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Photonic crystals boost light emissionMisha Boroditsky, Ell Yablonovitch

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Physics WorldJuly 1997

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Physica Wtorid July 1997 2 5

fusion alone can cool the spheres from apeak temperature of around 60 °C toroom temperature in a matter of nano-seconds. Indeed, time-delayed pump-probe measurements confirm that theoptical properties of the pop-up crystalfade within 3 ns, fast enough for a range ofoptical-switching applications.

The Pittsburgh group predict that thecolloidal crystals could diffract as much as97% of the incident light, but these firstcrystals have a diffractive efficiency of only2%. Lattice defects - all too familiar tothe makers of semiconductor devices - areimplicated as the culprits, since colloidal

crystals suffer from virtually all of thedefects that affect conventional crystals.New techniques are needed to minimizethese defects while maintaining the bene-fits of self-assembly.

Groups around the world are rising tothe challenge. Alfons van Blaaderen atUtrecht University and Pierre Wiltzius atBell Labs have demonstrated a particu-larly promising approach that is reminis-cent of conventional crystal epitaxy. Theyuse standard photolithography to deposita polka-dot pattern of holes on a glasssubstrate, where the symmetry and peri-odicity of the pattern reflects the desired

crystal structure. The small indentationsprovide a template for the first colloidallayer to align with, and subsequent layersare organized by the first layer to create alarge single crystal.

This technique could be used to orga-nize colloidal spheres with nonlinear opti-cal properties into a single crystal, andthe resulting structure could be preservedin a polymer gel. Such an approach couldallow the large-scale fabrication of ma-terials with optical properties that canbe rapidly switched and continuouslytuned, and promises a new generation ofoptical devices. •

Photonic crystals boost light emissionFrom Mlsha Borodttafcy and Ell Yabtonovftch inthe Departments of Physics and ElectricalEngineering, University of California, Los Angete3,US

FOR MANY years spontaneous emission oflight was considered a natural andimmutable property of radiating atoms.But we now know that spontaneousemission can be controlled by altering theenvironment of the atom: for example ametal mirror can be placed next tothe atom, or it can be enclosed inside ametallic cavity or a photonic crystal. Inthis way, the vacuum fluctuationsor zero-point electromagnetic fieldsin the atom's environment canbe manipulated.

Photonic crystals are artificialstructures that are periodic in one,two or three dimensions. Thesestructures provide a band gap forelectromagnetic waves, just as semi-conductors have a band gap, orenergy range, from which electronwaves are forbidden. Photoniccrystals can be used to suppressspontaneous emission in certaindirections and enhance it in others.Shanhui Fan and colleagues fromthe Massachusetts Institute ofTechnology have now shown howphotonic crystals could be used to createhighly efficient light-emitting diodes(LEDs), those ubiquitous little lamps thatturn up everywhere, from instrumentpanels to stadium-sized television displays(Phys. Rev. Lett. 1997 78 3294).

The sole purpose of LEDs is to emitspontaneous radiation. They consist of ap-n junction within a direct-gap semicon-ductor such as gallium arsenide, whichconverts electrons to photons with aninternal efficiency approaching 90—100%.Surprisingly, one of main factors thatlimits the performance of LEDs is the dif-ficulty of extracting light from materialsthat have a high refractive index, like gal-lium arsenide. Indeed, the escape cone oflight from LEDs is so small that we nor-mally see only 2% of the emitted light,while the rest bounces around inside the

device and is lost. The traditional ap-proach to overcoming this problem hasbeen to redirect the photons that are emit-ted outside of the escape cone. But theMIT scientists suggest that the "wrong"directions can be inhibited altogether bymaking the LED from a two-dimensionalphotonic crystal.

The researchers calculated the charac-teristics of a thin slab of gallium arsenidecontaining a two-dimensional array ofholes. A photonic band gap is createdwhen the ratio of hole diameter to thespacing of the holes is chosen correctly.

p-contact

n-contact

In this cavity-enhanced LED, an electromagnetic mode isconfined In three dimensions: the periodicity of the photoniccrystal prevents light escaping horizontally while total Internalreflection confines the light in the vertical direction. The activeregion Is restricted to the volume at the centre of the device.

