Dual epsilon-near-zero effect assisted broadband electro-optic modulation

A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.200093 discusses dual epsilon-near-zero effect assisted broadband electro-optic modulation.

 

With the continuous and rapid development of the information era, high-capacity, high-speed and parallel information processing will undoubtedly become an irreversible trend in the future. Active controlling the characteristics of the optical signal in amplitude, phase or polarization is essential for a broad application in diverse fields such as optical processing, optical interconnects, adaptive optics, image projection etc. Tremendous efforts have been made in the past decades towards realizing electrical addressed modulation by carrier dispersion, the Pockels, or quantum confined Stark effect. To meet the ever-increasing demands for modern photonic systems, advanced modulation solution should encompass not only the high speed and efficient operation, but also the low energy consumption and capability of large-scale integration. Therefore, electro-optic (E-O) modulation on all-silicon or silicon compatible platform has continued to be at the forefront of research in this field. The inherent weak E-O effect of silicon is major stumbling block to achieve efficient all-silicon modulation. The silicon modulation in the integrated optics domains has been struggled to satisfy large modulation depth, however at the expense of the speed because of the large footprint. In contrast, it becomes even more challenging to realize spatial light modulation relying on silicon E-O effect. By incorporating the plasmonic or high-Q resonance designs, several light-matter interaction (LMI) enhancing strategies have been proposed to achieve silicon-based spatial light modulation. Nevertheless, the former suffers from a serious Ohmic loss, while the latter with narrow spectral line-width are too sensitive to temperature drift. Alternately, the silicon compatible platform with hybrid integration of active medium facilitates the utilization of more efficient E-O mechanisms for free space light modulation. For instance, the liquid crystal on silicon (LCoS) is the most mature technology for spatial light manipulation and has been widely used on both the basic researches and practical applications. Because of the E-O response of the nematic liquid crystal is typically in the millisecond scale, the main limitation of LCoS technique lies on its slow switching speed.

Epsilon-near-zero (ENZ) effect based on the transparent conducting oxides (TCOs) has been emerged as a robust LMI enhancing scheme for all-optical switch, E-O modulation, perfect absorption/thermal emission and etc. In particular, the onset and offset of ENZ confinement on nanometer thick carrier accumulation/depletion layer can be dynamically controlled via classic MOS configuration on silicon platform. The ENZ effect leverages the combination of a local electric field enhancement and increased absorption in the material gated to ENZ state, revealing a robust electro-absorption (E-A) modulation. This makes the TCO as a compelling choice for hybrid integration on silicon to achieve more efficient E-O modulation for both the integrated and free-space optics. To date, several groups have studied and demonstrated silicon-based E-O modulator with small footprint and high speed using ENZ effects from the integrated TCOs materials. The E-O modulation was achieved upon tuning the carrier density of TCOs to match or mismatch the ENZ condition. Those previous studies have normally considered TCO as the sole active medium but seldom explored the carrier dispersion effect in silicon gate, which may hinder the possible contributions from E-O or even ENZ responses from silicon.

The research group of Prof. Chen from Jinan University propose a new spatial light modulation strategy which incorporates a dual material’s ENZ effect. While the free-carrier dispersion effect in silicon is frequently used in electro-refraction modulator, their results highlight the possibility to achieving similar ENZ confinement in silicon as the TCO materials. The proposed spatial light modulation scheme is based on a semiconductor-insulator-semiconductor (SIS) nano-capacitor. Imagining from a parallel plate capacitor, equal and opposite surface charges (Qs) will induced by an external voltage bias. According to Drude theory of free carrier, the complex permittivity for both the accumulation and inversion layers of the SIS structure are carrier concentration dependent. It’s possible to manipulate the permittivity crossover wavelengths (i.e. ENZ points) of the surface layers in the two different materials via external voltage biasing.

This work is entitled "Broadband Spatial Light Modulation with Dual Epsilon-Near-Zero Modes” and published in Opto-Electronic Advances issue 6 2022. The proof of concept device is based on a TCOs/insulator/silicon nanotrench configuration, in which the high aspect Si nanotrench array with an ultra-thin conformal coating of high k insulator assumes to be completely wrapped by Al-doped ZnO. The deep nanotrench array resembles a vertically aligned waveguide system with low optical loss when there is absent of external voltage biasing across the junction. Dual ENZ modes form alongside the nanotrench sidewall when applying a voltage bias, because of the surface carrier accumulation at the both side of the junction. These highly confined modes propagate in an elongated manner leading to very efficient electro-absorption. Compared to the plasmonic resonance strategy, the non-resonant and metal-less configuration allows for more efficient spatial light modulation with large absolute modulation depth and broad-band spectral operation from near- to mid-infrared.

The E-A modulation in a broad-band spectrum is clearly observable for their experimental demonstrated device. In order to clarify the underlying mechanisms, the paper establishes a rigorous theoretical model for the deep sub-micron SIS junction device, in which the spatially varied carrier dispersion was deduced by combining the Drude theory and quantum confinement model for the free carrier distribution. Their theoretical model suggests that the observed E-A modulation can be safely ascribed to the dual ENZ confinement effects arising both from the silicon and TCOs. Apart from the proof-of-concept demonstration with n-type silicon/insulator/TCO configuration operating in quasi-static state, the authors also point out that for pursuing high speed and large modulation depth simultaneously, the doping polarity has to be different in the two semiconductors of SIS capacitor (e.g., p-type silicon/insulator/TCO junction). They demonstrated that the proposed dual ENZ mediated E-O scheme with proper device optimization allows for high speed operation at the nanosecond timescale and large absolute modulation depth around 40-70% for broad spectrum from NIR to MIR.

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Article reference: Wen L, Nan XH, Li JX, David RSC, Hu X et al. Broad-band spatial light modulation with dual epsilon-near-zero modes. Opto-Electron Adv 5, 200093 (2022). doi: 10.29026/oea.2022.200093

Keywords: epsilon-near-zero / modulation / TCOs / electro-optic / light harvesting

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The Micro/Nano-Optoelectronic Device Group in the Institute of Nanophotonics at Jinan University is led by Professor Qin Chen and Professor Long Wen. The group is mainly engaged in the research of on-chip integrated optical sensing and detection, focusing on optical field manipulation to enhance sensitivity and on-chip microsystem integration for portable application. The group has developed on-chip direct electrical readout broadband optical sensor, chip-scale microspectrometer, reconfigurable photodetector, etc.

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