Yu-Jung Lu

Plasmonic Nanolaser Using Epitaxially Grown Silver Film

Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths. An atomically smooth layer of silver enhances the performance of nanophotonic devices. A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.

InGaN/GaN nanorod array white light-emitting diode

Conventional InGaN/GaN light-emitting diodes based on planar quantum wellstructures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaNquantum wells. Here we overcome the limitation by using self-assembled GaNnanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaNnanorod p - n junction, we are able to demonstrate natural white (color temperature ∼ 6000 K ) electroluminescence from InGaN/GaN nanorod arrays.

Nanoscale optical properties of indium gallium nitride/gallium nitride nanodisk-in-rod heterostructures.

III-nitride based nanorods and nanowires offer great potential for optoelectronic applications such as light emitting diodes or nanolasers. We report nanoscale optical studies of InGaN/GaN nanodisk-in-rod heterostructures to quantify uniformity of light emission on the ensemble level, as well as the emission characteristics from individual InGaN nanodisks. Despite the high overall luminescence efficiency, spectral and intensity inhomogeneities were observed and directly correlated to the compositional variations among nanodisks and to the presence of structural defect, respectively. Observed light quenching is correlated to type I1 stacking faults in InGaN nanodisks, and the mechanisms for stacking fault induced nonradiative recombinations are discussed in the context of band structure around stacking faults and Fermi level pinning at nanorod surfaces. Our results highlight the importance of controlling III-nitride nanostructure growths to further reduce defect formation and ensure compositional homogeneity for optoelectronic devices with high efficiencies and desirable spectrum response.

Dynamic Visualization of Axial p–n Junctions in Single Gallium Nitride Nanorods under Electrical Bias

We demonstrate a direct visualization method based on secondary electron (SE) imaging in scanning electron microscopy for mapping electrostatic potentials across axial semiconductor nanorod p-n junctions. It is found that the SE doping contrast can be directly related to the spatial distribution of electrostatic potential across the axial nanorod p-n junction. In contrast to the conventional SE doping contrast achieved for planar p-n junctions, the quasi-one-dimensional geometry of nanorods allows for high-resolution, versatile SE imaging under high accelerating voltage, long working distance conditions. Furthermore, we are able to delineate the electric field profiles across the axial nanorod p-n junction as well as depletion widths at different reverse biases. By using standard p-n junction theory and secondary ion mass spectroscopy, the carrier concentrations of p- and n-regions can be further extracted from the depletion widths under reverse biasing conditions. This direct imaging method enables determination of electrostatic potential variation of p-n junctions in semiconductor nanorod and nanowire devices with a spatial resolution better than 10 nm.

Correlation of doping, structure, and carrier dynamics in a single GaN nanorod

We report the nanoscale optical investigation of a single GaN p-n junction nanorod by cathodoluminescence (CL) in a scanning transmission electron microscope. CL emission characteristic of dopant-related transitions was correlated to doping and structural defect in the nanorod, and used to determine p-n junction position and minority carrier diffusion lengths of 650 nm and 165 nm for electrons and holes, respectively. Temperature-dependent CL study reveals an activation energy of 19 meV for non-radiative recombination in Mg-doped GaN nanorods. These results directly correlate doping, structure, carrier dynamics, and optical properties of GaN nanostructure, and provide insights for device design and fabrication.

Chain-network anatase/TiO2 (B) thin film with improved photocatalytic efficiency

A 3-dimensional chain-network anatase/TiO2 (B) was obtained via the basic hydrothermal treatment of a sandwich Ti/TiO2/Ti film on a glass substrate that was prepared from 16 nm anatase TiO2 nanoparticles. The Ti film was converted to the TiO2 (B) phase in a Teflon vessel containing a 10 M NaOH aqueous solution that was encapsulated in a stainless-steel autoclave and heated at 130 °C for 2 h. The TiO2 (B) then served as a binder layer that enabled the formation of pearl-necklace chains made of anatase TiO2 nanoparticles, and these chain-like structures thoroughly interpenetrated into the textured layer. Decomposition tests using methylene blue indicated that the chain-network anatase/TiO2 (B) mixed-phase film had a photocatalytic half-life that was 0.84 and 0.69 times shorter than those of as-prepared anatase TiO2 and P25, respectively. In addition, the intensity of the room temperature photoluminescence spectra of anatase TiO2 was 2.55-fold higher than that of the chain-network anatase/TiO2 (B). We thus conclude that the remarkable photocatalytic activity of the chain-network anatase/TiO2 (B) is attributed to the chain-network structural characteristics and a synergistic effect of the matching band gap potentials, which increases the transfer of photogenerated electrons and reduces electron-hole recombination.

Contrast between Surface Plasmon Polariton-Mediated Extraordinary Optical Transmission Behavior in Epitaxial and Polycrystalline Ag Films in the Mid- and Far-Infrared Regimes

In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

Dynamically controlled Purcell enhancement of visible spontaneous emission in a gated plasmonic heterostructure

Emission control of colloidal quantum dots (QDs) is a cornerstone of modern high-quality lighting and display technologies. Dynamic emission control of colloidal QDs in an optoelectronic device is usually achieved by changing the optical pump intensity or injection current density. Here we propose and demonstrate a distinctly different mechanism for the temporal modulation of QD emission intensity at constant optical pumping rate. Our mechanism is based on the electrically controlled modulation of the local density of optical states (LDOS) at the position of the QDs, resulting in the modulation of the QD spontaneous emission rate, far-field emission intensity, and quantum yield. We manipulate the LDOS via field effect-induced optical permittivity modulation of an ultrathin titanium nitride (TiN) film, which is incorporated in a gated TiN/SiO2/Ag plasmonic heterostructure. The demonstrated electrical control of the colloidal QD emission provides a new approach for modulating intensity of light in displays and other optoelectronics.The dynamic control of light emission from quantum dots is generally controlled via optical or electrical pumping. Here, Lu et al. electrically control the local density of states around a quantum dot to modulate its visible light emission properties.

All-color plasmonic nanolasers with ultralow thresholds

Diffraction-unlimited semiconductor nanolasers are demonstrated by using single shape-controlled InGaN/GaN core-shell nanorods as laser gain media on Ag films epitaxially grown on Si substrates. The use of atomically smooth Ag films allows us to fabricate low-loss plasmonic cavities for ultralow-threshold, continuous-wave (CW) nanolaser operation above liquid nitrogen temperature. Furthermore, the tunable band-gap energy of the InxGa1-xN alloy makes it possible to realize laser emission in the full visible spectrum.

Tunable optical response and purcell enhancement of gated plasmonic structures

We experimentally demonstrate plasmonic nanostructures that enable dynamic electrical control of the phase and/or amplitude of the plane wave reflected from the nanostructures. We also demonstrate dynamically controlled Purcell enhancement of spontaneous emission of InP quantum dots (QDs) coupled to plasmonic heterostructures.