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P r o f e ss o r A t w at e r ’ s g r o u p i s in t e r e s t e d in e l e c t r o n ic a ndp h o t o n ic mat e r ia l s f o r u s e in f u t u r e f u n c t i o n al d ev ic e s .D e v ic e mat e r ia l s r e s ear c h is i n t e r d is c i p l in ar y , in v o l v in gt h e o r e t i c a l an d ex p e r ime n t a l i s s u e s s p an n in g ap p l ie d p h y s i c s ,p h y s ic s , mat e r ia l s s c i e nc e , c h e mis t r y , an d e l e c t r ic al an dc h e mic al e n g i n ee r in g .

Nanostructure Electronics and Photonics

In the mesoscopic size regime, materials have size-

tunable properties intermediate between those of single

atoms and bulk solids. We are studying group IV semi-

conductor (Si and Ge) nanocrystals that behave electroni-

cally as ‘quantum dots,’ including nanoscale synthesis,

interface passivation, excited state decay, and localized

state carrier transport. We have also recently developed a

Si nanocrystal memory that is one of the highest perfor-

mance nonvolatile memories created to date. At the

nanoscale, optical materials are dominated by near-field

interactions. Electromagnetic energy transport can occurbelow the diffraction limit in nanoscale waveguides called

“plasmon wires” that consist of chains of closely spaced

metal particles yielding structures with optical functionality

that cannot be obtained in other ways at a length scale

<< 1 micron. Currently we are fabricating nanoscale plas-

mon waveguides and assessing their photonic perfor-

mance with near-field optical microscopy.

Photovoltaics

We are exploring two approaches to photovoltaics, the

direct generation of electric power from sunlight, including

designs with either ultrahigh efficiency or very low cost.

Advances in semiconductor wafer engineering enable us

to create structures with potential to achieve world-record

energy conversion efficiency (40–50%) in AlInGaP/

GaInAsP/InGaAs/Ge heterostructure solar-cell designs. To

achieve very low-cost cell designs, a crystalline silicon thin

film (1–30 microns) is grown at low temperatures with

large grain sizes on inexpensive substrates.

Active Oxide Materials

Epitaxial complex oxide films of BaTiO3 and related mate-

rials have interesting electromechanical and photonicdevice possibilities related to their piezoelectric, ferro-

electric, and photorefractive properties. Integration of

these materials in single crystal form with conventional

electronics is difficult. A new approach based on oxide film

growth on biaxially textured pseudosingle crystalline

magnesium oxide (MgO) templates enables a path to

integration of epitaxial oxide films on amorphous

substrates such as glass.

http://daedalus.caltech.edu/

Lef t image : t ransmiss ion e lec t ron m icrograp h o f s i li -

con nanoc rys ta l ar ray tha t fo rms t he f l oa t ing g a te o f

the nanoc rys ta l nonvo l a t ile m em ory dev i ce dep ic ted

schemat ica l l y a t upper r igh t . Nanocrys ta ls a re

fo rmed v ia gas -phase nuc l ea t ion and g row th as an

u l t ra f ine aeroso l tha t i s subsequent l y depos i ted in

the dev ice ’s ac t i ve reg ion . Us ing th is approach, we

have fab r ica ted nanoc rys ta l mem or ies w h ich a re

amo ng the h ighes t pe r fo rmanc e nonvo l a t ile mem ory

s t ruc tu res deve loped to da te .

M A TE R IA L S a n d S TR U CTU R ES f o r N A N O D E V IC E S : B E YO N D V L S I

HARRY ATWATER



We have been exploiting the relationship between solar

prominence and spheromak physics in laboratory experi-

ments. These experiments involve specially designed plas-

ma guns that eject multi-megawatt pulses of magnetized

plasma into a large vacuum chamber. The magnetic forces

driven by the tens or hundreds of kiloamperes of current

easily overwhelm all other forces (e.g., gravity, hydrody-

namic pressure) so that the plasma assumes a shape dic-

tated by the three-dimensional interaction of topologically

complex magnetic fields. The solar prominence simulation

and spheromak experiments differ mainly in the prescribed

symmetry of the initial magnetic field and current flow

pattern.

The solar prominence simulation experiment takes place

at a billion-fold reduction in both time and space scales

compared to solar eruptions (microseconds versus min-

utes and tens of cm versus 100,000 km). Nevertheless,

the laboratory simulation replicates the solar dynamics

because the relevant dimensionless parameters and topol-

ogy are similar. Erupting twisted, arch-shaped plasmas

have been produced and their dynamics have been inves-

tigated to provide insights regarding solar eruptions.

