Session V09: Nanowires and One-Dimensional Structures: Optical and Electronic Properties

Turning light-guiding nanowires into a fluorescence-based biosensing assay

In some designs, semiconductor nanowires act as nanoscale optical fibres. Their quasi-1D structure can allow for the coupling of photons, which are then guided along the central axis of the nanowire structure to be emitted through its tip. In this talk, I will discuss our collaborative effort to passivate the surface of nanowires with cleavage-activated fluorogenic peptides, thereby turning a lattice of GaP nanowires into a high-throughput biosensing assay. I will present our results showing that we are able to detect the presence and activity of enzymes free in solution. This detection happens via the increase in light intensity at the nanowire tips as the surface-tethered substrates are cleaved and become fluorescent. The generality of this finding is further improved with use of our newly developed triblock-copolymer surface chemistry, which enables both specific modification of nanowire surfaces and the blocking of undesired nonspecific adhesion (e.g. of quantum dots from solution).   

Size dependent surface band bending in GaN nanowires

Group III-nitride nanowires (NWs) are recently emerged as a potential candidate for the single nanostructure high-performance optoelectronic devices because of the unidirectional conduction of charge carriers and absence of extended defects. However, surface states play important role in nanostructure-based device performance. The role of surface states and Fermi-level pinning is still unclear for III-nitride semiconductors. Therefore, the present study intends to investigate the effect of surface states on the band bending of the unintentionally doped n-GaN NW using Kelvin probe force microscopy (KPFM) with high spatial resolution.
The KPFM measurements were carried out at high vacuum (~10^-7 mbar) to avoid the effect of surface adsorbents. Single-pass imaging was adopted for the simultaneous measurement of topography, and the contact potential difference (CPD). Several measurements were carried out for the NWs with different diameter and the corresponding surface band bending were calculated. The surface band bending value was found to depend on the diameter of the NWs. Diameter dependant surface band banding of the NWs is attributed to the variation in the density of surface states in the NWs.   

Electronic homogeneity of III-Nitride nanowire Light Emitting Diodes (LEDs) grown on amorphous and nanocrystalline metals

Conductive atomic force microscopy (c-AFM) is a powerful technique to probe electrical inhomogeneities at nanometer scale in as-grown nanowire ensembles without introducing uncertainty due to additional device processing steps. Using c-AFM, we investigated the nanoscale current uniformity of III-Nitride nanowire LEDs grown on Pt thin films and amorphous metal foil. In this talk, I will discuss the variation of the current distributions in GaN nanowire LEDs grown on p-Si and Pt thin films. By taking IVs on individual nanowires, we found that there is a reduction in the threshold voltage for the GaN nanowire LEDs grown on Pt films compared to those grown on p-Si.
  

Crop up of the B1l mode in high quality GaN nanowires due to isotopic disorder

The B_1l and B_1h vibrational modes of GaN are silent, i. e. they cannot appear neither in Raman nor in infra-red spectroscopies. However, the B_1l mode appears with a small linewidth in the Raman scattering spectra on high quality ultra-narrow GaN nanowires, while the B_1h mode does not. The simulltaneous appearance of the A₁(LO) and B_1l in the ultraviolet region indicates: a) that the B_1l mode is related to a resonance effect, i. e. Fröhlich interaction plays a role and b) the mechanism allowing the appearance of the B_1l does not allow the observation of the B_1h. After performing density functional theory calculations and discarding several options we have concluded that the only difference between both modes is the isotopic composition [1]. While Ga, which contributes to the B_1l mode, has mainly two isotopes, ⁶⁹Ga and ⁷¹Ga, N, the main contributor to the high frequency mode, has basically one isotope, i.e. it is isotopically ordered.
[1] C. R-F et al., Nano Lett. 18, 5091 (2018).   

