Photonic crystals (PCs, also known as photonic band gap materials) are optical materials with periodic changes in the dielectric constant on a length scale comparable to optical wavelengths. This periodical modulation of such property can be induced along one, two or three directions of space. This has similar influence on the propagation of light as atomic crystalline potential has on electrons. Thus, the dispersion relations of light can be described with band structures in a Brillouin zone in reciprocal space, and band gap s can be created for certain photon energies. PCs are currently being pursued to obtain a range of forbidden frequencies (a photonic band gap) in the optical region of the electromagnetic spectrum.
Parallelism - How Interference Between Scattered Light Waves Can Lead to Forbidden and Allowed Bands
A simple example consists of a periodic array of voids within a dielectric matrix material. Multiple interference between light waves scattered will eventually lead to some frequencies not being allowed to propagate, giving rise to forbidden and allowed bands, analogous to the electronic band gap of a semiconductor.
Periodicity, Photonic Band Gap Materials and Submicronic Structures
The energy dispersion relation for an electron in vacuum is parabolic with no gaps; when a periodical potential is present, gaps open. In a similar way, a periodic dielectric medium will present frequency regions where propagating photons are not allowed. Since the periodicity of the medium must be comparable to the wavelength of the electromagnetic waves to inhibit their propagation, photonic band gap materials in the optical or infrared domain require submicronic structures, which can be realized using standard nanofabrication technology.
Reasons for the Interest in 1-D, 2-D and 3-D Photonic Crystal Microstructures
The interest in the artificial one-, two- and three-dimensional PC microstructures has been growing tremendously in the last decade because of the deep implications both from a fundamental and a technological point of view. Actually, spectacular new optical properties of PCs have been predicted (e.g. lossless frequency selective mirrors, planar waveguides with sharp bends, high efficiency-surface light emitting diodes or low-threshold lasers).
How Combining Photonic Crystals, III-V Semiconductors and Silicon Materials Might Lead to Real Optoelectronic Integration
Many experimental efforts have been devoted to PCs based on III-V semiconductors due to the possibility of producing efficient optoelectronic devices (especially light sources such as solid-state lasers and LEDs) and systems. However, the possibility of combining photonic and electronic components on the same Si chip or wafer for real optoelectronic integration, is a strong motivation for Si-based microphotonics based on photonic crystals. In this framework, our research activity is devoted to the fabrication and characterization of Si-based one, two and three-dimensional photonic crystals.
One-Dimensional (1-D) Photonic Crystals
Focusing on one-dimensional photonic crystals, they basically consist of multilayer structures with a different refractive index constituting Distributed Bragg Reflectors (DBR) and Fabry-Pérot (F-P) microcavities. Actually, in the recent past, dielectric multilayers made of amorphous silicon-based alloys were realized as DBRs and F-P interference filters operating in the near-infrared region. Moreover, there is some report dealing with amorphous silicon alloy-based F-P microcavities structure, where the active layer consisted of Er-doped SiO2. Such a structure yielded noticeable enhancement of the photoluminescence (PL) intensity of the active optical medium, ascribed to the modified spontaneous emission induced in the optical cavity.
Research into Fabry-Pérot Microcavities Based on Amorphous Silicon Nitride, and Potential Industry Applications for this Research
In the recent past, we realized some prototypes of F-P microcavities entirely based on amorphous Silicon Nitride (a-Si1-xNx:H). These prototypes had appealing optical properties, such as noticeable spectral narrowing of the emission band (few nm), strong enhancement of the photoluminescence yield (more than one order of magnitude), and directionality of the radiation pattern. These results demonstrated the high potential of Si-based microcavities for optoelectronic applications, such as large area and high brightness light emitting devices with tunable emission and high spectral purity. The structures were deposited by 13.56 MHz Plasma enhanced Chemical Vapor Deposition (CVD) technique on which our research unit has a long-standing expertise, with particular specialist knowledge of amorphous silicon-based alloys.
Making Two-Dimensional (2-D) Photonic Crystals with Semiconductor Nanofabrication Technology
Prototypal structures of two-dimensional (2-D) photonic crystals (PCs) with submicrometric triangular and square lattices of air holes in dielectric media (silicon and silicon-based slabs in our case), are realized in our lab using semiconductor nanofabrication technology, such as e-beam lithography and dry/wet etching processing. The PCs are patterned on amorphous silicon-based layers deposited on films with high photoluminescence efficiency (e.g. under stoichiometric amorphous silicon-nitride, silicon-oxide and silicon-carbide), pre-grown on silicon wafers. All the film depositions are performed by PECVD techniques, and the sub-micrometric pattern is transferred to the substrate by means of dry etching process (plasma etching technique).
