Abstracts of Posters

Stationary Resonances and Mode Density Control in High Index Layers Fully Etched with a Periodic Microstructure. D. M. ATKIN, P. St. J. Russell, T. A. Birks, Optoelectronics Research Centre.
- - - A novel periodic thin-film dielectric resonator is proposed. An approximate analysis predicts the presence of localized modes with high Q-factors. The bandgap in transverse wavevector (at fixed optical frequency) created by the grating can also be used to reduce the number of guided modes that would otherwise appear in a layer of the same average refractive index. In this way, a fairly thick multi-mode waveguide can be made in which all but the highest order mode are suppressed, rendering the structure signal-mode. This could have applications in high power, single-mode, waveguide lasers where an increased mode volume would be very beneficial.

2D guides with PBG boundaries : mode structures. H. BENISTRY Laboratoire de Physique de la Matiere Condensee, Ecole Polytechnique, F-91128 Palaiseau Cedex, France.
- - - Guided modes in quasi-slab structures consisting of dielectric material surrounded by 2D PBG material (triangular array of holes in the same dielectric) are studied by the supercell method. Focussing on E modes (H transverse to the rods), three topics are outlined :
1 - Existence of a very narrow guide with mid-gap mode, much narrower than the classical quarte-wave layer of the 1D case.
2 - Role of the relative "phases" of the boundary corrugations in guide contradirectional first-odrer coupling properties.
3 - Symmetries of modes at Gamma point and its relationship with the mode energy versus guide width dependence.

PBG Structures in Fiber Form (Band-Gaps by the Kilometer). T. A. BIRKS, D. M. Atkin, G. Wylangowski, P. St. J. Russell, Optoelectronics Research Center, University of Southampton, Southampton SO17 1BJ, United Kingdom.
- - - We describe dielectric structures in the form of glass optical fibers, within which the dielectric constant varies periodically in the two transverse dimensions while being translationally invariant in the longitudinal dimension. Such a fiber is a two-dimensional photonic band gap (PBG) structure, because light propagation in the bulk of the material is forbidden for certain ranges of frequency and longitudinal wave constant. Any symmetry-breaking "defect" in the otherwise periodic structure can provide a site for the localization of light that cannot propagate elsewhere. Thus, the defect acts like the core of a fiber, but guiding light by Bragg reflection instead of the more usual total internal reflection.
Our first fiber is made from silica glass. To provide a contrast in the dielectric constant of 2.1:1 across the fiber, it incorporates periodically-located air-filled cavities. The unit cell of the structure is prepared on a large scale in the form of a hollow hexagonal rod. The fiber is made by a repeated process of drawing down and stacking in a honeycomb array. A defect can be introduced by making one element of the array different, or by omitting it altogether. The final fiber incorporates a central region of periodically disposed air holes spaced by between 1 and 2 $\mu$m. In principle, this fiber can be made in kilometer lengths, giving a plentiful supply of PBG material.
Latest results, potential applications, and work with other materials will be discussed.

This project is supported by the U.K. Defense Research Agency at Malvern.

Interaction of Different Scattering Mechanisms in a One-Dimensional Random Photonic Lattice. S. A. BULGAKOV, M. Nieto-Vesperinas, Instituto de Ciencia de Materiales, Sede B, C.S.I.C., Universidad Autonoma de Madrid, Spain.
- - - We have considered the concurrence of different scattering mechanisms, such as Bragg diffraction, single scatterer resonance and Bragg remnants effect, which take place in electromagnetic wave propagation through periodical and random sequences of slabs having different dielectric permittivity. This study has been made both by the plane wave expansion and by the transferring matrix methods. Analytical expressions allowing to predict band gap closing (or anti-gaps) have been obtained. Higher frequency bands have also been investigated.
It is shown that resonances in single scatterers constitute the strongest scattering mechanism and depend on parameter of the single scatterer only. Such phenomena suppresses all other effects when the wave frequency coincides either with the middle gap frequency or with the frequency of the Bragg remnants effect. The phenomenon of Bragg diffraction destroys the effect, being a resonant scattering effect, can bring to the scattering process more "reflectivity" than Bragg diffraction (in the sense of producing a smaller localization length), which actually occurs in a medium with small differences between the background and barrier dielectric permittivities. The particularities of each scattering picture result from the inter-play of the mentioned mechanisms, but the influence of each of them strongly depends on the concrete structure.

