Subwavelength pixelated CMOS color sensors based on anti-Hermitian metasurface.

The demand for essential pixel components with ever-decreasing size and enhanced performance is central to current optoelectronic applications, including imaging, sensing, photovoltaics and communications. The size of the pixels, however, are severely limited by the fundamental constraints of lightwave diffraction. Current development using transmissive filters and planar absorbing layers can shrink the pixel size, yet there are two major issues, optical and electrical crosstalk, that need to be addressed when the pixel dimension approaches wavelength scale. All these fundamental constraints preclude the continual reduction of pixel dimensions and enhanced performance. Here we demonstrate subwavelength scale color pixels in a CMOS compatible platform based on anti-Hermitian metasurfaces. In stark contrast to conventional pixels, spectral filtering is achieved through structural color rather than transmissive filters leading to simultaneously high color purity and quantum efficiency. As a result, this subwavelength anti-Hermitian metasurface sensor, over 28,000 pixels, is able to sort three colors over a 100 nm bandwidth in the visible regime, independently of the polarization of normally-incident light. Furthermore, the quantum yield approaches that of commercial silicon photodiodes, with a responsivity exceeding 0.25 A/W for each channel. Our demonstration opens a new door to sub-wavelength pixelated CMOS sensors and promises future high-performance optoelectronic systems.

Near-perfect broadband absorption from hyperbolic metamaterial nanoparticles.

Broadband absorbers are essential components of many light detection, energy harvesting, and camouflage schemes. Current designs are either bulky or use planar films that cause problems in cracking and delamination during flexing or heating. In addition, transferring planar materials to flexible, thin, or low-cost substrates poses a significant challenge. On the other hand, particle-based materials are highly flexible and can be transferred and assembled onto a more desirable substrate but have not shown high performance as an absorber in a standalone system. Here, we introduce a class of particle absorbers called transferable hyperbolic metamaterial particles (THMMP) that display selective, omnidirectional, tunable, broadband absorption when closely packed. This is demonstrated with vertically aligned hyperbolic nanotube (HNT) arrays composed of alternating layers of aluminum-doped zinc oxide and zinc oxide. The broadband absorption measures >87% from 1,200 nm to over 2,200 nm with a maximum absorption of 98.1% at 1,550 nm and remains large for high angles. Furthermore, we show the advantages of particle-based absorbers by transferring the HNTs to a polymer substrate that shows excellent mechanical flexibility and visible transparency while maintaining near-perfect absorption in the telecommunications region. In addition, other material systems and geometries are proposed for a wider range of applications.

Detection of optical activity with diode-integrated hyperbolic metasurfaces.

We present an analytical technique for designing integrated polarized light-emitting diodes (LEDs) and polarization-sensitive photodiodes (PD) based on hyperbolic metasurfaces (HMS) for the detection of optical activity. Leveraging effective medium theory and the scattering matrix method, we first derive the conditions for optimizing the transmission efficiency of an LED-integrated HMS and the absorption efficiency of a PD-integrated HMS. We then propose using a differential detection technique with orthogonally oriented PD-integrated HMS to measure optical activity in an extremely compact volume. Finally, we perform an estimation of system performance and find that, relative to state-of-the-art polarimeters, a reduction of complexity can be achieved without sacrificing resolution. The results hold merit for reducing the size and cost of polarimeters and associated polarimetric sensing systems, which play vital roles in the pharmaceutical and biomedical sciences.

High-Quality, Ultraconformal Aluminum-Doped Zinc Oxide Nanoplasmonic and Hyperbolic Metamaterials.

Aluminum-doped zinc oxide (AZO) is a tunable low-loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Due to its ultrahigh conformality and compatibility with semiconductor processing, atomic layer deposition (ALD) is a powerful tool for many plasmonic applications. However, despite many attempts, high-quality AZO with a plasma frequency below 1550 nm has not yet been realized by ALD. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The highly conformal nature of ALD is also exploited to coat silicon nanopillars to create localized surface plasmon resonances that are tunable by adjusting the aluminum concentration, thermal conditions, and the use of a ZnO buffer layer. The high-quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.

Extremely compact hybrid III-V/SOI lasers: design and fabrication approaches.

In this manuscript we discuss state of the art hybrid integration techniques and III-V/Si active components with an emphasis on hybrid distributed feedback (DFB) lasers for telecom applications. We review our work on ultra-compact III-V/Si DFB lasers and further describe design considerations and challenges associated with electrically pumped hybrid lasers. We conclude with a perspective on DFB lasers with extremely small footprint, a direction for future research with potential applications to densely-packed optical interconnects.