The spheromak experiment involves an advanced coaxial

magnetized plasma source that creates vortices of magne-

tized plasma in a controlled manner. Unlike previous

spheromak sources, the new source has a planar geome-

try which provides greater efficiency and also makes diag-

nostic access more straightforward. The planar spheromak

has topology and dynamics analogous to the accretion

disks associated with black holes; we plan to investigate

this relationship.

The plasmas in both experiments are diagnosed using

intensified gated CCD cameras with nano-second shutter

speeds, laser-induced fluorescence, magnetic probes, and

other advanced methods. Associated theoretical activities

are also underway. These include three-dimensional

numerical modeling of solar eruptions and investigations of

the excitation of magnetohydrodynamic waves (Alfven

waves) that occur when the plasma abruptly lowers its

magnetic energy by changing its magnetic topology.

http://ve4xm.caltech.edu/Bellan_plasma_page/

Pau l B e l l an an d h is g r o u p a r e s t u d y in gt h e 3 D t o p o l o g ic a l d y n amic s o fma g n e t i ze d p l a s ma s . Th i s s t u d y is r e l e v an tt o t w o r ad ic a l l y d i f f e r e n t ap p l i c a t i o n s

t h at , c u r io u s l y , ar e g o v e r n ed bye s s e n t i al l y t h e s a me p h y s ic s . Th e f i r s tap p l ic at i o n is t h e e r u p t i o n o f s o l arp r o min e nc e s , h u g e t w is t e d p l as ma-f i l l e dmag n et i c f l u x t u b es t h at p r o t r u d e f r o mt h e s u n ’ s s u r f a c e . No t o n l y ar e t h e s ee r u p t io n s a n in t r ig u in g my s t e r y int h e ms e l v e s , b u t t h e y a l s o a r e o fimp o r t a nt p r ac t ic al s ig n i f i c an c e b e c au s et h e p o w er f u l p l as ma b l as t s t h e y e j e c tc an , o n r ar e o c c as io n s , d amag e o rd e s t r o y s pac e c r a f t a nd in d u c e l a r g ec u r r e n t s in t e r r e s t r ia l e l e c t r ic a l p o w e rg r id s . Th e s e l ar g e c u r r e n t s c an c a u s e

g r id f a i l u r e p l u n g in g mi l l io n s o fp e o p l e in t o d ar k n es s . Th e s e c o n dap p l ic at io n i s t h e d e ve l o p me n t o ft h e s p h e r o mak c o n c e p t , a 3 Dv o r t e x -l ik e l ab o r at o r y p l as mac o n f ig u r at i o n t h at s e l f -o r g a n iz e so u t o f t u r b u l e n t i n s t ab i l i t i e s . Th es p h e r o mak o f f e r s a p o t e n t ia l l yat t r ac t i v e an d l o w -c o s t me t h o df o r c o n f in in g t h e h i g h - t e mp e r at u r ep l as ma r e q u ir e d f o r c o n t r o l l e dt h e r mo n u c l e ar f u s io n .

Labora tory s imula t ion

o f so l a r p romi nence .

Spherom ak in it i a t ion

us ing p lana r mag ne -

t ized p lasma so u rce .

3 D TO P O L O GIC A L D YN A M I C S o f M A G N E TIZ E D P L A S M A S

PAUL BELLAN



S chema t i c d raw i ng o f the A RC S spec t rom e te r p ropo sed fo r

construc t ion a t the Spa l la t ion Neutron Source. Red gr id

ho l ds de tec to r tubes , open d oo rs tow a rds r igh t show sca le .

Fu ltz i s the pr inc ipa l invest iga to r on a p ropo sa l to bu i ld the

ine las t i c neutron ins t rument dep ic ted in the f igure .

t h e D YN A M I C S a n d A R RA N G E M E N TS o f A TO M S

BRENT FULTZ

Most of what we know about atom arrangements in materi-

als comes from diffraction measurements, where an inci-

dent plane wave is directed into a sample and the angles

and intensities of the out-going diffracted waves are

detected. Fultz’s group is exploring the novel method of

γ -ray diffraction, in which an incident γ -ray is absorbed by

identical nuclei in a crystal. The decay of this nuclear

excitation creates a new γ -ray photon with the angular

distribution of a diffraction pattern. The physical process of

scattering is fundamentally different from that of x-raydiffraction, and therefore offers new opportunities for

studies of the atom arrangements in materials. For exam-

ple, we are using the chemical spectroscopic information

of Mössbauer spectra to control the phase and intensity of

wave emission from selected nuclei in a sample.