Partial Photonic Band Gaps at Bragg Directions in Polystyrene Colloidal Crystals-Work in Progress. C. CAMERON, Defense Research Agency, St. Andrews Road, Great Malvern, Worcs, WR14 3PS, United Kingdom.
- - - Colloidal suspensions of highly charged, mono disperse polystyrene spheres can be made to crystallize under appropriate conditions of charge, fill fraction, counterion concentration, and temperature. These colloidal crystals can conveniently be arranged to have lattice parameters comparable to optical wavelengths. The initial formation of randomly orientated crystallites characterized by iridescence from the opaque colloid can be encouraged to form clear single long range crystals by slight agitation of the cell. This process provides an opportunity to have a three-dimensionally periodic dielectric structure, which exhibits partial directional photonic band gaps. When the crystal is illuminated with monochromatic plane waves, we expect only to see scattering in Bragg directions. However, multiple scattering at crystal defects and from the particles themselves effectively randomizes the propagation direction of some of the light and we see a diffuse scattered background. This background is patterned with dark rings due to strong scattering out of directions where the Bragg condition is satisfied for the allowed values of the lattice vector in the particular crystal type. These dark rings have been previously viewed in X-ray diffraction as Kossel lines. Here we present our studies of these phenomena matching model data to what is observed in light scattering from crystalline suspensions of polystyrene colloids.

Magnetooptical Properties of PBG Materials (Theory). A. Boiko, Institute for Low Temperature Physics and Engineering, 47 Lenin Ave., Kharkov, 310164, Ukraine, A. EREMENKO, Institute for Single Crystals, 60 Lenin Avenue, Kharkov, 310001, Ukraine.
- - - Magnetooptical studies have proved to be valuable tool for investigating numerous properties of the photonic band-gap materials. The results of the theoretical studies of both intra- and interband magnetooptical transitions are performed in the present paper (one- and two-photon transitions were in the scope of our interest). The energies of basic magnetooptical resonances such as spin and cyclotron were calculated. The comparison with the situation in both narrow-gap and zero-gap materials is also provided. The results of the paper are able to help engineering in photonic band gap materials science.

Laser Driven X-Ray Sources and Applications. A. HENING, Atomic Physics Institute, Laser Division, P.O. Box MG-21, Bucharest, RO-76 900, Romania.
- - - In order to demonstrate the potential applications to nanolithography and due to its characteristics (coherence, monocromaticity) to the field of x-ray holography, the properties of x-ray source generated by focusing the beam of a commercially available excimer laser (KrF) onto a solid target have been investigated. Results and some application hints are reviewed.

Elastic Wave Band Structure and its Relation to Single Scattering. M. KAFESAKI (1), E. N. Economou (1), M. M. Sigalas (2), 1. Research Center of Crete, FORTH, P. O. Box 1527, 71110 Heraklio, and Department of Physics, University of Crete, Greece, 2. Ames Laboratory* and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011.
- - - The study of the elastic and acoustic wave scattering by a single sphere is used to analyze and to predict band structure results in composites consisting of periodically placed spheres in a host material.
It was found that classical wave propagation in a composite can be understood by considering two idealized channels: the first is using the host and the second is by hopping coherently from sphere to sphere using the resonances of the single scattering cross-section - in analogy with the LCAO in solids and molecules. Thus, widely separated strong resonances with a strong background in between (attributed to a rigid or soft sphere) in the single sphere scattering cross-section implies a wide gap in the periodic composite. It was found that a high density sphere embedded in a plastic produces cross-sections favorable for gaps. Detailed band structure results confirm this prediction.

*Operated for the U.S. Department of Energy by Iowa State University under contract no. W-7405-eng-82. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences.

Thermostimulate Optimization of IR-Photosensitive HgCdTe Crystal Parameters. L. KARACHEVTSEVA, A. Lyubchenko, Institute of Semiconductor Physics, Department of Semiconductor Materialogy, Kiev 252028, Ukraine.
- - - For technological optimization of CdHgTe crystals used for IR-photodetector, the mechanisms of principal (theoretical) photoelectric parameters restriction in the results of temperature and ultrasonic treatments were investigated. New phenomena and peculiarities of nonequilibrium processes were determined:

