Light on the future: creating the next generation in laser technology

Picture Credit: Srinivas reddy

Lasers are the heart of modern communication technology. As the industry strives continuously to improve the speed, efficiency and compactness of communication equipment, it is constantly on the look-out for new materials, techniques and methods that help the cause. Photonic crystal lasers have caught the attention of scientists and engineers for that very reason

Picture Credit: Srinivas reddy

Photonic crystals are artificial structures with a periodic variation of refractive index. This periodicity can be in one, two, or three orthogonal directions resulting in 1-D, 2-D and 3-D photonic crystals. As a result of the periodic nature of the structure, dispersion for light propagation in the crystal will split into a series of allowed and forbidden bands. This in turn leads to light propagation being prohibited either in certain directions or in all directions, arising from a pseudo-photonic stop-band or a complete photonic band-gap.

The density of electromagnetic modes at a given frequency-- the local density of states-- is reduced inside the photonic band gap and increased near the band edges. With an active medium present inside photonic crystals, it is possible to tune the emission characteristics of the active medium precisely in the required frequency ranges depending on the extent of overlap of the emission band with the photonic stop band. Enhancement in the emission can be achieved near the band edges due to increase in density of states and can yield lasing at low thresholds.
The challenge in fabricating 3-D photonic band gap materials for visible and near-IR wavelength range can be overcome by the method of self-assembly. Active materials can be incorporated into the photonic crystals and their emission characteristics can be analyzed to design photonic crystal lasers. Low-threshold lasing can be attained near the band edges of the photonic crystals. To achieve this choosing the proper colloidal diameter and the dye are the main parameters. Added to that the scattering losses due to the domain cracks also play a major role to estimate the lasing threshold and are unavoidable in the self-assembled crystals. At the IITB-Monash Research Academy in Mumbai, Research Scholar Srinival Reddy has been doing pioneering experimental work, fabricating and characterizing the self-assembled active photonic crystals as well as modeling their lasing threshold characteristics. His study also includes designing the best structures to achieve ultra-low threshold tunable lasers.
Developing the ability to confine and control light inside a fiber optic cable brought about a revolution in telecommunications. Being able to design materials that respond to light waves over a desired range of frequencies, either by perfectly reflecting them or by allowing them to propagate only in certain directions, or by confining them in a specified volume, will precipitate the next quantum leap in telecommunication and laser engineering.

Says Srinivas, "The research in photonic crystal drew my attention due to its potential to create next-generation solutions like all-optical integrated chips for optical computers and hyper-efficient solar cells".
We fabricated the active photonic crystals with face center cubic structure, using the Rhodamine-B doped polystyrene colloidal spheres using self-assembly method. Experimentally and theoretically demonstrated the suppression in the spontaneous emission within the bandgap and enhancement near the band edges are achieved.
The fabricated 3D PhCs are characterized in terms of the number of layers and losses due to scattering effects. These parameters were estimated by calculating the reflection spectrum of 3D PhC using the Korringa-Kohn-Rostoker method, combined with complex permittivity of the material. Both these quantities are important to select self-assembled PhCs for lasing applications. The role of thickness and scattering losses in designing the self-assembled photonic crystals is studied. We also studied the lasing threshold characteristics of the active opal photonic crystals. We estimated that the 30 number of layers in the active PhC are enough to achieve low-threshold lasing.
Working under the guidance of Prof. Malin Premaratne, Dr. Ivan Rukhlenko, Prof. Vijaya Ramarao and Prof. Subhabrata Dhar, Srinivas has proposed and analyzed a PhC heterostructure cavity consisting of a gain-medium doped 3D PhC sandwiched between passive multilayers.
The defect mode of the cavity formed by the passive multilayers is chosen to overlap with the band edges of the sandwiched 3D PhC. It is well known that the available density of states is higher at the defect mode frequency due to the multilayer cavity, and also near the band edge frequencies of the sandwiched 3D PhC. When the cavity defect mode becomes resonant with the band edge region of the 3D PhC, the net availability of the density of states for that mode increases as compared to a stand-alone configuration.
As a result, a drastic decrease in the lasing threshold is expected for these modes. A decrease of two orders of magnitude in the threshold gain was achieved as compared to a stand-alone 3D PhC. The proposed cavity design holds an immense potential for realizing miniaturized PhC based compact chip lasers with an ultra-low threshold.

Picture Credit: Srinivas reddy

This research project has enormous significance in the area of communication. The development of miniature low-threshold lasers, optical gates, optical switches etc are very useful in designing all-optical chips for optical computers.
By controlling the radiation and allowing propagating them in the required direction, it is possible to decrease the unwanted radiation losses, which is a major concern in communication networks at present. By designing all-optical integrated circuits, which includes components like optical rectifiers, optical switches using photonic crystals one can overcome the optical to electrical or electrical to optical conversion losses present in the present day communication systems. Moreover by integrating the photonic crystals with plasmonics the size of the components can be minized.
Integrating the photonic crystals with solar cells will yield compact modules with unique directional properties and increased efficiency. Exploiting the reduced group velocity (increase in light matter interaction time), which enhances the absorption of incident light waves of the propagating electromagnetic waves in the photonic crystals can increase the efficiency of solar cells.
Non-linear effects also can be enhanced in the photonic crystals due to increase in light-matter interaction time and can be useful in designing components like all-optical switches, optical rectifiers and optical transistors.
In short, the progress made at the IITB-Monash Research Academy has far-reaching implications in the areas of communication and solar energy harvesting because it is a major step towards the ability to design materials that can control the flow of light.
The IITB-Monash Research Academy is a Joint Venture between the IIT Bombay, India and Monash University, Australia. Opened in 2008, the IITB-Monash Research Academy operates a graduate research program located in Mumbai that aims at enhancing research collaborations between Australia and India. Students study for a dually-badged PhD from both institutions, and spend time during their research in both India and Australia.

Research scholar: M. Srinivas Reddy, IITB-Monash Research Academy

Project title: Photonic crystal antenna: Design and Fabrication

Supervisors: cnu.munige@gmail.com

Contact details: Prof. Malin Premaratne, Dr. Ivan Rukhlenko, Prof. Vijaya Ramarao, Prof. Subhabrata Dhar

Contact research@iitbmonash.org for more information on this, and other projects.