Postdoctoral Research Fellow, National University of Singapore
Materials behave differently at nano/micro scales
Lenses used in lasers, microscopes, telescopes are made up of brittle materials like glass, calcium fluoride, sapphire etc. With advances in optics the shapes of these lenses are becoming complex (Check your smartphone camera or flash lens) and the dimensions need to be strictly within given tolerance (less than 1 micrometer). Generally, the material is removed at nano and micro scale to avoid any cracks on the surface. Diameter of human hair is about 60 micrometer whereas we remove material at scale which is 1000 times smaller as compared to human hair. That's nano-scale and at this scale the material surface behaves as if it's soft but the whole block of material is still brittle. In short we make scratches and when thousands of scratches combined together, it produces desired shape. This is also called- ‘ductile-mode-machining’ of brittle materials. We use natural diamond tool (Yes! it’s expensive) as a cutting tool because it’s one of the hardest material and ideal for making scratches. But I think, this is a very inefficient way of removing the material as it takes days to produce some of the simple lenses. So, I use a novel approach in which two processes are combined- laser heating and then scratching to increase the material removal rate. As I heat the surface of the material it becomes even softer and I can remove more material and reduce the time required for fabrication. I also use some equations to find out how much time I should heat with laser without melting or damaging the surface. I use metal 3D printing to make specially designed tools for cutting. We found that this method is at least 2 times faster than the current methods. Therefore, I would like continue exploring so that I can make it even faster without loosing accuracy. Also, I want try this method on other materials and check its performance.
Abstract: Picosecond mid-IR USPL induced surface damage on a Gallium Phosphate (GaP) and Calcium Fluoride (CaF2) is reported. A semiconductor GaP and a dielectric material CaF2, that are transparent over3–10μm, were exposed to one picosecond mid-IR light (4.7μm) to investigate laser-induced surface morphological changes on the target The initiation of damage along the polishing scratch line of GaP and the random location of damage digs on the CaF2 suggests that the mid-IR picosecond laser-induced damage on targets started from intrinsic surface defects. Multiple pulse irradiations produced periodic corrugated surface structures (ripples) perpendicular to the polarization of light on both GaP and CaF2. In terms of the orientation and the spacing between ripples, observed ripples have common features with previously reported ripples.
Pub.: 01 Jul '07, Pinned: 18 Sep '17
Abstract: The cutting mode transition from ductile to brittle is related to the depth of cut for the machining of brittle materials. The critical depth of cut for ductile to brittle transition (DBT) during single-point diamond turning of single-crystal calcium fluoride was examined, and the effect of cutting direction, cutting speed, and tool rake angles were investigated. Results show that the cutting speed had a slight effect on the critical depth of cut for DBT, while negative rake angle tools yielded large critical depth of cut for DBT. The influence of cutting direction (crystallographic orientation) on the critical depth of cut for DBT was associated to fracture toughness (KC) of the materials. Higher KC values induced larger critical depth for DBT. Furthermore, periodic variations of KC values as a function of the crystallographic orientation correlated well with changes in critical depth ranging between 100 and 600 nm. This resulted in the successive emergence of brittle and ductile cutting regions when a nominal depth of cut of 0.5 μm was used, while it led to the formation of a smooth and homogenous surface with Ra of 2.838 nm at a nominal depth of cut of 0.1 μm.
Pub.: 28 Jun '16, Pinned: 18 Sep '17
Abstract: An optical microcavity, which stores light at a certain spot, is an essential component to realize all-optical signal processing. Single-crystal calcium fluoride (CaF2) theoretically shows a high Q-factor which is a desirable optical property. The CaF2 microcavity can only be manufactured by ultra-precision cylindrical turning (UPCT). The authors have studied UPCT of CaF2 and shown the influence of crystal anisotropy and tool geometry on surface roughness and subsurface damage. The study indicated that a smaller nose radius of the cutting tool led to shallower subsurface damage. Thus, it is inferred that a smaller nose radius compared to the previous nose radius (0.05 mm) can further reduce subsurface damage. Nevertheless, the mechanism that causes a difference in subsurface damage due to crystal anisotropy is not sufficiently clear. The influence of subsurface damage on microcavity performance is still unclear. In this study, the UPCT of CaF2 was conducted using a tool with a nose radius of 0.01 mm. The subsurface damage was investigated by transmission electron microscope (TEM) observation from the viewpoint of the change in crystal lattice arrangement. In our previous study, fast Fourier transfer (FFT) analysis was used for confirmation of change of crystal structure. In this study, FFT analysis was also used to quantitatively evaluate the depth of subsurface damage. In addition, inverse fast Fourier transfer (IFFT) was used to analyze change of crystal lattice arrangement clearly, which enables discussion of the influence of slip systems. Finally, optical microcavities are manufactured without any crack, and the influence of subsurface damage on microcavity performance is experimentally evaluated using a wavelength tunable laser and power meter.
Pub.: 01 Feb '17, Pinned: 18 Sep '17