Hongji Qi

Roughness evolution of ZrO2 thin films grown by reactive ion beam sputtering

A series of zirconium dioxide films were grown on rough borosilicate crown glass substrates with reactive ion beam sputtering technique. The evolution of the surface roughness was studied in the smoothing and roughening growth regimes using atomic force microscopy. By quantitative analysis of surface morphology, the interface width of the growth fronts was found to have a minimum during the deposition process. Dynamic scaling was observed for thicker films; the roughness exponent was found to be in the range of 0.7–0.9 and the growth exponent at approximately 0.4.

Nonpolarizing and polarizing filter design.

We describe a nonpolarizing filter design at oblique incidence and a polarizing filter design at normal incidence that use a uniaxially anisotropic layer. The phase thicknesses and the optical admittances of the layers are compensated for by the birefringent properties of a thin film at oblique incidence. This concept can be applied to the design of nonpolarizing bandpass and edge filters at oblique incidence and of polarizing beam splitters at normal incidence. Besides, the dependence of narrow-bandpass filters on normal incidence is discussed.

High-performance 4H-SiC based metal-semiconductor-metal ultraviolet photodetectors with Al 2 O 3 SiO 2 films

4H-silicon carbide (SiC) metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with Al2O3/SiO2 (A/S) films employed as antireflection/passivation layers have been demonstrated. The devices showed a peak responsivity of 0.12 A/W at 290 nm and maximum external quantum efficiency of 50% at 280 nm under 20 V electrical bias, which were much larger than conventional MSM detectors. The redshift of peak responsivity and response restriction effect were found and analyzed. The A/S/4H-SiC MSM photodetectors were also shown to possess outstanding features including high UV to visible rejection ratio, large photocurrent, etc. These results demonstrate A/S/4H-SiC photodetectors as a promising candidate for OEIC applications

Nanosecond laser pulse induced concentric surface structures on SiO 2 layer

We report the periodic concentric surface structures on SiO2 layer induced by a single shot nanosecond laser pulse at 1.06 μm. The fringe period of the structures ranges from 7.0 μm to 26.8 μm, depending on the laser fluence and the distance from central defect precursor. The size and depth of the damage sites increase almost linearly with the laser fluence from 19.6 J/cm(2) to 61 J/cm(2). Plasma flash was clearly observed during the damage process. We attribute the formation mechanism of the structures to the interference between the reflected laser radiations at the air/shock-front and the shock-front/film interfaces.

Damage threshold influenced by the high absorption defect at the film-substrate interface under ultraviolet laser irradiation

The laser-induced damage threshold (LIDT) of a single-layer coating at the nanosecond (ns) regime is obviously lower than an uncoated substrate or a high reflectivity coating coated by the same material. To elucidate this phenomenon, we demonstrate the LIDT of three types of samples at 355 nm with 8 ns. High absorption defects are found at the film-substrate interface by comparing their LIDTs and damage morphologies. These defects originate from the substrate and appear during the coating process. Simulation results show that these defects, coupled to the coating, are mainly responsible for decreasing the damage threshold.

Morphology, structure and optical properties in TiO 2 nanostructured films annealed at various temperatures

Amorphous TiO2 nanostructured thin films (TNFs) were deposited by the glancing angle deposition technique (GLAD) with an electron gun. All of the prepared TNFs were composed of discrete nanoscale columns, characterized by scanning electron microscope. With the annealing treatment, the pure anatase phase was transformed from amorphous TNFs. The morphological, structural and optical properties of TNFs under the annealing treatment were measured, and the evolvement mechanism was further analyzed. The optimum annealing temperature for the pure anatase precipitation of TNFs is about 400°C. The variation of morphology, chemical state and crystallization of TNFs also resulted in the shift of the transmittance spectra. The results show that appropriate post-annealing treatment can build fine nanostructures and pure anatase precipitation, which can support the applications and researches about the anatase phase TNFs powerfully.

Optical properties and microstructure of Ta 2 O 5 biaxial film

This study investigates the optical properties and microstructure of Ta(2)O(5) film deposited with the glancing angle deposition technique. The tilted nanocolumn microstructure, examined with scanning electron microscopy, induces the optical anisotropy of thin film. The optical properties of thin film are characterized with an inverse synthesis method. Based on the Cauchy model, the dispersion equations of optical constants of film are determined from the transmittance spectra measured at normal and oblique incidence over 400-800 nm. The starting values derived with an envelope method quicken the optimization process greatly. The dispersion of the principal indices N(1), N(2), and N(3) and the thickness d of thin film are presented statistically. A good agreement between the measured optical properties and theoretical calculation is obtained, which validates the model established for thin film produced by glancing angle deposition.

Defect analysis of UV high-reflective coatings used in the high power laser system.

By considering the rapid change of standing-wave electric-field and assuming the interface defect distribution, an improved model is developed to analyze the defect density distribution and assess the damage performance of high-reflective coatings. Two kinds of high-reflective coatings deposited by e-beam evaporation (EBE) and ion beam sputtering (IBS) techniques are analyzed with this method. The lower overall damage threshold is the major feature for the coatings deposited by IBS method according to the defect parameters extracted from the model. Typical damage morphologies of coatings are also measured and analyzed. The assumption of interface defects is supported by the damage behavior. The damage mechanisms of two high-reflective coatings are attributed to the formation of molten pool and mechanical ejection. The influence of the incident angle on the damage probability is also considered and numerically calculated. The defect analysis model improved here is suitable for high-reflective coatings.

Analysis of characteristic matrix for a uniaxially birefringent thin film

As the principal section consistent with the principal plane, electromagnetic propagation in a uniaxially birefingent thin film can be described with a concise 2 × 2 characteristic matrix, in which the refractive indices of the forward and backward propagating extraordinary rays are different and dependent. In this letter, based on Huygens construction, the refractive indices and effective optical admittances of the forward and backward propagating extraordinary rays are discussed further, and the 2 × 2 characteristic matrix is simplified. Furthermore, the input optical admittance, reflectance and transmittance of assembly is presented, just as an isotropic thin film. The result can be extended to the general case of multilayer uniaxially birefringent thin films with their optic axes in the incident plane.

Suppression of nano-absorbing precursors and damage mechanism in optical coatings for 3ω mirrors.

Damage precursors in the 3ω (351 nm) mirror for a high-power laser system are investigated as well as the relevant damage mechanisms. The precursors are classified into two ensembles according to the different laser resistance and damage features. The former is nano-absorbing precursors, which are sensitive to the standing wave electric field and vulnerable to the laser irradiation. The latter is submicrometer nodular defects, which have higher laser resistance and are sensitive to the adhesion strength between the fluoride coatings and oxide coatings. The damage due to nano-absorbing precursors is efficiently suppressed with the double stack design that screens the electric field in the oxides. Currently, the nodular seed is major originating from the Al2O3/SiO2 stack. Even for the same defect type and mirror, the final damage features are dependent on the local mechanical properties at the irradiation location. The investigations of the damage mechanisms provide a direction to further improve the laser-induced damage threshold of the 3ω mirror.