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Prof. Ivan Biaggio
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How can we build the best material for third-order nonlinear optics?

Third-order nonlinear optics describes phenomena where four photons simultaneously interact together, as an example when three photons combine to create a fourth one. This will cause a light-beam to undergo self-focusing, or it can give rise to the possibility of one light signal infuencing another.

Such effects efficiently happen in organic molecules. The fast and strong third-order optical nonlinearities of organic materials, together with the flexibility for tuning their chemical structure towards the optimization of functional properties, make them ideal for mediating the interaction between different optical waves in future photonics devices.

[Image: a DDMEBT film]

As an example, organic molecules for third-order nonlinear optics can be designed to have the first optical transition at wavelengths shorter than 700 nm, so that at telecommunication wavelengths they have an off-resonant response (away from two-photon absorption) that is electronic in origin and practically instantaneous, allowing in principle all-optical switching at THz speeds and beyond (in contrast, e.g., to silicon, where two-photon absorption and free carrier generation limit the response speed).

Unfortunately, the transition from laboratory demonstration of high molecular nonlinearities to the development of organic solid-state materials that reflects the molecular properties and can be efficiently combined with existing guided wave technology has proven difficult.

One fundamental challenge is this: How can we create a solid-state organic material (a piece of plastic) that has a high optical quality (low losses, low scattering) and that has the nonlinear optical properties one can expect when assembling the best organic molecules together?

That has been one of the main activities in our research group over the last few years.

The motivation is that the lack of ultrafast nonlinear optical materials that can be easily integrated with existing guided wave technology has consequences for what we can do with all-optical data processing. Developments in light-wave technology have exploded. There is now a wide availability of advanced integrated optical circuits on chips that can be fabricated on a large scale with nanometer precision, such as those based on the silicon photonics platform. Technology offers all kinds of waveguide structures, filters, microresonators, and various photonic crystal structures. All this could finally allow for flexible optical computing devices. But then the optical circuitry must also include elements that can provide for all-optical swithcing of optical signals and other active components.

[Image: the DDMEBT molecule]

Two related molecules with large intrinsic third-order susceptibilities. The TDMEE molecule has a planar structure. The DDMEBT molecule contains almost the whole structure of TDMEE as a subunit, but has an additional component that gives it a nonplanar structure. A three-dimensional view of DDMEBT is shown on the right.

It has been a long-established knowledge that rganic materials could in principle provide highly sensitive materials to create active optical elements, but doing so requires assembling the molecules into a high quality solid-state. Obviously, one possibility to to this are polymers (witness the high quality polycarbonate coatings on top of optical discs). But how do we use the best organic molecules with high third-order nonlinearities with polymers? The answer is that they need to be diluted in the polymer, and so we end up with potent molecules created via years of reasearch that then end up diluted in a relative low density, which seems to defeat the purpose of developing such strong molecules in the first place.

Ongoing research and development in our research group has focused on developing a new paradigm for creating third-order nonlinear optical materials based on organic molecules: Instead of large molecules or polymers, which are difficult to use, we focus on small molecules. Instead of diluting their properties in a polymer matrix, we look for a material that consists only of optimized molecules, with no wasted space in between. To develop these new ideas, we focused on molecules that have a nonlinearity that is large relative to their size, and which can be self-assembled into a dense bulk material (in our case by vapor deposition, but spin coating could also work). Such a dense assembly of small molecules is particularly advantageous for third-order nonlinear optics, which does not require any orientational order. In fact, a high optical quality can be achieved if we can make a molecular material that is amorphous, consisting of a dense assembly or randomly oriented molecules, like a glass. And compatibility with existing technology can be accomplished by exploiting the fact that small molecules can be sublimated without decomposition and using vapor deposition for the self-assembly process and for combining the molecules with existing integrated optical circuits. Vapor deposition allows to cover several square centimeters of a substrate with a very flat, very homogenous material that can enter and fill nanometer-size structures, without any significant interaction with the substrate

[Image: a DDMEBT film]

Thin films created by vapor-depositing the DDMEBT molecule.

Can this idea lead to a solid-state organic material that has both high optical quality and an off-resonant third-order nonlinearity three orders of magnitude larger than fused silica? If yes, such a material would provide for what silicon alone cannot do: all-optical switching at 1 Tbit/s data rates over propagation lengths of a few millimeters, without free carrier generation and two-photon absorption losses.

In our work, we have used different techniques such as Degenerate Four-Wave Mixing or Z-scan at various wavelengths from the visible to the near and far infrared, to study the ability of organic molecules to mediate multi-photon interaction at various wavelengths, from the off-resonant limit where the response is essentially instantaneous and no energy is deposited into the material, to the first two-photon resonance where two-photon excitation becomes very efficient.

While working on optimizing the third-order nonlinear optical response of smallmolecules, we have been able to demonstrate molecules that have a very large third-order nonlinearity despite their small size. This is very important because smaller molecules are easier to process into a dense solid state. The resulting very high specific third-order polarizability means that a high density supramolecular assembly of these molecules can have record-breaking nonlinearities. In addition, we have also investigated molecules whose nonlienar response approaches the maximum possible quantum limit, which means that they have a very high intrinsic third-order polarizability. Thanks to such molecules, we can create (by dense self-assembly) nonlinear organic films that have the required high nonlinearity and optical quality, and that can be used to fabricate integrated optics elements for all-optical switching, as an example a silicon-organic-hybrid device.



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