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Ultrafast Laser Chisels Lens Inside Glass

Photonics Spectra
Nov 2002
Stuart M. Hutson

In the classic magic trick, a magician asks a member of the audience to select a random card from a deck, and then seems to miraculously transport that card into a solid piece of ice or glass. Japanese researchers have performed their own version of the prestidigitation by forming a lens inside a block of silicon glass. No magic wand or sleight of hand was needed -- just a femtosecond laser.


The 400 x 400-µm Fresnel zone plate pattern was formed in a 3-mm-thick piece of silica glass by means of dot-by-dot ablation with a femtosecond laser.

Focusing ultrafast laser pulses on a specific area within a transparent object can induce nonlinear absorption at that spot, thus changing the local density -- sometimes even creating voids -- while leaving the rest of the material unscathed. Scientists have employed the technique to construct waveguides, couplers, gratings and types of three-dimensional data storage inside glass. The Japanese researchers, from Osaka University and the National Institute of Advanced Industrial Science and Technology in Osaka, have created a lens in the form of a 400 x 400-µm Fresnel zone plate.

Focusing ultrafast laser pulses on a specific area within a transparent object can induce nonlinear absorption at that spot, thus changing the local density -- sometimes even creating voids -- while leaving the rest of the material unscathed. Scientists have employed the technique to construct waveguides, couplers, gratings and types of three-dimensional data storage inside glass. The Japanese researchers, from Osaka University and the National Institute of Advanced Industrial Science and Technology in Osaka, have created a lens in the form of a 400 x 400-µm Fresnel zone plate.

Using a 130-fs Spectra-Physics Ti:sapphire laser, they ablated rings of empty space into the glass. The rings were formed in 1-µm, dot-by-dot increments by focusing the 800-nm, 0.4-µJ pulses through the 0.55-numerical-aperture objective lens of an Olympus microscope. The position of the ablated dot was determined by moving the 3-mm-thick block of glass with a two-dimensional positioning stage.

This method could have a wide variety of applications, such as coupling lenses to waveguides and creating compact imaging systems for CCD cameras. Wataru Watanabe, the principal author of the study, said that, before these applications become possible, however, the diffraction efficiency must be better than the 2 percent observed in this research. Work is under way to increase the efficiency through more precise ablation techniques, but he said there is more than one way to conjure a lens into a block of glass.

By creating a phase-reversal zone plate instead of the Fresnel plate, the researchers could reduce the zero-order light, increasing the diffraction efficiency. A phase-reversal zone plate would consist of rings of glass with an altered refractive index that would shift the phase of light passing through them by 1/4. This would reduce the zero-order light and increase diffraction efficiency. The construction of this plate would be very difficult, however, as the exposure time, scan speed and energy of the pulses would have to be precisely controlled.


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