Joseph S. T. Smalley

A frequency-addressed plasmonic switch based on dual-frequency liquid crystals

A frequency-addressed plasmonic switch was demonstrated by embedding a uniform gold nanodisk array into dual-frequency liquid crystals (DFLCs). The optical properties of the hybrid system were characterized by extinction spectra of localized surface plasmon resonances (LSPRs). The LSPR peak was tuned using a frequency-dependent electric field. A ∼4 nm blueshift was observed for frequencies below 15 kHz, and a 23 nm redshift was observed for frequencies above 15 kHz. The switching time for the system was ∼40 ms. This DFLC-based active plasmonic system demonstrates an excellent, reversible, frequency-dependent switching behavior and could be used in future integrated nanophotonic circuits.

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.

Temperature Dependence of the Spontaneous Emission Factor in Subwavelength Semiconductor Lasers

We perform a rigorous analysis of the temperature dependence of the spontaneous emission factor, β, in subwavelength semiconductor lasers. The analysis combines a recent formulation of the Purcell effect in semiconductor nanolasers with finite-element modeling and established theoretical models for temperature-dependent emission spectra. While the method is general, we apply it to a subwavelength metallo-dielectric nanolaser, and find that β of the dominant mode decreases sharply below a transition temperature. This result is found for both positive and negative thermo-optic coefficients of the semiconductor material, and occurs because of detuning between the dominant mode and peak emission. The analysis enables better understanding of nanolaser dynamics, as well as the design and characterization of high-β nanolasers.

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.

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.

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.

Amorphous Al 2 O 3 Shield for Thermal Management in Electrically Pumped Metallo-Dielectric Nanolasers

We analyze amorphous Al2O3 (α-Al2O3) for use as a thick thermally conductive shield in metallo-dielectric semiconductor nanolasers, and show that the use of α-Al2O3 allows a laser to efficiently dissipate heat through its shield. This new mechanism for thermal management leads to a significantly lower operating temperature within the laser, compared with lasers with less thermally conductive shields, such as SiO2. We implement the shield in a continuous wave electrically pumped cavity, and analyze its experimental performance by jointly investigating its optical, electrical, thermal, and material gain properties. Our analysis shows that the primary obstacle to room temperature lasing was the devices high threshold gain. At the high pump levels required to achieve the gain threshold, particularly at room temperature, the gain spectrum broadened and shifted, leading to detrimental mode competition. Further simulations predict that an increase in the pedestal undercut depth should enable room temperature lasing in a device with the same footprint and gain volume. Through the integrated treatment of various physical effects, this analysis shows the promise of α-Al2O3 for nanolaser thermal management, and enables better understanding of nanolaser behavior, as well as more informed design of reliable nanolasers.

Subwavelength semiconductor lasers for dense chip-scale integration

Metal-clad subwavelength lasers have recently become excellent candidates for light sources in densely packed chip-scale photonic circuits. In this review, we summarize recent research efforts in the theory, design, fabrication, and characterization of such lasers. We detail advancements of both the metallo-dielectric and the coaxial type lasers: for the metallo-dielectric type, we discuss operation with both optical pumping and electrical pumping. For the coaxial type, we discuss operation with all spontaneous emission coupled into the lasing mode, as well as the smallest metal-clad lasers to date operating at room temperature. A formal treatment of the Purcell effect, the modification of the spontaneous emission rate by a subwavelength cavity, is then presented to assist in better understanding the quantum effects in these nanoscale semiconductor lasers. This formalism is developed for the transparent medium condition, using the emitter-field-reservoir model in the quantum theory of damping. We show its utility through the analysis and design of subwavelength lasers. Finally, we discuss future research directions toward high-efficiency nanolasers and potential applications, such as creating planar arrays of uncoupled lasers with emitter densities near the resolution limit.

Luminescent hyperbolic metasurfaces

When engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature. Namely, a uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states. Using an unconventional multilayer architecture, we demonstrate luminescent hyperbolic metasurfaces, wherein distributed semiconducting quantum wells display extreme absorption and emission polarization anisotropy. Through normally incident micro-photoluminescence measurements, we observe absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission >0.9. We observe the modification of emission spectra and, by incorporating wavelength-scale gratings, show a controlled reduction of polarization anisotropy. We verify hyperbolic dispersion with numerical simulations that model the metasurface as a composite nanoscale structure and according to the effective medium approximation. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

Gain-enhanced high-k transmission through metal-semiconductor hyperbolic metamaterials

We analyze the steady-state transmission of high-momentum (high-k) electromagnetic waves through metal-semiconductor multilayer systems with loss and gain in the near-infrared (NIR). Using a semi-classical optical gain model in conjunction with the scattering matrix method (SMM), we study indium gallium arsenide phosphide (InGaAsP) quantum wells as the active semiconductor, in combination with the metals, aluminum-doped zinc oxide (AZO) and silver (Ag). Under moderate external pumping levels, we find that NIR transmission through Ag/InGaAsP systems may be enhanced by several orders of magnitude relative to the unpumped case, over a large angular and frequency bandwidth. Conversely, transmission enhancement through AZO/InGaAsP systems is orders of magnitude smaller, and has a strong frequency dependence. We discuss the relative importance of Purcell enhancement on our results and validate analytical calculations based on the SMM with numerical finite-difference time domain simulations.