Harnessing the magneto-optics of quantum wires for designing the optical amplifiers

Quantum wires occupy a unique status among the semiconducting nanostructures with reduced dimensionality
-- no other system seems to have engaged researchers with as many appealing features to pursue.
This letter aims at a core issue related with the magnetoplasmon excitations in the quantum wires
characterized by the confining harmonic potential and subjected to a longitudinal electric field and a
perpendicular magnetic field in the symmetric gauge. Despite the substantive complexity, we obtain the
exact analytical expressions for the eigenfunction and eigenenergy, using the scheme of ladder operators,
which fundamentally characterize the quantal system. Crucial to this inquiry is an
intersubband collective excitation that evolves into a magnetoroton -- above a threshold value of magnetic
field -- which observes a negative group velocity between maxon and roton. The evidence of negative group
velocity implies anomalous dispersion in a gain medium with the population inversion that forms the basis
for the lasing action of lasers. Thus, the technological pathway that unfolds is the route to devices
exploiting the magnetoroton features for designing the novel optical amplifiers and hence paving the way
to a new generation of lasers.   

Magneto-Chiral Anisotropy in Carbon Nanotubes

Chiral conductors exist in two forms which are mirror images of each other and have different handedness. The nonlinear term of current in powers of voltage is an odd function of magnetic field and its coefficient in chiral conductors depends on the handedness and electron-electron interactions. Due to the importance of electron-electron interactions in carbon nanotubes and their large magneto-chiral anisotropy, they are one of the best candidates to study nonlinear magneto-transport in chiral conductors. In this work we use ultra clean suspended carbon nanotubes to investigate electrical magneto-chiral anisotropy in these materials, which can be used as a probe to deduce the electron-electron interaction strength and handedness of the tubes. Using photocurrent spectroscopy, we identify our nanotubes’ structure to compare the transport data with the existing theories.   

Ultrafast Stimulated Raman Scattering in Carbon Nanotubes

We perform ultrafast transient absorption spectroscopy on (6,5) carbon nanotubes in aqueous solution, using a near-IR pump and supercontinuum near-IR probe. We observe stimulated Raman scattering between the pump and probe light involving the nanotube G phonon mode, and examine the dependence of this Raman feature on the pump and probe wavelengths and pump fluence. We find that the stimulated Raman process is enhanced when the probe energy is near the nanotube exciton resonance, allowing us to investigate exciton-phonon coupling in the nanotubes.
  

Modification of Second Harmonic Generation Signals From Multiferroic Nanofibers Under Changing Magnetic Field

Multiferroic materials have recently attracted widespread attention as a novel class of materials due to their diverse multifunctional properties and applications. In particular, multi-phase structures are easier to synthesis and more common at room temperature in comparison to the single-phase materials. The hybrid structures investigated in this study are Janus nanofibers consisting of two hemi-cylinders of ferroelectric barium titanate (BTO) and ferrimagnetic cobalt ferrite (CFO). The mechanism involved in the magnetoelectric (ME) coupling between the two phases for this particular system is strain. In this study, we used the optical second harmonic generation (SHG) technique to monitor changes in the ferroelectricity of the BTO half while modifying the magnetization of the CFO half. The optical investigation was performed on an ensemble of pre-aligned nanofibers. In addition, the polarizations of the SHG signal were analyzed at different magnetic field orientations. We observed a clear signature of the SHG polarization rotation as the magnetic field direction was changed. This result suggests the existence of a measurable ME coupling between the two phases of this hybrid structure at room temperature which is essential for realizing devices with multiferroic applications.   

Intrinsic quality of InSb NWs grown in template pores

InSb NWs were grown in two different types of templates: nanoporous anodic aluminum oxide (AAO) and polycarbonate. In this talk, the difference in the structural quality of the as-grown NWs in both cases will be presented. NWs grown in commercially available AAO templates were found to have rough surfaces attributed to non-uniform pores in the membrane. On the other hand, InSb NWs grown in the polycarbonate track-etched membranes showed very uniformly, smooth surfaces. Both types of NWs were characterized by Raman spectroscopy, which showed different Raman spectrum. The strong defect related peak in the polycarbonate-grown InSb NWs attest to the fact that they are possibly amorphous. Temperature-dependent measurements on these NWs also show very different behavior. The NWs grown in the polycarbonate membrane show Mott Variable Range Hopping in the low-temperature regime and Arrhenius (thermally activated) transport in the high-temperature regime. This is contrary to the AAO template grown InSb NWs which shows thermally activated conduction mechanism in both low and high-temperature regime.   

