Ali M. Adawi

Photopatterning, etching, and derivatization of self-assembled monolayers of phosphonic acids on the native oxide of titanium.

Electron-hole pair formation at titania surfaces leads to the formation of reactive species that degrade organic materials. Here, we describe the degradation of self-assembled monolayers of alkylphosphonic acids on the native oxide of titanium following exposure to UV light. The rate of degradation was found to decrease as the length of the adsorbate molecule increased. Increasing order in the monolayer, resulting from the enhanced dispersion forces between longer adsorbates, impedes the progress of oxygen-containing molecules to the oxide surface and slows the rate of oxidation. Rates of degradation on titanium oxide are substantially greater than rates of degradation on aluminum oxide because of the photocatalytic effect of the titanium oxide substrate. Micrometer-scale patterns may be fabricated readily using a UV laser in conjunction with a mask, and nanometer-scale patterns may be fabricated using a scanning near-field optical microscope coupled to a UV laser. Photodegraded adsorbates may be replaced by contrasting molecules to yield chemical contrast. Such patterned materials have been utilized to fabricate patterns from polymer nanoparticles. The resist character is switchable--at lower exposures, the monolayer behaves as a positive tone resist, but at higher exposures, it exhibits negative tone behavior. Patterned samples may also be utilized as resists for solution-phase etching of the underlying substrate.

Optical nanolithography using a scanning near-field probe with an integrated light source

An ultracompact near-field optical probe is described that is based on a single, integrated assembly consisting of a gallium nitride (GaN) light-emitting diode (LED), a microlens, and a cantilever assembly containing a hollow pyramidal probe with a subwavelength aperture at its apex. The LED emits ultraviolet light and may be used as a light source for near-field photolithographic exposure. Using this simple device compatible with many commercial atomic force microscope systems, it is possible to form nanostructures in photoresist with a resolution of 35 nm, corresponding to λ/10.

Strong coupling in high-finesse organic semiconductor microcavities

We report the fabrication of high-finesse strongly coupled microcavities composed of a polystyrene film doped with the dye tetraphenyl–porphyrin zinc positioned between two high reflectivity dielectric mirrors. The bottom mirror was deposited by plasma enhanced chemical vapor deposition, and was composed of 11 λ/4 thick (silicon oxide/silicon nitride) pairs. The organic layer was deposited on to this by spin coating. Finally, the top mirror was deposited by thermal evaporation and consisted of 12 λ/4 thick (tellurium oxide/lithium fluoride) pairs. Such cavities are characterized by Q factors of between 440 and 620. Strong coupling was evidenced via white light reflectivity measurements. Due to the high cavity Q factor, a Rabi splitting of 135 meV at resonance was very clearly resolved.

Comparison of intraband absorption and photocurrent in InAs/GaAs quantum dots

We present a comparative study of mid-infrared absorption and photocurrent measurements of self assembled InAs/GaAs quantum dots. A thermally activated bound/state–bound-state transition, as well as bound–wetting-layer and bound–continuum transitions are identified. By analyzing the temperature dependence of these transitions using absorption and photocurrent spectroscopies we are able to explain the previously reported discrepancies between the two measurement techniques. The activation energy (≅100 meV) for the bound–bound transition indicates that thermal escape of electrons occurs directly to continuum states. Evidence for preferential escape within the inhomogeneous distribution of dots is presented.

Strong in-plane polarized intraband absorption in vertically aligned InGaAs/GaAs quantum dots

We present a midinfrared absorption study of an n–i–n structure containing two planes of strongly coupled In0.5Ga0.5As self-assembled quantum dots. We find that the dominant absorption occurs for light polarized in the growth plane (E//xy), contrasting with uncoupled dots which exhibit stronger absorption for light polarized in the growth direction (E//z) in the same energy range. Results from the coupled dot sample indicate that the confinement length in the growth direction is increased, lowering the energy of the s-like ground state and hybridizing px,y-like excited states and wetting layer states. A significantly increased normal incidence photocurrent signal is measured for the sample containing coupled dots, relative to samples containing up to 30 layers of uncoupled dots, confirming the enhancement of the normal incidence absorption.

Single molecule spectroscopy of red- and green-emitting fluorene-based copolymers.

Single molecule fluorescence spectroscopy is used to study the optical properties of two polymers: a fluorene-based statistical copolymer that contains a low fraction (10%) of a red-emitting thiophene group, and the green-emitting polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole). These polymers were studied when isolated at a low concentration in a polymer matrix (either polynorbornene or polystyrene). For the red-emitting polymer, we compare the relative emission intensity from the green-emitting benzothiadiazole groups with the red-emitting thiophene. We find that red emission from the thiophenes is significantly suppressed in the single molecule regime, suggesting that interchain energy transfer dominates intrachain processes in such polyfluorene copolymers. We then use fluorescence spectroscopy and polarization anisotropy measurements to show that the conformations of both polymers are dependent on their host matrix, adopting a more collapsed, globular conformation in polystyrene and a more extended chain conformation in polynorbornene.

Improving the light extraction efficiency of red-emitting conjugated polymer light emitting diodes

We demonstrate a significant improvement in the external electroluminescence efficiency of red-emitting polymer light emitting diodes (LEDs) by modifying the optical structure of the device. By using a cathode composed of a thin (5nm) film of calcium backed with an optically thick film of silver, we measured improvements in the external efficiency of polymer LEDs by a factor of 1.6 times compared to a device using a cathode composed of calcium backed with lower reflectivity aluminum. By incorporating the LED into a microcavity structure (to form a resonant cavity LED) it is possible to obtain additional (but rather smaller) improvements in external efficiency of the order of 1.15 times, compared to a standard LED utilizing the same cathode. By combining high reflectivity cathode∕mirror materials with a low finesse cavity structure, we show that the external efficiency of a LED can be improved by as much as 1.8 times compared to a standard (noncavity) LED. Our results are in good agreement with those of th...

Opal photonic crystals infiltrated with chalcogenide glasses

Composite opal structures for nonlinear applications are obtained by infiltration with chalcogenide glasses As2S3 and AsSe by precipitation from solution. Analysis of spatially resolved optical spectra reveals that the glass aggregates into submillimeter areas inside the opal. These areas exhibit large shifts in the optical stop bands by up to 80 nm, and by comparison with modelling are shown to have uniform glass filling factors of opal pores up to 40%. Characterization of the domain structure of the opals prior to infiltration by large area angle-resolved spectroscopy is an important step in the analysis of the properties of the infiltrated regions.

Continuously tuneable optical filters from self-assembled block copolymer blends

We demonstrate that two symmetric high molecular weight diblock copolymers, of differing molecular weights, can be blended together and subsequently shear aligned to form one photonic structure without macrophase separation. The lameller period depends on the composition of the blend and gives a photonic structure that is easily tuneable in the wavelength range (λpeak = 400–850 nm).

Efficient out-coupling and beaming of Tamm optical states via surface plasmon polariton excitation

We present evidence of optical Tamm states to surface plasmon polariton (SPP) coupling. We experimentally demonstrate that for a Bragg stack with a thin metal layer on the surface, hybrid Tamm-SPP modes may be excited when a grating on the air-metal interface is introduced. Out-coupling via the grating to free space propagation is shown to enhance the transmission as well as the directionality and polarization selection for the transmitted beam. We suggest that this system will be useful on those devices, where a metallic electrical contact as well as beaming and polarization control is needed.