Session L32: Focus Session: Nano-Optics, Semiconductor and Metal Nanostructures

Linear Optical and SERS Study on Metallic Membranes with Subwavelength Complementary Patterns

An efficient technique is developed to fabricate optically thin metallic films with subwavelength patterns and their complements simultaneously. By comparing the spectra of the complementary films, we show that Babinet's principle nearly holds in the optical domain. A discrete-dipole approximation can qualitatively describe their spectral dependence on the geometry of the constituent particles and the illuminating polarization. Using pyridine as probe molecules, we studied surface-enhanced Raman spectroscopy (SERS) from the complementary structure. Although the complementary structure posses closely related linear spectra, they have quite different near-field behaviors. For hole arrays, their averaged local field gains as well as the SERS enhancements are strongly correlated to their transmission spectra. We therefore can use cos$^{4}\theta $ to approximately describe the dependence of the Raman intensity on the excitation polarization angle $\theta $, while the complementary particle arrays present maximal local field gains at wavelengths generally much bigger than their localized surface plasmonic resonant wavelengths.   

Optically controlled patchy modification of metal nanoparticles

It is well known that that metal nanostructures strongly concentrate light intensity at hot spots located at sharp corners or in narrow gaps, either due to plasmonic resonances or the lightning-rod effect. This is exploited a several important applications, such as surface enhanced Raman spectroscopy (SERS) and apertureless NSOM. We propose using this phenomenon to induce photochemical reaction on the surface of metal nanoparticles, leading to differential, or patchy, functionalization of the particles. We have functionalized gold and silver nanoparticles fabricated using nanosphere lithography with ligands that contain o-nitrobenzyl functional groups. Upon absorption of a photon, these compounds cleave off, leaving behind a modified surface. Differential functionalization will be demonstrated by comparing the rate of photoreactions at the hotspots (measured with SERS) to the average cleavage rate (measured with FTIR).   

TDDFT studies of plasmonic excitations in small transition metal-doped gold chains

We apply a TDDFT approach to study the absorption spectra of pure Au chains and those doped with transition metal (TM) atoms (Ni, Rh, Fe) up to 24 atoms. We find that for gold chains with more than 10 atoms a collective plasmon mode is formed whose intensity grows with the number of atoms. The plasmon energy approaches asymptotically a value of ~0.6eV when the number of atoms is about 20. However, in the chains with odd number of atoms, an additional low-energy excited state close to the plasmonic peak is found which can be related to an excitation at the chain edge. Doping with TM atoms also leads to the formation of additional plasmon peaks close in energy to the main one, especially pronounced in the case of Ni- doped chains. We compare the results for the optical absorption spectrum of the system in the case of doping by different TM atoms and the role of the d-electron states of these atoms in formation of the additional plasmon peaks.   

Ultrafast photoconductive response of LaAlO$_{3}$/SrTiO$_{3}$ nanoscale photodetectors

Conducting AFM lithography can be used to create nanoscale field effect transistors at the LaAlO$_{3}$/SrTiO$_{3}$ interface.\footnote{C. Cen et al., Nature Material, 7, 2136(2008)}$^,$\footnote{C.Cen et al., Science, 323, 1026 (2009)} Such devices exhibit gatable photoconductive response, which spans from visible to near-infrared regime.\footnote{P.Irvin et al., Nature Photonics advanced online publication, 14 Nov.2010 (DOI 10.1038/nphoton.2010.238)} By implementing the pump-probe measurement with a home-built femtosecond laser, we observe an ultrafast nonlinear optical response of these nanoscale photodetectors. We explore the feasibility of these devices for molecular-scale THz spectroscopy applications.   

Infrared and Terahertz Nanoscopy

During the last years, near-field microscopy based on elastic light scattering from atomic force microscope tips (scattering-type scanning near-field optical microscopy, s-SNOM [1]) has become a powerful tool for nanoimaging of local dielectric material properties [2-5] and optical near fields of photonic nanostructures [6-8]. After an introduction of s-SNOM, I will discuss recently developed applications in materials sciences and nanophotonics. I will focus particularly on IR and THz imaging at wavelengths $\lambda $ around 10 and 118 $\mu $m, where we typically achieve a wavelength-independent resolution better than 40 nm, corresponding to $\lambda $/250 and $\lambda $/3000, respectively [3]. Using metal-coated tips, the strong field enhancement at the tip apex probes the local dielectric properties of a sample, allowing for the simultaneous recognition of materials and free-carrier concentration in semiconductor nanodevices [3] and nanowires [5]. Quantitative free-carrier mapping is enabled by near-field plasmon-polariton spectroscopy, which can be also applied to study strain-induced changes of carrier concentration and mobility [4]. Nanoscale imaging of strain and nanocracks in ceramics can be achieved by near-field infrared phonon-polariton spectroscopy [4]. I will also discuss the capability of s-SNOM to image the vectorial near-field distribution of photonic nanostructures. In this application, a dielectric tip scatters the near fields at the sample surface. I will discuss how the amplitude and phase-resolved measurement of different near-field components allows for mapping of the polarization state in nanoscale antenna gaps [8], of near-field modes in loaded infrared gap antennas [7] and of mid-infrared energy transport in nanoscale transmission lines. \\[4pt] [1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004). \\[0pt] [2] R. Hillenbrand et al., Nature 418, 159 (2002). \\[0pt] [3] A. Huber et al., Nano Lett. 8, 3766 (2008). \\[0pt] [4] A. Huber et al., Nature Nanotech. 4, 153 (2009). \\[0pt] [5] J.M. Stiegler et al., Nano Lett. 10, 1387 (2010). \\[0pt] [6] T. Taubner et al. Science 313, 1595 (2006). \\[0pt] [7] M. Schnell et al., Nature Photon., 3, 287 (2009). \\[0pt] [8] M. Schnell et al., Nano Lett., 10, 3524 (2010).   

