A survey of polarization-dependent optical phenomena in semiconductor and metal nanowires and nanorods is presented. Due to a large dielectric constant mismatch between nanostructures and their environment, the amplitude of the optical electric field inside the former depends drastically on the angle between the direction of light polarization and the nanostructure axis. As a result, optical absorption, photoconductivity, and nonlinear photoresponse in semiconductor structures are strongly anisotropic, with the maximal value for the parallel light polarization. In metal structures, absorption anisotropy depends on the light frequency, and for that close to the transverse plasmon frequency is maximal for the perpendiculat light polarization. Luminescence emitted by semiconductor nanowires and nanorods is strongly polarized along their axis. Joint action of polarization effects in absorption and luminescence results in the polarization memory, when luminescence of a random ensemble of nanorods is polarized in the same direction as the exciting light.
Pillar-array based optical cavities have unique properties, e.g., having a large and connected low dielectric index space
(normally air space), having a large percent of electric field energy in air and standing on a substrate. These properties
make them well suitable to make ultra compact and highly sensitive label-free optical sensors to detect bio-/chemical
reactions. We designed, fabricated, and measured a silicon-on-insulator pillar array microcavity that possesses a quality
factor as high as 27,600. We studied its sensitivity for both bulk index change and surface index modification. As a bulk
index sensor, for environmental refractive index change of 0.01, a resonance peak wavelength shift of 3.5 nm was
measured. As a surface index sensor, the simulations show, for a coating with thickness of 1 nm, the resonance
wavelength shifts as large as 2.86 nm. Combining with a sharp 0.06 nm wide resonance peak, our pillar-array sensor is
able to resolve ultra small bulk and surface refractive index changes caused by target molecules.
A Quantum-dot saturable absorber mirror (QD-SAM) has been fabricated by the molecular beam epiiaxy (MBE) technique. Preliminary measurements show that our QD-SAM is a very promising candidate for passive mode-locking a fiber laser or a solid state laser with wavelength in the range of 970-1090nm. The 22%-33% dips in the reflectivity spectrum are observed, which are attributed to quantum dot absorption, indicating the potential for a large modulation
depth and hence generation of ultra-short laser pulses through mode-locking.
In recent years, semiconductor nanodots have been actively used for biolabeling. We propose using alternate composite nanostructures consisting of a semiconductor size-quantized core covered by a nanometer-thick Au shell, having two principal advantages over purely semiconducting nanodots: (i) reduction of toxicity due to a complete Au coverage of the cores containing potentially poisonous Cd, Se, or Pb; (ii) amplification of exciting and/or emitted light by plasmon effects in a metallic shell which will increase the imaging efficiency. Theoretical calculations show that the optical absorption and emission spectra have several peaks corresponding to interband transitions in the core, and the two plasmon modes in the Au shell. When the energy of interband transitions coincides with one of the plasmon peaks, the resonant electromagnetic field in the core is enhanced which should result in amplification of the luminescence
intensity. Especially effective amplification can be reached if the frequencies of the exciting and emitting light both match two plasmon peaks. Experimental measurements were performed with composite nanostructures containing CdSe-ZnS cores fabricated by the organo-metallic method, followed by deposition of the gold shell using thermal decomposition of a Au (I) precursor. These revealed a multimodal structure of the absorption and luminescence spectra, good tunability, high intensity, and narrow emission linewidth. The dependence of spectra on the thickness of Au shell was investigated. The measurements were performed in different biological media and demonstrated stability and environment-insensitivity - a prerequisite for biolabeling.
Polarization phenomena in the optical absorption and emission of metallic, semiconducting or composite nanowires are considered theoretically. Most nanowire-based structures are characterized by a dramatic difference in dielectric constant ε between the nanowire material and environment. Due to image forces caused by such ε mismatch in nanowire structures, coefficients of their absorption and emission become essentially different for light polarized parallel or perpendicular to the nanowire axis. As a result, the intensity and spectra of absorption, luminescence, luminescence excitation, and photoconductivity in nanowires or arrays of parallel nanowires are strongly polarization-sensitive. In light-emitting nanowire core-shell structures, the re-distribution of a.c. electric field caused by the image forces may result in essential enhancing of core luminescence in frequency regions corresponding to luminescence from the semiconducting core or when the frequency of optical excitation coincides to the frequency of the plasmon resonance in the metallic shell. Random nanowire arrays acquire some properties typical for nematic liquid crystals. In such arrays, the effect described above may result in "polarization memory", where polarization of luminescence is determined by the polarization of the exciting light.
Photonic crystals consisting of semiconductor nanowire arrays grown using a metal catalyzed vapor-liquid-solid (VLS) method are excellent candidates for photonic elements and devices, such as micro-cavities, due to the high dielectric constant contrast and high aspect ratio. In addition, it is easy to control the crystal structure by patterning the metal catalysis, and the versatility of composition of nanowires (including II-VI, III-V and ternary III-V) makes the integration of optical components in diversified wavelength ranges possible. Here we use a Plane-Wave-Expansion (PWE) method and Finite Difference Time Domain (FDTD) technique toinvestigate the optical properties of nanowire based photonic crystals. It is found that arrays consisting of nanowires with radius at or below the edge of the effective single-wire confining range for a stand alone Fabry-Perot cavity can still form a high-Q value cavity with single mode operation. Our results will help to extend the concept of the-state-of-art 1-D distributed bragg reflector (DBR) and distributed feedback (DFB) lasers into 2-D ones with a working range from ultraviolet to near infrared.
