Question

# Light and 10 nm gap

What would happen if a light wave is obstructed by 10 nm gap? would it pass through it? would it diffract?

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• Garen Kwan · Nanyang Technological University
Perhaps if you could set up the experiment in a drawing, it would help in giving a better answer, but based on what you have described, I think you are referring to the gap being in plane and parallel to the direction of propagation of the light wave. In that case, only thin film interference effects would occur, easily solved using the Fresnel equations.

But if you wanted diffraction, you would need the gaps to be spaced perpendicular to the direction of propagation of the light wave right? And what is the size of your gap? 10nm too?
• Tejabhiram Yadavalli · SRM University
dear garen, thank you for the explanation. Yes it is a question that raised in my mind when i was reading about optical lithography. It said when the diameter of the gap in the mask is less than half the wavelength of the light, the patterns would start generating diffraction defects on the photo resist. so suddenly i had this doubt as to what would happen if the propagating light were to be obstructed by something that is as small as 10 nm diameter hole. the best of answers that i found was that the particle nature of light exists till the diameter of the obstruction is more than half of the wavelength, but behaves like a wave once this barrier is broken.
• Garen Kwan · Nanyang Technological University
Ah, I see. The lithography effect is best described by Fraunhofer and (I can't remember the other name) diffraction. Now this occurs in the case where there is a gap between the mask and the resist as you have just described. I suppose that is what you are reading on right?

Now if the hole were 10nm in diameter, it wouldn't affect the photons going straight through, but in a real world experiment, there would be some light entering the hole from a non-perpendicular direction and those would result is some diffraction. So indeed, it would be possible to observe diffraction, and it would probably look like Fresnel's rings.
• Tejabhiram Yadavalli · SRM University
wow.. thats nice. That is exactly what i was reading. beautiful phenomenon.
• Josep Canet-Ferrer · University of Valencia
Hello Tejabhiram,
I think Garen gave you a good answer, but may be would you like also read a bit about near-field optics. Notice it is hard to simulate of the scattering of light in front of sub-wavelenght aperutre, but for the case of well defined geomentries asround appertures it could be well known.
• Martin Cryan · University of Bristol
Hi, this is a really interesting problem that I don't think is fully understood yet. Below are some people who have done experiments for similar problems.

There is a connected area known as optical nanoantennas - good example paper attached. We're looking at using these for improving solar cells and optical nanosensors.

http://www.bris.ac.uk/eeng/research/pho/people/mjc.html

Finite Difference Time Domain modelling is one way to analyse these types of problems : http://ab-initio.mit.edu/wiki/index.php/Meep

Regards

Martin

Extraordinary optical
transmission through
sub-wavelength hole arrays
T. W. Ebbesen*†, H. J. Lezec‡, H. F. Ghaemi*, T. Thio*
& P. A. Wolff
NATURE |VOL 391 | 12 FEBRUARY 1998

Analysis of the transmission process through
single apertures surrounded by periodic
corrugations
A. Degiron and T.W. Ebbesen
9 August 2004 / Vol. 12, No. 16 / OPTICS EXPRESS 3694

Optical Antennas, Palash Bharadwaj, Bradley Deutsch, and Lukas Novotny, Advances in Optics and Photonics 1, 438–483 (2009) doi:10.1364/AOP.1.000438
If the 10nm hole is like a 'tube' instead of in a 'thin disc', the light will not pass through at all. It will probably induce some plasmon effect at the entry point. If it is a 10nm hole in a thin disc, the thickness of the disc being in 30-100nm, then again surface plasmon effect can be seen. Again the material of the disc (or tube) will matter. The diffraction effects you are observing on the photoresist are probably due to surface plasmons. Here are some papers you will find useful.

(1) "Extraordinary optical transmission through sub-wavelength hole arrays" Nature 391, 667-669 (12 February 1998)
(2) "Direct evidence for surface plasmon-mediated enhanced light transmission through metallic nanohole arrays" Nano Lett. 2006 Sep;6(9):2104-8
• Raj Kumar · Central Scientific Instruments Organization
Hi,
Hope you got some direction from above discussion. In case of sub-wavelength apertures diffraction of light indeed takes place and it is a combined effect of many processes like surface plasmon polaritons, diffractive scattering at the aperture edges, tunneling etc. This is a quite hot topic for researchers and not fully understand yet. In this case it is not expected to generate separate observable diffraction fringes and only single fringe will be formed (i.e. almost uniform intensity will be on the other side. This minima (darkness) or maxima (brightness) depends upon the path matching condition for the incident wavelength. This is also dependent on nature of aperture material, properties of incident light, thickness of the aperture strips.
• Tejabhiram Yadavalli · SRM University
Wow..thanks sir. The above papers were really useful.
• Mircea Pop · Babeş-Bolyai University
If light reaches an obstacle, among others phenomene, it suffers dffraction. The difraction region is comparable to the wavelenght of the radiation. If one has a slit and the light passes through it, it suffers diffraction at both edges of the slit. If the slit is smaller than the wavelenght the diffraction paterns become predominant and the light doesn't seem to act as a continuum ray but as a common wave. So, even if diffraction happents in all cases, when the dimmension of the obstacle becomes comparable (in dimensions) to the wavelenght, diffraction becomes significative. For a 10 nm gap you can't get a "shadow" in visible light - you can only get a very strange diffraction patern.

#### Contributors (7) See all

• ##### Raj Kumar
Central Scientific Instruments Organization