Single GaN nanowire polariton luminescence

Polariton emission from a single GaN nanowire in the strong coupling regime has been investigated in the temperature range of 200-300 K. GaN nanowires grow in the wurtzite structure with the c-axis along the growth direction. The polariton dispersion characteristics are determined from angle-resolved reflectivity measurements. The light emission characteristics measured as a function of incident optical power density reveal a distinct non-linear behavior and threshold, accompanied by a sharp decrease in linewidth over an order of magnitude and a small blue-shift that is ascribed to polariton-polariton interactions. Angle-resolved photoluminescence measurements above threshold indicate polariton cooling to the bottom of the lower polariton branch, triggered by the onset of stimulated scattering which is characterized by a fast relaxation time as obtained from time resolved photoluminescence measurements. Emission above threshold is linearly polarized. Second order correlation measurements and interferomtery indicate significant bunching below threshold and a coherent emission above threshold. These measurements indicate a coherent emission. Photon lasing due to carrier population inversion is observed at higher pump power densities.   

Photoreflectance spectroscopy of single GaAs/GaP Core-shell Nanowires

We present a direct observation of the light hole (lh) and heavy hole (hh) valence band splitting in highly strained GaAs/GaP core/shell nanowires obtained by photoreflectance (PR) from a single nanowire. The NWs were prepared by Au nanoparticle (100 nm) catalyst-assisted MOCVD growth with two different shell thicknesses, where the induced strain is controlled varying the core/shell ratio. They were then dispersed on silicon for the PR measurement. The spectra show a $\sim $140eV splitting of the lh and hh bands for two different wires. Raman spectroscopy was carried out on the same growths in order to measure the hydrostatic and shear strain [1]. From the measured strain we calculate the hh and lh splitting and find them to be in reasonable agreement with PR. \\[4pt] [1] M. Montazeri, et. al., Nano Letters 10, 880-886 (2010).   

Photoreflectance measurements of single wurtzite InP nanowires

We have carried out photoreflectance measurements from a single semiconductor nanowire for the first time to our knowledge. We show that photoreflectance is an easy, quick and nondestructive technique which could be used to study the electronic band structure of a single semiconductor nanowire at both room and low temperatures. We have used photoreflectance to study electronic band structure of single wurtzite InP nanowires at room and low temperatures. Nanowires were grown by MOCVD using 100nm Au-nanoparticle catalysts. Derivative like features in the photoreflectance spectrum around the fundamental gaps allow us to extract energies of 1.50eV, 1.53eV and 1.70eV for A, B and C excitons of wurtzite an InP nanowire at low temperature. These values are compared to values obtained by photoluminescence-excitation and photocurrent measurements. Supported by the NSF ({\#}0701703, {\#}0806700 and {\#}0806572) and the Australian Research Council.   

Investigation of Electronic Structure in Wurtzite InP Nanowires

We use photoluminescence excitation (PLE), time-resolved photoluminescence (PL), and CW photoluminescence to investigate the electronic structure of wurtzite InP nanowires (NWs) as a function of diameter (30, 50, 100~nm) at 10 K. The NWs were prepared by Au catalyst-assisted MOCVD growth with a 420\r{ }C growth temperature and a V/III ratio of 700. A tunable Titanium-Sapphire laser was used to excite the nanowire sample. PL from the NWs show a dominant defect line near 840nm (1.475eV) that obstructs the view of the free exciton line which should be around 824nm (1.504eV). PLE was performed by measuring the intensity of the defect emission as a function of the excitation laser. The laser was polarized parallel and perpendicular to the nanowire and the PL was collected with circular polarization. PLE spectra show three peaks for the A, B and C hole bands (\textit{APL} \textbf{97}, 023106-2010). Polarization measurements may probe optical selection rules. Support for this work was provided by the NSF (0701703, 0806700 and 0806572) and the Australian Research Council.   

Photoluminescence in Strain-Engineered Si/SiGe Three Dimensional Nanostructures

The effect of strain on the degeneracy of energy band minima in composition-controlled Si/SiGe nanostructures with high germanium content ($\sim $ 50{\%}) is studied by low temperature photoluminescence (PL) spectroscopy, ultra-high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy measurements. PL spectra obtained from selective excitation of the multilayered nanostructures show a reduction in the strained-silicon fundamental energy bandgap and a splitting of energy levels presumably associated with partial removal of two-fold degeneracy of the SiGe valence band. PL kinetics recorded using different excitation wavelengths show dramatically different PL lifetimes, ranging from $\sim $ 2 $\mu $s to $<$ 10 ns. We show that it is possible to obtain high quantum efficiency luminescence at 1.3-1.6 $\mu $m.   

State filling dependent tunneling in hybrid InAs/GaAs-InGaAs/GaAs dot-well structures

A strong dependence of quantum dot (QD) - quantum well (QW) tunnel coupling on the energy band alignment is established in hybrid InAs/GaAs - In$_{x}$Ga$_{1-x}$As/GaAs dot-well structures by changing the QW composition to shift the QW energy through the QD wetting layer (WL) energy. Due to this coupling a rapid carrier transport from the QW to the QD excited states takes place. As a result, the QW photoluminescence (PL) completely quenches at low excitation intensities. The threshold intensities for the appearance of the QW PL strongly depend on the relative position of the QW excitonic energy with respect to the WL ground state and the QD ground state energies. These intensities increase by orders of magnitude as the energy of the QW increases to approach that of the WL due to the increased efficiency for carrier tunneling into the WL states as compared to the less dense QD states below the QW energy.