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.

Colloidal upconversion nanocrystals enable low-temperature-grown GaAs photoconductive switch operating atλ = 1.55µm.

Microwave photoconductive switches, allowing an optical control on the magnitude and phase of the microwave signals to be transmitted, are important components for many optoelectronic applications. In recent years, there are significant demands to develop photoconductive switches functional in the short-wave-infrared spectrum window (e.g.λ = 1.3-1.55µm) but most state-of-the-art semiconductors for photoconductive switches cannot achieve this goal. In this work, we propose a novel approach, by the use of solution-processed colloidal upconversion nanocrystals deposited directly onto low-temperature-grown gallium arsenide (LT-GaAs), to achieve microwave photoconductive switches functional atλ = 1.55µm illumination. Hybrid upconversion Er3+-doped NaYF4nanocrystal/LT-GaAs photoconductive switch was fabricated. Under a continuous waveλ = 1.55µm laser illumination (power density âˆ¼ 12.9 mWµm-2), thanks to the upconversion energy transfer from the nanocrystals, a more than 2-fold larger value in decibel was measured for the ON/OFF ratio on the hybrid nanocrystal/LT-GaAs device by comparison to the control device without upconversion nanoparticles. A maximum ON/OFF ratio reaching 20.6 dB was measured on the nanocrystal/LT-GaAs hybrid device at an input signal frequency of 20 MHz.

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.

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.