Reza Sanatinia

Surface Second-Harmonic Generation from Vertical GaP Nanopillars

We report on the experimental observation and analysis of second-harmonic generation (SHG) from vertical GaP nanopillars. Periodic arrays of GaP nanopillars with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed a strong dependence of the SHG intensity on pillar diameter. Analysis of surface and bulk contributions to SHG from the pillars including the calculations of the electric field profiles and coupling efficiencies is in very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP nanopillars.

High Optical Quality InP-Based Nanopillars Fabricated by a Top-Down Approach

Dense and uniform arrays of InP-based nanopillars were fabricated by dry etching using self-assembly of colloidal silica particles for masking. The pillars, both single and arrays, fabricated from epitaxially grown InP and InP/GaInAsP/InP quantum well structures show excellent photoluminescence (PL) even at room temperature. The measured PL line widths are comparable to the as-grown wafer indicating high quality fabricated pillars. A stamping technique enables transfer with arbitrary densities of the nanopillars freed from the substrate by selectively etching a sacrificial InGaAs layer.

Modal Engineering of Second-Harmonic Generation in Single GaP Nanopillars

We report on modal dispersion engineering for second-harmonic generation (SHG) from single vertical GaP nanopillars/nanowaveguides, fabricated by a top-down approach, using optical modal overlap between the pump (830 nm) and SHG (415 nm). We present a modal analysis for the SHG process in GaP nanopillars and demonstrate efficient utilization of the longitudinal component of the nonlinear polarization density. Our SHG measurements show quantitatively the presented model. We experimentally demonstrate that polarization beam shaping and field distribution modification of the radiated SHG light, at nanometer scale, can be achieved by tuning the pillar diameter and linear pump polarization. SHG from single pillars can be used as femtosecond nanoscopic light sources at visible wavelengths applicable for single cell/molecular imaging and interesting for future integrated nanophotonics components. While this work focuses on GaP nanopillars, the results are applicable to other semiconductor nanowire materials and synthesis methods.

GaAs nanopillar arrays with suppressed broadband reflectance and high optical quality for photovoltaic applications

We report on fabrication and optical characterization of GaAs nanopillar (NP) arrays, obtained using a combination of low-cost mask generation by self-assembled silica particles (nanosphere lithography) and dry etching. Tapered structures (conical and frustum NP arrays) are fabricated by appropriate optimization of process parameters. Significant suppression of surface reflectance is observed for both geometries over a broad wavelength range. Simulations, based on finite difference time domain (FDTD) method, show good agreement with reflectivity measurements and serve as a guideline for design of NPs and understanding their interaction with light. A combination of wet chemical etching and sulfur–based passivation of GaAs NPs, results in more than one order of magnitude enhancement in PL intensity and recovery of PL line-width, which is very promising for photovoltaic applications.

Silicon micro-structure and ZnO nanowire hierarchical assortments for light management

We present fabrication and optical characterization of Si microstructure-ZnO nanowire (NWs) hierarchical structures for light management. Random and periodic hierarchical structures constituting Si micro pillar or micro pyramid arrays with overgrown ZnO NWs have been fabricated. Inexpensive colloidal lithography in combination with dry and wet chemical etching is used to fabricate Si microstructures, and ZnO NWs are grown by hydrothermal synthesis. The periodic Si micro pyramid-ZnO NWs hierarchical structure shows broadband antireflection with average reflectance as low as 2.5% in the 300-1000 nm wavelength range. A tenfold enhancement in Raman intensity is observed in this structure compared to planar Si sample. These hierarchical structures with enriched optical properties and high surface to volume ratio are promising for photovoltaic (PV) and sensor applications.

Aluminum-Induced Photoluminescence Red Shifts in Core–Shell GaAs/AlxGa1–xAs Nanowires

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/Al(x)Ga(1-x)As nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All Al(x)Ga(1-x)As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ~66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

Experimental quantification of surface optical nonlinearity in GaP nanopillar waveguides

We report on surface second-order optical nonlinearity in single GaP nanopillars (nanowaveguides). The relative contribution of optical nonlinearity from the surface and the bulk is resolved by mode confinement analysis and polarization measurements. By investigating the thickness of nonlinear region at the surface of nanopillars, we estimated the nonlinear coefficient to be ~15 times higher at the surface with respect to the bulk. The presented results are interesting both from the fundamental aspects of light-matter interaction and for future nonlinear nanophotonic devices with smaller footprint.

Nanostructuring of GaAs with tailored topologies using colloidal lithography and dry etching

The authors report on the fabrication of GaAs nanopillars with different profiles/topologies using colloidal lithography and dry etching. GaAs nanopillars with different shapes and dimensions were successfully fabricated using inductively coupled plasma reactive ion etching. Two different etch chemistries CH4/H2/Cl2 and Ar/Cl2 were investigated. The fabricated nanopillar arrays had a typical period of ∼500 nm, and the depths could be varied from a few nanometers to 4 μm. The CH4/H2/Cl2 chemistry with optimized gas flows and plasma powers is shown to produce nanopillars with smooth sidewalls compared to those fabricated with the Ar/Cl2 chemistry. The GaAs nanopillar arrays have appreciably lower reflectivities in the measured wavelength range from 400 to 850 nm and are typically one order of magnitude lower compared to planar GaAs, which shows their potential for photovoltaic applications.

Surface second harmonic generation from silicon pillar arrays with strong geometrical dependence.

We present experimental demonstration and analysis of enhanced surface second harmonic generation (SHG) from hexagonal arrays of silicon pillars. Three sets of Si pillar samples with truncated cone-shaped pillar arrays having periods of 500, 1000, and 2000 nm, and corresponding average diameters of 200, 585 and 1550 nm, respectively, are fabricated by colloidal lithography and plasma dry etching. We have observed strong dependence of SHG intensity on the pillar geometry. Pillar arrays with a 1000 nm period and a 585 nm average diameter give more than a one order of magnitude higher SHG signal compared to the other two samples. We theoretically verified the dependence of SHG intensity on pillar geometry by finite difference time domain simulations in terms of the surface normal E-field component. The enhanced surface SHG light can be useful for nonlinear silicon photonics, surface/interface characterization, and optical biosensing.

Thermal conductivity of epitaxially grown InP: experiment and simulation

The integration of III–V optoelectronic devices on silicon is confronted with the challenge of heat dissipation for reliable and stable operation. A thorough understanding and characterization of thermal transport is paramount for improved designs of, for example, viable III–V light sources on silicon. In this work, the thermal conductivity of heteroepitaxial laterally overgrown InP layers on silicon is experimentally investigated using microRaman thermometry. By examining InP mesa-like structures grown from trenches defined by a SiO2 mask, we found that the thermal conductivity decreases by about one third, compared to the bulk thermal conductivity of InP, with decreasing width from 400 to 250 nm. The high thermal conductivity of InP grown from 400 nm trenches was attributed to the lower defect density as the InP microcrystal becomes thicker. In this case, the thermal transport is dominated by phonon–phonon interactions as in a low defect-density monocrystalline bulk material, whereas for thinner InP layers grown from narrower trenches, the heat transfer is dominated by phonon scattering at the extended defects and InP/SiO2 interface. In addition to the nominally undoped sample, sulfur-doped (1 × 1018 cm−3) InP grown on Si was also studied. For the narrower doped InP microcrystals, the thermal conductivity decreased by a factor of two compared to the bulk value. Sources of errors in the thermal conductivity measurements are discussed. The experimental temperature rise was successfully simulated by the heat diffusion equation using the FEM.