26 Nov 2002
Femtosecond lasers can now create 3D structures as small as human cells and beat the diffraction limit to generate sub-wavelength microstructures. Phillip Hill finds out more about the latest developments at Laser Zentrum Hannover.
From Opto & Laser Europe December 2002
"Two-photon polymerization is a true 3D fabrication technique that allows the production of 100-200 nm structures, or micrometre structures - as in the Venus - with 100-200 nm resolution," explained Boris Chichkov, head of LZH's strategy group. "In this sense we can speak about nanostructuring with lasers."
Simple but powerful The principle behind 2PP is simple but extremely powerful. First, femtosecond laser pulses are tightly focused onto a liquid resin which is transparent in the infrared. The pulses initiate a chemical process - two-photon polymerization - that converts the liquid resin into a solid. This process is confined to a highly localized area at the focal point, owing to the nonlinear dependence of the two-photon absorption rate on the laser intensity.
Jesper Serbin at LZH is carrying out the work on the 2PP technique in close co-operation with a group based at Germany's Fraunhofer-Institut für Silicatforschung (ISC). ISC develops and makes organically modified ceramic (ORMOCER), which is used in the 2PP process. ORMOCERs, a new class of photosensitive inorganic-organic hybrid polymer, are made by sol-gel synthesis with molecular-level mixing of different components. According to Chichkov: "It is remarkable that properties of these hybrid polymers can be tuned from those that are characteristic of organic polymers into those that are similar to inorganic glasses."
Medical applications ORMOCERs have a great deal of potential. They can be used as storage-stable, photo-polymerizable resins and come with a host of attractive properties: an adjustable refractive index (in the 1.47-1.56 range); high optical transparency with low losses at data and telecommunications wavelengths (less than 0.06 dB/cm at 830 nm, less than 0.2 dB/cm at 1310 nm, and less than 0.6 dB/cm at 1550 nm); exceptional thermal and mechanical properties; high chemical resistance; and relatively low cost.
Just a handful of groups worldwide is working on 2PP, according to Chichkov. "In all experiments so far, commercial acrylate or epoxy-based resins have been used. The hybrid polymers developed by ISC have much better mechanical, optical, thermal and chemical properties. What we have demonstrated for the first time is the 3D structuring of ORMOCERs with a 100-200 nm resolution."
LZH employs a Ti:sapphire laser with a repetition rate of 80 MHz, a pulse duration of 80 fs, and a wavelength of 780 nm to irradiate the ORMOCER. Femtosecond laser pulses are focused by a 100x immersion lens microscope objective, which has a numerical aperture of 1.4 and is filled with a refractive index-matching oil. Owing to the threshold behaviour of the 2PP process, a resolution beyond the diffraction limit can be realized by controlling the laser pulse energy and the number of applied pulses.
Since the optical losses of ORMOCERs are lower than 0.6 dB/cm at data and telecommunications wavelengths, the 2PP technique is very attractive for the fabrication of micro-optical components and devices. "These will be better than existing devices, since the 2PP technique allows the fabrication of 3D integrated optical components with a higher level of complexity and functionality than at present," commented Chichkov. " Moreover, with this technique we can fabricate periodic structures such as photonic crystals [figure 3]."
Chichkov believes that the prospects for using femtosecond laser pulses are rosy indeed, especially in areas where lithographic techniques cannot be applied.
Smaller than ever In addition to studying two-photon polymerization, LZH is investigating the ablation of materials using femtosecond pulses. Frank Korte at LZH has been able to fabricate sub-wavelength structures in metals and dielectrics using a commercial kilohertz Ti:sapphire laser system. Infrared pulses with a duration of 30 fs and an energy of 1 mJ are focused onto the material using a 36x/0.5 reflective Schwarzschild objective. Owing to the nonlinear nature of the interaction of femtosecond laser pulses with transparent materials, it is possible to produce high-quality sub-diffraction-limited structures at the target surface.
To fabricate the periodic nanostructure in a sapphire crystal shown in figure 2, 10 laser pulses were used for each hole. The total processing time for 216 holes was 30 s (including the time required for the sample positioning). This structure can be considered as a prototype of a 2D photonic crystal.
LZH has also applied the technique to metals. Traditionally, when tightly focused femtosecond pulses are focused onto a metal surface, the hole made is always accompanied by surface deformations around its edge. LZH says that it can avoid this effect by using a special imaging technique. The result is sub-micron holes with very sharp edges.