, rectification demands an extremely asymmetrical current–voltage curve. A layered metal–graphene–insulator–metal makes this feasible especially when an SPP-TW, between the graphene and the insulator, establishes a Seebeck effect resulting in the desired asymmetric characteristic. When sampling, the rectified direct tunneling current is desirable to avoid thermionic emission (inherently slow), as well as to induce the SPP-TW in a better form from the outside of the junction. A cooling mechanism that preserves a high electric conductivity on the top metal contact is required. In this theoretical work, we performed a simulation study of how a sheet of graphene is able to enhance the thermal behavior of the receiver under study.

The design process of a photoconductive antenna (PCA), which emits efficiently in the electromagnetic terahertz range, demands some considerations that are discussed through this work. In this work, several essential characteristics of a photoconductive antenna made with LT-GaAS are studied by means of well established commercial software (COMSOL 5.3 [ 1 ]). An approach to the efficiency is also made through the study of geometry, the laser illumination position, the substrate doping distribution, the direction of the bias applied to the semiconductor, the matching impedance at the laser operating frequency and, finally, the plasmonics effects or penetration laser enhancement due to the use of nano antennas. We study and compare two kinds of structures, one which is quasi-bidimensional or planar and the other which is vertical. Additionally, the photoconductive antennas are also modeled by using a simplified equivalent circuit which helps to understand the antennas’ performance. Therefore, some fundamental parameters, like the transient capacitance between the metal contacts are also studied. Furthermore, we introduce an optimized vertical design which achieves the best results.

By using acoustic emission (AE) it is possible to control deviations and surface quality during micro milling operations. The method of micro milling is used to manufacture a submillimetre waveguide where micro machining is employed to achieve the required superior finish and geometrical tolerances. Submillimetre waveguide technology is used in deep space signal retrieval where highest detection efficiencies are needed and therefore every possible signal loss in the receiver has to be avoided and stringent tolerances achieved. With a sub-standard surface finish the signals travelling along the waveguides dissipate away faster than with perfect surfaces where the residual roughness becomes comparable with the electromagnetic skin depth. Therefore, the higher the radio frequency the more critical this becomes. The method of time-frequency analysis (STFT) is used to transfer raw AE into more meaningful salient signal features (SF). This information was then correlated against the measured geometrical deviations and, the onset of catastrophic tool wear. Such deviations can be offset from different AE signals (different deviations from subsequent tests) and feedback for a final spring cut ensuring the geometrical accuracies are met. Geometrical differences can impact on the required transfer of AE signals (change in cut off frequencies and diminished SNR at the interface) and therefore errors have to be minimised to within 1 µm. Rules based on both Classification and Regression Trees (CART) and Neural Networks (NN) were used to implement a simulation displaying how such a control regime could be used as a real time controller, be it corrective measures (via spring cuts) over several initial machining passes or, with a micron cut introducing a level plain measure for allowing setup corrective measures (similar to a spirit level).

We determine the physical parameters of the outer atmosphere of a sample of eight evolved stars, including the red supergiant {\alpha} Scorpii, the red giant branch stars {\alpha} Bootis and {\gamma} Crucis, the K giant {\lambda} Velorum, the normal M giants BK Virginis and SW Virginis, and the Mira star W Hydrae (in two different luminosity phases) by spatially resolving the stars in the individual carbon monoxide (CO) first overtone lines. We used the Astronomical Multi-BEam combineR (AMBER) instrument at the Very Large Telescope Interferometer (VLTI), in high-resolution mode (${\lambda}/{\Delta}{\lambda} {\approx} 12000$) between 2.28 and 2.31 ${\mu}m$ in the K band.The maximal angular resolution is 10 mas, obtained using a triplet telescope configuration, with baselines from 7 to 48 m. By using a numerical model of a molecular atmosphere in a spherical shells (MOLsphere), called PAMPERO (an acronym for the "Physical Approach of Molecular Photospheric Ejection at high-angular-Resolution for evOlved stars"), we add multiple extended CO layers above the photospheric MARCS model at an adequate spatial resolution. We use the differential visibilities and the spectrum to estimate the size ($R$) of the CO MOLsphere, its column density (${N_{CO}}$) and temperature (${T_{mol}}$) distributions along the stellar radius. The combining of the ${\chi^2}$ minimization and a fine grid approach for uncertainty analysis leads to reasonable ${N_{CO}}$ and ${T_{mol}}$ distributions along the stellar radius of the MOLsphere.

