Christophe Fumeaux

Characterization of the Propagation Properties of the Half-Mode Substrate Integrated Waveguide

The propagation properties of the half-mode substrate integrated waveguide (HMSIW) are studied theoretically and experimentally in this paper. Two equivalent models of the HMSIW are introduced. With the first model, equations are derived to approximate the field distribution inside and outside the HMSIW. Using the second model, an approximate closed-form expression is deduced for calculating the equivalent width of an HMSIW that takes into account the effect of the fringing fields. The obtained design formulas are validated by simulations and experiments. Furthermore, the attenuation characteristics of the HMSIW are studied using the multiline method in the frequency range of 20-60 GHz. A numerical investigation is carried out to distinguish between the contributions of the conductive, dielectric, and radiation losses. As a validation, the measured attenuation constant of a fabricated HMSIW prototype is presented and compared with that of a microstrip (MS) line and a substrate integrated waveguide (SIW). The SIW is designed with the same cutoff frequency and fabricated on the same substrate as the HMSIW. The experimental results show that the HMSIW can be less lossy than the MS line and the SIW at frequencies above 40 GHz.

Nanometer thin-film Ni-NiO-Ni diodes for detection and mixing of 30 THz radiation

Abstract We report on the realization and the experimental study of thin-film Ni–NiO–Ni diodes with integrated infrared antennas. These diodes are applied as detectors and mixers of 28-THz CO 2 -laser radiation with difference frequencies up to 176 GHz. They constitute a mechanically stable alternative to the point-contact MOM diodes used today in heterodyne detection of such high frequencies. Thus, they represent the extension of present millimeter-wave and microwave thin-film and antenna techniques to the infrared. Our thin-film Ni–NiO–Ni diodes are fabricated on SiO 2 /Si substrates with the help of electron-beam lithography at the IBM Research Laboratory (Ruschlikon, Switzerland). We have reduced the contact area to 110 nm×110 nm in order to achieve a fast response of the device. This contact area is in the order of those of point-contact diodes and represents the smallest ever reported for thin-film MOM diodes. The thin NiO layer with a thickness of about 35 A is deposited by sputtering. Our thin-film diodes are integrated with planar dipole, bow-tie and spiral antennas that couples the incident field to the contact. The second derivative I″ ( V ) of the nonlinear I ( V ) characteristics at the bias voltage applied to the diode is measured at a frequency of 10 kHz. It determines the detection and second-order mixing performed with the diode for frequencies from dc to at least 30 THz. The I″ ( V ) characteristics exhibit for low bias voltage V bias a linear dependence, which is followed by a saturation and a maximum for high V bias . The zero-bias resistance of the diode is in the order of 100 Ω. It is not strictly inversely proportional to the contact area of the diode. The first application of our thin-film diodes was the detection of cw CO 2 -laser radiation. The measured dc signal generated by the diode when illuminated with 10.6- μ m radiation includes a polarization-independent contribution, caused by thermal effects. This contribution is independent of the contact area and of the type of integrated antenna. The polarization-dependent contribution of the signal originates in the rectification of the antenna currents in the diode by nonlinear tunneling through the thin NiO layer. It follows a cosine-squared dependence on the angle of orientation of the linear polarization, as expected from antenna theory. For the linearly polarized dipole and bow-tie antennas, the maximum detection signals are therefore measured for the polarization parallel to the antenna axis. Bow-tie antennas with a half length of 2.3 μ m generate the highest detection signals. The full length of these antennas corresponds to 3/2 of the wavelength of the incident 10.6- μ m radiation in the supporting Si substrate. The relevance of the substrate wavelength confirms that our antennas are more sensitive to the radiation incident from the substrate side. The time of response of our thin-film diode is not limited by the speed of the electron-tunneling effect, but by the RC time constant of the diode circuitry. Thus, the overall best performances are attained by the diodes with the smallest contact areas and corresponding capacitances. The study of the polarization response of our integrated asymmetric spiral antennas revealed the contribution of an unbalanced mode propagating on the antenna arms beside the fundamental balanced mode. The imbalance is caused by the reactive impedance of the diode and by the asymmetry of the antenna arms in the feed region. In addition, the response of the diode is influenced by reflection of the antenna currents near the end of the spiral arms. The resulting polarization of our spiral antenna is therefore not the expected circular polarization, yet an elliptical polarization with an axial ratio in the order of 0.12. Furthermore, we have demonstrated the presence of two distinct additive thermal effects besides the fast antenna-induced contribution by the measurement of the response of our thin-film diodes to 35 ps optical-free-induction decay (OFID) CO 2 -laser pulses. The measured characteristic times of these two relatively slow relaxations are τ 1 ≈100 ns and τ 2 ≈15 ns. These exponential relaxations observed are explained by thermal diffusion in the SiO 2 and in the Ni layers of our structures. These time constants show that thermal effects influence mixing processes at low difference frequencies. For the first time, the operation of thin-film diodes as mixers of 28-THz radiation was demonstrated. Difference frequencies up to 176 GHz have been measured when the diode was irradiated by two CO 2 -laser beams and microwaves generated by a Gunn oscillator working at 58.8 GHz. These difference frequencies were generated in mixing processes from the second to the fifth order. These experiments were performed with thin-film Ni–NiO–Ni diodes with the minimum contact area of 0.012 μ m 2 and integrated resonant bow-tie antennas. The transmission of the high-frequency signals to the spectrum analyzer was accomplished using integrated rhodium waveguides and flip-chip connections. The diode and the antenna were irradiated through the substrate, taking advantage of the better sensitivity of the antenna to radiation incident from the substrate side. The dependence on the linear polarization of the mixing signal matches almost perfectly the ideal cosine-squared dependence predicted by antenna theory for bow-tie antennas. A ratio of the mixing signals for the polarization parallel to the axis vs. the cross-polarization of over 50 was attained. The signal-to-noise ratios of our mixing signals demonstrate the potential of our type of diodes to respond to even higher carrier and difference frequencies. Also higher-order mixing can be achieved with our thin-film diodes.

