Ultrafast Laser Laboratory

Femtosecond Laser Micromachining

Laser machining is a process that uses a focused optical light beam to selectively remove materials from a substrate to create a desired feature on or internal to the substrate. The process is noncontact yet it has high spatial confinement. Comparing to other mechanical machining techniques, laser machining exhibits low heat deposition to the working piece. The process generally relies on the linear optical absorption and plasma formation mechanisms. However, conventional laser machining, cw or pulsed, can not create micron-sized structures because linear optical absorption of the materials often lead to heat deposition, micro-cracks and small collateral damage to the surrounding is unavoidable.

Femtosecond laser micromachining is a rapidly advancing area of ultrashort laser applications. It utilizes the ultrashort laser pulse properties to achieve an unprecedented degree of control in sculpting the desired microstructures internal to the materials without collateral damage to the surroundings. Using femtosecond rather than picosecond or nanosecond light pulses, laser energy is deposited into small volumes by multiphoton nonlinear optical absorption followed by avalanche ionization. Because typical heat diffusion time is in the order of nanosecond to microsecond time scale whereas the electron-phonon coupling time of most materials are in the picosecond to nanosecond. Therefore when laser energy is deposited at a time scale much shorter than both the heat transport and the electron-phonon coupling, the light-matter interaction process is essentially frozen in time. The affected zone altered from solid to vapor phase and to plasma formation almost instantaneously. Unlike conventional laser machining, femtosecond laser machining reduces collateral damages to the surroundings. Because the machining process is not dependent on the linear absorption at the laser wavelength, virtually any dielectric, metals, and mechanically hard materials can be machined by the same laser beam.

 

By careful control of the laser intensity, one can produce only permanent refractive index modification on the work piece. Direct writing of optical waveguides in three-dimension becomes possible. More importantly, passive and active optical devices can potentially be fabricated directly in three-dimension using one laser system in one processing step.

Because of the nature of the nonlinear optical fabrication process, features size smaller than the diffraction limit can be created internal to the substrate at various depths. 3-dimensional submicron sized structures can be defined to produce miniature photonic components, read only memory chips, and hollow channel waveguides that may be of interest in the area of optical communication network, optical data memory, and biological optical chips. Perhaps the most encouraging of all, clinical trial of femtosecond laser eye surgery on human is now in progress.

Despite of the various potential applications of femtosecond laser micromachining, the precise mechanism of the process is still under investigation. Various groups are studying practical issues such as reliability, stability, and lifetime effects of the fabricated devices. We have developed an online diagnostic technique for the micromachining of various materials. It was found in one of our early studies that the third-order nonlinear susceptibility, which is dipole-allowed on all materials, was efficiency enhanced by the non-uniformity of a medium either in the linear refractive index or in the nonlinear susceptibility. A surprisingly strong optical third-harmonic (THG) is generated in the forward direction only at the material interface. This THG is maximized when the interface is located at the beam waist of the light pulse. The onset of the THG signal with input laser energy can be use to establish the threshold for the creation of the micromachined features on the sample. Furthermore, the same laser beam but at different intensity levels can be used to create and then probe the micromachined structures at various depth in the substrate on the fly. Hence a novel 3-D optical read and write scheme is proposed and demonstrated.

 

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For more information or preprint request contact Thomas Y. F. Tsang