A major thrust of our research is investigating how atom

vibrations affect the entropy and thermodynamic stability of

materials. The concept of “vibrational entropy” is new to

the materials science community, and its importance was

unexpected. Our group is now measuring phonon spectra

of materials by inelastic neutron scattering to understand

the reasons for differences in vibrational entropy of differ-

ent solid phases. Recent studies have identified effects on

vibrational dynamics from chemical disorder and local dis-

tortions around impurity atoms. The field of inelastic neu-

tron scattering is a broad one, and a field that will grow in

the United States with the construction of the Spallation

Neutron Source.

http://www.caltech.edu/~matsci/btf/Fultz1.html

Th e g r o u p h e a de d by B r e n t Fu l t z i s s t u d y in g t h ea r r an g e me n t s an d d yn ami c s o f at o ms in mat e r i al s b ys c at t e r in g me t h o d s u s in g x -r ay s , e l e c t r o n s , n eu t r o n s ,an d γ -r ay s . C o h e r e n t e l as t i c s c a t t e r in g , f o r e xamp l e ,r e v ea l s at o m ar r an g e me n t s . C o h e r e n t in e l as t i cs c at t e r in g p r o v i de s t h e e n er g y -w av e l e n g t h r e l at io n s h i p so f e l e me n t ar y e xc i t at io n s i n s o l id s . Id e nt i f y in g t h ep o s i t i o n s o f at o ms an d t h e ir mo v e me n t s i s a g e n e r alt h e me f o r Fu l t z an d h is g r o u p .



Th e g o al o f t h e r e s e ar c h i n Pr o f e ss o r Go d d ar d ’ s g r o u p (t h eM at e r ia l s an d P r o c e s s S imu l at io n C e n t e r o r M S C in t h e B e c k manIn s t i t u t e ) is “ d e no v o ” o r f i r s t p r in c ip l e s e n g in ee r in g . Th e y de v e l o pt h e o r y an d s imu l at i o n t o o l s t o p r e d ic t t h e f u n d ame n t a l p r o p e r t i eso f mat e r ia l s an d o f man u f ac t u r in g p r o c e s s e s . To c o n ne c t t h ee no r mo u s r an g e o f s c a l e s f r o m an g s t r o ms t o yar d s an d f r o mf e mt o s e c o n d s t o y e ar s , t h e y u s e a h i e r ar c h y o f s imu l at io n s w h ic hs t a r t w i t h q u a n t u m me c h a n ic s ( e l e c t r o n s ) , an d mo v e t h r o u g h as e q u e nc e o f s u c c e s s iv e l y c o ar s e r l e v e l s , e ac h u s in g p ar ame t e r san d c o n c e p t s av e r ag e d f r o m t h e f in e r l e v e l s . H is s t u d e n t s o b t a ind e g r e e s in c h e mis t r y , mat e r ia l s s c i e n c e , p h y s ic s , ap p l ie d p h y s i c s ,c h e mic a l e n g in e er in g , b io c h e mis t r y , an d b i o l o g y .

Our goal is to describe the properties of chemical, biologi-

cal, and materials systems directly from first principles

(without empirical data). Our strategy is to build from

quantum mechanics (QM) to practical engineering design

and processing through a hierarchy of more approximate

methods suitable for longer length and times scales as

indicated above, including molecular dynamics (MD) and

mesoscale dynamics, to connect ultimately to macroscopic

dynamics.

Our research is equally focused on developing new meth-

ods and on applying these methods to applications impor-tant in the industrial and commercial sectors. Our method

development focuses on extending the methods of QM

and MD to higher accuracy on larger systems, on develop-

ing the connections from QM to MD that describe reac-

tions, and on averaging atomic quantities to describe the

meso scale. The new methods, validated by application to

problems, are well characterized experimentally.

We then apply these methods to critical problems in chem-

ical, biological, and materials systems. Current research

efforts are directed to the following materials, among oth-

ers: Semiconductors (dopant diffusion in nanoscale Si,

properties of Si/SiO2 interface, epitaxial growth of GaN);

Ceramics (structures, phase diagrams, catalysts); Metal

Alloys (plasticity, dislocations, crack propagation, spall,

glass formation); Polymers (structures and properties of

dendrimers, gas diffusion, surface tension); and

Biochemical Materials (structure and function of proteins,

non-natural amino acids for biopolymers).

The applications of our work have proven useful in a vari-

ety of domains, including catalysis (methane activation,

metathesis, ammoxidation of alkanes), nanotechnology(carbon nanotubes, bio-nanotechnology), and environmen-

tal engineering (selective encapsulation, humic acid, par-

ticulates).

Most of our projects involve collaborations with experimen-

talists at Caltech, other universities, national laboratories,

and industry.

http://www.wag.caltech.edu/

The H ierarch ica l Mu l t isca le s t ra teg y i s used by

the God da rd g roup to co nnec t f i r st p r i nc ip l es

s imula t ion and des ign o f m ater ia ls w i th

mac roscop ic m anu fac tu r ing and c ha rac te r iza -

t ion . App l ica t ions inc lude senso rs ( i ll us t ra ted

is C 9 H 19OH bound to an o l fac to ry recep to r ) ,

nano techno logy (an a r ray o f bucky tub es i s

show n), and pro cess ing (d i ffus ion o f B in S i) .

