Chang Kwon Hwangbo

Nano‐tailoring the Surface Structure for the Monolithic High‐Performance Antireflection Polymer Film

Both single-layer and multilayer antirefl ection (AR) coatings between a bulk substrate and air have been used extensively to reduce surface refl ection. These coating methods utilize the destructive interference between the lights refl ected from the interfaces to minimize the total intensity of the refl ected light. [ 1 , 2 ] However, the AR characteristics obtained with the coating methods can only be observed for a limited wavelength range and for an incidence angle close to the normal incidence. These drawbacks of the conventional coating methods can be suffi ciently overcome either by the graded refractive index (RI) layers method or by the antirefl ective structure (ARS) method. [ 3 , 4 ] The graded RI layers method and ARS method utilize optical impedance matching at two interfaces at air and substrate by making the RI of the AR layer vary gradually from the RI of air(n o ) to that of the substrate(n sub ) along the normal direction. In contrast with the coating methods, the graded RI layers method and ARS method were theoretically predicted to show broadband AR behavior of up to one order of magnitude and omnidirectional AR for an incidence angle of up to 80 degrees. [ 3 ]

Design of omnidirectional high reflectors with quarter-wave dielectric stacks for optical telecommunication bands

A design for omnidirectional high reflectors with quarter-wave dielectric stacks in the optical telecommunication band that uses conventional optical thin-film design theory is described. The omnidirectional bandwidth is derived in units of wavelength and investigated as a function of its high- and low-refractive-index values in the near infrared. The results show that the high refractive index should be larger than 2.26 for an omnidirectional high reflector and that the low refractive index for maximum onmidirectional bandwidth should be approximately 1.5. It is shown that one can design broad-bandwidth omnidirectional high reflectors for S, C, and L bands for optical telecommunication simply by connecting the band edges of omnidirectional high reflectors.

Optical inhomogeneity and microstructure of ZrO 2 thin films prepared by ion-assisted deposition

The index inhomogeneity and the microstructure of ZrO(2) films prepared by Ar-ion-assisted deposition are investigated. The results show that as the Ar-ion momentum transferred to the growing film increases, the average refractive index increases, the vacuum-to-air spectral shift becomes almost zero, the sign of relative inhomogeneity transits from negative to positive, and the void fraction of the top layer next to air becomes smaller than that of the bottom one. These optical properties result from the improved packing density and denser outer region next to air. The Ar-ion bombardment also induces the changes in microstructure of ZrO(2) films, such as the preferential (111) orientation of cubic phase, increase in compressive stress, and reduction of surface roughness.

Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared

Perfect absorption (PA) of incident light is important for both fundamental light-matter interaction studies and practical device applications. PA studies so far have mainly used resonant nanostructures that require delicate structural patterning. Here, we realize tunable and broadband PA in the near-infrared region using relatively simple thin film coatings. We adjust the growth condition of an ITO film and control its epsilon-near-zero (ENZ) wavelength. We show that this results in highly tunable PA in the telecommunication window. Then, using an ITO multilayer of different ENZ wavelengths, we demonstrate broadband PA that covers a wide range of near-infrared wavelengths. The use of ENZ coatings makes PA adjustable during the film growth and does not require any structural patterning afterward. It also facilitates the chip-scale integration of perfect absorbers with other device components. Broadband PA relaxes the single wavelength condition in previous PA studies, and thus it is suitable for many practical device applications, including sensors, photodetectors, and energy harvesting devices.

Wideband circular polarization reflector fabricated by glancing angle deposition.

We demonstrate a wideband circular polarization reflector fabricated as cascades of helical films with different pitch thickness by using glancing angle deposition (GLAD) technique. The full-width-at-half-maximum bandwidth of this reflector is measured from the reflectance spectra and is found about 200 nm indicating the feasibility of wideband reflector. A helical TiO(2) film with three sections, each of different pitch thickness, is also studied. It shows three Bragg peaks at different wavelengths. To select appropriate material for this circular reflector, the optical properties of 5-turns TiO(2), ZrO(2), and Ta2O(5) helical films and the porosity effect on the TiO(2) helical film are investigated.

