Optically Controlled Gain Modulation for Microwave Metasurface Antennas.

Over the past decade, metasurfaces (MTSs) have emerged as a highly promising platform for the development of next-generation, miniaturized, planar devices across a wide spectrum of microwave frequencies. Among their various applications, the concept of MTS-based antennas, particularly those that are based on surface wave excitation, represents a groundbreaking advancement with significant implications for communication technologies. However, existing literature primarily focuses on MTS configurations printed on traditional substrates, largely overlooking the potential benefits of employing photosensitive substrates. This paper endeavors to pioneer this novel path. We present a specialized design of a modulated MTS printed on a silicon substrate, which acts as a photosensitive Ka-band surface wave antenna. Remarkably, the gain of this antenna can be time-modulated, achieving a variance of up to 15 dB, under low-power (below 1 W/cm²) optical illumination at a wavelength of 971 nm. This innovative approach positions the antenna as a direct transducer, capable of converting an optically modulated signal into a microwave-modulated radiated signal, thus offering a new dimension in antenna technology and functionality.

Millimeter-Wave Permittivity Variations of an HR Silicon Substrate from the Photoconductive Effect.

The photoinduced microwave complex permittivity of a highly resistive single-crystal silicon wafer was extracted from a bistatic free-space characterization test bench operating in the 26.5-40 GHz frequency band under CW optical illumination at wavelengths of 806 and 971 nm. Significant variations in the real and imaginary parts of the substrate's permittivity induced by direct photoconductivity are reported, with an optical power density dependence, in agreement with the theoretical predictions. These experimental results open the route to ultrafast system reconfiguration of microwave devices in integrated technology by an external EMI-protected and contactless control with unprecedented performance.

Experimental Microwave Complex Conductivity Extraction of Vertically Aligned MWCNT Bundles for Microwave Subwavelength Antenna Design.

This paper reports the extraction of electrical impedance at microwave frequencies of vertically aligned multi-wall carbon nanotubes (VA MWCNT) bundles/forests grown on a silicon substrate. Dedicated resonating devices were designed for antenna application, operating around 10 GHz and benefiting from natural inductive/capacitive behavior or complex conductivity in the microwave domain. As obtained from S-parameters measurements, the capacitive and inductive behaviors of VA MWCNT bundles were deduced from device frequency resonance shift.

Gallium-nitride-based plasmonic multilayer operating at 1.55 µm.

In this Letter, we have designed and fabricated a III-V semiconductor multilayer based on surface plasmon resonance (SPR) operating at the telecom wavelength. Optimization of the optogeometrical parameters and the metal/semiconductor layers required for this novel structure was conducted accurately by theoretical tools using the Maxwell equations. Technological fabrication of the device and its experimental characterizations using an evanescent coupling configuration was performed: the results have confirmed the existence of SPR associated to a sharp width response. This study could be a first step in the design of new plasmonic-semiconductor-based optical devices such as modulators and switches.

Electrically tunable liquid-crystal wave plate using quadripolar electrode configuration and transparent conductive polymer layers.

We present the realization of an electrically tunable wave plate, which uses a nematic liquid-crystal (LC) phase retarder that allows fast and continuous control of the polarization state. This device is built using a quadripolar electrode design and transparent conductive polymer layers in order to obtain a uniform electric field distribution in the interelectrode area. With this realization, we obtain a high degree of control of the orientation of the electric field and, consequently, of the LC director. Indeed, this modulator outperforms classical bipolar LC cells in both optical path variation (>4 microm) and LC rotation speed (0.4 degrees/micros).

Evidence of ultra low microwave additive phase noise for an optical RF link based on a class--a semiconductor laser.

The additive RF phase noise of a microwave photonics link is measured using, as the optical source, a semiconductor laser operating in the class-A regime. The relative intensity noise of this laser being below the shot noise relative level, the phase noise floor of the link is shown to be shot noise limited, -152 dBc/Hz in our experimental conditions. As a result, the phase noise floor evolves as the inverse of the detected photocurrent, pushing the limits of performance to the availability of high power photo-detectors. Below 6 kHz from the carrier frequency at 3GHz, some noise, in excess with respect to the shot noise limit, is observed but remains lower than -110 dBc/Hz at 100 Hz offset frequency. This residual noise originates mainly from environmental noise and can be reduced by isolating the laser from acoustic/electromagnetic perturbations.