5 pags., 5 figs., 2 tabs., A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH-He and CH-He complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C–H stretching vibrations are recorded in the range of 2720–2800 cm. The spectrum of CH-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm, and thus confirms it to have a T-shaped structure. For CH-He, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions., This work has been supported via Collaborative Research Centre 956, sub-project B2, funded by the Deutsche Forschungsgemein- schaft (DFG, project ID 184018867) as well as DFG SCHL 341/15–1 (‘‘Cologne Center for Terahertz Spectroscopy”) and AS 319/2–2. JLD acknowledges partial financial support from the Agencia Estatal de Investigación (AEI) through grant FIS2016- 77726-C3-1-P and from the European Research Council through grant agreement ERC-2013-SyG-610256-NANOCOSMOS.

The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4x8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry., Comment: Presented at 2018 SPIE Astronomical Telescopes + Instrumentation, Austin, Texas, USA. See www.planetformationimager.org for more information

A simple method to control the nucleation and morphology of (4-N,N-dimethylamino-4-N-methyl-stilbazoliumtosylate) DAST crystals is explored. At equal concentrations of oleic acid and DAST in methanol, pure DAST crystals with an irregular hexagonal shape are obtained by a solvent evaporation method. The influence of oleic acid on facilitating the growth in specific faces is investigated. The purity of the grown crystal is investigated by powder XRD, NMR and Raman spectroscopy analyses. As a major improvement, we present a method where a preference for the growth of one of the desired faces (010) in the final morphology of the DAST crystal is possible which would be attractive for terahertz generation and detection studies.

We present concept and first experimental lab results for a low-cost near-infrared heterodyne interferometer based on commercial 1.55μm fiber components with relative phase-stabilization between both telescopes. After a demonstration with 14”-telescopes, the concept should be upgradable to larger numbers of mid- or large-class telescopes. Given that the employed fiber phase stabilization scheme should enable the operation of long baselines, we discuss the applicability of this concept for long-baseline, high telescope number systems (scalability of the concept) and mid-infrared wavelengths. This could finally result in contributions to the design of the large infrared Planet Formation Imager which is being proposed currently.

The Planet Formation Imager (PFI) project aims to image the period of planet assembly directly, resolving structures as small as a giant planet's Hill sphere. These images will be required in order to determine the key mechanisms for planet formation at the time when processes of grain growth, protoplanet assembly, magnetic fields, disk/planet dynamical interactions and complex radiative transfer all interact - making some planetary systems habitable and others inhospitable. We will present the overall vision for the PFI concept, focusing on the key technologies and requirements that are needed to achieve the science goals. Based on these key requirements, we will define a cost envelope range for the design and highlight where the largest uncertainties lie at this conceptual stage., 14 pages, 5 figures, Proceedings of SPIE 2016

The Planet Formation Imager (PFI) Project has formed a Technical Working Group (TWG) to explore possible facility architectures to meet the primary PFI science goal of imaging planet formation in situ in nearby star- forming regions. The goals of being sensitive to dust emission on solar system scales and resolving the Hill-sphere around forming giant planets can best be accomplished through sub-milliarcsecond imaging in the thermal infrared. Exploiting the 8-13 micron atmospheric window, a ground-based long-baseline interferometer with approximately 20 apertures including 10km baselines will have the necessary resolution to image structure down 0.1 milliarcseconds (0.014 AU) for T Tauri disks in Taurus. Even with large telescopes, this array will not have the sensitivity to directly track fringes in the mid-infrared for our prime targets and a fringe tracking system will be necessary in the near-infrared. While a heterodyne architecture using modern mid-IR laser comb technology remains a competitive option (especially for the intriguing 24 and 40{\mu}m atmospheric windows), the prioritization of 3-5{\mu}m observations of CO/H2O vibrotational levels by the PFI-Science Working Group (SWG) pushes the TWG to require vacuum pipe beam transport with potentially cooled optics. We present here a preliminary study of simulated L- and N-band PFI observations of a realistic 4-planet disk simulation, finding 21x2.5m PFI can easily detect the accreting protoplanets in both L and N-band but can see non-accreting planets only in L band. (abridged -- see PDF for full abstract), Comment: 12 pages, 4 Figures, Proceedings of SPIE 2016