Dielectric resonator nanoantennas at visible frequencies

Drawing inspiration from radio-frequency technologies, we propose a realization of nano-scale optical dielectric resonator antennas (DRAs) functioning in their fundamental mode. These DRAs operate via displacement current in a low-loss high-permittivity dielectric, resulting in reduced energy dissipation in the resonators. The designed nonuniform planar DRA array on a metallic plane imparts a sequence of phase shifts across the wavefront to create beam deflection off the direction of specular reflection. The realized array clearly demonstrates beam deflection at 633 nm. Despite the loss introduced by field interaction with the metal substrate, the proposed low-loss resonator concept is a first step towards nanoantennas with enhanced efficiency. The compact planar structure and technologically relevant materials promise monolithic circuit integration of DRAs.

Measurement of the resonant lengths of infrared dipole antennas

The resonant lengths of infrared dipole antennas at 10.6 and 3.39 lm are experimentally investigated. For this purpose, submicron-sized microbolometers coupled to dipole antennas with lengths between 0.7 and 20 lm were fabricated on a SiO2-on-Si substrate. The response of the detector to 10.6 lm radiation shows a first resonance for an antenna length between 1.0 and 2.5 lm. A subsequent zero and a second attenuated resonance are observed as the antenna length increases. Similar behavior is observed for illumination at 3.39 lm, with a first resonance occurring at a length shorter than 1 lm. The results permit evaluation of an eAective dielectric permittivity and shows the eAect of the surface impedance of the metal on the propagation of current-wave on the antenna. The resonance behavior is further studied by changing the irradiation conditions of the detectors. Air-side and substrate-side illumination exhibit identical resonant antenna lengths, but diAerent eAciencies of power collection. The antenna patterns as a function of incident angle have also been measured at 10.6 lm, showing a transition from a primary broadside lobe to the development of side lobes for longer antennas. Finally, an antenna response is measured at visible frequencies. Our measurements point out similarities, as well as diAerences, between infrared antennas and their counterparts at microwave frequencies, and provide insights useful for the design optimization of planar infrared antennas. ” 2000 Elsevier Science B.V. All rights reserved.

