Our Plans

1. Our Plans • Today : Review of Science & technology of Light • Dec 8: Review of the Exam #3 material (ch. 9,10,13) • (additional office hours – web page) • Dec. 10: Exam #3 (exam scores & preliminary grades will be posted on Dec. 12); • Dec. 15, 5PM: Extra credit Project Deadline, accepting projects, G1B20, 1:30-4PM; Dec. 18: Final grades;

2. Light at Work • Invisibility: • Is this possible? Yes!!! • How it works & when we can buy our invisibility clothes? • Energy from light: • Solar cells & solar cell paints; • Lasers: • What they are & how they work; • Laser tweezers: moving things with light without touching; • Laser applications: science, technology, & everyday life; • Holography & Diffraction Gratings

3. Is this possible??? http://www.youtube.com/watch?v=JKPVQal851U

4. Invisibility cloaks made of metamaterials http://www.youtube.com/watch?v=Ja_fuZyHDuk http://www.telegraph.co.uk/scienceandtechnology/science/sciencenews/3353461/Harry-Potter-invisibility-cloak-a-step-closer-to-reality.html

5. How the cloaks of invisibility work: Recall that we did not see the glass rod immersed in vegetable oil because there was no reflected light from the glass-oil interface First cloak of invisibility demonstrated to work at a particular wavelength of light No rays reflected from the cloak-surrounded object - it can not be seen & is invisible

6. Demonstrate to work at wavelength larger than that of visible light so far

7. Negative refraction air n>0 Unusual bending of rays of light n<0

8. Where would we see the fish if water had negative refraction index Note that the Snell’s law of refraction still works at such interfaces

9. Wave propagation through negative-index medium & air interface Recall the “Army of marching soldiers” analogy of bending light

10. Structured metamaterials that can achieve negative refraction Model Image of a nano-fabricated material The size of these features has to be much smaller than the wavelength of light

11. Transforming solar energy to electricity Solar Cells

12. solar cells for energy by converting sunlight directly into electricity. The sun radiates ~1000W per square meter (see the map), so a 10 x 10 cm solar cell is exposed to nearly 10 watts of radiated power. Depending on the quality of the cell, it can produce an electrical output of 1 - 1.5 watts.

18. Light shining from this side:

19. Principal scheme of a solar cell A photovoltaic cell comprises P-type and N-type semiconductors with different electrical properties, joined together. The joint between these two semiconductors is called the "P-N junction.“ Sunlight striking the photovoltaic cell is absorbed by the cell. The energy of the absorbed light generates particles with positive or negative charge (holes and electrons), which move about or shift freely in all directions within the cell. The electrons (-) tend to collect in the N-type semiconductor, and the holes (+) in the P-type semiconductor. Therefore, when an external load, such as an electric bulb or an electric motor, is connected between the front and back electrodes, electricity flows in the cell.

20. Construction of a typical solar cell

21. How it works? Photovoltaic technology is actually quite simple: The conventional solar cells comprise two adjoining semiconductor layers that are equipped with separate metal contacts and have each been doped, thus creating an “n” layer (n = negative) with a surplus of electrons and below that, a “p” layer (p = positive) with an electron deficiency. Due to the difference in concentration, the electrons flow from n into the p area, thus creating an electrical field, or “space charge zone”, inside the semiconductor structure. The Photovoltaic Effect The upper “n” layer in a solar cell is so thin that the photons from sunlight can penetrate it and can only discharge their energy to an electron once they are in the space charge zone. The electron that is activated in this manner follows the internal electrical field and thus travels outside of the space charge zone and reaches the metal contacts of the “p” layer. When an electrical load is connected, the power circuit is closed: the electrons flow across the electrical load to the solar cell’s rear contact and then back to the space charge zone. This effect is called the “photovoltaic effect” (derived from ‘‘Phos’’, the Greek word for light and the name of the physicist Alessandro Volta). An inverter, the “heart” of the system, converts the direct current (DC) produced by the solar cells into alternating current (AC).

22. Laser vs. other light sources • Rays: Waves: Laser Laser Flashlight Flashlight Light bulb Light bulb

23. Principal components & how lasers work 1. Gain medium2. Laser pumping energy3. Mirror (100% reflection) 4. Output coupler mirror (98-99% reflection);5. Laser beam

24. Light at Work • The term “LASER" is an acronym for Light Amplification by Stimulated Emission of Radiation. • Laser light is spatially coherent: either emitted in a narrow, low-divergence beam, or can be converted into one with the help of lenses. • Lasers are emitting light with a narrow monochromatic wavelength spectrum. • Laser in a research lab:

25. Compare different light sources: Intensity-distribution curve of light from a incandescent lamp Intensity-distribution curve of light from a White fluorescent tube Laser

26. Holography • Use monochromatic (single-color) light source; • Record information about both amplitude and phase of the light creating an image

27. Holography: an experimental setup There are holograms on most driver's licenses, ID cards and credit cards. They're two-dimensional surfaces that show absolutely precise, three-dimensional images of real objects (unlike regular images).

28. Recorded image (similar to a diffraction grating): This is what we see: Photograph of a hologram in front of a diffuse light background - 8x8mm

31. Holography: principles & examples Holography is a photographic process which does not capture an image of the object being photographed, as is the case with the conventional technique, but rather records the phases and amplitudes of light waves reflected from the photographic film. The phases are recorded as interference patterns produced by the reflected light and a reference coherent light (from the same laser). Each point on the hologram received light reflected from every part of the illuminated object and, therefore, contains the complete visual record of the object as a whole. When the hologram obtained from the development of a film exposed in this way is placed in a beam of coherent light, two sets of strong diffracted waves are produced - each an exact replica of the original signal bearing waves that impinged on the plate when the hologram was made. One set of diffracted waves produces a virtual image, which can be seen by looking through the hologram. It appears in a complete three-dimensional form with highly realistic perspective effects. In fact, the reconstructed picture has all the visual properties of the original object.