Investigating surface migration energy barriers of self-assembly ZnO nanorods by multi-optical methods

Post-annealing treatment plays an important role for self-assembly (SA) metal oxide by providing thermal energy for oxygen atoms to overcome the migration energy barrier. With theoretical methods, it’s hard to calculate the height of migration energy barrier from the surface to bulk which depends on depth. Due to the high surface-to-volume ratio of SA ZnO nanorods (ZNRs), the optical properties and structural evolution on the surface obtained by ultraviolet/X-ray photoemission spectroscopy (UPS/XPS) and photoluminescence (PL) could be well investigated and reveal mutual agreement in all results in PL, XPS and UPS. In this work, we first demonstrated structural variation on the surface of SA-ZNRs by scanning over a range of annealing temperatures and durations by rapid thermal annealing (RTA) system and then characterized the multi-optical properties by integrating the results of PL, XPS and UPS. Finally, the model of migration energy barriers reveals the well ZNRs formed at 570°C and the further oxidation process with formation of hydroperoxide on the Zn-rich surface of ZNRs at 640°C.   

Maximizing Photonic Response with Simple Resonating Structures in Various Geometries

Resonances are all over us in multiple forms concerning the collaboration of two (or more) parts in order to maximize an output. In Photonics, the effect is interpreted as an optimally balanced regime between electric and magnetic response in domains with different optical axes or opposite-sign permittivities. In this talk, we examine simple structures of different geometries serving various purposes from large wave polarization conversion to efficient absorbance and maximal scattering.

Firstly, we consider a planar structure consisting of two slabs positioned back-to-back with different thicknesses but the same binary multilayered texture. By rotating in-plane the two slabs, we aim at tilting the linearly polarized incident field by 90 degrees; it is a particularly challenging task when working with such simple configurations.

Secondly, an isolated core-shell nanowire, where the resonance occurs between the metallic cylinder and the semi-conducting coat, is regarded. The potential of these realistic designs to operate as switches, sensors or as components in filtering and tagging optical systems, is also identified by evaluating the corresponding frequency responses.   

Dark vertical conduction of cavity-embedded semiconductor heterostructures

We present a linear-response theory for the electronic transport along the growth direction of an arbitrary semiconductor heterostructure coupled to a photonic resonator in its vacuum state (no real photons are injected or created). To ensure a coupling between intersubband and photonic excitations, the cavity field is assumed to be polarized along the growth direction. We point out how the light-matter and electron-electron interactions hybridize the (confined) many-body ground state with the continuum, showing how this can affect the conduction properties in the growth direction. Our findings are relevant, for instance, in the development of cavity-embedded quantum-well infrared photodetectors (QWIP).   

Visible Frequency Zero-Index Metasurfaces for 3D Augmented Reality Displays

Arrays of gratings can be multiplexed to create multi-view backlights, where each super-pixel is composed of several angularly directive beams. However, real applications of this technology are hampered by a limited field of view. In particular, diffractive optics are incapable of efficiently radiating light perpendicular to the plane of the array. This causes a hole in the center of the field of view because the refracted beam intensity from a diffraction grating coupler at wavelengths near cut-off. The grating is unable to produce a beam at the broadside, due to a standing wave formed in the plane of the array.
To overcome this limitation, we have developed metasurface-based pixel. The design consists of an array of high-index dielectric pillars, which exhibit simultaneous electric and magnetic resonances. The resulting metasurface supports leaky waves above the light line, which efficiently radiate into the broadside at telecom and visible frequencies. The Dirac-cone metasurface can produce beams over a continuous range of angles around the broadside due to its linear dispersion at the center of the Brillouin zone. By carefully tuning the interference of multiple bound modes, we can independently control the radiative efficiency, beam width, and beam angle.