Brandon Demory

Carrier dynamics in site- and structure-controlled InGaN/GaN quantum dots

We report on the carrier dynamics in InGaN/GaN disk-in-a-wire quantum dots with precisely controlled location and structural parameters, including diameter, thickness and material composition. We measured the time-integrated and time-resolved spectra and the second-order correlation function of the photoluminescence from quantum dots with diameters ranging from 19 nm to 33 nm at temperatures of 10 K to 120 K. The influence of the small fluctuations in structural parameters, most importantly the quantum dot thickness, on the optical properties are also investigated through statistical correlations among multiple optical properties of many individual quantum dots. We found that in a single dot the strain-induced polarization field and the strain relaxation at the sidewall form a potential barrier to protect the exciton from reaching the sidewall surface. However, the exciton can overcome this potential barrier and recombine nonradiatively at the surface through two mechanisms: tunnelling through the barrier quantum mechanically and hopping over the barrier by attaining sufficient thermal energy. The former (latter) mechanism is temperature insensitive (sensitive) and dominates nonradiaitve exciton decay at low (high) temperatures. We also found that despite the good uniformities in structural parameters, all optical properties still exhibit inhomogeneities from dot to dot. However, all these inhomogeneities can be modeled by simply varying the potential barrier height, which also explains the observed correlation curves among all optical properties. Finally, we found that the biexciton-to-exciton quantum efficiency ratio, which determines the probability of multi-photon emission, can be tuned by adjusting the potential barrier height and the temperature, suggesting a new way to achieve single photon emission at high temperatures.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photons polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

Monolithic integration of individually addressable light-emitting diode color pixels

Monolithic integration of individually addressable light-emitting diode (LED) color pixels is reported. The integration is enabled by local strain engineering. The use of a nanostructured active region comprising one or more nanopillars allows color tuning across the visible spectrum. In the current work, integration of amber, green, and blue pixels is demonstrated. The nanopillar LEDs exhibit an electrical performance comparable to that of a conventional thin-film LED fabricated on the same wafer. The proposed platform uses only standard epitaxy and a similar process flow as a conventional LED. It is also shown that the emission intensity can be linearly tuned without shifting the color coordinate of individual pixels.

Reducing inhomogeneity in the dynamic properties of quantum dots via self-aligned plasmonic cavities

A plasmonic cavity is shown to greatly reduce the inhomogeneity of dynamic optical properties such as quantum efficiency and radiative lifetime of InGaN quantum dots. By using an open-top plasmonic cavity structure, which exhibits a large Purcell factor and antenna quantum efficiency, the resulting quantum efficiency distribution for the quantum dots narrows and is no longer limited by the quantum dot inhomogeneity. The standard deviation of the quantum efficiency can be reduced to 2% while maintaining the overall quantum efficiency at 70%, making InGaN quantum dots a viable candidate for high-speed quantum cryptography and random number generation applications.

Integrated parabolic nanolenses on MicroLED color pixels

A parabolic nanolens array coupled to the emission of a nanopillar micro-light emitting diode (LED) color pixel is shown to reduce the far field divergence. For a blue wavelength LED, the total emission is 95% collimated within a 0.5 numerical aperture zone, a 3.5x improvement over the same LED without a lens structure. This corresponds to a half-width at half-maximum (HWHM) line width reduction of 2.85 times. Using a resist reflow and etchback procedure, the nanolens array dimensions and parabolic shape are formed. Experimental measurement of the far field emission shows a HWHM linewidth reduction by a factor of 2x, reducing the divergence over the original LED.

Color mixing from monolithically integrated InGaN-based light-emitting diodes by local strain engineering

Additive color mixing across the visible spectrum was demonstrated from an InGaN based light-emitting diode (LED) pixel comprising red, green, and blue subpixels monolithically integrated and enabled by local strain engineering. The device was fabricated using a top-down approach on a metal-organic chemical vapor deposition-grown sample consisting of a typical LED epitaxial stack. The three color subpixels were defined in a single lithographic step. The device was characterized for its electrical properties and emission spectra under an uncooled condition, which is desirable in practical applications. The color mixing was controlled by pulse-width modulation, and the degree of color control was also characterized.

Enhanced single photon emission from quantum dots coupled to localized surface plasmons

The coupling between excitons and localized surface plasmons in two-level systems can lead to enhanced spontaneous emission and was studied using site-controlled InGaN quantum dots. The dynamics and the Purcell effect were measured and analyzed.

Single photon emission from site-controlled InGaN quantum dots up to 90 K

Semiconductor quantum dots (QDs) have diverse quantum photonic applications [1] due to their atomic-like properties, characterized by discrete, optically active energy states. Most work to date has been based on III-As and III-P materials, which face severe limitations in operating temperature due to their small band offsets and exciton binding energies. On the other hand, QDs have been typically fabricated by the Stranski-Krastanow growth which forms dots at random locations and creates large inhomogeneity in size and spectral distribution, making controlled coupling of QDs with cavities or another QD difficult. Site-controlled InGaN QDs can address both issues. [2] Compared to III-V QDs, InGaN can alleviate the operating temperature limitation thanks to its large exciton binding energy. [3] In this paper, we report single photon emission up to 90 K from site-controlled InGaN QDs, the highest temperature to date for site-controlled QDs to our best knowledge.

Effects of strain relaxation on luminescent properties of InGaN/GaN nanorods from 2D to 0D transition

We characterized luminescent properties of InGaN nanodisks in both quantum well and dot regimes. The luminescent efficiency increases as strain is relaxed in the quantum well but peaks at the transition from well to dot.

Enhancement of spontaneous emission rate in an InGaN quantum dot coupled to a plasmonic cavity

An enhanced spontaneous emission rate was experimentally observed from a single semiconductor quantum dot, exhibiting photon antibunching, coupled to a silver plasmonic cavity.