M A T E R I A L S a n d P R O C E S S S I M U L A T I O N C E N T E R

WILLIAM A. GODDARD III



N O N - E Q U I L I B R I U M a n d M E T A S T A B L E M A T E R I A L S

WILLIAM JOHNSON

P r o f e s s o r J o h n s o n ’ s g r o u p c o n d u c t s r e s e ar c h o n no n -e q u i l i b r iu man d me t a s t ab l e mat e r ia l s . D u r in g t h e p as t d e c a d e , t h e y h av ed e v e l o p e d u nu s u al me t al l i c a l l o y s w h ic h f a i l t o c r y s t a l l i z e du r in gs o l id if ic a t i o n at l o w c o o l in g r at e s , t h u s f o r min g “ b u l k ” g l as s e s .R e s ear c h o n t h e l iq u i d al l o y s i n c l u d e s f u n d ame n t a l s t u d i e s o fr h e o l o g y , at o mic d if f u s i o n , c r y s t al l i z at i o n k in e t i c s , l i q u id / l iq u idp h a s e s e par at io n , an d t h e g l as s t r an s it i o n . R e s ear c h o n t h e s o l id“ g l as s y” mat e r ia l s in c l u d e s s t u d i e s o f e l as t i c p r o p e r t i es , an dme c h a n is ms o f d e f o r mat i o n , f l o w , an d f r ac t u r e . Th e g r o u p h asd e ve l o p e d c o mp o s i t e mat e r ia l s w h i c h e mp l o y a me t al l ic g l as s mat r ixt o ac h i e ve u nu s u al c o mb in at i o n s o f p r o p e r t i es f o r s t r u c t u r a le n g i ne e r in g ap p l i c at io n s .

Conventional metallic materials have a crystalline structure

consisting of single crystal grains of varying size arranged

in a microstructure. Such structures are produced by the

nucleation and growth of crystalline phases from the

molten alloy during solidification. By contrast, certain oxide

mixtures (e.g., silicate glasses), have such sluggish crystal

nucleation and growth kinetics, that the liquid can be read-

ily undercooled far below the melting point of crystals

(e.g., a quartz crystal). At deep undercooling, these oxide

melts undergo a “glass transition” and freeze as vitreous

solids. Our group has developed multicomponent metal

alloys which vitrify with the same ease as observed in sili-cate melts. These bulk metallic glasses (BMGs) have

unusual properties. They are typically much stronger than

crystalline metal counterparts (by factors of 2 or 3), are

quite tough (much more so than ceramics), and have very

high strain limits for Hookean elasticity (see left-hand fig-

ure above). As a new class of engineering materials,

BMGs offer an opportunity to revolutionize the field of

structural materials with combinations of strength, ductility,

toughness, and processability outside the envelope

achievable using current technology.

On the scientific side, the development of BMG alloys has

made possible detailed fundamental studies of the under-

cooled liquid state and the glass transition in metallic sys-

tems. Quite unexpectedly, it has been found that the liquid

BMG alloys exhibit atomic transport and rheological char-

acteristics very different from simple metals and previously

thought to be unique to oxide/silicate materials. Further,

the traditional theory of crystal nucleation has been found

to be inapplicable in these materials and the development

of new theory based on chemical kinetics is in progress.

Professor Johnson is currently heading a multi-universityDoD project to develop structural amorphous metal for use

in such diverse areas as aircraft, autos, sports products,

and medical implants. The project includes collaborations

with several companies (e.g., Boeing and General Motors).

http://www.its.caltech.edu/~matsci/wlj/Johnson.html



E N G IN E E R IN G o f t h e P H O TO N : N A N O -S C A L E O P TO E L E C TR O N I C D E V I C E S

OSKAR PAINTER

Current research efforts are focused on the interestingways in which light propagates within microfabricated high-

contrast periodic dielectric and metallic structures. The use

of periodic structures to engineer electromagnetic wave

propagation has a rich history dating back to some of the

early work on microwave radar technologies during the

Second World War and more recently to the design of

Distributed Feedback Lasers and Fiber Bragg Gratings

which have become integral components of the fiber-optic

telecommunication industry. Today these ideas have been

reborn in the form of photonic bandgap (PBG) materials or

photonic crystals (PC), in which high-contrast periodic

dielectric and metallic structures are used to create such

strong dispersion as to open up frequency windows withinwhich the propagation of light is entirely forbidden.