Arrays of free-standing ZnSe nanowires of length 8-10 μm and diameter 60-150 nm were fabricated by Au-catalyzed vapor-liquid-solid growth. Electron microscopy showed that these were high quality single crystal nanowires. Photoluminescence (PL) measurements of the as-grown nanowires were characterized by weak near band edge emission and strong defect-related emission. The effect of post-growth annealing on the PL spectra under both Zn-rich and Se-rich conditions were studied. Annealing under a Zn-rich atmosphere was found to significantly enhance the near band edge emission and suppress deep-level emission, resulting in spectra dominated by the near band edge emission. On the other hand, annealing in a Se-rich atmosphere had the reverse effect, resulting in spectra dominated by deep level emission.
Highly oriented gallium arsenide (GaAs) nanowires are grown on GaAs (100), (110) and (111) substrates by molecularbeam epitaxy using vapor-liquid-solid growth. The preferred growth direction of the nanowires is <111>. GaAs nanowire arrays are grown using a number of approaches such as nanochannel alumina template, gold colloid, and
patterns fabricated using focused ion beam. Large interwire separation in the range of submicron can be obtained using the later two methods, which is required for applications in photonic devices such as photonic crystals.
Molecular beam expitaxial (MBE) grown GaAs at low substrate temperature (LT-GaAs) possesses a unique combination of properties (i.e., semi-insulation and short carrier lifetime) that has led to a variety of electronic and photonic device applications. In this paper, we report on the optical characterization of LT-GaAs, including carrier lifetime, photoreflectance (PR), and surface photovoltage (SPV) measurements. The undoped LT-GaAs samples were grown using our ow custom designed MBE system at the following substrate temperatures: 200 degree(s)C, 250 degree(s)C and 300 degree(s)C. These sample were then annealed at 7000-850 degree(s)C in a rapid thermal annealing (RTA) system. The PR spectra revealed that the PR amplitude depends strongly on the carrier lifetime, while the PR spectral broadening of near bandgap peak depends strongly on the internal field non-uniformly caused by buried Schottky barriers around the As precipitates. Above bandgap SPV measurements revealed a unique SPV spectrum compared with that for bulk GaAs. Carrier lifetime was measured for LT-GaAs samples grown at 200, 250, and 300 degree(s)C, respectively, and annealed at 700 degree(s)C for 30 seconds, and the corresponding carrier lifetimes at 1.5, 2.2, and 12 ps.
We study the island size distributions of Xi1-xGex/Si(001) (x equals 0.4 - .07) islands of varying Ge fractions and thicknesses by ultrahigh vacuum chemical vapor deposition. The island size distributions of the percolating islands obey a dynamic scaling hypothesis admitting only one length scale governing the growth, in the limit of large island sizes. Although bimodal distributions are found in coherent islands at large misfit strain, due to the large stress concentration at island perimeters; faulted dislocation loops forming as islands grow remove this stress concentration. This re-establishes a unimodal distribution,, reclaiming the scaling hypothesis. We show that the misfit strain is renormalized and, thus, is not essential in determining the size distribution. We also demonstrate evidence for Smoluchowski ripening mechanism occuring during growth. Finally, we discuss implications of these issues on achieving a uniform Xi1-xGex/Si(001) island distribution, which is crucial for technological applications.
We report on passivation of AlxGa1-xAs/GaAs surfaces using different sulfur and chlorine based treatments: These include ammonium sulfide solution, arsenic sulfide vapor and hydrochloric acid treatments. Enhancements in the intensity of near band-gap photoluminescence (PL) peaks, coupled with peak half-width reduction on treatment were attributed to a reduction in the density of surface states. Pre-etching using sulfuric acid- and ammonium hydroxide-based solutions prior to sulfur passivation was also found to contribute significantly to the overall success of a passivation treatment. The best sulfur-passivation results for all x (0 < x < 0.38) were found when sulfuric acid-peroxide-deionized water (Caros) solution pre-etching was followed by ammonium sulfide solution treatment at 65 degree(s)C for 25 min.
The effect of Laser Processing (LP) on defect complex formation and dissociation in ITC-GaAs was investigated by Surface Photovoltage (SPV), Photoluminescence (PL) and C-V profiling measurements. We propose that the reduction in hole concentration on LP samples is caused by a complexing of CAs and a laser-induced primary point defect. A clear correlation was found between the introduction of this complex and sample characteristics: these include, enhanced SPV signals, reduced CAs related PL intensities and a nonuniform distribution of ionized acceptors. In this exploratory work, we have demonstrated the unique influence of LP on the distribution of isolated acceptors and the free carrier concentration.
We report on XeCl excimer (308 nm) laser-assisted dry etching ablation (LADEA) of InP in a Cl2/He atmosphere. The InClx layer produced by the chlorination process was ablated for laser fluences below 80 mJ/cm2; this was determined to be the ablation threshold of InP for our experimental conditions. We studied the influence of Cl2/He mixture pressure, laser fluence and number of pulses on the etch rate of InP. Experiments were carried out with 5% and 10% concentrations of chlorine and at a laser repetition rate of 5 Hz. Employing linearly polarized light with an appropriate choice of experimental parameters (based on the afore mentioned studies), gratings were patterned by laser-induced coherent modulation of the beam at the semiconductor surface. We have also, for the first time, combined diffraction with LADEA to develop regular shaped features on semiconductor surfaces. Using this technique, 0.3-0.5 micrometers gratings were developed on the InP surfaces by an array of rectangular apertures. This approach offers the potential for fabrication of damage-free micro- or nano-structures, as well as substrates with patterns suitable for selective area deposition/epitaxy.