Florencio Garcia-Santamaria

Self-assembly approach to optical metamaterials

Photonic crystals can be viewed just as a subclass of a larger family of material systems called metamaterials in which the properties largely derive from the structure rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings for further templating other materials with photonic applications for instance. The tortuous route from materials to devices might perhaps find reward in the ease and low cost of fabrication of these materials. In this paper we present a review of recent work and work under way in our laboratory tending towards synthesis based on self-assembly to realize metamaterials in the optical range. This comprises the formation of the templates (opals) and subsequent synthesis of guest materials such as semiconductors, metals and insulators. The possibility of further processing allows additional two-dimensional and quasi-two-dimensional patterning for the design of new structures. In this paper we show how the raw matter can be checked for quality and learn how to use its optical properties to evaluate application potential. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined. We show some examples where opals are used to pattern the growth of other materials with photonic applications (such as metals and semiconductors) and developments leading to both vertical and lateral engineering are shown.

Optical surface resonance may render photonic crystals ineffective

In this work, we identify and study the presence of extremely intense surface resonances that frustrate the coupling of photons into a photonic crystal over crucial energy ranges. The practical utility of photonic crystals demands the capability to exchange photons with the external medium; therefore, it is essential to understand the cause of these surface resonances and a route to their elimination. We demonstrate that by modifying the surface geometry, it is possible to tune the optical response or eliminate the resonances to enable full exploitation of the photonic crystal.

Are artificial opals non-close-packed fcc structures?

The authors report a simple experimental method to accurately measure the volume fraction of artificial opals. The results are modeled using several methods, and they find that some of the most common yield very inaccurate results. Both finite size and substrate effects play an important role in calculations of the volume fraction. The experimental results show that the interstitial pore volume is 4%–15% larger than expected for close-packed structures. Consequently, calculations performed in previous work relating the amount of material synthesized in the opal interstices with the optical properties may need revision, especially in the case of high refractive index materials.

Wavefunction engineering in core-shell semiconductor nanocrystals: from fine-tuned exciton dynamics and suppressed Auger recombination to dual color electroluminescence

Using semiconductor nanocrystals (NCs) one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. As a result, NCs exhibit important physical properties which, in combination with the chemical stability and solution processability, make this class of functional materials particularly appealing for several technological fields, such as solid-state lighting, lasers, photovoltaics, and electronics. Generally, the tunability of their physical properties is achieved through particle-size control of the quantum confinement effect. Wavefunction engineering adds a degree of freedom for manipulating the physical properties of NCs by selectively confining the carriers in specific domains of the material, thereby controlling the spatial overlap between the electron and hole wavefunctions. This design has been applied to several material systems in different geometries and has been shown to successfully control the emission energy and recombination dynamics as well as to reduce nonradiative Auger recombination, a process in which, as a consequence of strong spatial confinement, the energy of one electron-hole pair is nonradiatively transferred to a third charge carrier. The focus of this presentation is on nanocrystal heterostructures that comprise a small CdSe core overcoated with a thick shell of wider-gap CdS. These quasi-type II structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over 500 meV)1, and are a remarkable class of model compounds for investigating the influence of nanoengineered electron-hole overlap on the exciton fine structure.2 We indeed recently showed that this quasi-type II motif can be used to tune the energy splitting between optically active (“bright”) and optically passive (“dark”) excitons due to strong electron-hole exchange interaction, which is typical of quantum-confined semiconductor nanocrystals. This design provides a new tool for controlling excitonic dynamics including absolute recombination time scales and temperature and magnetic field dependences separately from the confinement energy. As a result of reduced Auger recombination, in combination with essentially complete suppression of energy-transfer in thick-shell NCs films, we recently fabricated bright, monochrome LEDs based on these nanostructures. Our results indicate that the luminance and efficiency can be improved dramatically by increasing the shell thickness without detrimental effects of increased turn-on voltage.3 Detailed structural and spectroscopic studies reveal a crucial role of interfaces on the Auger recombination process ion these heterostructures. Specifically, we observe a sharp transition to Auger-recombination-free behavior for shell thickness ~1.8-2.5 nm, accompanied by the development of an intense phonon mode characteristic of a CdSeS alloy.4 These results suggest that the likely reason for suppressed Auger recombination in these nanostructures is the “smoothing out” of the otherwise sharp confinement potential due to formation of a graded interfacial CdSeS layer between the CdSe core and the CdS shell, as was recently proposed by theoretical calculations by Cragg and Efros.5

Materials aspects of opals as photonic crystals

Opals have only a relatively recent history as photonic materials. The tortuous route from materials to devices might perhaps find compensation in the case and low cost of fabrication. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined and developments leading to possible devices shown. We perform a study of the influence of finite size effects on the optical properties of the samples prior to comparison of experimental results and calculated energy bands, which may be determined along desired directions in reciprocal space. The role of intrinsic defects in the optical properties of the samples has been investigated. The optimization of the procedure to grow accurate amounts of amorphous silicon and germanium by CVD free of contamination in opals has been tackled. Multilayer systems of both semiconductors have been fabricated. Finally we present a method to fabricate opal based photonic crystals containing 2D cavities.

Photonic band engineering in opals by growth of Si/Ge multilayer shells

The optimization of the procedure to grow accurate amounts of amorphous silicon and germanium by chemical vapor deposition (CVD) free of contamination in opals has been performed. The samples have been optically characterized and results agree with theoretical calculations of band structures. Multilayer systems of both semiconductors have been fabricated. Samples have been optically characterized and observed with a scanning electron microscope. Selective removal of germanium with aqua regia has proven to be possible. Theoretical calculations show that subtle variations of the topography may give rise to important effects (flat bands, pseudogap openings, etc). As an example, a photonic band structure with a complete photonic band gap between the 5th and 6th band has been provided along with a method to obtain it. It would be impossible to discuss all the possible structures that could be obtained from samples with different number of layers and materials forming them. However, there are many interesting topographies that could be fabricated in a relatively straightforward manner following the techniques described here.

Growth of amorphous Si/Ge multilayer shells in opals

The optimization of the procedure to grow accurate amounts of amorphous silicon and germanium by CVD free of con-tamination in opals has been performed. The samples have been optically characterized and results agree with theoretical calculations of band structures. Multilayer systems of both semiconductors have been fabricated. Samples have been optically characterized and observed with a scanning electron microscope. Selective removal of germanium with aqua regia has proven to be possible. Theoretical calculations show that subtle variations of the topography may give rise to important effects (flat bands, pseudogap openings, etc). As an example, a photonic band structure with a complete photonic band gap (cPBG) between the 5th and 6th band has been provided along with a method to obtain it. It would be impossible to discuss all the possible structures that could be obtained from samples with different number of layers and materials forming them. However, there are many interesting topographies that could be fabricated in a relatively straightforward manner following the techniques described here.

Opal-like photonic crystal with diamond lattice

In this contribution, a method to fabricate a diamond structure with a complete PBG in the near infrared is proposed. The procedure starts by building an opal composed of two types of microspheres (organic and inorganic) in a body-centered-cubic symmetry by means of a micro-robotic technique. Then, the organic particles may be selectively removed to obtain a diamond structure of inorganic particles. Once this structure is assembled its filling fraction may be controlled by sintering. Subsequently this template can be infiltrated with an adequate high refractive index material. In this way, the method can be extended to make diamond inverse opals of, for instance, silicon with gap to mid gap ratios as large as 13% for moderate filling fractions. An overview of micromanipulation as well as previous experimental results will be offered to show the feasibility of this method.