Extraordinary optical properties of a superconducting periodic multilayer in near-zero-permittivity operation range

Optical properties in a one-dimensional periodic multilayer structure consisting of superconducting and dielectric films are rigorously studied. Due to the temperature- and wavelength-dependent refractive index in superconducting films, the design of multiple-resonance Fabry–Perot resonators can be achievable without physically introducing any defects to break the periodicity of the structure. These extraordinary optical properties, different from the regular all-dielectric Bragg reflectors, are mainly attributed to the operation in the near-zero-permittivity range of superconductors. Additionally, while the reflection bandwidth tends to enlarge with increasing angle of incidence for transverse magnetic polarization, it remains unchanged with band edges shifted for transverse electric polarization.

TiNx layer as an antireflection and antistatic coating for display

Abstract In this study TiN x films were fabricated by a r.f. magnetron sputtering apparatus and the optical, electrical and structural properties were investigated. X-Ray photoelectron spectroscopy (XPS) measurements showed a Ti 2p 3/2 peak at 456 eV after etching the oxidized film surface so that the formation of TiN bonding was confirmed. As the nitrogen gas flow rate increased, the XRD analysis showed that the TiN films were polycrystalline and the ratio of (200) to (111) intensity peak increased. The sheet resistivity of the films varied with the N 2 flow rate and was influenced by the film thickness. Optical constants of TiN x films were measured by the spectroscopic ellipsometer and also calculated by fitting the measured reflectance to the Forouhi–Bloomer dispersion relation. From the measured optical constants the thickness of SiO 2 layer was calculated for broadband AR coating in the visible region. Reflectance of a two-layer coating, TiN x and SiO 2 , on glass substrate was below 0.5% in the visible wavelength and the maximum transmittance was approximately 62% at 520 nm.

Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance.

The optical monitoring method is described to compensate for the thickness error of nonquarterwave layers of dielectric multilayer filters, using optical admittance during deposition. Stability is confirmed by computer simulation of random thickness error generation in layers. In addition, a band split filter consisting of 61 nonquarterwave and nonperiodic layers is deposited in the proposed method, resulting in high spectral performance, as the design requires.

DC reactive magnetron sputtering with Ar ion-beam assistance for titanium oxide films

Titanium oxide thin films were deposited by DC reactive magnetron sputtering with Ar ion-beam assistance at low oxygen partial pressure and long target-to-substrate distance. The optical and structural properties of them were investigated by the measurement of transmittance and reflectance, atomic force microscope, and X-ray diffraction. The results show that the Ar ion-beam-assisted DC reactive magnetron sputtering for titanium oxide thin films induces the higher packing density, lower absorption, and smoother surface than the conventional DC reactive magnetron sputtering, suggesting that it can be employed in deposition of optical dielectric coatings.

Control of resonant wavelength from organic light-emitting materials by use of a Fabry-Perot microcavity structure.

We report the fabrication of Fabry-Perot microcavity structures with the organic light-emitting material tris-(8-hydroxyquinoline) aluminum (Alq3) and derive their optical properties by measuring their photoluminescence (PL) and absorption. Silver and a TiO2-SiO2 multilayer were used as metal and dielectric reflectors, respectively, in a Fabry-Perot microcavity structure. Three types of microcavity were prepared: type A consisted of [air[Ag[Alq3]Ag]glass]; type B, of [air[dielectric[Alq3]dielectric]glass]; and type C, of [air[Ag[Alq2]dielectric]glass]. A bare Alq3 film of [air[Alq3]glass] had its PL peak near 514 nm, and its full width at half-maximum (FWHM) was 80 nm. The broad FWHM of a bare Alq3 film was reduced to 15-27.5, 7-10.5, and 16-16.6 nm for microcavity types A, B, and C, respectively. Also, we could control the PL peak of the microcavity structure by changing the spacer thickness, the amount of phase change on reflection, and the angle of incidence.