This new focus on optical PBG materials has spawned a

great deal of interest in work on the control of light emis-

sion from materials placed within PBG structures. It has

long been realized that the spontaneous emission of radia-

tion from an excited state of matter depends critically upon

the electromagnetic environment in which it is placed. One

may thus imagine using PBG materials to significantly alter

the way in which radiation is emitted from that in free

space. Using a variety of micro and nano-fabrication tech-

niques to create wavelength scale features in semiconduc-tor materials we have been able to realize this goal. By

forming optical cavities in which light is trapped within

modal volumes approaching the theoretical limit of a cubic

half-wavelength (some hundredths of a cubic micron),

electrons and holes within the semiconductor material are

forced, when they recombine, to emit light into a single

resonant mode of the cavity. Ongoing projects involve the

design, fabrication, and characterization of semiconductor

laser sources based upon this technology, and more fun-

damental studies of the interactions of electrons and pho-

tons within these ultra-small volume single-mode optical

cavities.

As with some of the earlier applications of periodic struc-

tures, our research group is also looking at utilizing the

more fundamental aspects of photonic crystals, that being

their dispersive properties. Present research involves the

design and fabrication of different planar photonic crystal

structures for wavelength division multiplexing (WDM)

applications, non-linear optics, and high-density planar

lightwave circuits.

http://www.aph.caltech.edu/people/painter_o.html

A bove : A n a r ray o f nanome te r sca le lase rs

fo rmed in p l ana r pho ton i c c rys ta ls . Inse t show s

a magn i f ied image o f th ree lasers w i th in the

array , the ve ry cent er reg ion (bare ly v is ib le ) o f

each cav i t y rep resen t ing the a rea to w h i ch the

l igh t i s ac tua l ly c onf ined.

Th e r e s ear c h i n p r o f e s s o r p ain t e r ’ sg r o u p c e n t e r s a r o u n d t h e e n g in ee r in go f t h e p r o p ag at i o n o f l i g h t w i t h i nmat e r ia l s t o c r e at e n ew o p t o e l e c t r o n icd e v ic e s w i t h in c r e as e d f u n c t io n a l i t y a n dd en s i t y . A r e as o f c u r r e n t in t e r e s tin v o l v e s e mic o n d u c t o r l as e r s ,mic r o c av it y p h y s ic s , a n d p l an arl ig h t w av e c i r c u i t s .

Lef t : F in i te -d i f fe rence

t ime -do ma i n s imu l a t ion o f

a l oca lized resonan t m ode

in a hexagona l photon ic

crys ta l . Rea l Spac e ( top )

a n d M o m e n t u m S p a c e

( b o t t o m ) .



T H E P H Y S I C S O F S T R U C T U R E a n d F U N C T I O N

ROB PHILLIPS

Nanomechanics

Tools such as the atomic-force microscope and optical

tweezers have made it possible to examine mechanics at

the level of individual molecules in the biological setting

and at the level of single defects in crystalline materials.

Phillips’ group aims to construct models of nanomechani-

cal phenomena such as the mechanics of molecules such

as DNA and proteins, with emphasis on problems such as

the packaging of DNA both in viruses and eucaryotic cells,

the mechanics of ion channels and the ways in which pro-teins unfold in the presence of an applied force. In the

context of materials, similar efforts are underway which

aim to uncover the relation between dissipation in small

scale structures (such as the micron sized cantilevers built

and examined in the group of Professor Michael Roukes)

and the defects that populate these materials.

Dynamics of Molecules and Defects

One of the traditional tools for examining the behavior of

isolated molecules or individual defects is molecular

dynamics in which the trajectories of individual atoms are

computed. Work in the Phillips group is aimed at develop-

ing alternatives to the full brute force molecular dynamics

in which only subsets of the original full set of atomic

degrees of freedom are retained. With these methods in

hand, the objective is to build higher level models of

macromolecular assemblies such as ion channels and

molecular motors which will capture the essential features

of these systems without having to pay the price of full

atomistic simulation. These methods have analogous

applications in the setting of more traditional materialswhere we aim to determine the kinetic properties of

defects such as dislocations, cracks and grain boundaries

and to build effective equations of motion for such defects

which relinquish all further reference to the underlying

atomic coordinates.

http://www.me.caltech.edu/faculty/phillips.html

This f i gure show s one o f seve ra l way s in wh ich the Ph il li ps group in

co l labora t ion w i th tha t o f K laus Schu l ten ’s g roup a t the Un ivers i ty o f

Illino i s is a t tem p t i ng to cons t ruc t “ coa rse -g ra ined ” rep resen ta t ions o f

macromolecu les . Th is f i gure shows the coarse gra in ing o f an a lan ine

res idue and the w ay tha t t h i s res idue f i ts in to an a lpha he l ix . The

ob j ec t i ve o f th i s w o rk i s to bu i ld reduc ed m ode l s o f mac rom o l ecu la r

func t i on w h i ch do no t requ i re fu l l a tom ic reso l u t ion .