Displacement Sensor Based on Diamond-Shaped Tapered Split Ring Resonator

Split-ring resonators (SRRs) are ideal structures for the realization of compact high-sensitivity and high-resolution sensors due to their high-quality factor resonance, compact size, and high sensitivity to changes in the constituent materials and physical dimensions. This paper presents a displacement sensor based on a diamond-shaped tapered SRR coupled to a coplanar waveguide. Two significant improvements over previous designs are reported. Firstly, the proposed sensor has higher dynamic range and linearity for displacement sensing. Secondly, compared with previous designs, where the displacement changes both the resonant frequency and depth of the transmission notch, the proposed sensor has a fixed resonant frequency. This is an important improvement since the sensor can be operated at a single fixed frequency and bypass the need for a frequency-sweeping microwave source and measurement system such as an expensive network analyzer. It is shown that, while preserving the compact size, the proposed sensor also benefits from a lower operating frequency. The design principle and simulation results are validated through measurement.

Wearable Textile Half-Mode Substrate-Integrated Cavity Antenna Using Embroidered Vias

A wearable textile antenna based on the fundamental mode of a half-mode substrate-integrated semicircular cavity is presented. The antenna operates around 5 GHz and is manufactured with two layers of silver fabric conductors, on the top and bottom of a low-permittivity low-loss foam with minimal water absorption. The vias are realized with five passes of conductive yarn with a diameter of 0.12 mm placed every 1 mm. Their precise arrangement is obtained using computerized embroidery. Compared to other antenna concepts, this design features minimal manufacturing complexity as it does not require accurate patterned cutting of textile conductors. Good isolation from the human body and robustness in terms of deformation are further key characteristics of this structure. This letter provides design guidelines and investigates realistic issues associated with the manufacturing technique. A good agreement between simulated and measured performance with a gain of 7.2 dB validates the concept.

Comparison of the Radiation Efficiency for the Dielectric Resonator Antenna and the Microstrip Antenna at Ka Band

The radiation efficiency of a dielectric resonator antenna (DRA) and a microstrip antenna (MSA) at Ka band is investigated numerically and experimentally. For direct comparison, one cylindrical DRA and one circular disk MSA were designed with similar feeding networks for operation at around 35 GHz. The efficiency of both devices was measured using the directivity/gain (D/G) method and the Wheeler cap method, in both of which the losses in the test system and the feeding structure were taken into account for calibration purposes. A good agreement between measured and simulated results for both methods is found, when considering the effect of the sampling interval and cross-polarization in the D/G method and the effect of the metallic cap size in the Wheeler cap method. It is finally demonstrated that the radiation efficiency of the DRA is significantly higher than that of the MSA at millimeter wave frequencies.

A generalized local time-step scheme for efficient FVTD simulations in strongly inhomogeneous meshes

A new generalized local time-step scheme is introduced to improve the computational efficiency of the finite-volume time-domain (FVTD) method. The flexibility of unstructured FVTD meshes is fully exploited by avoiding the disadvantage of a single short time step in the entire mesh. The great potential of this scheme is fully revealed in the FVTD simulation of electromagnetic (EM) problems with both large and fine structures in close proximity. The scheme is based on an automatic partition of the computational domain in subdomains where local time steps of the type 2/sup /spl lscr/-1//spl Delta/t(/spl lscr/=1,2,3,...) can be applied without violating the stability condition. Interfaces between subdomains are reduced to a generic two-level system which requires a very limited number of time interpolations during the FVTD iteration, therefore resulting in a very simple and robust technique. The application of local time stepping to three-dimensional EM problems demonstrates a significant speed-up of the computation without compromising the accuracy of the results.

