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Developments in Optics and Optical Components

May 29, 2013
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Additional Questions & Answers from the Webinar below:

Speakers:

Dr. Robert R. McLeod
Associate Professor and Graduate Director
Electrical, Computer and Energy Engineering Department
University of Colorado at Boulder




3-D Gradient-Index Polymer Optics

Gradient index (GRIN) optics are traditionally fabricated via diffusion of a liquid species through a solid host material such that the refractive index depends on the local concentration of the diffusing species. The distribution of the index is thus controlled by Fick's Law, strongly limiting the complexity of the optical element. Additionally, diffusion time increases as the square of feature size, putting an upper limit on GRIN size. In this talk I will describe materials and lithography processes that enable the fabrication of GRIN optics with arbitrary 2-D and 3-D structure covering scales from ~200 nm to cm. The key is a photopolymer material in which diffusion is locally controlled by a photo-initiated polymerization reaction. By proper selection of monomers and processing conditions, index change up to 0.1 has been demonstrated. Unusual lens function (e.g. extended depth of focus) and 3-D photonic devices will be used to illustrate the process.

1. What are the applications for GRIN optics?

Well, virtually any place you would use traditional surface optics you *could* use a GRIN, so it’s a cost vs. performance tradeoff.  GRINs offer the designer increased degrees of freedom, so can be exploited even in traditional lens trains to reduce the number of elements, weight and cost. I think they really shine when you think about applications that just can’t be done with surface features only. Examples are Bragg holograms, waveguides and nontraditional lenses that exploit the 3D distribution of index. An example of the last is the endoscope lens I showed that produces a Bessel beam with the focus far from the back of the lens – this is not possible with just surface features.

2. Can you let us know the damage threshold of these lenses?

I actually don’t know that. However, polymer optical waveguides used in the telecom industry support several watts of IR in a 10 micron core. So this level of power density is possible. It does take optimization of your polymer, particularly paying attention to photolabile species such as reaction fragments on the photoninitiator.

3. What are the polymer materials you are using when you copolymerize? Do you know what the Abbe of the GRIN is after copolymerization? What is the amount of time it takes to generate a sample that is 1cm3 using the raster scan method and also using the intensity pattern method?

a) We use a urethane matrix and acrylate monomers as the high-index writing species. Some of our conference publications have the formula and we are submitting several new journal articles which will discuss the formula and its response in great detail.

b) The Abbe number should be very close to that of polyurethane since this is 95% of the formulation, so roughly 35.

c) The raster scan system exposes lenses in about 1 second. We want this fast so that the material experiences the exposure as being essentially instantaneous (that is, fast in comparison to the response time of the material which is dominated by diffusion). I’m not quite sure what you mean by intensity pattern method, but if you mean exposure of a large area, then it’s limited by the dose required which is roughly 100 mJ/cm2. This works out to approximately a second, again.

4. What is the typical dn/dT for your materials?  How about the thermal coefficient of expansion?  Interesting talk.

a) Again, the physical properties are dominated by the “matrix” polymer which is 95% of the formulation. In this, that is polyurethane, which has a very high dn/dT, typically -5x10-4 per degree.

b) The CTE of these materials is high, typically several hundred ppm/degree C. This is because we are using them above their glass transition temperature. That is, they are rubbery. However, we have recently shown that the matrix can be cross-linked after development to raise the Tg (see D. P. Nair, N. B. Cramer, J. C. Gaipa, M. K. McBride, R. R. McLeod, R. Shandas, C.N. Bowman, “Two-Stage Reactive Polymer Network Forming Systems,” Advanced Functional Materials 2012, 1-9, 2012.) This will dramatically reduce the CTE to be typical of glassy polymers such as PMMA.

5. Where can you buy appropriate polymer for photochemistry and what is the cost per liter?

Dupont and, recently, Bayer Materials are the only commercial suppliers I know. They supply materials in films which are appropriate for holography. We formulate our own materials from commercially available components (e.g. Sigma Aldrich). Some of our conference papers have the full formulation and we are about to publish journal articles that will go into some detail. A simple analytic chem lab with balances, hot plates and vacuum jars is sufficient to make your own materials.

6. Is it possible to fabricate plano-plano RGRIN lenslet arrays using your process?

Yes, all of the lenses I showed were plano/plano.

7. When is it better to create/use holographic optical element vs. designed GRIN ?

That’s a big question, starting with what you mean by “better”. HOEs can be stamped at low cost and, as a surface feature, can be lightweight and cheap. But they are inherently dispersive and so have challenges in broad band applications. I would say that HOEs, surface curvature and GRIN are all elements in the designer toolbox. The choice of when to use one or more of them depends strongly on the cost, performance and manufacturability requirements of the design. The goal of my talk was to introduce the idea of polymer GRIN as a new element in that toolkit.



