Effect of porosity on thickness uniformity of MgF2 films on spherical substrates.

Simulation based on Knudsen's law shows that film thickness uniformity above 99% can be realized on spherical substrates with optimized profiles of shadowing masks. However, a type of optical thickness nonuniformity is revealed when the masks are applied for thickness correction of MgF2 films experimentally. The optical thickness nonuniformity depends on steepness of the spherical surfaces and reaches 5% approximately for surfaces with CA/RoC = 1.22. Porosity of the MgF2 film is superimposed on Knudsen's law to interpret the optical thickness nonuniformity. For theoretical simulation, the influence of porosity on optical thickness distribution is characterized by a new parameter that describes nonlinear dependence of deposition rate on cosine function of molecular injection angles in Knudsen's law. Utilizing the optimized deposition model, optical thickness uniformity of MgF2 films approaching to or above 99% has been achieved for surfaces of different steepness in a single coating run.

Performance optimization of 193 nm antireflective coatings with wide incident angle ranges on strongly curved spherical substrates.

A method to optimize the spectral performance of 193 nm antireflective (AR) coating with a broad range of angle of incidence (AOI) on strongly curved spherical substrates is described. In this method, the actual film thickness on test plates for single-layer LaF3 and MgF2 films are corrected by measuring the relationship between the film thickness on test plates and that on quartz crystal microbalance. Interface roughness in multi-layer AR coating is obtained from atomic force microscopy measurements and its effect on the spectrum of the multi-layer is taken into account in this method by being simulated as a homogeneous sublayer. Porosities of the sublayers in AR coatings are obtained by reversely engineering the residual reflectance of the coatings/substrate/coating stacks. The obtained refractive indices and thicknesses in the multilayer are then used for analysis and optimization of the spectrum of 193 nm AR coatings. For strongly curved spherical surfaces, spectrum uniformity of the AR coating is optimized by taking into consideration simultaneously the merit functions at different positions of spherical substrates. This work provides a general solution to the performance optimization of 193 nm AR coatings with broad AOI range and on strongly curved spherical substrates.

Pulse stretcher with two beamsplitting elements for excimer laser pulses.

A pulse stretcher with two beamsplitting elements and an optical ring cavity is proposed. With a two-beamsplitting-element configuration, the temporal profile of the stretched output pulse is effectively improved, and the transient peak power is significantly reduced. By optimizing the radius of curvature of the concave mirrors used in the ring cavity, the spatial profile of the stretched pulse is kept nearly unchanged. The performance of the pulse stretcher is experimentally investigated with a 193 nm excimer laser. By optimizing the splitting ratios of the two beamsplitting elements, the pulse width is extended from 15.5 ns to 46.2 ns with a stretching ratio of 2.89; the transient peak power of the stretched pulse is reduced to 27% of that of the input laser pulse; an energy transmission efficiency of 87.8% is achieved for the proposed pulse stretcher with two beamsplitting elements.

Anomalous optical Anderson localization in mixed one dimensional photonic quasicrystals.

Anomalous optical Anderson localization (AOAL) in mixed one-dimensional (1D) photonic quasicrystals with matching impedance is obtained when the average refractive index of left- and right-handed layers is approximately zero. The transport properties of ordinary and anomalous localization are investigated and compared. The difficulties in expressing analytically the scaling factors of the mixed photonic quasicrystals are illustrated from Hamiltonian-map analysis. An approach based on transfer matrix method is proposed to simulate the localization behavior. From the simulation, it is found that the narrow distribution of the phase shift is responsible for AOAL in the mixed photonic structures. The scaling factors of AOAL decrease with the broadening of the phase shift distribution. The maximum phase shifts of the mixed photonic structures determine the lower boundary of the anomalous localization.

Tamm plasmon-polariton with negative group velocity induced by a negative index meta-material capping layer at metal-Bragg reflector interface.

Influence of a negative refractive index meta-material (NIM) capping layer on properties of Tamm plasmon-polariton at the interface of metal-Bragg reflector structure is investigated. Conditions for excitation of the plasmon-polariton is determined from reflectivity mapping calculation and analyzed with cavity mode theory. For specific thicknesses of capping layers, Tamm plasmon-polariton with negative group velocity is revealed in a wide region of frequency. Different from backward optical propagation induced by negative effective-group-refractive-index in dispersive media, negative group velocity of Tamm plasmon-polariton results from opposite signs of cross-section-integrated field energy and Poynting vector.

Optimization of thickness uniformity of optical coatings on a conical substrate in a planetary rotation system.

