Femtosecond pulses generate microstructures

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

Researchers at Laser Zentrum Hannover (LZH), Germany, believe they have developed two new optical technologies that will enable the fabrication of 100 nm structures. The team's first technique involves two-photon polymerization (known as 2PP) of photosensitive materials. This can produce complicated 3D microstructures and nanostructures, such as the miniature Venus statue shown in figure 1. The second method uses sub-diffraction-limited ablation to create nanostructures (see box).

"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.

When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows the fabrication of any computer-generated 3D structure by direct laser "recording" into the resin. The non-irradiated liquid resin can be dissolved in alcohol, leaving behind the polymerized copy of the computer model. Compared with conventional photolithography, which is a planar process, 2PP offers a genuine 3D volume microfabrication method.

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.

The material is also bio-compatible and can be used for medical and biological applications. "In medicine, the materials can be used as implants," explained Chichkov. "With the 2PP technique, we can fabricate, for example, arbitrary structured 3D medical implants and drug-delivery devices."

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.

To date, LZH has fabricated a number of 3D microstructures using the technique, such as miniature Venus statues measuring from 10 µm to 1 cm high. To accelerate the fabrication process, only the outer shell of the statue can be initially polymerized. The unwanted liquid resin surrounding it is then washed away before the statue is irradiated with ultraviolet light for the final polymerization of the inner body.

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.

For more information Contact Boris Chichkov at ch@lzh.de or visit the LZH website at www.lzh.de.