Like 3D printing, nanoscale structures can be created by making a 2D slice at a time, and stacking slices together from bottom up carefully. A mature technology called laser direct writing (DLW) has been the workhouse in this field. Utilizing an ultrafast femto- or picosecond laser to induce multiphoton absorption of the photoresist in solution is the key principle (Figure 1, top). Since it is a multiphoton process, the polymerization will happen only at the focal point of the laser, even the focal point is embedded deep in the solution. With this “point construction capability”, you are actually free to build up any kind of structure. The resolution, or the pixel size (voxel) is defined by how tight you can focus the light beam.
Focusing the light into a tight voxel isn’t a trivial business, especially in the direction of light propagation due to the nature of the diffraction. But it is needed if we want to create even finer structures. Researchers in Martin Wegener’s group are using novel techniques to improve this. It is called stimulated emission depletion (STED) and is adopted from fluorescence microscopy. In short, by overlapping laser pulses of two different lasers in space and time, they are able to defeat the diffraction limit and create a voxel smaller than you can achieve with only one laser.
Another new trick is called dip-in DLW, as shown in figure 1 (bottom). In traditional 3D-DLW, the height of the structures is limited due to the finite working distance of the microscope objective lens. In addition, due to aberrations arising from the refractive-index mismatch between glass substrate and photoresist, the voxel size will vary. This can be overcome by dip-in 3D DLW. In this methodology, the liquid photoresist itself is used as the immersion fluid between the microscope lens and the substrate. Refractive-index mismatch is not an issue any more! As a result, structures of even up to millimeters overall height combined with sub-micrometer feature sizes become possible. This allows researchers to build bigger structures to check their mechanical properties.
“Black silicon” is another interesting topic. Silicon is the main surface component of solar cell. Due to its high index of reflection, most of the sunlight is reflected back. So if we can make it black, more solar energy can be absorbed and used. Scientists from Minghui Hon’s group have found a unique way to make the silicon surface black. Here is the recipe: Laser ablation on the silicon surface + coating the surface with nanoparticles. By carefully controlling the power, repetition rate, and the scanning rate of the lasers, they can create pyramid like structures, which trap the light. In addition, by coating the surface with alloy of nanoparticles (mix of 10 nm Cu, 5 nm Ag, and 15 nm Au). The broadband reflectivity can be lower 4%.
Silicon surface can also be patterned by lithography. With the help of laser interference lithography (LIL), it is possible to generate pattern like uniform moth-eye structure and bi-hybrid moth-eye structure (figure 2). When coated this pattern with alloy of nanoparticles, the reflectivity can be below 1% from visible to IR wavelength. For detailed study on this topic, you can follow this link.
There are many more 3D structures to explore. For example, 3D parabolic mirror arrays are created to increase solar cell efficiency and assist the collimation of LED. Another technique called pattern-integrated interference lithography (PIIL). It integrates the interference lithography and superposed pattern mask imaging into a single-exposure step. It is promising to be applicable to future nano- and microelectronics.
With this excitement, we are all looking forward to seeing more optical magic molding the future of material science.
Posted: 10/10/2013 1:59:16 PM by
By Frank Kuo
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