We report the experimental fabrication of optical elements with femtosecond pulses. The laser source we adopted is a low power Ti: sapphire laser oscillator, with a central wavelength of 790 nm and pulse duration of 100 fs. Positive-photoresist-coated film acts as the sacrificial material. To obtain the optical elements, three microobjectives with high numerical aperture 0.25 and 0.1 were used to focus the light beam of femtosecond laser. Due to the extreme high intensity of the tightly focused femtosecond laser beam, nonlinear effect occurred between photoresist and the laser pulses, which enable the ablation of the photoresist. In the experiments, we use a translational stage that hold the sample by a pump through a ventage. Various gratings and phase plates are fabricated by this method. The obtained gratings patterns are checked with a conventional optical microscopy. The fabricating widths and depths are measured with the Taylor Hobson equipment. With the same method, photomask for microelectronics can also be fabricated. From the experimental results, we see that the fabrication of the different microobjectives can be achieved with this method. This technique can be applied to the fields of microoptics and microelectronics. The mechanism between femtosecond laser and photoresist is also investigated. The processing mechanics is considered as laser ablation. Fabrication of optical elements with femtosecond laser reflects a new trend for fabrication of microoptical elements.
We demonstrate experimentally fabrication of optical elements with femtosecond pulses. The laser source we adopted is a low power Ti: sapphire laser oscillator, with a central wavelength of 790 nm and pulse duration of 100 fs. Positive-photoresist-film-coated glass substrate acts as the sacrificial material. Due to the extreme high intensity of the tightly focused femtosecond laser beam, nonlinear processing occurred between photoresist and the laser pulses, which enable the sub-micron feature processing. In the experiments, we use a translational stage that is controlled by a computer to accurately move for fabrication of optical elements with high precision. Various gratings and phase plates are fabricated by this method. The obtained gratings patterns are checked with a conventional optical microscopy. The fabricating widths and depths are measured with the Taylor Hobson equipment. With the same method, photomask for microelectronics can also be fabricated. From the experimental results, we see that a high processing precision and the feature size exceeding the diffraction limit can be achieved with this method. This technique can be applied to the fields of microoptics and microelectronics. The mechanism between femtosecond laser and photoresist is also investigated. The processing mechanics is considered as laser ablation and nonlinear two-photon absorption phenomenon. Fabrication of optical elements with femtosecond laser reflects a new trend for fabrication of microoptical elements.
In fabrication of a fine optical element, femtosecond laser is an attractive experimental tool because it avoids the splutter effect to damage the nearby lines. However, the wavelength of the usual Ti:sapphire laser is insensitive to the widely-used photoresist in microlithographic industry. In this paper, we introduce a new method with femtosecond doubled-frequency laser by use of a BBO crystal to fabricate optical gratings and chromium photomasks. The laser source is the Ti:sapphire laser with a central wavelength of 790 nm and its doubled-frequency laser is obtained through the BBO crystal whose wavelength (395nm) is within the sensitive exposure range of the photoresist. This enables us to fabricate fine optical elements with the normal photolithographic technique. In the experiment, we use a translator that is controlled by a computer to accurately move for fabrication of optical elements with high precision. In contrast to the other techniques, our approach has the higher quality and precision, for femtosecond laser works faster than the material’s thermal diffusion, i.e., without splutter effect that yields the clear edge of the optical element. Moreover, it also makes the fabrication processing simplified. Experiments are given to verify that this method should be highly interesting for the fabrication of fine binary optical elements.
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