This band gap inhibits spontaneous emis-sion within the plane of the slab - which isthe wrong direction for light extraction -while the semiconductor continues toemit light normal to the slab and into freespace. The MIT calculations are the firstto use thin two-dimensional photoniccrystals - previous studies only consideredthe infinitely thick case, which does notrelate to practical devices.

But it would be even better if we couldenhance the spontaneous emission. Asearly as 1946 Edward Purcell predictedthat an atom in a cavity will radiate fasterthan an atom in free space. This impliesthat, in addition to preventing emission inthe wrong directions, we can also enhancethe emission in the "right" directions.One-dimensional cavity-enhanced LEDs- in which the cavity is made from a pair

of parallel mirrors - were first proposed in1992 by Fred Schubert, then at LucentTechnologies, and the most impressive ofthese devices were recently demonstratedby Hans de Neve and colleagues at theUniversity of Ghent in Belgium.

If cavity enhancement could be ex-tended to two and three dimensions, thetotal rate of radiative recombinationwould significantly increase, allowingfaster communications signals and prov-iding more effective competition againstresidual non-radiative recombination.Moreover, the output would resemble that

from a laser: the light could be con-centrated into a single mode, whichcould couple efficiently to opticalfibres. Such LEDs could be usefulfor their raw output efficiency, aswell as for high-speed, broad-bandoptoelectronic communications.

Three-dimensional cavity en-hancement can provide a ten-foldincrease in the spontaneous emis-sion rate, which means that it wouldnot be essential to suppress spon-taneous emission in the wrong dir-ections. Indeed, an open cavity thatdoes not inhibit spontaneous emis-sion can perform almost as well as acompletely closed cavity that doesinhibit spontaneous emission. This

means that a full photonic band gap maynot be necessary, although it might help inthe cavity design.

The promise is great, but there aresome practical problems. The layer ofactive material must be restricted to thesmall cavity volume where the enhance-ment takes place, which may require adifficult patterning and regrowth step inproducing the semiconductor layer. Inaddition, most materials used in optoelec-tronics do not work well when they have afree surface near the region that generateslight. These problems can be solved inpart by using a two-dimensional structurein which the holes do not penetrate intothe active layer at the bottom of the slab(see figure).

Other material systems may also easethese restrictions on device fabrication.

2 6 Physics Worid July 1997

For example, Lionel Kimerling at MIT isdeveloping silicon LEDs doped witherbium. These are more easily adapted tothe concentration of active material withinthe cavity. Finally, the two new families of

III-V materials - gallium nitride andindium gallium aluminium phosphide -could provide much better surface proper-ties. All this work shows that photonicband-gap materials provide many possibil-

ities for controlling spontaneous emissionin semiconductors. More experimentaltests are sure to follow, and these photoniccrystals may soon be finding their way intopractical devices. D

Superlattice lasers go to longer wavelengthsFrom Manfred Helm in the Institute forSemiconductor Physics, University of Unz, Austria

SEMICONDUCTOR lasers are one of the keydevices in modern optoelectronics andoptical communications. Most work atwavelengths from the visible to the near-infrared (0.5-2 |im), but semiconductorlasers operating in the mid-infrared (2-20 |im) are usefulfor gas-sensing applicationssuch as pollution monitoring, orthe control of industrial andchemical processes.

These longer wavelengthlasers are mainly based on lead-salt semiconductors, but theiruse is limited due to high costs,problems with reliability, lack ofcompatibility with other opto-electronic materials and lowoperating temperatures. A newtype of semiconductor laser hasnow been developed by a groupat Bell Laboratories in NewJersey (G Scamarcio et aL 1997Science 276 773). It is based on asemiconductor superlattice andprovides a high-power output inthe mid-infrared.

Conventional semiconductor lasersessentially consist of a p-n diode. A negat-ive voltage applied to the n-doped regionextracts electrons from the n-type regionand holes from the p-type region, injectingthem towards the junction, where theyrecombine by emitting a photon. Thus thephoton energy is determined by theenergy gap between the conduction andvalence bands of the semiconductor.