The methylene radical (CH2) was very important to Gerhard Herzberg. We have carried out high-resolution spectroscopic measurements on two energetically low-lying, pure rotational transitions of methylene in its ground vibrational–electronic state at frequencies near 2 THz. One of the transitions — the NKaKc = 211 ← 202 multiplet — belongs to ortho-CH2 and is centered at 1.954 THz. The other rotational transition — the NKaKc = 110 ← 101 multiplet — belongs to para-CH2 and is centered at 1.915 THz. Since the ground electronic state of methylene has 3B1 symmetry, the rotational transitions are split into three fine-structure components, while the ortho transitions are split additionally by hyperfine structure. In total, we have measured 29 new lines, six for the para transition and the remainder for the ortho transition. The newly measured lines can be added to high-resolution spectral data obtained by ourselves and others at lower frequencies. Progress towards a global fit is discussed. Key words: spectroscopy, THz frequencies, radicals, methylene, interstellar medium.

We have studied the NKaKc = 111-202 rotational transition of the ortho-methylene radical CH2 in its ground vibrational and electronic ( 3B1) state. Centered near 943 GHz, this pure rotational transition is split by fine-structure interactions into three components, which lie at 931, 945, and 956 GHz and are characterized by the quantum numbers J = 1-2, 2-3, and 0-1, respectively. In turn, each fine-structure component breaks up into a number of hyperfine components. A total of eight new hyperfine components were measured to a precision of about 20 kHz and analyzed as part of a global data set with previously known data at lower frequencies. Since the 111-202 transition is energetically rather low (110 K) compared with the previously measured 404-313 interstellar transition, it should be more widely observable in interstellar clouds, either through an atmospheric window from the ground or, in the near future, with SOFIA and the Herschel Space Observatory.

We present a new scheme for stabilizing diode lasers with optical feedback. This setup was designed with the intention to realize photonic local oscillators in terahertz receivers by means of mixing the radiation of two lasers on a LTG–GaAs mixer. Due to a novel second-order Littrow configuration the output beam has a circular shape and its direction is independent of laser frequency. Side mode and spontaneous emission background suppression is achieved with an intracavity etalon. Our results fully satisfy the requirements for heterodyne receivers with hot electron bolometer (HEB) mixers, and there are good prospects that the more demanding requirements for SIS-mixers can be also fulfilled.

We are presenting first experimental results for subsystems of a low-cost near-infrared heterodyne interferometer concept based on commercial 1.55μm fiber-components with relative phase-stabilization between both telescopes, a shot noise limited heterodyne scheme with ambient temperature operated photodiodes, an ultra-coherent fiber laser, and a ROACH-based correlator. After we worked on a first demonstration with two 14” amateur telescopes on Betelgeuse, the concept should be upgradable to connect mid- or large-class telescopes, also given that the employed fiber phase stabilization scheme will enable the operation of long baselines.

A central component of a sideband separating (2SB) receiver is the quadrature hybrid which splits the incoming radio frequency signal into two branches with a 90°-phase shift. Its fabrication, however, is one of the factors limiting the operational bandwidth and the maximum frequency at which a 2SB can be built. In this paper a 100%-photonic approach to produce this split is presented. In this way, wider operational bandwidth and a higher maximum operational frequency could be achieved. We also present the first experimental results of a proof-of-concept implementation at 55 MHz that studied the phase stability and controllability of this approach using a commercial 90-degree optical hybrid.

Sideband-separating receivers are usually preferred in the presence of high atmospheric noise. However, one of the most important figures of merit for this kind of receiver, the sideband ratio, is still low and typically around 10 dB. This is because keeping low amplitude and phase imbalances over large RF and IF frequencies is extremely difficult. However, by introducing a digital back-end that mimics the performance of an IF-hybrid, such imbalances can be calibrated out. We have recently presented a digital sideband-separating receiver, working at the W band, that can achieve sideband ratios well above 35 dB. Here we extend this work by demonstrating that it can also be applied to receivers that incorporate a second down-conversion stage with the same performance.

Rotational transitions of D2O with J

Extensive mid-infrared lasing action by water molecules is observed in a supersonic plasma expansion contained inside an optical supercavity. An ion–electron dissociative recombination mechanism is proposed, that may also be relevant to atmospheric and astrophysical environments.

We report the measurement of two new (H2O)3 bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy. Both bands have been assigned to torsional (“pseudorotational”) transitions and are highly perturbed by Coriolis interactions. The 42.9 cm−1 band corresponds to the k=±2←±1 transition while the 65.6 cm−1 band corresponds to the k=±2←0 transition. A model Hamiltonian is derived which allowed a global fit of 361 VRT transitions of these two new bands and the previously reported torsional band at 87.1 cm−1. Each of the bifurcation tunneling components is accurately described. This global fit represents a complete description of the VRT transitions of (H2O)3 up to 150 cm−1, and complements our similar treatment of the (D2O)3 torsional dynamics.