Patrice Genevet
Research Associate
Professor Federico Capasso's Group
Harvard University





Photonic Metasurfaces

Recent advances in nanotechnology have stimulated the development of new approaches across many major disciplines including photonics, energy harvesting and biophysics. Genevet will give an overview of the recent contributions of Federico Capasso's group to the field, with special emphasis on the development of ultrathin nanostructured optical components. The wavefront of light can be controlled without relying on gradual phase shifts accumulated during propagation (as in the case of classical refractive materials such as lenses or prisms), but instead with abrupt phase discontinuities introduced into the light path over the scale of a wavelength. Interfaces decorated by resonant nanostructures to generalize the classical laws of reflection and refraction will also be considered. The versatility of these interfacial phase discontinuities can be exploited to create ultrathin devices that focus light, function as waveplates, and even generate optical field with complex wavefronts.

1. Seems like the fill factor on your antennae might be less than unity; what are physical limits to the scattering efficiencies for your m=1 versus incident field (function of polarization, etc.)?

The filling factor is indeed an important parameter. The more you compact the elements, the higher will -in principle- be the overall scattering efficiency of your interface. I said in principle because when the antennae start to be too close to each other, the scattering properties are modified compared to a single and isolated element. You have to find a good compromise between filling factor and near-field coupling. Of course one can increase the filling factor and calculate the response when the elements are strongly coupled but the design of the whole interface becomes more complex.

2. What is the resolution of an optical system made with the V antennas at 8 microns?

I am not entirely sure that i understand the question since in general the resolution refers to how you can resolve the details in an object after being imaged using, for example, a lens (which we didn't fabricate for the mid-IR).

But let's say: The resolution in phase is in principle be very accurate since you can design an antenna to have a very specific scattering phase response. The fact that the elements are sub-wavelength, you can address phase jumps with sub-wavelength transverse separation which to me is impossible to achieve with classical refractive materials. So now the question is, what can we do with such an accurate control of the phase ? Well, the future will tell us whether there are or not applications which require sub-wavelength phase addressing.

Now, in the near IR, the flat lens that we fabricated has the same resolution as any refractive lenses which the same NA and the same focal distance. If you like, we just compacted the effect of thick classical devices within only 50 nm. In terms of optical throughput, the classical elements are up to now much more efficient than our metasurfaces.

3. What gradient geometries are you capable of producing? Axial, Radial, Spherical, Parabolic, etc?

You can arrange the elements along the interface to create any type of phase and eventually amplitude profiles. Since we designed each element individually i.e. we calculated the response as if each element was alone along the interface, you can arrange them one by one to create any type of phase distributions.

So, i would say, you basically can do whatever you have in mind !!! Of course as long as the elements don't interact too much (see question 1).

4. When is it better to create/use holographic optical element vs. designed GRIN ?

It depends what type of applications you are targeting. If you have little signal and you need to use efficient devices, you definitely want to use a more conventional approach such as refractive materials of GRIN materials. If you are willing to lose some of the incident optical power, but you are worried about the thickness and the weight of your system (for example for embedded systems), then you might want to consider using ultrathin devices based on phase discontinuities. The latter have another important advantage: we can account, in a single interface, for the phase retardation of several optical components.



Bios:

Dr. McLeod's research group at the University of Colorado specializes in the interaction of light and organic materials with applications to lithography, integrated optics, computational imaging and cellular biology. He has held research and management positions at Lawrence Livermore National Lab, Siros Technologies (a holographic data storage startup) and JDS Uniphase. He holds graduate degrees from Montana State University (MSEE 1985), University of California (MS Applied Science 1989) and the University of Colorado (PhD 1995).

Patrice Genevet was born in Nice, in France, in 1982. He received his PhD degree in Physics from the university of Nice-Sophia-antipolis, France, 2009. During his PhD, he demonstrated that two coupled micro-resonators can operate as a cavity soliton laser. He shows that such devices are able to emit simultaneously several localized and bistable sources of coherent radiation, the cavity solitons, and that each of these are independently controllable with an external optical beam. He also showed that localized structures can bifurcate towards self-sustained and independent optical vortices which feature four among the most intriguing properties of light (all of them self-sustained): bistability, spatial localization, coherence and vorticity.

In 2009, he joined the Capasso's group at Harvard University in collaboration with Prof. M.O. Scully in Texas A&M University to work on nonlinear plasmonics and metasurfaces. By designing nano-antennas disposed at an interface, he and his co-workers developed a new approach to manipulate the light, controlling the phase, the amplitude and the polarization of a beam without relying on propagation effects. Since 2011, he is research associate at Harvard University in the group of Prof. Capasso. His research interests include semiconductor lasers, nanophotonics, plasmonics, metamaterials, metasurfaces, nonlinear optics and nonlinear dynamics.
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