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Engineering & Science California Insriture of Technology Engineering & Science Spring 1994 In this issue Plastics that Bend Light A Brie/History o/OVRO India's Forgotten Army Caltech's first radio observatory begins to take shape in the Owens Valley in the late 1950s. One of the 90·foot antennas of the interferometer stands virtually complete in the bakground, while the pedestal of the second waits to grasp in its thumb·and· forefinger-like mount the other dish, whose frame is just getting started in the fore· ground. California Institute~ of Technology Engineering & Science Spring 1994 Volume LVII, Number 3 2 1\ny Color You Like - by Douglas L. Smith Some crystals change color or bend light in response to an electric field. A Caltech-JPL-industry collaboration uses basic chemistry to design plastics that do the same thing, only faster. 8 The Owens Valley Radio Observatory: Early Years - by Marshall H. Cohen The U.S. lagged behind Europe and Australia in radio astronomy until the late 1950s, when Caltech erected two giant dishes at the foot of the Sierra Nevada. 24 Farrer Park - by Peter Ward Fay Backed by the Japanese, an Indian army, now largely forgotten, fought the British during World WarH. Though it was crushed on the battlefield, it hastened the coming of independence. Departments 32 Lab Notes: But Only Dogs Can Hear It; The Gamma Gambit 39 Books: The Strands ofa Life by Robert 1. Sinsheimer 40 Letters On the cover: These brightly colored vials are filled with solu· 42 Random Walk tions of synthetic organic dyes. These molecules' elecfronic Engineering & Science (ISSN 0013-7812) is published quarterly, William M. Whitney structures allow them Fall; Winter, Spring, and Summer, at the California Institute of President of the Alumni Association to absorb certain Technology, 1201 East California Boulevard,Pasadena, California 91125. Annual subscription $8.00 domestic; $20.00 foreign air Thomas W. Anderson wavelengths of light Vice President for Institute Relations very strongly, giving mail; single copies $2.00. Third class postage paid at Pasadena, California. All rights reserved. Reproduction of material contained Robert 1. O'Rourke them their intense herein forbidden without authorization. © 1994 Alumni Associa­ Assistant Vice President for Public Relations colors. Sometimes tion, California Institute Of Technology. Published by the these structures also California Institute of Technology and the Alumni Association. make the molecules Telephone: 818-395-3630. Postmaster: Send change of address to interact with light in Caltech 1-71, Pasadena, CA 91125. considerably more PICTURE CREDITS: Cover, 2 - Bob Paz; inside front STAFF: Editor-Jane Dietrich unusual ways that. cover, 15, 16, 18 - John Bolton; 4 - Doug Smith; 5-7 - Managing Editor - Douglas Smith may be vital to the Seth Marder; 10 - R. W. Porter; 12 - Tom Harvey/Caltech Copy Editors - Michael Farquhar, fiber·optic information superhighway of the Archives; 13 - Caltech Archives; 13, 16 - Dick Read; 17- Danielle Gladding, Julie Hakewill future. Story on U.S. Navy; 26-28,31- Prem Sahgal; 32 - Linda Weavers; Production Artist - Barbara Wirick page 2. 33, 34, 38 - Michelle Owen; 36 - Sky & Telescope, Eric Business Manager - Debbie Bradbury Wemhoff and Al Ratner; 37, 44-NASA; 43 - White Circulation Manager - Susan Lee House; inside back cover - Paul Chodas and Don Yeomans Photographer - Robert Paz At!y~olor You Like On the fiber- optic lanes of the information superhighway, nonlinear-optic materials may by Douglas L. Smith one day provide the off ramps and cloverleaf inter­ changes. If you shine a red light through a pane of But if the molecule is already polarized, glass, it comes out red. But shine that same applying an electric field in the direction of light through a nonlinear-optic material, and it polarization can shove the electrons a considerable might come out blue. Or it might come out red, distance. Applying the same field in the opposite but at a funny angle. A nonlinear-optic material direction pushes the electrons against the tide, modifies light passing through it in a way that and they won't move nearly as far. Now the plot depends on the light itself, among other things. of net polarization versus the applied field is steep On the fiber-optic lanes of the'information super­ in one direction and flat in the other-a nonlin­ highway, nonlinear-optic materials may one day ear, asymmetric response. This effect, called provide the off ramps and cloverleaf interchanges, "hyperpolarization," is relatively weak, becoming routing data-laden pulses of light to their proper noticeable only in the presence of strong fields. destinations. Seth Marder, a member of the tech­ There, however, the effect becomes quite power­ In Marder's lab, nical staff at Caltech's Jet Propulsion Laboratory ful, since it varies with the square of the field's nonlinea.... optical compounds are and a member of the Beckman Institute