We report an automatic frequency control (AFC) for an optically pumped far infrared (FIR) ring laser applicable for high resolution THz sideband spectroscopy by mixing a fraction of the laser power and a harmonic of a phase-locked synthesizer on a planar Schottky diode. We achieve a relative frequency accuracy of about 0.5 kHz rms at 803 GHz (15NH3) and about 1 kHz rms at 1626.6 GHz (CH 2F 2) over hours of lock time. The absolute frequency accuracy is estimated to be about 5 kHz at 1626.6 GHz.

We describe the design, construction and first experimental results with an open triangle FIR ring resonator with Michelson interferometer output coupling, which we combined with a Brewster-windowless 10W CO2 pump laser to a compact laser package of 1m length. The achieved stability of power and frequency of this system is very high, typically on the order of ±0.01% over several hours and

We are presenting a low-cost near-infrared heterodyne interferometer based on 1.55im fiber-components, amateur telescopes and a 3-GSPS-ROACH-based correlator. While first performance is estimated to be sufficient for the brightest stars, we expect science-relevant astronomical performance given various improvements we are working on, as stabilization of fiber coupling, detectors near the quantum limit requiring lowest possible local oscillator power, and fiber line-length correction. These will enable the investigation of extremely long baselines, adaption of existing medium-class telescopes, and testing at the pointing-telescope slots of ALMA, and/or the extension to three of more baselines.

Die hohen FIR-sowie Pumpstrahlverluste in FIR-Lasern mit dielektrischem Wellenleiter und Hybridauskoppler bewirken, das diese ublicherweise nahe der Laserschwelle betrieben werden. Vor diesem Hintergrund haben wir die Bilanz aller Laserprozesse in einem offenen Gausresonator untersucht, in dem die Pumpstrahlung mit moglichst hoher Gute und moglichst hohem Uberlapp zum FIR-Strahl abgebildet wird. Wegen des aufgrund der grosen FIR-Wellenlange notigen grosen Resonatorquerschnitts wird die auf Diffusion zu den Gefaswanden beruhende Vibrationsabregung eventuell zu einem Problem. Die Rechnungen zeigen aber, das man auf der IR-Seite mehr gewinnt, als man auf der FIR-Seite einbust. Gerade bei Gasen mit schwacher Pumpabsorption ist dieser Effekt schr stark. Die Analyse ergibt eine optimale Resonatorlange von ca. 0.7 m fur ungunstige Molekulparameter. Basierend auf diesen Untersuchungen haben wir einen Ringlaser mit offenem Resonator und variabler Interferometerauskopplung aufgebaut. Wir diskutieren erste experimentelle Ergebnisse mit diesem System, die bestatigen, das es auf wesentlich mehr Laserlinien z. B. als Lokaloszilator fur einen Heterodyne-Empfanger eingesetzt werden kann.

More efficient continuous-wave photonic nearinfrared mixers as terahertz sources are investigated with the motivation to develop a universal photonic local oscillator for astronomical submillimeter/terahertz receiver systems. For this, our group has developed new concepts for vertically illuminated traveling-wave (TW) photomixers. The new device called TWUni- Travelling Carrier photodiodes (TW-UTC PD) was simulated, modeled and shall be optical/terahertz tested at the Electrical Engineering Department of the University of Chile, whereas device fabrication is performed at the MC2 cleanroom facility at Chalmers University of technology. We are reporting on first progresses in this direction.

The authors have fabricated and characterized photomixers based on high energy nitrogen-ion-implanted GaAs. For material optimization and annealing dynamics in MSM photodetector structures, 400 keV implantation energy was used with an ion dose of 1times1016 cm-2 . For photomixer structures, 3 Me V energy was used to implant N + ions into GaAs substrates, with an ion concentration dose of 3times1012 cm-2. The N+-implanted GaAs photomixers exhibit improved output power in comparison to their counterparts, photomixers fabricated on low-temperature-grown GaAs. The highest output power was 2.6 muW at 850 GHz and about 1 muW at 1 THz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+ -implanted GaAs the material of choice for efficient high power sources of terahertz radiation

