Periodic nanowires are observed from (001) orientation of Si and GaAs when the surfaces are irradiated interferentially by high power laser pulses. These nanowires are self-assembled and can be strain-free while their period is consistent with interference period. The nanowire morphologies are studied by atomic force microscopy. The observed period between nanowires depends on the wavelengths used and interference angle. The nanowire width increases with laser intensity. The narrowest nanowires observed have the width smaller than 20 nm, which is more than 10 times smaller than the interference period.
We report a direct method of fabricating high density nanodots on the GaAs(001) surfaces using laser irradiations on the surface. Surface images indicate that the large clumps are not accompanied with the formation of nanodots even though its density is higher than the critical density above which detrimental large clumps begin to show up in the conventional Stranski-Krastanov growth technique. Atomic force microscopy is used to image the GaAs(001) surfaces that are irradiated by high power laser pulses interferentially. The analysis suggests that high density quantum dots be fabricated directly on semiconductor surfaces.
Interferential irradiation of high power laser pulses can produce arrays of periodic nanostructures on surfaces. Patterning Si wafers directly by high power laser pulses indicates that the trench depth is limited to the laser pulse intensity. We present our recent studies on direct laser patterning of polystyrene coated Si wafers, which are irradiated interferentially by high power laser pulses. Polystyrene films were formed on silicon wafers with thickness controlled based on a previously developed method. Interferential irradiations of laser pulses are applied on the polystyrene coated Si wafer. The laser pulse intensities are varied along with other interferential parameters such as interference angle and laser wavelengths of 532, 355, and 266nm. The polystyrene film is dissolved to expose the patterned Si surfaces. Atomic force microscopy (AFM) images from the patterned Si surfaces indicate that the area covered with the films has trenches deeper than those on bare Si wafers patterned at the same laser intensity. Furthermore, studies of AFM images indicate that the thicker the polystyrene coating, the deeper the trenches that are produced by direct laser patterning Si surfaces. The enhancement and modification due to polymer films may enhance the security features by improving the quality of holograms.
The so-called Stranski-Krastanov (S-K) growth technique is useful to fabricate quantum dots in large quantity. However, it is limited to hetero-epitaxial systems because the S-K growth method requires a lattice mismatch generally larger than 2% such as in InGaAs quantum nanostructures. We present a study on direct laser fabrication of a strain-free selfassembled GaAs nanostructures on GaAs(001) surfaces in a molecular beam epitaxy (MBE) growth reactor in-situ. This self-assembly is due to the rapid thermal relaxation of materials heated at the interference maxima lines that are created by overlapping two laser pulses interferentially on the epitaxial growth front inside an MBE growth reactor. The morphologies of the GaAs nanostructures are characterized by atomic force microscopy and field emission scanning electron microscopy (FESEM) while their stoichiometry has been characterized by low voltage energy dispersive X-ray spectroscopy that is coupled with FESEM. The morphological study indicates that the width and length of nanodots are a few tens of nanometers while their height is around ten nanometers. The nanodot dimensions are much smaller than the interferential period and the wavelength of laser used but comparable to findings in our recent reports of quantum dots produced by direct laser annealing. For the stoichiometry study of the nanostructures, low electron voltages less than 5 kilovolts have been used in order to enhance the surface sensitivity of the resulting X-ray fluorescence due to the small inelastic mean free path of electron (~ 4 nm at 3 kV) in GaAs. The stoichiometric analysis indicates that the relative gallium content increases with size. However, the nanodots’ arsenic content as well as the relative Ga composition reaches to those of GaAs substrate when the dot size becomes smaller than 100 nm. The chemical analysis suggests a novel route of strainfree semiconductor nanodots.
The propagation of solitons along the interface between two dielectric nonlinear media was investigated theoretically extensively in the 1980s but never realized experimentally. Recently we predicted that the required small index differences between the media and hence solitons can be created at the interface between continuous and periodic discrete media consisting of arrays of weakly coupled waveguides. Our theoretical analysis has predicted the existence of stable solitons with power thresholds both in the centre and at the edge of the Brillouin zone. We have observed both of these discrete surface solitons with power thresholds in both Kerr and quadratically nonlinear media. Spatial solitons with fields in neighboring channels either in phase or pi out of phase with one another have been identified.
Discrete nonlinear optical systems exhibit unique properties unknown from wave propagation in bulk materials. Among them are the possibilities to form highly localized discrete solitons and the ability of a wide beam to propagate without diffraction and modulational instability across the array. The interaction between a highly localized discrete soliton and a non-diffracting beam has potential applications for all optical routing and switching. We present our results on the experimental investigation of this kind of beam interactions in a one-dimensional AlGaAs array at a wavelength of 1550 nm. A discrete soliton, almost completely confined to a single waveguide, was excited and the interaction with a wide beam of the same or orthogonal polarization was studied. We confirmed that the wide beam is able to drag the soliton over multiple waveguides towards itself while the soliton is able to maintain its original, highly confined shape. The outcome of the coherent interaction depends on the power of the wide beam and the relative phase between the two beams. This phase-dependence is due to linear interference in the case of co-polarized beams and due to four-wave mixing for orthogonally polarized beams.
Carrier transfer in low-density InAs/InP dot arrays with a multi-modal dot size distribution is studied by means of steady-state photoluminescence. The transition from saturation of the inter-dot carrier transfer to the unsaturated regime is surely observed by analyzing the shape of the luminescence signal for decreasing excitation densities. We unambiguously show that larger size dots provide a competing but saturable relaxation channel for smaller quantum dot ground states.
Self-assembled InP/InAs/InP quantum wires have been successfully stacked for 10 vertical periods and characterized based on photoluminescence (PL) studies. Compared with single-period quantum wires, unique behaviors appear in the PL spectra and some fundamental effects have been observed. Through the detailed analyses of the PL shapes, linewidths, and polarizations at different pump wavelengths, pump intensities and sample temperatures, it is evidenced that the wire width and subband energy gradually decrease while the average wire thickness increases from the bottom period to the top one, period by period. Meanwhile, the average wire width gradually decreases. In addition, from the bottom to the top period the size fluctuation within each period decreases. Furthermore, above certain temperature or pump intensity all the quantum wires are vertically coupled among one another. Following these results, new growth conditions have been suggested, which can be essential to improving the optical quality of these self-assembled quantum wires.
We have observed first-, second- and third-order quasi- phase-matched second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. We have measured phase- matching curves and identified all the peaks. The linewidth for the first order is limited only by wave-vector mismatch. We have demonstrated two-order-of-magnitude enhancement solely using quasi-phase-matched multilayers. We have also achieved cavity-enhanced quasi-phase-matched second-order and non-phase-matched second-harmonic generation from GaAs/Al<SUB>0.8</SUB>Ga<SUB>0.2</SUB>As multilayers. We have determined the element of the second-order susceptibility tensor used for quasi-phase matching. We have measured the conversion efficiencies and discussed possibilities for further enhancements.