$\bullet$ Experimentally and theoretically minimum of lifetime concentration dependence were obtained when equilibrium carrier concentration was changed due to modification of acceptor concentration (mercury vacancies). It was established that such a minimum in wide donor concentration range corresponded to compensation level K$=$0.5.
$\bullet$ For "weak" p-type conductivity crystals in regions of low magnetic fields for holes, the simple analytical Hall coefficient dependence R$=$f(b) was justified. These permitted the control of electron and hole concentrations, and mobilities for most hard n, p range in materials with high mobilities ratio.
$\bullet$ For correct formation of numerical criteria of physical parameters restriction by extended defects, the crystals with varied concentration of growth dislocations were investigated. The mathematical model was developed for quantitative analysis of recombination activity of micro-and nano-sized inclusions in semiconductors. Recombination-sized effects dealing with dependence of characteristic diffusion length on defect size and configuration were obtained. It was established that dislocation critical concentration which characterized material quality corresponded to interdislocation distance R$=$0.2 L (L-diffusion length) which confirmed recombination-sized effect.
$\bullet$ The important acoustic treatment results are generation and ``cure" of mercury vacancies in HgCdTe crystals. Such processes depend on acoustic energy intensity and initial vacancy concentration.

Induced Transparency in Potassium by an Internally Generated Field. M. E. Movsessian*, T. Efthimiopoulos, M. KATHARAKIS, N. Merlemis, Physics Department, University of Crete, Iraklion, Greece.
- - - Results related to an electromagnetically induced transparency in atomic potassium are presented where a strong dye laser was used to excite the 6S state by two photon absorptions. The internally-generated field corresponding to the transition 6S-4P couples with the incoming laser to produce a coherent four wave mixing radiation originating from the 4P state. For strong excitation, the internally generated radiation induces an absorption window in the transition from the ground state to the 4P state, and enhances the efficiency of the produced radiation by modifying the nonlinear susceptibility.

* Permanent address: Institute of Physical Research, Armenian Academy of Science, Yerevan, Armenia.

Coupled Gap Solitons. V. V. KONOTOP Departamento de Fisica, Universidade da Madeira, Praca do Municipio, 9000, Funchal, Portugal, and G. P. Tsironis, FORTH, P. O. Box 1527, 71110 Heraklion, Crete, Greece.
- - - Small width of the stop gap results in the coupling of linear carrier waves associated with the adjacent gap edges. In the presence of nonlinearity this coupling leads to creating of new solitonic states moving with the finite velocity and having the amplitudes related to the gap width. The theory of such coupled solitons is developed.

Wave Propagation in Nonlinear Photonic Band Gap Materials. Q. LI, C. T. Chan, K. M. Ho, C. M. Soukoulis, Ames Laboratory* and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011.
- - - We study theoretically the propagation of EM wave through periodic structures consisting of dielectric layered materials with intensity dependent dielectric constant. We find the transmission properties are strongly modulated by both frequency and intensity in the presence of nonlinearity, and the transmission diagram in the $\omega$-A$_{out}$ plane exhibits distinctive features depending on whether the Kerr coefficient is positive or negative. These features, though complicated, can be explained through analysis of periodic orbits of the corresponding nonlinear mapping. Bistability and multistability are found to manifest most strongly around the upper band edges and between the basins of stable periodic orbits. Resonance transmissions via soliton formation are analyzed through a simple mechanical analogy. We also discuss the switching threshold and the feasibility of making a switch utilizing such a structure.

*Operated for the U.S. Department of Energy by Iowa State University under contract no. W-7405-eng-82. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences.

EM Field Distributions in Complex Dielectric Structures. L. MARTIN-MORENO (1), P. M. Bell (2), F. J. Garcia-Vidal (2), J. B. Pendry (2), 1. Instituto de Ciencia de Materiales (CSIC), Universidad Autonoma de Madrid, Cantoblanco, 28049, Madrid, Spain, 2. Condensed Matter Theory Group, The Blackett Laboratory, Imperial College, Prince Consort Road, London SW.
- - - Recently, there has been renewed interest in the electromagnetic (EM) properties of complex dielectric materials. An outcome of this interest has been the application of novel methods to solve Maxwell's equations. One of these methods, the transfer matrix technique, which is particularly suitable for the study of the propagation of EM waves is presented elsewhere in this conference. In this work we show how it is possible to apply it to the study of several properties, such as energy loss of charged particles passing close to corrugated surfaces, absorption of light in metallic colloids and enhanced Raman scattering. As an example we study these properties in an array of metallic cylinders and explore how they vary as a function of the cylinders packing fraction.

