This award supports theoretical research and education at the interface of condensed matter physics, quantum optics, and photonics. The PI and coworkers have developed a novel self-consistent laser theory, which finds the steady-state of a laser with a cavity of arbitrary complexity and openness without doing time-dependent simulations. The theory treats the openness of the cavity exactly and the non-linear interactions in the cavity to all orders with a single, well-justified approximation. Recent comparisons to exact time-dependent numerical simulations of the lasing equations confirm the accuracy and validity of the approach. The PI has applied the theory to treat diffusive random lasers, a system which resisted conventional approaches because of its extreme openness. Applications of the theory to shape design of dielectric cavity lasers, to speckle correlation measurements in media with gain and to tunable random lasers will be studied. In addition, a full statistical theory of random lasers will be developed, to characterize their frequency distribution, number of lasing modes, the stability of their frequencies and modal intensities under changes in pump and impurity configuration. Finally, the theory will be extended beyond the semiclassical limit to include field quantization effects, which determine the laser linewidth and noise properties. A long-term goal is to develop a realistic laser simulation code based on the theoretical model developed by this work, which could have very wide technological impact.
NONTECHNICAL SUMMARY This award supports theoretical research and education with potential impacts on the fields of quantum optics and photonics. The research supported by this award has the potential to change our basic understanding of lasers, which are used in a wide range of technologies including CD players and some surgical tools. Unlike a light bulb, lasers generate light waves in which each wave is in lock-step with another. This property of coherence enables device technologies that exploit the phenomenon of interference for their operation. This research project develops a theory for a new kind of laser, random lasers which, like other lasers, emits coherent light, but unlike other lasers, in random directions. Ordinary lasers use strategically placed precision mirrors to trap laser light in a gain material to amplify the light. In contrast, light in random lasers is trapped in the gain material by the laser light being scattered from randomly positioned scattering centers in that material. This notion of localizing light has analogies to electrons being trapped in localized quantum mechanical states in imperfect materials ? a phenomenon studied in condensed matter physics with aspects that keep the problem at the frontiers of the field. The PI?s research has potential impact on newly developed lasers that are about a ten-millionth of an inch in size and has many potential practical applications, including detecting injuries in human tissues, revealing chemical impurities in water, and determining the authenticity of documents.
The PI aims to develop a realistic laser simulation code based on this theoretical model, which would have wide technological impact, with the code then used for laser design and optimization. This research lies at the cutting edge of science and optical engineering, and will possible real world applications that would benefit society.
This multi-year project made critical contributions to a new quantitative theory of lasers for the purposes of understanding and designing novel laser systems. Laser light sources play a crucial role in modern communications and information processing technology, as well as in many industrial processes. While the most basic types of lasers date back to the sixties and seventies, there continues to be enormous progress in developing new laser systems for specific technological and scientific purposes. In particular there has been an explosion in the development of micro and nano-structured lasers with much more complex resonators for trapping and amplifying light, and with novel types of gain media for producing light. Conventional laser theory was formulated for the simpler macroscopic laser systems that were all that existed prior to the past two decades of invention. Hence these theories were not general enough to treat the current generation of micro and nanolasers. The theory developed in this project, Steady State Ab Initio Laser Theory (SALT), fills this gap, and unifies the description of all lasers through a single universal set of coupled non-linear equations. In addition we have developed a set of computational algorithms to solve these equations and predict the important properties of lasers that determine their suitability for various applications. Specifically, for a given amount of input energy (pump power) to the laser, SALT can predict how much output power the laser will product in the form of light, how many different frequencies of light it will generate, and exactly how the output power will be distributed in space. This requires the treatment of large non-linear effects that were beyond the reach of previous theories. An extension of SALT to include the effects of quantum fluctuations, predicts the linewidth of the laser (i.e the degree to which its frequencies are uncertain). SALT is already being used in the design of laser sources for medical imaging applications. It is our hope that SALT will become the standard approach to almost all laser design and simulation going forward, and will also provide a unified framework for teaching the principles of laser systems to future generations of scientists and engineers. The development of SALT led to an exciting conceptual "spin-off" from this project: a new idea of controlling absorption of light. We showed that if one starts with a laser, containing an amplifying gain medium, and replaces the gain medium with an absorbing medium with the same amount of loss (typically this is quite a small amount) that the resulting optical device would perfectly absorb specific input patterns of light, but would hardly absorb at all other input patterns of light. Such a device, which is the "time-reversed" twin of the original laser was termed a "Coherent Perfect Absorber" (CPA). The basic principle of the CPA was confirmed experimentally in collaboration with colleagues. It is now being explored for applications in communications, biomedical imaging, spectroscopy and sensing.
