Advanced Manufacturing Laboratory    



Ultrafast Laser Processing of Dielectrics and Semiconductors


Ultrafast Laser Processing of Transparent Non-Crystalline Solids

Ultrafast laser processing of transparent non-crystalline solids employs non-linear absorption to induce structural changes within the target material interior without affecting its surface. This process has potential use in transmission welding of glasses, microfabrication of optical microdevices (waveguides) and three dimensional optical data storage. Altering the structure using a femtosecond laser in transparent materials occurs when the laser beam is tightly focused into the bulk of the sample. The laser intensity is concentrated within the focal volume and induces non-linear absorption of the deposited energy through multiphoton and avalanche ionization.   

Current research issues involve the study of the laser induced rearrangement of the ring structures of amorphous fused silica. Spatially resolved Raman spectroscopy and Rayleigh scattering are employed as non-destructive characterization techniques to show local densification and relative volume fraction changes of the ring structures within the affected region.



Femtosecond/nanosecond Laser-Induced Surface Texturing and Simultaneous Crystallization of Hydrogenated Amorphous Silicon (a-Si:H)

Amorphous silicon thin films have been considered for use in solar cell applications because of their significantly reduced cost compared with bulk crystalline silicon. However, their overall efficiency and stability are less than that of their bulk crystalline counterparts.

This project is to use both femtosecond and nanosecond (excimer) laser processes of a-Si:H to solve the two disadvantages simultaneously. Both lasers produces nano-/micro-meter size spikes which can be used for production of periodic structures on the sample surface, and simultaneous crystallization occurs in a one-step process. Optical absorption is enhanced by light trapping via multiple reflections through the surface geometry changes, and the formation of a mixture of crystalline silicon and a-Si:H after crystallization suggests that the overall stability can be potentially increased. Both laser-based treatments of a-Si:H show a promising methodology for thin-film solar cell fabrication, and compared to femtosecond laser, excimer nanosecond laser is more desirable for producing large grains with low defects and preventing hydrogen from diffusing out through a step-by-step process.