Challenges in realizing ultraflat materials surfaces

Enhanced technique for photochemical nano-polishing of a rough quartz surface: the numerical calculation of profile evolution

The enhanced technique of quartz surface nano-local etching is considered. The enhancement of an evanescent field above surface protrusions and, as a result, an increase in the rate of quartz nano-local etching, are proposed. The possibility to reduce the amount of etch products filled in rough surface troughs and control the optimal rate of the surface nano-polishing process is achieved. The dependences of the quartz surface profile evolution on the initial values of surface roughness parameters, on the refractive index of the medium containing molecular chlorine and contacting the quartz surface, and on the wavelength of radiation illuminating this surface are shown.

Profile evolution during photochemical nano-polishing of a rough quartz surface under direct illumination

A model for photochemical polishing of a rough quartz surface has been developed that uses the near field rising above a given surface under the action of electromagnetic radiation. The model allows solving the problem of predicting the results without the need for time-consuming calculations. Simulation of the etching of the quartz surface profile in the spatial frequency domain is used. The features of the surface profile evolution are analyzed, and the dependences of this evolution on the initial correlation length and standard deviation of a rough quartz surface profile as well as radiation wavelength are shown.

Realization of red shift of absorption spectra using optical near-field effect

In many applications such as CO₂ reduction and water splitting, high-energy photons in the ultraviolet region are required to complete the chemical reactions. However, to realize sustainable development, the photon energies utilized must be lower than the absorption edge of the materials including the metal complex for CO₂ reduction, the electrodes for water splitting, because of the huge amount of lower energy than the visible region received from the sun. In the previous works, we had demonstrated that optical near-fields (ONFs) could realize chemical reactions, by utilizing photon energies much lower than the absorption edge because of the spatial non-uniformity of the electric field. In this paper, we demonstrate that an ONF can realize the red shift of the absorption spectra of the metal-complex material for photocatalytic reduction. By attaching the metal complex to ZnO nano-crystalline aggregates with nano-scale protrusions, the absorption spectra by using diffuse reflection of the metal complex can be shifted to a longer wavelength by 10.6 nm. The results of computational studies based on a first-principles computational program including the ONF effect provide proof of the increase in the absorption of the metal complex at lower photon energies. Since the near-field assisted field increase improves the carrier excitation in the metal-complex materials, this effect may be universal and it could applicable to CO₂ reduction using the other metal-complex materials, as well as to the other photo excitation process including water splitting.

Surface improvement of organic photoresists using a near-field-dependent etching method

Surface flattening techniques are extremely important for the development of future electrical and/or optical devices because carrier-scattering losses due to surface roughness severely limit the performance of nanoscale devices. To address the problem, we have developed a near-field etching technique that provides selective etching of surface protrusions, resulting in an atomically flat surface. To achieve finer control, we examine the importance of the wavelength of the near-field etching laser. Using light sources at wavelengths of 325 and 405 nm, which are beyond the absorption edge of the photoresist (310 nm), we compare the resulting cross-sectional etching volumes. The volumes were larger when 325 nm light was employed, i.e., closer to the absorption edge. Although 405 nm light did not cause structural change in the photoresist, a higher reduction of the surface roughness was observed as compared to the 325 nm light. These results indicate that even wavelengths above 325 nm can cause surface roughness improvements without notably changing the structure of the photoresist.

Angstrom-scale flatness using selective nanoscale etching

The realization of flat surfaces on the angstrom scale is required in advanced devices to avoid loss due to carrier (electron and/or photon) scattering. In this work, we have developed a new surface flattening method that involves near-field etching, where optical near-fields (ONFs) act to dissociate the molecules. ONFs selectively generated at the apex of protrusions on the surface selectively etch the protrusions. To confirm the selective etching of the nanoscale structure, we compared near-field etching using both gas molecules and ions in liquid phase. Using two-dimensional Fourier analysis, we found that near-field etching is an effective way to etch on the scale of less than 10 nm for both wet and dry etching techniques. In addition, near-field dry etching may be effective for the selective etching of nanoscale structures with large mean free path values.

Optically controlled magnetic-field etching on the nano-scale

Electric and magnetic fields play an important role in both chemical and physical reactions. However, since the coupling efficiency between magnetic fields and electrons is low in comparison with that between electric fields and electrons in the visible wavelength region, the magnetic field is negligible in photo-induced reactions. Here, we performed photo-etching of ZrO₂ nano-stripe structures, and identified an etching-property polarisation dependence. Specifically, the etching rate and etched profiles depend on the structure width. To evaluate this polarisation-dependent etching, we performed numerical calculations using a finite-difference time-domain method. Remarkably, the numerical results revealed that the polarisation-dependent etching properties were determined by the magnetic field distributions, rather than the electric field distributions. As nano-scale structures induce a localised magnetic field, the discovery of this etching dependence on the magnetic field is expected to introduce a new perspective on advanced nano-scale structure fabrication.