Modal amplification in active waveguides with hyperbolic dispersion at telecommunication frequencies.

We present a method for studying amplification of electromagnetic modes in active, circularly symmetric waveguides with hyperbolic dispersion. Using this method, we obtain a closed-form expression for the modal threshold condition. We find that modal amplification is possible in a region of the radius-wavelength phase-space with small enough radius so that propagation of the mode is permitted while modal energy and phase counter-propagate. At telecommunication frequencies, such a situation is achievable only when the absolute value of the real metal permittivity exceeds that of the active dielectric. We validate our theoretical conclusions with numerical simulations that explain the threshold condition in terms of an energy balance between the longitudinal and radial components of the electric field.

Tensor of the second-order nonlinear susceptibility in asymmetrically strained silicon waveguides: analysis and experimental validation.

We theoretically consider the existence of multiple nonzero components of the second-order nonlinear susceptibility tensor, χ(2), generated via strain-induced symmetry breaking in crystalline silicon. We determine that, in addition to the previously reported χ(xxy)(2) component, the χ(yyy)(2) component also becomes nonzero based on the remaining symmetry present in the strained material. In order to characterize these two nonlinearities, we fabricate Fabry-Perot waveguide resonators on 250 nm thick silicon-on-insulator wafers clad with 180 nm of compressively stressed (-1.275 GPa) silicon nitride. We measure the shifts in these devices' modal effective indices in response to several bias electric fields and calculate the χ(eff,xxy)(2) and χ(eff,yyy)(2) nonlinear susceptibility tensor elements induced by the breaking of the guiding material's inversion symmetry. Through the incorporation of finite element simulations encompassing the theoretical distribution of strain, the applied bias field, and the optical modes supported by the waveguide geometry, we extract two phenomenological scaling coefficients which relate the induced optical nonlinearities to the local strain gradient.

Invariance of optimal composite waveguide geometries with respect to permittivity of the metal cladding.

We optimize the threshold gain for cylindrical composite (semiconductor-dielectric-metal) waveguides (WGs) with various metal claddings. We show that the optimal dielectric width is invariant with respect to the imaginary part of the permittivity of the metal, εM'', and weakly dependent on the real part, εM'. To explain this behavior, we compare optimal geometries of WGs with different semiconductor permittivities, εG'. Results from these comparisons indicate that the optimal effective index parallels the optimal threshold gain in its relation to εM. We use our results to heuristically propose an analytical expression for the optimal threshold gain that approximates the numerical solution to within a factor of two over the range of explored εG'. Finally, we use data from our optimizations to obtain approximate analytical expressions for the optimal dielectric width and threshold gain as functions of the total WG radius.

Purcell effect in sub-wavelength semiconductor lasers.

We present a formal treatment of the modification of spontaneous emission rate by a cavity (Purcell effect) in sub-wavelength semiconductor lasers. To explicitly express the assumptions upon which our formalism builds, we summarize the results of non-relativistic quantum electrodynamics (QED) and the emitter-field-reservoir model in the quantum theory of damping. Within this model, the emitter-field interaction is modified to the extent that the field mode is modified by its environment. We show that the Purcell factor expressions frequently encountered in the literature are recovered only in the hypothetical condition when the gain medium is replaced by a transparent medium. Further, we argue that to accurately evaluate the Purcell effect, both the passive cavity boundary and the collective effect of all emitters must be included as part of the mode environment.

High contrast modulation of plasmonic signals using nanoscale dual-frequency liquid crystals.

We have designed and simulated a dual-frequency liquid crystal (DFLC) based plasmonic signal modulator capable of achieving over 15 dB modulation depth. The voltage-controlled DFLC is combined with a groove and slit configuration and its operation is discussed. Using the finite-difference time domain (FDTD) method, simulations were conducted to discover the groove-slit separation distance that enabled a practically useful modulation depth for the two states of the DFLC. Moreover, we have shown that significant improvement in modulation depth can be achieved by addition of a second groove to the design structure. Additionally, a performance analysis indicates a switching energy on the order of femtojoules and a switching speed on the order of 100 microseconds. Results of this investigation can be useful for the future design, simulation, and fabrication of DFLC-based plasmonic signal modulating devices, which have application in electro-optical and all-optical information systems.