Mahi R. SinghWestern Science

Polaritonic, plasmonics, photonic and optoelectronic properties of nanomaterials

Recently there is considerable interest in finding a new class of materials which can be used to fabricate faster and smaller photonic and optoelectronic devices since semiconductor industry has reached its highest limit. In this research project we study photonic, plasmonics and optoelectronic properties of nanomaterials made form polaritonic/photonic band gap materials and doped with nanoparticles. These materials have energy gap in their energy spectrum. Energy gap in polaritonic/excitonic materials appears due to the phonon/exciton-photon coupling [fig 1a/1b]. In other words radiation signals in polaritonic materials are carried out by an admixture of photons and optical phonons/excitons rather than photons. On the other hand the energy gap in photonic materials is due the scattering of photons with periodicity of the dielectric constant (fig 1c). There is an analogy between polaritonic and photonic materials and semiconductors which have an energy gap in their electronic energy spectrum. When the energy of radiation signals lies within the band gap they are reflected. However, when the energy lies outside the band gap radiation signals propagate within these materials. Examples for photonic materials are photonic crystals and example for polaritonic materials are quantum dots and wires, III-V, II-VI semiconductors, polymers, dispersive materials; oxides, halides, organic and inorganic materials etc. Recently we have proposed fabrications of nanowires, nanowells, nanocavities, and nanofibers from these materials. To make electronic devices from semiconductors one has to dope n-and p-type (i.e. phosphorous, boron) impurities in semiconductors. Similarly, to make new types of photonic and optoelectronic devices from optical materials we have to dope them with an ensemble of nanoparticles such. Nanoparticles such as quantum dots, quantum wells [4a] and ZnO nanoparticles [4b] have been doped in these materials recently. To understand the optoelectronic properties of these materials we have applied probe and control lasers.This is a new research area which has been mainly started by my group. In this research area we are one of the leading research group in the world.2-1



We have proposed for the first time that polaritonic nanowires and nanowells can be fabricated from polaritonic materials [6]. For example, a nanowire can be fabricated by embedding a polaritonic material (i.e. MgO) into another polaritonic material (i.e. GaP) (fig. 2). It is considered that the embedded material has a smaller band gap than that of the host material. Because of this band gap engineering, polaritons are co


nfined in the embedded material and have bound states. The radius of the wire is taken in the order of several hundred nano

metres. The nanowires are also doped with an ensemble of multi-level nanoparticles. We have predicted many new phenomena. For example, the one-polariton absorption spectr

um splits into many absorption peaks due to the strong coupling between quantum dots and bound polariton states and has several transparent and absorption states. Therefore, polaritonic nanowire can be switched from one transparent state to another and hence polaritonic switches can be fabricated. We have also developed a theory of nonlinear two-polariton phenomena in polaritonic materials when the nanoparticles are interacting with each other via dipole-dipole interaction (DDI). We discovered that the two-photon absorption can be turned ON (one-peak) and OFF (two-peak) when the decay resonance energy of the three-level nanoparticles is moved within the lower energy band (see fig.3). These are very interesting and important discoveries which can be used to make polaritonic switches and transistors.

Plasmonics: Plasmonic is the study of collective motions of electrons in metallic nanomaterials. We proposed new types of metallic nanowires and waveguides made from metallic photonic crystals [7]. They are made from embedding a periodically arranged metallic spheres into a host dielectric or metallic photonic crystal. The advantage of using metallic crystals is that their 2-4 energy gap can be manipulated by the plasmon frequency [8]. There are several methods for the fabrication of metallic crystals [9]. Plasmon and photon resonances lie in the same spectral range. Therefore, this unique property can be used to produce new types of the plasmonic devices. Due to the plasmon-photon coupling, photons are localized within the metallic structure and are reflected from the dielectric crystal. The nanowire is doped with nanoparticles which are interacting with the localized photons. It is found that number of bound photons states within the structures depends on the plasmon frequency. Numerical simulations of the absorption coefficient predict that the position of a transparent state can be switched by changing the frequency of plasmons. We have also found a new switching mechanism due to plasmon in metallic crystals doped with four-level quantum dots. It is found that when pump laser is applied and plasmon coupling is absent the system has one minimum (i.e. transparent state) between two absorption peaks (fig 4a). In the presence of the plasmon coupling the system has three peaks and two minima (two transplant states) (fig 4b). For certain experimental conditions two transparent states can be merged into the one transparent state (fig. 4c). This means that the system can be switched from two transparent states to one and vice versa. The present study can be used to make new types of plasmonic devices such as plasmonic switches, transistors, light sources, integrated optical circuits, incandescent application lamp and thermal photovoltaic power generation.

