We experimentally demonstrated graphene plasmon resonant absorption in mid-IR by utilizing an
array of graphene nanoribbon resonators on SiO2 substrate. By tuning resonator width we probed
the graphene plasmons with λ<sub>p</sub> ≤ λ0/100 and plasmon resonances as high as 0.240 eV (2100 cm<sup>-1</sup>) for 40 nm wide nanoresonators. Resonant absorption spectra revealed plasmon dispersion as well
as plasmon damping due to the interaction of graphene plasmons with the surface polar phonons in
SiO<sub>2</sub> substrate and intrinsic graphene optical phonons. Graphene nanoribbons with varying
widths enabled us to identify the damping mechanisms of graphene plasmons and much reduced
damping was observed when the plasmon resonance frequencies were close to the substrate polar
phonon frequencies. Then, by direct ebeam exposure of graphene nanoresonators, we effectively
changed the carrier density and caused red-shift of the plasmon spectra. This work will provide
insight into light-sensitive, frequency-tunable photodetectors based on graphene’s plasmonic
We report transmission spectroscopy results from the mid- to far-infrared on graphene, grown by chemical vapor
deposition (CVD) on Cu. Similar results have been reported by several groups and their substrates of choice were
thermal Si dioxide, quartz, or SiC, where strong phonon absorption results in transmission blocking bands in midinfrared.
Silicon wafers (thickness ~ 500 μm), on the other hand, have transmission extending out to about 100 cm-1
when the doping level is low. Therefore, we choose to use Si wafers as the carrier substrates for transferred CVD
graphene. The complex refractive index of the Si substrate is measured by infrared spectroscopic ellipsometry. As a
result, continuous spectra (without blocking bands) in the range of 400 to 4000 cm-1 are obtained and they are modeled
by free carrier absorption (the Drude model) and interband transitions (considering the Pauli blocking.) From these, the
carrier density, carrier mobility, sheet resistivity, intraband scattering rate, and graphene layer number can be inferred. In
the far-infrared range, the absorption is dominated by the intraband free carrier absorption and it mainly results from the
interband transition in the mid-infrared range. Having continuous spectra using the Si substrates gives us the advantage
to model the whole spectral region (from far-infrared to mid-infrared) accurately.
Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were investigated. It was found that the same PL band is characteristic for different AlN materials mentioned above; however, in the nanostructured materials (nanorods, nanotips and nanopowder) the intensity of UV-blue PL is remarkable lower than in the bulk material (ceramics). In the case of nanostructured AlN materials, excitation of the oxygen-related defect is mainly realized through the energy transfer from the host material (electron/hole or exciton processes) to the defects and this mechanism prevails over the mechanism of direct defect excitation.
Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD and thermal
CVD will be presented, along with their relation and difference to thin film counterparts. While there is a growing
acceptance of the low band gap (0.6-0.7 eV) of InN, the optical properties of the actual samples still suffered,
presumably due to the difficulty in obtaining high-quality samples and/or controlling their defect and carrier
concentrations. However, the free-standing nanobelts can be nearly defect-free, allowing an excellent opportunity for
fundamental investigations on unique dimensionality. InNNBs show photoluminescence (PL) in IR with peak width of
14 meV, the sharpest reported to date for InN. Interestingly, with increasing excitation intensity, InNNBs reveal an
anomalously large blueshift in PL, compared to thin films; along with a decrease in the phonon frequencies as evident by
Raman measurements. Surface band bending, converse piezoelectric effect, and photoelastic effects are employed to
explain these behaviors. As for InGaN NWs, both In-rich and Ga-rich ternary nanowires have been synthesized by
simply varying growth temperature. Morphological and structural characterizations reveal a phase-separated
microstructure wherein the isovalent heteroatoms are self-aggregated, forming self assembled quantum dots (SAQDs)
embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich nanowires,
which has been explained by proposing a multi-level band schema.
Biomimetic structures provided important clues for nano-synthesis in pursuit of enhanced performances. Here, we
report a wide angle and broadband antireflection is observed on a
6-inch silicon nanotip array (SiNTs) substrate
fabricated using a single step electron cyclotron resonance plasma etching technique. This subwavelength structure
consists of the SiNTs with apex and bottom diameter of ~5 nm and ~200 nm, respectively, length of ~1600 nm and
density of 10<sup>9</sup>/cm<sup>2</sup>. This aperiodic array of SiNTs with geometry designed in the sub-wavelength level to demonstrate a low hemispherical reflectance of < 1% in the ultraviolet to infrared region. The antireflection property holds good for a wide angle of incidence and both, s and p, forms of polarizations of light. The effective refractive index distribution
related to the structure of SiNTs is built. The equivalent three-layered thin films with gradient refractive index can be
applied in interpretation of the low reflection phenomenon. The equivalent admittance of the system is shown to be near
that of air even the wavelength is varied from 400 nm to 800 nm (or angle of incidence is varied from 25 to 70 degree).
The configuration to have broadband and wide-angle antireflection is different from the previous design because the
equivalent rare film adjacent to air in our case is much thinner than the requirement proposed by J. A. Dobrowolski. This
near ideal antireflection property suggests enhanced performances in renewable energy, and electro-optical devices in defense applications.
Optical and structural properties of InGaN/GaN multi-quantum well (MQW) structures with different well width, influenced by the nano-structural features in the MQWs, were investigated by optical measurements of photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL), as well as structural analysis methods, such as high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) measurements. Due to the quantum confined Stark effect (QCSE), larger Stokes shift is induced with larger well width. Thermally activated carrier screening model is established to well describe the so-called S-shaped spectral shift with temperature. Inhomogeneous line-width broadening induced by piezoelectric field is found to be dominant at low temperature, while homogeneous line-width broadening due to phonon scattering takes over at higher temperature. Additionally, two activation energies are extracted from the Arrhenius plot of PL intensity. One is assigned to be the exciton binding energy and the other one the confinement energy of electrons in the quantum well. TRPL study further indicated that the radiative lifetime was decreased with the decreased well width. All these are associated with the In-composition fluctuation and nano-structures in the MQWs.
Indium nitride (InN) film was successfully grown on the Si(111)substrate. The growth rate of InN film can be enhanced about four times by a novel-designed MOCVD system with a NH<SUB>3</SUB> pre-cracking device, in which the NH<SUB>3</SUB> was fed through a quartz tube passing over a high temperature (650-850 degree(s)C) graphite. A maximum growth rate of about 6 (mu) m/hr in our system was achieved due to high cracking efficiency of NH<SUB>3</SUB>. The growth temperature of substrate widely ranged from 350to 600 degree(s)C provides more flexible conditions to improve the film quality. The X-ray diffraction peaks of 31.7 degree(s) and 65.5 degree(s) were obtained from the (0002) and (0004) InN respectively, indicating (0001)-oriented hexagonal InN was epitaxially grown on the silicon(111) substrate.