on strength. Only molecules lacking a center of purified by running campus, and Joseph Perry (PhD '84), leader of symmetry are hyperpolarizable in this manner­ them through silica· the optoelectronic materials group at JPL and a a symmetrical molecule would have an equal gel·filled columns like this one. visiting associate at the BI, lead a group that has desire to polarize in either direction. (This kind created a whole new class of substances with the of hyperpolarization is what's called a second­ highest nonlinear-optic properties yet found. order effect-there are other, higher-order Light is an electromagnetic wave, so it's no hyperpolarizations beyond our scope here.) surprise that it interacts with electrically charged The interplay of polarization and hyperpolar­ things, including the electrons in a molecule. As ization has several useful consequences. You can light passes through a molecule, the molecule's bend light by manipulating the material's refrac­ electrons ride the waves like boats at anchor. As tive index. When a beam of light hits a molecule the electrons surge against their moorings, the at the edge of a chunk of matter and sets its elec- molecule becomes polarized-it acquires a slight . trons bobbing, the oscillating electrons them­ negative charge in the direction the electrons are selves generate an electromagnetic field at the moving, and a slight positive charge in the region same frequency as the original wave. The new they've vacated. In materials such as glass, the field travels to the next molecule, and the light electrons move back and forth with equal facility, propagates through the material. Each molecule and the distance an electron moves in either imposes a tiny time lag as its electrons respond­ direction is proportional to the wave's amplitude, in essence, the light slows down. This time lag, at least to a first approximation. Thus a plot of relative to that for light through air, is the mate­ polarization versus the applied field is a straight rial's refractive index. (This is why a straw stick­ line-a linear, symmetric response. ing out of a glass of water appears bent: light Engineering & Science/Spring 1994 3 fundamental frequ~l:lcYWith other frequencies, Water an effect that comes in v~ryhandy when working with lasers. A laser en:tifs light at one specific frequency that depeqds on what the laser is made of. But this frequehcy generall¥ isn't the one that a researcher needs, so nonlinear-optic crystals are An overhead view widely used to "tune" lasers to the right frequen­ Straw's Straw's of a glass of water. apparent real cy-say, a particular spectroscopic resonance of Light travels by the position position quickest path (solid some molecule. Outside the lab, surgeons need arrows), and since short-wavelength lasers to cut tissue, while light travels more longer wavelengths work better to cauterize slowly in water than in air, the quickest the incision. path is not the short· To date, all commercial nonlinear-optic mate­ est path (shaded rials have been inorganic compounds such as arrow). But we know Air that light travels in potassium dihydrogen phosphate and lithium straight lines, so we niobate. But their nonlinear effects aren't that see the straw in a I Il' ;Z large, whereas some organic molecules are highly different location from where it actually is. Observer polarizable and thus should show strong effeq:s. And while lithium niobate and its kin polariz~----' ~ in part by schlepping heavy ions back and forth, organic molecules polarize by whisking feather­ weight electrons around. Electrons are nimble travels one-third slower in water than in air. enough to keep up with the highest frequen­ Light reaches your eye by the fastest possible cies-a very big advantage, says Marder, when route rather than the shortest possible route, and you consider that the information superhighway's as a consequence takes the path that minimizes structural engineers want electro-optic switches the time spent traveling through the slower capable of operating 10 billion times a second. medium.) Applying a second, nonoscillating "If you tried to do that with an inorganic," says field slows the wave further-the electrons have Perry, "its efficiency drops way off. It becomes to fight harder to get back to their initial position very power-hungry." Organics have other advan­ to begin the next oscillation. As the second field tages-they can be formed into films and sheets, intensifies, the refractive index gets higher, until or molded into any shape you like; and best of all, eventually the light beam emerges from the the molecule's properties can be customized by . material in a radically different direction. Thus, altering its chemical composition. Lured by these laser signals could be shunted from one fiber­ promises, folks have been tinkering with organic optic line to another by applying a switching molecules for the past two decades.
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