Photonics and optoelectronics: The study of photonics and optoelectr2-5onics deals with matter, light and their interactions. Recently we have proposed a new class of materials called photonic nanowires and nanowells made from photonic crystals doped with an ensemble of nanoparticles [10]. Traditional nanowires are fabricated by doping a high refractive index material into a low index material [11a].

However, these structures are fabricated by embedding a photonic crystal into another photonic crystal (fig 5a, 5b). These5-6 types of structures have been fabricated by some researchers [12b]. We have chosen that the band gap of embedded crystal lies within the band gap of the host crystal. This band gap engineering bound photons within the embedded crystal. The bound photon states energies, density of states, the absorption coefficient and refractive index are calculated. It is found that the number of bound states in the wire depends on the size and band gaps of crystals. It is considered that doped nanoparticles are not only interacting with bound photons but also with each other via the dipole-dipole interaction. Many useful new phenomena are predicted [11]. For example it is found that when a resonance energy of nanoparticles lies near a bound photon states the absorption spectrum switched from one peak to two-peak (Fig 6). This is due to the strong coupling between the nanoparticles and the bound photons in the wire. It has also been predicted that due to the dipole-dipole interaction the nonlinear two-photon absorption can2-9 be switched from one peak (fig 7a) to two-peak (fig 7b, 7c) spectrum. We have also studied the quantum coherence and interference phenomena in linear [12] and nonlinear photonic crystals [13] doped with nanoparticles. We have predicted many new effects such as switching, inhibition of s2-7pontaneous emission; the photon-atom bound state, giant refractive index; anomalous electromagnetically induced transparency and the anomalous Stark effect. The present findings can be used to make new types optoelectronic based on one- and two- photon switching.


Electronic and Bio Nanomaterials: We have also developed theoretical models for charge and spin transport due to equilibrium and non-equilibrium processes in semiconductor nanowires and quantum wells [14]; DNA nanowires and DNA derivatives [15]. The theory is able to explain the existing experiments. Excitons phase transitions in semiconductor heterostructures has also been studied.

Photonic Heterostructures

We study the photonic, plasmonics and optoelectronic properties of nanoparticles and DNA molecules doped in optical heterostructures (nanowires, nanowells and nanocavities). Optical heterostructures are fabricated by embedding a linear or nonlinear polaritonic (photonic) material into other polaritonic (photonic) materials. The band gap of the embedded materials lies with the band gap of the host material. Heterostructures fabricated from metals and biomaterials will be also comprehensively studied. Doped nanoparticles are not only interacting with each other via dipole-dipole interaction but also interaction with bound polaritons, photons and plasmons states of the heterostructures. This research area can be called as nano-photonics, nano-polaritonics and nano-plasmonics. This is a new research and has a p2-9otential application in making new types of optoelectronic, photonic, biotronic and plasmonics devices such as switches in the range of femto- and atto-seconds. We will also include in our study other types of heterostructures which are fabricated by embedded two or more optical materials (Fig). The embedded material can be also a metallic structure or DNA wires. We will apply a probe laser to study the absorption and emission properties of these systems. A control laser will be applied to manipulate the coupling between nanoparticles and heterostructures. These systems are very complex where lasers, nanoparticles and heterostructures are interacting with each others. Some groups have fabricated optical fibers by embedding a dielectric material into a photonic crystal . A comprehensive study of the following major topics is proposed.