For a coating machine with a planetary rotation system and counterrotating shadowing mask configuration, a shadowing mask was designed using a numerical optimization algorithm to control the thickness uniformity of optical coatings formed on conical substrate. Single-layer magnesium fluoride (MgF(2)) and antireflective (AR) coating at 193 nm were fabricated on a convex conical substrate holder (with diameter 225 mm, apex angle 140 deg, and height 41 mm) by thermal evaporation. Thickness distribution determined from the transmittance spectra of single-layer MgF(2) thin films on BK7 slices showed that uniformities better than 99.3% were experimentally achieved with the designed counterrotating shadowing mask. From the reflectance spectra, uniform optical performance was also obtained for the 193 nm AR coating deposited on fused-silica substrates.

Microstructure-related properties of magnesium fluoride films at 193nm by oblique-angle deposition.

Magnesium fluoride (MgF2) films deposited by resistive heating evaporation with oblique-angle deposition have been investigated in details. The optical and micro-structural properties of single-layer MgF2 films were characterized by UV-VIS and FTIR spectrophotometers, scanning electron microscope (SEM), atomic force microscope (AFM), and x-ray diffraction (XRD), respectively. The dependences of the optical and micro-structural parameters of the thin films on the deposition angle were analyzed. It was found that the MgF2 film in a columnar microstructure was negatively inhomogeneous of refractive index and polycrystalline. As the deposition angle increased, the optical loss, extinction coefficient, root-mean-square (rms) roughness, dislocation density and columnar angle of the MgF2 films increased, while the refractive index, packing density and grain size decreased. Furthermore, IR absorption of the MgF2 films depended on the columnar structured growth.

Theoretical design of shadowing masks for uniform coatings on spherical substrates in planetary rotation systems.

A straightforward theoretical routine is proposed to design shadowing masks which are used for preparing uniform coatings on flat as well as strongly curved spherical substrates with large diameters in planetary rotation system. By approximating a spherical substrate in planetary rotation to a corresponding flat substrate in simple rotation around the revolution axis, the initial shape of a shadowing mask is determined. The desired uniformity for the spherical substrate is further realized through expanding appropriately the arc length of the initial shadowing mask. Utilizing the shadowing masks designed with the theoretical routine, film uniformities better than 97% are experimentally achieved for large-diameter spherical substrates with ratios of clear aperture to radius of curvature range from approximately -1.0 to 1.3.

Laser spot position dependence in photothermal mode cooling of a microcantilever.

We explore the laser spot position (LSP) dependence of photothermal mode cooling in a microcantilever-based Fabry-Perot cavity. Experiments on photothermal cooling demonstrate that the direction of photothermal backaction on the first two cantilever modes is LSP dependent, which can be either parallel or antiparallel. A theoretical analysis of this LSP-dependent effect identifies the parallel and the antiparallel coupling regions along the lever. Simulation results are in quantitative agreement with our experimental observations. We conclude that the cooling limit imposed by photothermal mode coupling can be surmounted by operating in the parallel coupling region.

Molecular orientation and lattice ordering of C60 molecules on the polar FeO/Pt(111) surface.

C(60) molecules assemble into close packing layer under the domination of the intermolecular interaction when deposited onto Pt(111)-supported FeO layer kept at 400 K. From corresponding high resolution scanning tunneling microscopy (STM) image, a kind of C(60) molecular orientational ordering stabilized by the intermolecular interaction is revealed as C(60)/FeO(111)-(√133 × âˆš133) R17.5° structure and determined from the commensurability between the C(60) nearest-neighbor distance and the lattice of the underlying oxygen layer. Moreover, due to the inhomogeneously distributed work function of the underlying FeO layer, the C(60) molecular electronic state is periodically modulated resulting in a bright-dim STM contrast. In addition, one coincidence lattice ordering is determined as 8 × 8 superstructure with respect to the C(60) primitive cell, which overlays a 3 × 3 moiré cell of the underlying FeO layer.

Molecular orientations and interfacial structure of C60 on Pt(111).

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13 × âˆš13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3 × 2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.

Coverage-dependent structures of cobalt-phthalocyanine molecules adsorbed on Cu(001) surface.

The morphologies, self-assembly structures, and stability of cobalt-phthalocyanines (CoPc) molecules adsorbed on Cu(001) with coverage ranging from 0.2 monolayer (ML) to 1.6 ML are investigated by ultrahigh-vacuum low-temperature scanning tunneling microscopy (UHV LT-STM) at liquid nitrogen temperature. Upon increasing the deposition of CoPc molecules various structures, such as isolated adsorption, quasi-hexagonal structure, square root(29) x square root(29) structure, are well characterized by the corresponding high-resolution STM images. The CoPc-CoPc intermolecular interaction and CoPc-substrate interfacial interaction dominate the structural evolutions. For the coverage higher than 1 ML, CoPc molecules preferentially locate on top of the molecules underneath and organize into square root(58) x square root(58) structure. As more and more CoPc molecules adsorb on the first layer, in some square root(58) x square root(58) regions molecular insertion leads to the formation of the square root(29) x square root(29) domain to effectively decrease the energy of the whole system.