Photomixers as tunable terahertz local oscillators

This work reports on the development of the photomixing technology and its immediate application to realize a tunable coherent source in the terahertz (THz) frequency range with an unprecedented bandwidth. An extensive experimental study of low-temperature-grown gallium arsenide (LT- GaAs) and ion-implanted GaAs as photomixing materials is performed in order to determine the optimal material parameters and fabrication conditions. Defect Engineering allows to create photoconducting materials with outstanding properties for THz signal generation. The type and concentration of semiconductor defects has a critical importance in the performance of the material used for photomixing. In LT-GaAs, defects are highly dependent on the arsenic beam equivalent pressure (BEP), growth and anneal temperature. Unfortunately, the growth temperature at which an LT- GaAs sample shows optimal properties lacks very often of fabrication reproducibility. In contrast to LT-GaAs, the defects created in ion-implanted GaAs can be tailored by varying the implantation dose and energy. In order to achieve a given concentration of defects, Monte Carlo simulations were performed to determine optimal implantation conditions. The precise control over implantation dose and energy allows to overcome the reproducibility limitations of LT-GaAs. Photomixers were fabricated patterning Ti/Au interdigitated electrodes by electron beam lithography on the feed point of different planar antenna designs (resonant dipoles and broadband logarithmic spirals). Electromagnetic simulations of the radiating structures are shown. In addition, semiconductor simulations were performed, revealing the build-up of space charge regions next to the electrodes. The problematic of space charge formation is analyzed and discussed. Experiments with optimized photomixers demonstrate successfully pumping of astronomical heterodyne receivers at 450 GHz with a superconductor-insulator-superconductor (SIS) mixer and at 750 GHz with a hot-electron-bolometer (HEB) mixer. The double sideband (DSB) noise temperature of the astronomical receiver pumped by a photomixer and by a solid state local oscillator (both measured at an intermediate frequency band of 2 to 4 GHz) were identical (Treceiver = 170 K). In addition to the photomixing results, the issue of frequency stabilization of free-running lasers is covered. Experiments were performed using an optical comb generator as a relative frequency reference. Under the frequency lock condition, the beat signal fulfilled the linewidth requirements for the photomixing system to be used as a local oscillator for heterodyne receivers in radio astronomy

Group delay in THz spectroscopy with ultra-wideband log-spiral antennae

We report on the group delay observed in continuous-wave terahertz spectroscopy based on photomixing with phase-sensitive homodyne detection. We discuss the different contributions of the experimental setup to the phase difference \Delta\phi(\nu) between transmitter arm and receiver arm. A simple model based on three contributions yields a quantitative description of the overall behavior of \Delta\phi(\nu). Firstly, the optical path-length difference gives rise to a term linear in frequency. Secondly, the ultra-wideband log-spiral antennae effectively radiate and receive in a frequency-dependent active region, which in the most simple model is an annular area with a circumference equal to the wavelength. The corresponding term changes by roughly 6 pi between 100 GHz and 1 THz. The third contribution stems from the photomixer impedance. In contrast, the derivative (d\Delta\phi / d\nu) is dominated by the contribution of periodic modulations of \Delta\phi(\nu) caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we discuss the Fourier-transformed spectra, which are equivalent to the waveform in a time-domain experiment. In the time domain, the group delay introduced by the log-spiral antennae gives rise to strongly chirped signals, in which low frequencies are delayed. Correcting for the contributions of antennae and photomixers yields sharp peaks or "pulses" and thus facilitates a time-domain-like analysis of our continuous-wave data.Comment: 7 pages, 7 figure

Meander dipole antenna to increase CW THz photomixing emitted power

The success in conquering the terahertz (THz) gap is subject to some facts such as maximizing the emitted power. Traditionally resonant antenna designs for continuous-wave (CW) THz photomixing include a RF choke to compensate the capacitive part of the photomixer and an antenna with a very high input impedance at its resonance to match the low value of the photomixer conductance. This communication considers that the antenna itself can provide this large impedance margin needed to directly match the photomixer, so that the RF choke can be avoided. The meander antenna constitutes an excellent candidate to achieve that goal based on the Active Integrated Antenna concept to improve both matching and radiation efficiencies. The main objective is to maximize the total efficiency and, as a consequence, the THz emitted power. A prototype working at 1.05 THz is designed and manufactured and results show a 6 dB output power improvement when compared with a conventional log-periodic antenna.This work was supported by Consolider CSD 2008–00068. The work of J. Montero-de-Paz was supported by the Spanish Education Minister under the program FPU (AP2009–4679) . The work of E. Ugarte-Muñoz was supported by the Spanish Economy Minister under the program FPI (BES2010–037676).Publicad

Giant natural optical rotation from chiral electromagnons in a collinear antiferromagnet

In NiTe$_3$O$_6$ with a chiral crystal structure, we report on a giant natural optical rotation of the lowest-energy magnon. This polarization rotation, as large as 140 deg/mm, corresponds to a path difference between right and left circular polarizations that is comparable to the sample thickness. Natural optical rotation, being a measure of structural chirality, is highly unusual for long-wavelength magnons. The collinear antiferromagnetic order of NiTe$_3$O$_6$ makes this giant effect even more peculiar: Chirality of the crystal structure does not affect the magnetic ground state but is strongly manifested in the lowest excited state. We show that the dynamic magnetoelectric effect, turning this magnon to a magnetic- and electric-dipole active hybrid mode, generates the giant natural optical rotation. In finite magnetic fields, it also leads to a strong optical magnetochiral effect.Comment: 9 pages, 4 figure

Application of operational modal analysis method in the Monastery of San Jeronimo (Seville, Spain)

The use of Operational Modal Analysis is establishing as an acceptable tool for the update of numerical models. Through its application, mechanical properties of structural elements, as stiffness or mass, can be obtained with great accuracy. The analysis of the current structural state of the Monastery of San Jerónimo, an important piece of heritage located in Seville (Spain), is presented in this paper. The monastery dates from the XV century. The cloister has been analysed by OMA in order to update a FE model, to subsequently carry out the structural assessment of the Monastery

High-power terahertz emitter arrays

This manuscript proposes the combination of several photomixing THz sources for overcoming the power limitations of this technology. Each element is placed in the gap of a balanced antenna. All the elements conform a 2D rectangular array. For avoiding issues related to having out-of-focus sources when using one electrically large silicon lens, two different strategies are considered: increasing the density of elements by using a compact design and using an array of dielectric rod waveguide antennas instead of the lens. Experimental considerations are also provided for two prototypes manufactured in the 1550 nm window. Measured power level will be shown at the conference

High-power terahertz emitter for a communication link: The chessboard array

This manuscript proposes a high-power terahertz source that consists on a large amount of coherently driven photomixing THz sources. Each element is placed in the gap of a bow tie antenna. The resulting array defines a chessboard-like layout which maximize the density of devices. This novel concept is validated through full-wave simulations. Experimental considerations are also provided for a prototype manufactured for the 1550 nm window. Measured power level will be shown at the conference