Optical Tweezer Nanofabrication

Once assembled, tweezer-organized structures can be fixed in place, for instance by sintering or gelling. The tweezers themselves can be used in this process. In particular, the intense illumination at an optical tweezer's focus is ideal for driving photochemical reactions. If the reaction rate depends strongly on intensity, the resulting spatially-resolved photochemistry can yield features smaller than the wavelength of light.

Figure 4: The diffraction-limited focus of an optical tweezer is ideal for spatially localized photochemistry. (a) A photopolymerized sculpture whose finest features are roughly 100 nm across. From Ref. (47). (b) Solid model of a photopolymerized turbine with submicrometer features. (b) The actual turbine shown suspended in water, from Ref. (48). (d) Fine lines of MoS₂ deposited on a glass substrate by photoreduction of an aqueous salt solution, from Ref. (49). (e) A three-dimensional fluorescent polymer structure embedded in a colloidal crystal, from Refs. (50)] and [(51).

The first such application of optical tweezers involved the spatially resolved photo-oxidation of biological materials such as chromosomes (52,53), essentially creating a scalpel from light. Optical scalpels and scissors have been used for surgery on living cells (54,16) as well as for ablating sub-wavelength structures into microscopic substrates (55).

Spatially-resolved photochemistry using optical tweezers has been used to fabricate small complex three-dimensional structures such as the examples in Fig. 4. Figures 4(a), 4(b) and 4(c) show three-dimensional plastic structures created by multiphoton photopolymerization in scanned optical tweezers. The smallest features in Fig. 4(a) are about 100 nm across. The tiny turbine in Figs. 4(b) and 4(c) not only was created in this way, but also was trapped and spun on its axis with an optical tweezer (48). Arrays of interlocking turbines and gears assembled and driven by light already have been demonstrated (48). Still other photochemical transformations provide opportunities for optical tweezers to create three-dimensional electronic and photonic structures. The fine lines of MoS₂ in Fig. 4(d) were patterned on glass by photoreduction of aqueous salts, and similar results have been obtained in silver, gold, and oxidized copper (49).

Beyond creating structures de novo, spatially-resolved photochemistry also can be used to modify pre-existing structures. The three-dimensional optical waveguide structure in Fig. 4(e) demonstrates this principle. Here, a self-assembled crystal of colloidal silica spheres was perfused with a photosensitive precursor and selectively patterned with an optical tweezer to create the embedded polymer structure shown in Fig. 4(e) (50). Filling the gaps with a high-index material and then dissolving away the spheres and polymer would leave a tweezer-drawn waveguide pattern embedded in the otherwise self-assembled photonic crystal (56). This hybrid approach to creating hierarchically structured materials could sweep away many of the practical hurdles that have prevented self-assembled systems from making bigger inroads in photonics and electro-optical systems (51).

Using large numbers of optical tweezers to organize prefabricated nanometer-scale parts and simultaneously to stitch them together with spatially-resolved photochemistry would yield a whole new category of hierarchically-structured materials and devices. Such scale-spanning heterostructures would provide the building blocks for sensors, photonic devices, and a host of other technologies. Hierarchically structured micromechanical systems hold similar promise for optomechanical and microfluidic applications. In this case, optical trapping also solves the outstanding problem of actuating such small devices. Some aspects of this solution involve the unusual and counterintuitive properties of traps created with newly discovered modes of light.