Th e g r o u p h e ad e d by Ro b P h i l l i p s i s b u i l d in g

t h e o r e t ic a l mo d e l s o f t h e c o n n e c t i o n b e t w ee ns t r u c t u r e an d f u n c t i o n in t h e s e t t in g o f b o t hb io l o g ic al s y s t e ms as w el l as t r ad it i o n a lmat e r ia l s . On e o f t h e k e y t h e o r e t ic a l t o o l su s e d in t h e P h i l l i p s g r o u p is s ys t e mat i c c o a r s eg r ain in g w h ic h a l l o w s f o r a c o n ne c t i o n b e t w e enat o mis t ic an d c o n t i nu u m d es c r ip t io n s . A c e n t r a lt h e me in t h e P h i l l i p s g r o u p i s t h e u s e o fme t h o d s l ik e t h o s e d es c r ib e d ab o v e t o e x amin en an o s c a l e me c h an ic s in p r o b l e ms r an g in g f r o mt h e pac k in g o f D NA in t o v ir u s e s t o t h ed is s ip at io n in mic r o n s i ze d c an t i l e v e r s .



P H O T O N I C C R Y S T A L S

AXEL SCHERER

A xe l S c h e r e r an d h is g r o u p h av e d e v e l o p e d s o me o f t h e b u i l d in gb l o c k s wh i c h a r e ne c e s s ar y f o r d e f in in g p h o t o n ic i n t e g r at e dc i r c u i t s b as e d o n p h o t o n ic c r y s t a l s . Wi t h t h e s e i t w i l l b ep o s s ib l e t o g e n er at e , r o u t e , f i l t e r , o r d e t e c t l i g h t w i t h i n v er ys mal l a r e as o n a p h o t o n i c c r y s t al c h ip . To d e mo n s t r at e s u c hp h o t o n ic i n t e g r at i o n , t h e Sc h e r e r g r o u p i s me as u r in g t h ec o u p l in g f r o m w av e g u id e s in t o c av it i e s wi t h g e o me t r ie s s u c h ast h e o n e s h o w n in t h e f ig u r e ab o v e . A no t h e r o u t c o me f r o m w o r ko n s i n g l e -d e f e c t p h o t o n ic c r y s t a l c av it i es i s t h e d e s ig n o f h i g h -Q o p t i c a l c av it ie s in w h ic h t h e max imu m o f t h e o p t ic al f ie l d l i e s

in an ai r h o l e w i t h i n a p h o t o n i c c r y s t al . S u c h c av it i e s , wh i c hd is p l ay Q v al u e s in e x c e s s o f 2 0 , 0 0 0 , ar e id ea l l y s u i t e d t oap p l i c a t i o n s in s t r o n g c o u p l in g e xp e r ime n t s , a s ar e r e q u i r e d f o rq u a n t u m c o mp u t a t io n . Th e g r o u p is p r e s en t l y me as u r in g t h e c o l dQ va l u e s o f s u c h c av it i es i n o r d e r t o e va l u at e t h e p r e s en t l imi t so f r e al i s t i c man u f ac t u r ab l e s t r u c t u r e s .

The past rapid emergence of optical microcavity devices,

such as Vertical Cavity Surface Emitting Lasers (VCSELs)

can be largely attributed to the high precision over the

layer thickness control available during semiconductor

crystal growth. High-reflectivity mirrors can thus be grown

with subnanometer accuracy to define high-Q cavities in

the vertical dimension. Recently, we have shown that it is

possible to microfabricate high-reflectivity mirrors by creat-

ing two- and three-dimensional periodic structures. These

periodic “photonic crystals” can be designed to open up

frequency bands within which the propagation of electro-

magnetic waves is forbidden, irrespective of the propaga-

tion direction in space, and thus define photonic

bandgaps. When combined with high-index contrast slabs

in which light can be efficiently guided, microfabricated

two-dimensional photonic bandgap mirrors provide us with

the geometries needed to confine and concentrate light

into extremely small volumes and to obtain very high field

intensities. Our group is working to use these “artificially”

microfabricated cavities in functional nonlinear optical

devices, such as lasers, modulators, add/drop filters,

polarizers, and detectors.

http://nanofab.caltech.edu/

Scann ing e lec t ron

mi c rog raph o f a

pho to n ic c rys ta lw avegu i de .

H igh-reso lu t ion l i thography and h igh- index con-

t ras t w avegu i des a llow us to fab r ica te co mp ac t

add / d rop f i lt e rs and sw itches .