Mechanically Tunable Dielectric Resonator Metasurfaces at Visible Frequencies.

Devices that manipulate light represent the future of information processing. Flat optics and structures with subwavelength periodic features (metasurfaces) provide compact and efficient solutions. The key bottleneck is efficiency, and replacing metallic resonators with dielectric resonators has been shown to significantly enhance performance. To extend the functionalities of dielectric metasurfaces to real-world optical applications, the ability to tune their properties becomes important. In this article, we present a mechanically tunable all-dielectric metasurface. This is composed of an array of dielectric resonators embedded in an elastomeric matrix. The optical response of the structure under a uniaxial strain is analyzed by mechanical-electromagnetic co-simulations. It is experimentally demonstrated that the metasurface exhibits remarkable resonance shifts. Analysis using a Lagrangian model reveals that strain modulates the near-field mutual interaction between resonant dielectric elements. The ability to control and alter inter-resonator coupling will position dielectric metasurfaces as functional elements of reconfigurable optical devices.

Experimental demonstration of reflectarray antennas at terahertz frequencies.

Reflectarrays composed of resonant microstrip gold patches on a dielectric substrate are demonstrated for operation at te rahertz frequencies. Based on the relation between the patch size and the reflectio n phase, a progressive phase distribution is implemented on the patch rray to create a reflector able to deflect an incident beam towards a predefine a gle off the specular direction. In order to confirm the validity of th e design, a set of reflectarrays each with periodically distributed 360 ×360 patch elements are fabricated and measured. The experimental results obta ined through terahertz time-domain spectroscopy (THz-TDS) show that up to n early 80% of the incident amplitude is deflected into the desired directi on at an operation frequency close to 1 THz. The radiation patterns of the reflec tarray in TM and TE polarizations are also obtained at different frequen cies. This work presents an attractive concept for developing components a ble to efficiently manipulate terahertz radiation for emerging terahertz com munications. OCIS codes:(300.6495) Spectroscopy, terahertz; (110.5100) Phased-a rray imaging systems; (240.6645) Surface differential reflectance. References and links 1. D. G. Berry, R. G. Malech, and W. A. Kennedy, “The reflectarr ay antenna,” IEEE Trans. Antennas Propag. 11, 645–651 (1963). 2. J. Huang and J. Encinar, Reflectarray Antenna . Wiley-IEEE Press, 2008. 3. J. P. Montgomery, “A microstrip reflectarray antenna elem ent,” Antenna Applications Symposium, University of Illinois (1978). 4. D. M. Pozar and T. A. Metzler, “Analysis of a reflectarray an tenna using microstrip patches of variable size,” Electron. Lett.29,657–658 (1993). 5. D. C. Chang and M. C. Huang, “Multiple-polarization micro strip reflectarray antenna with high efficiency and low cross-polarization,” IEEE Trans. Antennas Propag. 43,829–834 (1995). 6. J. P. 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Antennas Propag., 1, 289–293 (2007).Reflectarrays composed of resonant microstrip gold patches on a dielectric substrate are demonstrated for operation at terahertz frequencies. Based on the relation between the patch size and the reflection phase, a progressive phase distribution is implemented on the patch array to create a reflector able to deflect an incident beam towards a predefined angle off the specular direction. In order to confirm the validity of the design, a set of reflectarrays each with periodically distributed 360 × 360 patch elements are fabricated and measured. The experimental results obtained through terahertz time-domain spectroscopy (THz-TDS) show that up to nearly 80% of the incident amplitude is deflected into the desired direction at an operation frequency close to 1 THz. The radiation patterns of the reflectarray in TM and TE polarizations are also obtained at different frequencies. This work presents an attractive concept for developing components able to efficiently manipulate terahertz radiation for emerging terahertz communications.