Saturday, June 25, 2011

Invisibility Cloaks – when light images nothing

Fig. 1. The blue stuff is the polymer
and golden stuff is gold used to SEM
image the structures. The upper one as
they point out is for reference while the
polymer in the lower rectangle contains the
3D features that give invisibility.
Ref: Joachim Fischer, Tolga Ergin, and Martin
Wegener, "Three-dimensional polarization
-independent visible-frequency carpet
invisibility cloak,"
Opt. Lett. 36, 2059-2061 (2011)

Invisibility cloaks are always interesting; no matter how they do it. Some people use ideas for camouflage like imaging and then displaying the background behind the invisible person on the cloak, effectively rendering the individual see-through.  Others use optical metamaterials.

Metamaterials are artificial materials that could be made to demonstrate un-natural properties like a negative refractive index, unusual optical frequency tuning capabilities, non-linear properties, etc. Optical metamaterials used to obtain invisibility are made up of sub-wavelength structures that bend light away from the object so as to render it “invisible”.

Earlier work on metamaterials started off at microwave frequencies. For optical wavelengths much finer nano-structure was needed. Soon the technology progressed to optical frequencies, but provided invisibility only along 2 dimensions. So if you looked along the 3rd dimension, the person wouldn’t be invisible. Then last year someone managed to do it for 3D. But it was still in a restricted range of wavelengths of 1000s of nanometers.

But I recently read articles here and here, that report a group at Karlsruhe Institute of Technology's Center for Functional Nanostructures (CFN) that have managed to write small enough features in their cloak that they can now guide light around it. Their team uses a combination of Stimulated Emission Depletion (STED) with direct laser writing to produce the polymer based, fine 3-D nano-structure. Their cloak works in non-polarized red light.

I realize one gets a lot of coverage for research like this. But we rarely hear about the effort. There is a lot of detail involved in work like this.. math, derivations, calculations, fabrication, chemistry, SEMs, microscopy.. Here is their paper in Optics Letters (finally out) for more detail. They show angle dependence, wavelength dependence, comparisons with ray-tracing simulations...

Fig. 2. This figure taken from their paper demonstrates the visibility
and invisibility of the vertical bars in the center of each rectangular
block for different wavelengths. The bars are visible in the reference
(upper rectangles) and invisible in the cloaked rectangles (lower ones).
Ref: Joachim Fischer, Tolga Ergin, and Martin Wegener, "Three
-dimensional polarization-independent visible-frequency
carpet invisibility cloak," Opt. Lett. 36, 2059-2061 (2011)

All that effort condensed into a 3 page easy-to-read optics-letter! But this is really the kind of work engineers and scientists do.. and love to do! So it's way cool to see coverage of the kind of work we love! :) Check it out! (If you can.. I wish every research paper were open and unlocked.)

Three cheers for all the good news coverage scientists and engineers get! And some more cheers for absolutely every individual involved in the effort!

Friday, June 17, 2011

Imaging Rods In-vivo

Rods are the retinal cells that we use to see in dim light conditions. They are much smaller than cones which are used to see during the day and in bright light settings. The Williams lab at the University of Rochester pioneered the use of Adaptive Optics to image and resolve cone photoreceptors, blood vessels, retinal pigment epithelial cells and ganglion cells. They have shown much work in the areas of color vision, light sensitivity and disease detection. But rods were hard to image, and when then were seen there was much jubilance.

Recent work by the Rochester team shows repeatable imaging of rods in the human retina! How did they do it? Better design! Alf Dubra’s design has reduced astigmatism in both the pupil and image planes for better adaptive optics wavefront correction and improved imaging performance. They have two papers (here and here). The first shows a lot of good imaging results. The second one is super for system details.

The Williams Lab is where I did much of my PhD research. So any fun stuff from there is always interesting to me! And an actual online video with interview and all!!! Now that I simply have to put on my blog! :)

Friday, June 10, 2011

Maskless Lithography

One of the benefits of living in Silicon Valley is the number of events and activities in the area. One recent addition to my regular talk schedule is the monthly meeting held by the Northern California Chapter of OSA.

Last month they had invited Eric Hansotte from Maskless Lithography to give us a talk. When I looked him up online (of course that's what everyone does).. the first thing that showed up was - this Lasers Rock concert at CLEO/QELS in 2010! A guitar-playing scientist certainly rocks!

It was cool to meet Eric over dinner and listen to him speak about his work at Maskless Lithography. ML makes direct-write digital imaging lithography products for the PCB manufacture industry. Digital Lithography eliminates the need for a mask.

At ML they use a Digital Micromirror Device (DMD) based system which performs multiple exposure scans of the PCB, producing an effective gray-level "dosing" of the exposures. Stronger exposures have a tighter impulse response whereas low level exposures have a broader impulse response. These widths control the exposed feature width and lets them obtain features with finer spacing.

What is interesting is that a super-sharp impulse response would not provide enough variation in its base width (at different gray-levels) for this technique to work. So they actually prefer some amount of blur in their system. 

Good resolution + good speed + flexibility to update the pattern on a whim! Now who might own a projector we can take apart...

Hello World!

I am Sapna Shroff. I'm a Research Scientist at Ricoh Innovations Inc., Menlo Park, CA, a research lab for Ricoh Company Limited, Japan. I graduated from the University of Rochester and work in optics and imaging. I was recently asked by OSA to consider writing a blog. So here we are! I hope to write about Optics and Imaging, experiences working in the field, general news, and fun stuff. Let's see how this goes.

Hope your time spent here is fun. Else click away on one of the links on my blogroll! ;)