Mi c rog raph show ing a 4 0 -m ic ron d iame te r s ilica m ic rosphe re t ha t i s

doped w i th the rare ear th erb ium. Upon incorpora t ion in to s i l i ca , e rb iumion izes to t he 3+ s ta te and ex h ib its d ipo le t rans i t ions in the gre en and

nea r i n fra red . In the m i c rog raph o ne o f these t rans i t ions has be en exc ited

by op t i ca l pum p i ng th rough a f i be r tape r. The tape r can be see n i n the

micrograph as the s l igh t l y ou t-o f - focus hor i zonta l l i ne . The green r ing

em iss ion f rom the sphe re co r responds to a fundam en ta l w h i spe r ing

ga l le ry m ode o f t he sp here . Th is par t i cu la r sphere i s a lso las ing in the 1.5

micron band ( the impor tan t te lecom band) . The las ing emiss ion i s e f f i -

c i en t ly coup led o n to the same f ibe r tape r used fo r op t i ca l pum p i ng .

In order to confine optical radiation in compact resonant

structures, an ordinary optical fiber is prepared having a

tapered region by pulling the fiber in flame. The tapered

region has a narrow waist (typically a few microns in diam-

eter) permitting access to the electromagnetic field by

evanescent coupling in the region around the taper. The

taper-to-sphere coupling is extraordinarily efficient with

99.8% optical power transfer possible from the fiber to

microsphere resonant modes. It is important to understand

that this coupling is completely reversible. In particular,

once energy is coupled into the high-spatial-diversity

microsphere system, it can be recovered into the single-

mode fiber guide with equally high efficiency. As a result,

the coupling offers a high-efficiency link between the tech-nologically important single-mode fiber medium and the

spatially complex silica microsphere. We have confirmed

this in our work on a spheres attached to two fiber tapers.

In addition, the high Q of the microsphere modes allows

each spatial mode to reside at a precise optical frequency.

This provides a convenient “modal address” mechanism

based on optical frequency. Overall, these structures pro-

vide a possible way of harnessing spatial and spectral

attributes of light in a compact and intrinsically fiber-optic

compatible package. The highest Q structures are also

optically nonlinear at low power (100s of microwatts) so

that complex control functions are possible based on the

Kerr nonlinearity of silica. Device applications being pur-

sued include a four-port filter that resonantly couples opti-

cal power between two fiber cables as well as microsphere

lasers. In the latter device, a single optical fiber is used

both to convey optical pump power to a microsphere (to

create a lasing inversion) and to collect the lasing emis-

sion. The figure shows a micrograph of such a micro-

sphere laser. The green equatorial ring is excited state

emission associated with the lasing whispering gallery

mode. Beyond these device applications, we are also

studying the far-reaching implications of coupled-micro-

sphere systems (i.e., photonic molecules) in which reso-

nant power is injected to a multi-sphere system by wayof fiber tapers and then allowed to circulate among the

coupled modes of the system. The additional degrees of

freedom in such a system could one day be used to

create compact switching nodes for manipulation of light.

http://www.its.caltech.edu/~vahalagr/

Th e ab i l i t y t o c o n f in e o p t i c a l r ad iat i o n in c o mp ac t r e s o n an t s t r u c t u r e sis o f c e n t r a l imp o r t an c e t o t h e c o n s t r u c t i o n o f o p t i c a l f i l t e r s , l as e r s ,an d a v ar ie t y o f me as u r e me n t s y s t e ms . It i s w el l k n o w n t h at s p h e r ic a ld ie l e c t r i c s t r u c t u r e s w i l l e f f ic i e n t l y c o n f in e r ad iat i o n a s s o -c a l l e d“ w h is p e r in g g a l l e r y ” mo d e s . Th e s e mo d e s d e r iv e t h e i r n ame f r o m t h e irac o u s t ic a l an al o g u e (f i r s t d e s c r ib e d by Ray l e ig h ) . Th e r e s o n at o r Q(s t o r ag e t ime n o r mal iz ed b y t h e o p t i c a l p e r io d ) p r o v i de s a c o n v en ie n tme as u r e o f c o n f in eme n t t i me . In w av e l e ng t h -s c a l e r e s o n at o r s t r u c t u r e s ,Q val u e s o f 1 0 0 0 ar e c o n s id e r e d e xc e l l e n t . I t h as b e e n f o u n d t h atd ie l e c t r i c mic r o s p h e r e s o f s il ic a s u p p o r t w h is p er in g g a l l e r y mo d e sw it h Q v a l u e s as l ar g e as 1 0 b i l l i o n . S u c h Q va l u e s c o u l d p r o v eu s e f u l i n p r e c i s io n me as u r e me n t s y s t e ms an d in o p t i c a l c o mmu n i c at io n s .E f f ic i en t o p t i c a l p o w e r c o u p l in g t o t h e s e s y s t e ms is p o s s ib l e u s in g ame t h o d p i o n e er e d at C al t e c h i n t h e Vah a l a g r o u p .