Using Transmittance, Reflectance Spectroscopy and Photoluminescence to Characterize Fabricated Structures
Optical characterizations are used for verifying the photonic-band nature for well-defined directions and light-polarizations in the visible-near infrared range (400 nm-1550 nm). Transmittance and reflectance spectroscopy allow us to verify the predictions of our photonic-band theoretical modelling. Stationary photoluminescence measurements allow us to further characterize our fabricated structures, and to check spectral and angular control of the emission of the underlying luminescent films. This study is performed with the aim of improving the light extraction of surface-light-emitting-diodes.
Manipulating Photons in Photonic Band Gap Structures via the Use of Artificial Defects
By introducing artificial defects into photonic band gap structures, it should be possible to manipulate photons. For example, photons can propagate through a linear defect within a 2-D pattern. This phenomenon may be used in ultra-small optical devices for optical communications.
Making Photonic Crystal Waveguides with Silicon Dioxide and Silicon Layers
In particular, waveguides with near-zero reflection and low loss through sharp bends, can be realized by photonic crystals. In detail, photonic crystal single-mode waveguides can be designed and fabricated by laminating silicon-dioxide layers and then Si layers onto Si substrates. A single-line-defect can be introduced in the patterned hole lattice, with 90° and 120° bends; this creates a localized band that falls within and is guided by the photonic band gap. The small refractive indexes of the lower and upper cladding layers (silicon dioxide and air, respectively), allow a vertical light confinement.
Two-Dimensional (2-D) Photonic Band Gap (PBG) Structures
The photonic band gap (PBG) structures considered in our work are two-dimensional slabs with a square or a triangular lattice structure. The structures make use of the effort of a two-dimensional PBG to confine the light in the in-plane direction, and a large refractive index contrast to confine the light in the vertical direction. In our experiments, different lattice constants were chosen, from 1mm to 0.5 mm.
Three-Dimensional (3-D) Photonic Crystals
3-D photonic crystals are generally realized with two methods:
· Nanolithography: are obtained PCs with few defects, but a step-by-step process is expensive
· Self-Assembly: is fast, simple and cheap method which allows thin and wide crystals, but does not permit to control the defects presence.
We use a self-assembly method starting by colloidal silica nanospheres. The colloidal dispersion of silica takes months to file orderly. In fact, in presence of the only gravity force, the thermic velocity under gravitational acceleration is given by vter(grav) = (2 a2 D g) / 9 n: where D is the density difference between particle and liquid, a is the particle radius, and n is the viscosity. So a lot of materials and growth techniques are used to deposit opals.
Materials and Growth Techniques
· Polystyrene spheres (650 nm), purchased,
· Silica spheres (330 nm), synthesised according to the Stober recipe.
· Electrophoretic deposition,
· Vertical deposition,
· Deposition by ultrasonic device,
· Sedimentation obtained onto glass and silicon substrates using ultrasonic device at room temperature (about 10 min),
· Vertical deposition or deep coating.
The vertical deposition is the technique giving better results.
Direct Opals and Inverse Opals
In our research experiments, direct opals are obtained by aqueous solution of polystyrene spheres (~ 0.2% dil. in H2O) assembled onto glass substrate are reported. Those were grown for 2 days at T= 55 C; Polystyrene nanospheres (diameter=330 nm, dev. 2%). By infiltrating the direct opal and removing the original ordinate structure, we obtain inverse opals. Inverse opals are structures in which the ratio between the dielectric constants gives rise to light modes.
Deposition on Indium Tin Oxide with Silicon Opals Placed on a Conductive Substrate
Another interesting system which we are focusing on is deposition on ITO (Indium Tin Oxide), where silicon opals deposited on a conductive substrate will be infiltrated with liquid crystals in order to obtain electric bias. The synthesis and the deposition are made in the Politecnico of Turin, while the liquid crystals infiltration concerns the European Laboratory for Non-linear Spectroscopy (LENS) of Sesto, Fiorentino. The opals are made on a SiO2 buffer layer deposited by PECVD and which is 100nm thick. Future research will focus on the fabrication of crystals using centrifuge infiltration of silica opal with silicon.