Second Harmonic Scattering from Sites of a Crystalline Lattice. J. MARTORELL, R. Corbalan, Department de Fisica, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain, R. Vilaseca, J. Trull, Department de Fisica I Enginyeria Nuclear, Universitat Politecnica de Catalunya, C/Colom 11, 08222 Terrassa (Barcelona), Spain.
- - - Experimental evidence of second harmonic generation in a macroscopically centrosymmetric lattice formed by spherical particles of optical dimensions is presented. Second harmonic light is scattered from the surface of these spherical particles. A simple theoretical model based on scattering in the Rayleigh-Gans approximation, indicates that constructive interference of light scattered at the second harmonic frequency leads to a plane wave front propagating in the direction of the incident beam. The implications of the addition of defects in a controlled manner will be discussed in the framework of the photonic band gap theory. Along these lines we will discuss in detail the simple case of a truncated periodic lattice in one dimension. We will present additional experimental results showing enhancement and suppression of the radiation of a dipole sheet oscillating at the second harmonic frequency embedded in a 1-dimensional periodic structure with a defect.

Dispersion, Tunability and Applications of Defect Modes in Photonic Band-Gap Structures. R. D. PECHSTEDT, P. St. Russell, T. A. Birks, F. D. Lloyd-Lucas, Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom.
- - - Electronic surfaces states, whose energies lie within the semiconductor band-gap, are normally regarded as a nuisance to be avoided in semiconductor lasers. This is particularly true of micro-pillar laser arrays where the surface area is large. Similarly, intra-photonic-band-gap interface or surface states will reduce the effectiveness of a photonic bandgap material by introducing high Q-factor resonances into which unwanted spontaneous emission and lasing can occur [1,2]. These photonic defect states are not, however, always undesirable. Examples include: i) the DFB laser mode supported by a structural defect at the center of a uniform Bragg mirror - the resonant frequency of this mode lies within the photonic bandgap of the Bragg mirror [3]; ii) the surface-guided Bloch modes confined at the surface of multilayer stacks [4] - these have attractive potential applications in waveguide mode filtering [5]; and iii) Bragg waveguide modes in which total internal reflection is replaced by Bragg reflection between two multilayer stacks. A general feature of defect modes is a phase velocity that is highly sensitive both to optical frequency, and to "strength" of the local aperiodicity that defines the defect. Small compositional and structural changes can radically alter the position of the mode within the stop-band, providing an effective tuning mechanism. Owing to these and other unique properties, defect modes may provide the basis for the development of a versatile new family of optoelectronic devices.

[1] R. D. Meade, K. D. Brommer, A. M. Rappe and J. D. Joannopoulos, "Electromagnetic Bloch waves at the surface of a photonic crystal," Phys. Rev. A 44 (10961-10964) 1991.
[2] D. R. Smith, R. Dalichaouch, N. Kroll, S. Schultz, S. L. McCall and P. M. Platzman, "Photonic band structure and defects in one and two dimensions," J. Opt. Soc. Am. 10 (314-321) 1993.
[3] R. P. Stanley, R. Houdr=E9, U. Oesterle, M. Ilegems and C. Weisbuch, "Impurity modes in one-dimensional systems: The transition from photonic band gaps to microcavities," Phys. Rev. A 48 (2246-2249) 1993.
[4] P. Yeh, "Optical Waves in Layered Media," Wiley, New York (1988).
[5] P. St. J. Russell, T. A. Birks and F. D. Lloyd-Lucas, "Photonic Bloch Waves and Photonic Band Gaps," in Confined Electrons and Photons: New Physics and Applications, eds. E. Burstein and C. Weisbuch, Plenum Press, 1994.

Photonic Band Gap Structures for Opto-electronic Devices. P. RIGBY, IRC Semiconductor Materials, Dept. Eng. Sc., Oxford University, Oxford, United Kingdom.
- - - The techniques of electron beam lithography and electron cyclotron resonance (ECR) plasma etching have been used to fabricate 2-dimensional periodic structures in GaAs/AlGaAs waveguides. The structure chosen is a triangular lattice of air cylinders because it is expected to show a full photonic band gap in-plane. Three different approaches have been investigated for writing circles: by single point exposures or as pixellated areas in positive resist, and in negative resist, by writing lines defining the regions between the circles.

Photonic Band Structure of a Triangular Lattice of Vertical Cavities (A VCSEL Array). D. SIEVENPIPER, M. Goertemiller, C. Lam, E. Yablonovitch, Electrical Engineering Department, University of California, Los Angeles, CA 90095-1594.
- - - We investigate the two-dimensional photonic band structure of a triangular lattice of vertical cavity etalons. Each etalon consists of a series of alternating dielectric layers forming a pair of Bragg reflectors which are arranged into a resonant cavity. A triangular array of such cavities forms a periodic lattice of coupled resonators possessing a lateral photonic band structure. This band structure is explored using transmission measurements in the microwave regime and models are presented to explain the electromagnetic intercavity interactions.