We present the measurement and analysis of five new (D2O)5 bands via vibration-rotation-tunneling (VRT) spectroscopy as well as a preliminary description of a second (H2O)5 band. The vibrationally averaged rotational constants of all five fitted bands agree well with those from the two previously observed (D2O)5 bands and confirm that the pentamer averages to a symmetric, quasi-planar structure on the time scale of our experiment. While the spectrum of the first two bands, located at 50.7 cm(-1) (1.52 THz) and 27.3 cm(-1) (0.82 THz) are indicative of unperturbed oblate rotors, the three remaining (D2O)5 bands centered at 47.7 cm(-1) (1.43 THz), 45.4 cm(-1) (1.36 THz), and 45.0 cm(-1) (1.35 THz) are severely perturbed by first-order Coriolis coupling. This represents the first observation of this perturbation in the perdeuterated water pentamer, as well as the first observation of transitions between degenerate states of the torsional-puckering manifold. Unlike transitions from the (H2O)5 band observed by Brown et al. at 89.0 cm(-1) and the 103.8 cm(-1) band that we report here, none of the individual rovibrational transitions of any of the five (D2O)5 bands demonstrate spectral splittings due to bifurcation tunneling. We conclude, through careful analysis of these water pentamer bands, that at least three torsional-puckering manifolds have been probed and that the lowest-energy manifold is highly compacted. A plausible water pentamer torsional-puckering correlation diagram is proposed, though additional experimental data are required to unambiguously establish the energies of the torsional-puckering levels.

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3 MeV energy to implant N+ ions into GaAs substrates with an ion concentration dose of 3x10(12) cm(-2). The N+-implanted GaAs photomixers exhibit improvements in the output power in comparison with their counterparts, photomixers fabricated on low-temperature-grown GaAs. The maximal output power was 2.64 mu W at 850 GHz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+-implanted GaAs the material of choice for efficient high power sources of terahertz radiation. (c) 2006 American Institute of Physics.

A pump experiment of two astronomical heterodyne receivers, a superconductor- insulator-superconductor (SIS) receiver at 450 GHz and a hot-electron-bolometer (HEB) receiver at 750 GHz, is reported. A low-temperature-grown GaAs metal-semiconductor-metal photonic local oscillator (LO) was illuminated by two near infrared semiconductor lasers, generating a beat frequency in the submillimeter range. I-V junction characteristics for different LO pump power levels demonstrate that the power delivered by the photomixer is sufficient to pump a SIS and a HEB mixer. SIS receiver noise temperatures were compared using a conventional solid-state LO and a photonic LO. In both cases, the best receiver noise temperature was identical (T-sys=170 K). (c) 2006 American Institute of Physics.

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal (MSM) contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers. (c) 2006 American Institute of Physics.

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion concentration of similar to 3x10(12) cm(-2). The resulting material exhibited 110 fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350 nm and 950 nm. In comparison to their counterparts (photomixers fabricated on low-temperature-grown GaAs) the N+-implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100 mA/W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N+-implanted GaAs the material of choice for efficient optoelectronic photomixers. (c) 2005 American Institute of Physics.

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 mu W at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices. (C) 2005 American Institute of Physics.

A new type of geometry for terahertz traveling-wave photomixers, vertically pumped free-space by two detuned continuous-wave diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured as a first result, which meets power requirements for superconducting heterodyne mixer devices.

We present the first results obtained with our new dual frequency SIS array receiver SMART The instrument is operational since September 2001 at the KOSMA 3m telescope on Gornergrat near Zermatt/Switzerland. The receiver consists of two 2×4 pixel subarrays. One subarray operates at a frequency of 490 GHz, the other one at 810 GHz. Both subarrays are pointed at the same positions on the sky. We can thus observe eight spatial positions in two frequencies simultaneously. For the first year of operation we installed only one half of each subarray, i.e. one row of 4 mixers at each frequency. The receiver follows a very compact design to fit our small observatory. To achieve this, we placed most of the optics at ambient temperature, accepting the very small sensitivity loss caused by thermal emission from the optical surfaces. The optics setup contains a K-mirror type image rotator, two Martin-Puplett diplexers and two solid state local oscillators, which are multiplexed using collimating Fourier gratings. To reduce the need for optical alignment, we machined large optical subassemblies monolithically, using CNC milling techniques. We use the standard KOSMA fixed tuned waveguide SIS mixers with Nb junctions at 490 GHz, and similar Nb mixers with Al tuning circuits at 810 GHz. We give a short description of the front end design and present focal plane beam maps, receiver sensitivity measurements, and the first astronomical data obtained with the new instrument.