Nanometer Material Processing Using NSOM-delivered Femtosecond Laser Pulses
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Nanometer Material Processing Using NSOM-delivered Femtosecond Laser Pulses Chen-Hsiung Cheng and Ming Li Panasonic Boston Laboratory, Panasonic Technology Company, Matsushita Electric Corporation of America Cambridge, MA 02142, USA. ABSTRACT Nanometer-scale surface topology modification has been demonstrated using NSOM (near-field scanning optical microscope) delivered femto-second pulses. The ablation laser has a pulse width of 150 femto-second and wavelength of 387-nm. The laser pulses are coupled into the free end of a multimode optical fiber that a nanometer-size NSOM probe was fabricated on the other end with small orifice. The transmitted laser pulses from the probe orifice illuminates and machines the substrate surface when the probe is in near-field range of the substrate surface. The produced feature on Silicon surface is as least 200-nm deep with hole diameter around 200-nm. Near-field coupling of the laser has the potential to achieve ablation of feature size less than diffraction limit. Using NSOM delivery method also allows us to take advantage of nanometer metrology in precision surface ablation or other type of preformed surface modification. The ability of monitoring surface topology of substrate in real time enables us to accomplish the in-situ surface processing. We have demonstrated the technique of drilling 200-nm air holes on a pre-formed 600-nm wide wave guide. This method can be used to fabricate one-dimensional photonic crystal on a waveguide in ambient environment. The experiment design and performance evaluation will be discussed. INTRODUCTION Nanometer laser machining techniques recently have been used to fabricate photonic band gap materials1-4. Typically femto-second pulses are used to achieve those nanometer scale results. Although sub-wavelength features can be fabricated using free space beam delivery, it is often found difficult. Furthermore, position precision of free space delivery is usually questionable. We will discuss briefly the two major limitations using ultra-fast laser pulses to drill nanometer scale holes in free space. First of all, the fundamental limitation of diffraction restricts the laser spot size to larger than half of the wavelength in free space optics. Secondly, it is difficult to precisely direct the beam onto target. The same diffraction limit mentioned above also limits how small optical microscopy can resolve a feature. It is impossible to do in-situ nanometer machining using traditional optical microscope. A metrology method that can be applied in air and coexist with laser drilling technology is necessary for precision machining on pre-formed nanometer structures.
A new laser-machining technology is being developed to fabricate sub-wavelength, or nanometer scale, features in ambient condition with nanometer precision. To preserve the advantages of ultrafast laser ablation, we use 150 femto-second pulses at wavelength of 387-nm. However, there are two essential advances in our new technique. To improve sub-wavelength machining, we use a nanomete
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