A Study of Multistability in Semiconductor Superlattices. N. G. SUN, G. P. Tsironis, Work done in collaboration with Q. L. Yu and K. M. Ho. Physics Department, University of North Texas, Denton, TX 76203.
- - - We use a simple nonlinear Kronig-Penney model to study multistability and discontinuity in the current-voltage characteristics of doped semiconductor superlattices in a homogeneous electric field. Nonlinearity in our model enters through a self-consistent potential used to describe the interaction of the effective electrons with charge accumulation in the doped layers. We show how multistability is affected by different nonlinear strengths, and discuss the corresponding changes in the optical properties of the superlattices.

Wavelength-Tunable Femtosecond Diagnostic Methods for Characterizing Photonic Bandgap Device Performance. C. N. TZILIGAKIS, K.-Y. Lim, G. S. Petrich, L. A. Kolodziejski, E. P. Ippen, S. Fan*, P. R. Villeneuve*, J. D. Joannopoulos*, Department of Electrical Engineering and Computer Science, *Department Physics, Center for Materials Science and Engineering and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139.
- - - A major area of potential applications for photonic bandgap materials is in the development of new devices with improved characteristics in the optical regime. Sub-micron structures are being fabricated for the near-infrared wavelengths of interest, and wavelength tunable femtosecond (WTF) techniques are being developed to determine the actual device behavior.
In our poster, we will describe a WTF experimental setup for characterizing one dimensional air-bridge photonic bandgap devices in the 3--5 micron range. Such devices are currently under fabrication at MIT. They consist of a dielectric waveguide periodically punctuated with holes and suspended in air, thus forming an air-bridge structure which exhibits a bandgap in the 3--5 micron wavelength range. The introduction of an appropriate defect region (absence of a hole) in the mid dle of the periodic structure creates a microcavity characterized by a high Q resonance placed inside the bandgap. In the fabrication of the structures, GaAs is used for the waveguide and Al$_x$Ga$_{1-x}$As for the underlying substrate. More details on these devices will be on our poster. The experimental setup we are using to test these structures takes advantage of the unique features of femtosecond Ti:sapphire lasers and Optical Parametric Oscillators (OPO's). By synchronously pumping an OPO with a Ti:sapphire laser, two well-synchronized, femtosecond pulse trains are obtained with mW average power tunable in the 1--2 micron wavelength range. Femtosecond pulses at longer wavelengths can then be obtained by difference frequency generation in a AgGaS$_2$ nonlinear crystal. Wavelength conversion up to the 3--5 micron regime is possible with average powers of hundreds of microwatts available for coupling into the photonic bandgap structures.

OPAL. Using Transfer Matrices as a Computational Tool in Photonics. A. J. WARD, J. B.Pendry, P. M. Bell, Condensed Matter Theory Group, Imperial College, London, L. Martin-Moreno, Instituto de Ciencia der Materiales (CSIC) Madrid.
- - - The Transfer Matrix method has been exploited to calculate the transmission and reflection coefficients and the band structure for a wide range of photonic crystals and other systems. In this poster, we sketch how these techniques can be extended. We show how the transfer matrix can be used to calculate dispersion surfaces for photonic crystals which, in turn, can be excellent tools in the search for a photonic insulator. We also show how the transfer matrix method can be applied to a wider range of system geometries than the usual cubic systems.

Localization in Highly Anisotropic Systems. I. ZAMBETAKI, E. N. Economou, Research Center of Crete/FORTH, P.O. Box 1527, 71110 Heraklion Crete, Greece, Q. Li, C. M. Soukoulis, Ames Laboratory* and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011.
- - - Recently, the effects of anisotropy in disordered systems have received considerable attention, due to the fact that high-T$_c$ superconductors are highly anisotropic. There is a strong intra-plane coupling and a very weak inter-plane coupling. We have studied the localization properties of this model by the transfer matrix technique for both propagation directions, to check if the predictions of the one-parameter scaling theory is valid. We have also studied the localization behavior of weakly coupled disordered chains, to check the 1D-to-3D behavior. The numerical results are compared with the self-consistent theory of localization.

*Operated for the U.S. Department of Energy by Iowa State University under contract no. W-7405-eng-82. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences.

Last updated: Thu Jun 8 17:22:06 CDT 1995
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