F I B E R - C O U P L E D M I C R O S P H E R E F I L T E R S a n d L A S E R S

KERRY VAHALA



We have studied extensively the physics of semiconductor,

distributed feedback (DFB) lasers, including the effects of

propagation in optical fiber, using a new measurement

technique developed in our group. Bragg gratings and opti-

cal fiber have been used to increase and flatten the sys-tem response of high-modulation-speed signal propaga-

tion, the basis of a new type of transmission system known

as dispersion supported transmission (DST). During recent

years, polarization mode dispersion (PMD) has become a

major limiting factor of optical communication systems.

PMD refers to signal distortion due to polarization effects.

Our PMD research has resulted in a new theory and novel

methods for PMD compensation. We are currently examin-

ing possible integrated implementations for a PMD com-

pensator resulting from the new theory. We have recently

developed a new pulse measurement setup based on

time-resolved optical gating and dispersive propagation

(DP-TROG), enabling us to characterize a pulse complete-ly in amplitude and phase from monolithic mode-locked

lasers designed and fabricated by our group. We have pro-

posed a new method for super-high-speed A/D conversion

(>20 Gbit/sec) based on a combination of semiconductor

mode-locking techniques and wavelength division multi-

plexing (WDM) which exceeds the maximum conversion

rate of current state-of-the-art electronics. We have also

developed a novel fiber ring laser. The laser has orders-of-

magnitude better frequency stability and noise perfor-

mance than semiconductor DFB lasers. It handily outper-

forms a semiconductor DFB laser in digital transmission

tests, even at 10 Gbit/sec. This laser is now developed

commercially for the DWDM market by Orbits Lightwave,

Inc., a Caltech-based start up. Our group has a large, on-

going effort in the field of optical microstructures, including

photonic crystal-based devices, microresonators, and mod-ulators. In 1999, we successfully demonstrated for the first

time lasing in thin-film photonic bandgap-based microcavi-

ties. Lasing from a single photonic lattice point defect was

shown—possibly the smallest modal volume semiconduc-

tor laser ever demonstrated. Multiple microcavities can be

coupled together to form a coupled resonator optical wave-

guide. For a given optical power input this is expected to

enhance the optical intensity by a factor of hundreds or

even thousands, increasing the efficiency of nonlinear opti-

cal processes. Coupling optical waveguides to resonators

makes possible the ability to improve performance of mod-

ulators and switches by orders of magnitude. Using the

concept of critical coupling, we have designed a modulatorwith sub-1-volt half-wave modulation voltage. We are cur-

rently exploring implementations in semiconductors and

electro-optic polymers.

http://www.its.caltech.edu/~aphyariv/

P r o f e s s o r A mn o n Yar iv an d h is r e s ear c h g r o u p h av e p io n e er e d t h e f ie l d

o f o p t o e l e c t r o n ic s an d o p e n ed u p ne w f ie l d s o f s t u d y . D is t r ib u t e df e ed b ac k (D FB ) s e mic o n d u c t o r l as e r s , in t e g r at e d o p t o e l e c t r o n ic c i r c u i t s ,o p t ic a l p h as e c o n j u g at i o n , ex t e r n al c av it y t u n ab l e s emic o n d u c t o r l as e r s ,q u a nt u m w e l l i n f r a r e d p h o t o d e t e c t o r s (Q WIP s ), an d al l -f i b e r ad d /d r o pf i l t e r s h av e a l l f o u n d t h e ir b e g in n in g s in t h i s r e s ear c h g r o u p . To d ay ,ad van c e s c o n t i nu e t o b e mad e in t h e r e s ear c h t h r u s t s o f an al o g u e an dd ig i t al s ig n a l p r o p ag at io n in f i b e r s , p h o t o n i c c r y s t al d e v ic e s , f ib e rl as e r s , an d s o u r c e s f o r w av e l e n g t h d iv is io n mu l t ip l e x in g (WD M ).

E lec t ron m ic rog raph o f a

pho to n ic c rys ta l nanocav -

i ty laser.

t h e Q U A N TU M E L E C TR O N I C S a n d S O L I D S TA TE L A B O R ATO R Y

AMNON YARIV

B E R pe r fo rm ance com par ison

be tw een the f ibe r r i ng lase rand a co mm erc i al DFB semi -

conductor laser . The DFB

laser requ i res tw ice the

rece i ved pow er in o rde r to

ach i eve the same b it e r ro r

ra te as t he f iber laser . The

inse t show s the op t i ca l spec -

t ra , revea ling tw o -o rde rs -o f -

m a g n i tu d e b e t t e r s id e - m o d e

supp ress ion fo r the f iber laser .

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