The introduction of epitaxial tech-niques in the 1970s made it possible togrow ultrathin layers of different semi-conductor materials. Depending on thematerial composition and the band gapof neighbouring layers, electrons can beconfined in square potentials calledquantum wells. The energies of the elec-trons in these wells are restricted to dis-crete energy states, as predicted by basicquantum mechanics.

It was soon realized that quantum-wellstructures could provide the basis for asemiconductor laser that uses only onetype of carrier (e.g. electrons and noholes), provided that a population inver-sion could be achieved between some ofthe energy levels. The wavelength would

first demonstrated by the Bell Labs groupin 1994 (see Physics World July 1994p24). To date, this group has producedquantum cascade lasers that operatebetween 4 and 11 (im.

The group has used a slightly differentconcept in the new laser: the active ele-ments are made from semiconductor

800

light -*y\/v

GalnAs P°s i t i on AllnAs 0.2 0.4 0.6 0.8wavevector k2

1.0

1 The conduction band structure of the superlattice in real space (left)and reciprocal space (right) showing the two lowest minibands.Electrons in the ground state (blue region) are excited to level 4 andthen quickly relax to level 3. A photon Is emitted when electrons relaxinto the lower miniband via a population inversion (levels 3 to 2) andthe electrons then return to the ground state (level 1).

superlattices rather than quantum wells. Asuperlattice is a periodic sequence of thinlayers of two different materials. Thiscreates a one-dimensional band structurewithin the conduction band, and repre-sents the simplest realization of energybands in a solid (figure 1). The one-dimensional bands are called "minibands"to distinguish them from the conductionand valence bands, and the gaps betweenthem are called minigaps.

The superlattice laser exploits electrontransitions between the two lowest mini-

2 The band structure of the superlattice laser whenbe determined by the spacing of the u n d e r bias. Most of the voltage drops across theenergy levels, which in turn depends on injector, which removes electrons from the lowerthe layer thickness. This type of device, miniband of one superlattice and Injects them intonow called a quantum cascade laser, was the upper miniband of the next superlattice.

bands. This transition is located at A, = nldin reciprocal space, where kt is thewavevector perpendicular to the layersand d is the superlattice period. From theband structure, it is clear that a superlat-tice could work in a similar way to a con-ventional atomic four-level laser: electronscould be pumped from the ground state

Qevel 1) into level 4, quicklyrelax to level 3, and build up apopulation inversion betweenlevels 3 and 2. After a photon isemitted, electrons must berapidly removed from level 2into the ground state.

Fortunately, the relaxationtimes in a superlattice fulfil all therequirements for establishing apopulation inversion: the timestaken to relax from levels 4 to 3and 2 to 1 are much shorterthan from 3 to 2 because the re-laxation (via emission of opticalphonons) within a miniband isfaster than between two mini-bands. Moreover, in 1995 Ishowed that the transition atkM = jdd is up to five timesstronger than transitions betweenquantum-well levels.

The laser structure is built up fromlayers of gallium indium arsenide (Gao 47Irio 53As) and aluminium indium arsenide(Al(j 48In0 52As) grown by molecular beamepitaxy on an indium phosphide sub-strate. A crucial point is that the lasermust be pumped by electrical injection:this implies that there must be a voltagedrop across the superlattice, which couldmodify or even destroy the band struc-ture (figure 2).To avoid this problem, theBell Labs team has not made the laserstructure from a single superlattice, butfrom 25 repetitions of an eight-periodsuperlattice with a period of 5.3 run.

The superlattices are separated by an"injector" that helps to remove electronsfrom the first miniband and inject theminto the second miniband of the nextsuperlattice. This is achieved by using a"graded-gap" superlattice in which theratio of barrier thickness (AllnAs) to wellthickness (GalnAs) increases along theinjecting path. The result is that a quasi-miniband forms in the injector when avoltage is applied (figure 2), while theinjector exhibits high electrical resistancewhen there is no voltage. This behaviourguarantees that most of the voltage dropis across the injector and that the bandstructure in the superlattice remains fairlyflat, as required for the operation of the