Meta-materials & META-DEVICES
The ability to engineer non-existent optical and electronic properties into materials broadly makes up the field of research into metamaterials. Within this broad topic, our research thrusts have focused on three of the most popular and widely used material platforms, namely: Silicon, Silicon Nitride (SiNx) and III/Vs such as InGaAsP/InP. Utilizing state-of-the-art fabrication techniques at UCSD’s Nano3 cleanroom facilities as well as those at the Centre of Magnetic Recording Research (CMRR), our group has successfully demonstrated several “first of their kind” results including, Demonstration of electronically-tunable metal-semiconductor-metal metamaterials, Demonstration of gain-incorporated hyperbolic meta-surfaces.
Electronic Metamaterials with Tunable Second-order Optical Nonlinearities
The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. Traditionally, metals have been employed to enhance nonlinear optical interactions through feld localization. Here, inspired by the electronic properties of materials, we introduce and demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM) metamaterial that exhibits a large and electronically tunable effective second-order optical susceptibility (χ(2)). The induced χ(2) originates from the interaction between the third-order optical susceptibility of the semiconductor (χ(3)) with the engineered internal electric field resulting from the two metals possessing dissimilar work function at its interfaces. We demonstrate a five times larger second-harmonic intensity from the MSM metamaterial, compared to contributions from its constituents with electrically tunable nonlinear coefficient ranging from 2.8 to 15.6pm/V. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space invariant and -variant nonlinear impulse response.
Silicon Nitride
In recent years, silicon nitride has emerged as a leading candidate for on-chip CMOS compatible photonics. This is primarily on account of the fact that the platform offers ultra-low loss, wide transparency and ease of integration. Starting with the work on Kaz Ikeda [1,2], our group was on the first to explore silicon nitride waveguides and their linear and nonlinear optical properties. Using PECVD fabrication facilities at UCSD nano3, our group demonstrated the presence of bulk-nonlinearities in stoichiometric silicon nitride thin-films [3]. With state-of-the-art optical setups available for in-waveguide and free-space second-harmonic generation, measurements were/can be performed on both silicon nitride thin-films (using pico-second pulsed laser sources) and waveguides (using CW sources), showing values as high as 2pm/V for the nonlinear coefficient.
Currently, the group’s work in this thrust is focused on exploring the field of silicon-rich-nitride (SRN) which involves engineering the silicon content in silicon nitride films deposited using PECVD, to modify its nonlinear properties. While there have been results exploring third-order nonlinearities (n2) in these films, the nature (magnitude, wavelength-dependence) of their second-order nonlinearities remain largely unexplored and hence ripe for academic research.
Engineered nonlinearties in silicon waveguides
Silicon being a centrosymmetric material, does not exhibit a second-order nonlinear susceptibility χ(2). As such, important on-chip applications like linear optical modulation and wave-mixing are often not possible and or inefficient. Starting in 2006, straining waveguides using a stresser layer of silicon nitride, was demonstrated as a possible candidate to “induce” this nonlinearity into silicon waveguides, albeit with a relatively small magnitude of 15pm/V. Thereafter, and over a span of 8 years, results showing “electro-optic” modulation and or second-harmonic generation in such waveguides were reported and attributed to strain induced second-order nonlinearity.
In 2014, our group published, the very first experimental results [1] on a detailed tensorial analysis on strained silicon waveguides. Anomalies, primarily the non-linear nature of the observed electro-optic shift were reported on in the same publication casting doubts on true nature of the “induced” nonlinearity. Subsequently, our group undertook a detailed study to quantify and qualify any or all causes of this apparent nonlinearity.
Primarily, the interface between the dielectric cladding (SiNx, SiO2 and Al2O3) and semiconductor (Si) waveguide was studied particularly from the stand-point of its effects on the observed electro-optic properties and passive properties of these waveguides. We showed how a high density of “fixed-charges” does exist at the dielectric-semiconductor for most commonly used dielectric clads [2]. Furthermore, it was shown both theoretically and experimentally that these fixed charges can cause a large redistribution of free-carriers inside the semiconductor waveguide and effect both active (in terms of the slope of the electro-optic response) and passive (in terms of the observed loss) properties [2,3]. The observed electro-optic shift in strained silicon waveguides was then shown conclusively to be resulting from the capacitively induced free-carrier plasma dispersion effect rather than the Pockels effect. Lastly, it was also clearly demonstrated that the enhanced second-harmonic generation observed in strained-silicon waveguides was a result of the high electric-field induced close to the periphery of the semiconductor waveguide due to the interface fixed-charges. This high electric-field interacts with silicon’s third-order susceptibility χ(3), to create an apparent enhanced second-order susceptibility.
Future work on this thrust involves using the capacitively induced free-carrier plasma dispersion effect to model and demonstrate an efficient capacitive silicon-dielectric modulator based on a slot waveguide scheme.
In 2014, our group published, the very first experimental results [1] on a detailed tensorial analysis on strained silicon waveguides. Anomalies, primarily the non-linear nature of the observed electro-optic shift were reported on in the same publication casting doubts on true nature of the “induced” nonlinearity. Subsequently, our group undertook a detailed study to quantify and qualify any or all causes of this apparent nonlinearity.
Primarily, the interface between the dielectric cladding (SiNx, SiO2 and Al2O3) and semiconductor (Si) waveguide was studied particularly from the stand-point of its effects on the observed electro-optic properties and passive properties of these waveguides. We showed how a high density of “fixed-charges” does exist at the dielectric-semiconductor for most commonly used dielectric clads [2]. Furthermore, it was shown both theoretically and experimentally that these fixed charges can cause a large redistribution of free-carriers inside the semiconductor waveguide and effect both active (in terms of the slope of the electro-optic response) and passive (in terms of the observed loss) properties [2,3]. The observed electro-optic shift in strained silicon waveguides was then shown conclusively to be resulting from the capacitively induced free-carrier plasma dispersion effect rather than the Pockels effect. Lastly, it was also clearly demonstrated that the enhanced second-harmonic generation observed in strained-silicon waveguides was a result of the high electric-field induced close to the periphery of the semiconductor waveguide due to the interface fixed-charges. This high electric-field interacts with silicon’s third-order susceptibility χ(3), to create an apparent enhanced second-order susceptibility.
Future work on this thrust involves using the capacitively induced free-carrier plasma dispersion effect to model and demonstrate an efficient capacitive silicon-dielectric modulator based on a slot waveguide scheme.