The prinicple of Stimulated Emission of Radiation forms the basis of laser operation. Building on the 1900 work of Max Planck who first demonstrated that the energy of electromagnetic waves could be describled as discrete packets, or quanta , with energy proportional to frequency of the wave, Albert Einstein postulated the theory of Stimulated Emission. The then-new science of quantum mechanics defines discrete allowable energy states which electrons might occupy; an electron orbiting an atom in a higher, or "excited" energy state would eventually decay spontaneously to a lower energy state, releasing a quantum of energy in the process in the process. A large population of excited electrons would emit these quanta more or less randomly, a phenomenon known as fluorescence.

Einstein postulated that one of these quanta (later called a photon) striking an excited electron would cause the electron to decay, releasing a second photon identical in wavelength, phase and direction. In a population of atoms with excited electrons, a cascade of identical photons could be generated in this manner.

In the absence of an obvious application, stimulated emission was regarded as a curiosity for almost 30 years, but World War II spurred interest in the generation of microwaves for radar. Responding to the demand for very short (millimeter domain) microwaves, American physicist Charles Townes working at Bell Laboratories and Russian physicists Nikolay Basov and Alexander Prokhorov at the Lebedev Institute of Physics, independently proposed in the early 50's that microwaves could be efficiently generated by the stimulated emission of molecules. In 1954, Townes, working with James P. Gordon and Herbert Zigler at Columba University, built the first device producing Microwave (Molecular) Amplification by the Stimulated Emission of Radiation, or MASER from an energized stream of ammonia gas. (Townes, Basov, and Prokhorov shared the 1964 Nobel Prize for their work in this field)

By 1958,Townes and his colleague Arthur Schawlow had published a scientific paper and filed patents proposing that an "optical MASER" could be used to produce infrared or even visible light. However, just a few months before, Gordon Gould, then a graduate student also working at Columbia University, independently proposed the concept of an optical resonator using mirrors at either end of a linear cavity, allowing a gain medium to be optically pumped to maintain a population inversion and produce a collimated, coherent beam of light. Gould recorded his sketches and calculations in his laboratory notebook, referring to his device as a LASER (Light Amplification by the Stimulated Emission of Radiation). This notebook was to be the focus of a thirty year battle over patent rights to lasers and laser applications.

Inspired by Townes' and Schawlow's 1958 paper, Theodore Maiman, working at the Hughes Research Center, built the first laser using a ruby rod with mirrored ends optically pumped by a coiled photographic flash lamp sealed inside an aluminum cylinder. He announce this achievement at a press conference in New York City in July, 1960, and was dismayed at press reports of the invention of a "death ray".

What made Maiman's accomplishment especially remarkable was that he demonstrated the relative ease with which a laser could be constructed. He also introduced the concept of pulsed laser operation (until that time, the focus was on constructing a continuous wave "optical MASER), capable of delivery of large amounts of energy over very short intervals, vastly expanding potential applications including photomechanical interactions and non-linear optics.

An explosion of interest in lasers followed, and during the next few years, the Helium-Neon (HeNe) gas laser (Ali Javan, Bell Labs, 1961), the Nd:Glass laser (Elias Snitzer, American Optical, 1961), the first semiconductor laser (Robert Hall, GE Labs, 1962), the CO2 laser (Kumar Patel, Bell Labs, 1963) had been demonstrated.

As early as 1961, pioneers such as Dr. Leon Goldman began research on the interaction of laser light on biologic systems, including early clinical studies on humans, and his first paper "Pathology of the Effect of the Laser Beam on the Skin" was published in 1963. Goldman went on to explore various laser applications including tattoo removal, treatment of vascular and pigmented lesions, and helped develop the Ruby, Argon, and Copper Vapor lasers for medical use. Dr. Goldman went on to found the American Society for Lasers in Medicine and Surgery.

Interest in medical applications was intense, but low and sometimes erratic power output and the relatively poor absorption of these red and infrared wavelengths in tissue led to inconsistent and disappointing results in early experiments. The exception was the introduction of Ruby Laser clinical systems retinal surgery in the mid-60's. In 1964, the Argon Ion Laser was developed. This continuous wave 488nm gas laser was easy to control, and it's high absorption by hemoglobin made it well suited to retinal surgery. Clinical systems for treatment of retinal diseases were soon available, all but displacing the ruby laser for this application.

Experiments with the newly developed CO2 laser at American Optical Corporation in 1965 led to the introduction of the first surgical laser system in 1967. The CO2 laser is a continuous wave gas laser, emitting infrared light at 10600nm in an easily manipulated, focused beam that 's well absorbed by water. Because soft tissue consists mostly of water, researchers found that a CO2 laser beam could cut tissue like a scalpel, but with minimal blood loss. The surgical uses of this laser were investigated extensively from 1967-1970 by pioneers such as Drs. Thomas Polanyi, Geza Jako, Stanley Stellar, M. Stuart Strong and Charles Vaughn, as well as Leon Goldman, and by the the early 70's, use of the CO2 laser in ENT and gynecologic surgery became well established in academic and teaching hospitals.

By the late 70's and early 1980's, smaller, more powerful lasers were being deployed in community hospitals and even physician's offices. Most of these systems were CO2 lasers used for cutting and vaporizing, and Argon lasers for opthalmic use. Nd:YAG and KTP laser systems were used by some hospitals for the new field of laparoscopic surgery. However, these "second generation" lasers were all continous wave (CW) systems, tending to cause significant non-selective heat injury, requiring a long "learning curve", and offering only a marginal advantage of traditional treatment modalities.

The single most significant advance in the use of medical lasers was the principle of "Selective Thermolysis, first published in 1983 by R. Rox Anderson and Simon Parrish, who showed that applying laser energy in suitably timed pulses, rather than continuously, would allow selective destruction of abnormal or undesired tissue while leaving surrounding normal tissue undisturbed. The first lasers to fully exploit this principal of "selective thermolysis" were the Pulsed Dye Lasers introduced in the late 1980's for the treatment of port wine stains and strawberry marks in children, and shortly after, the first Q-switched medical lasers for the treatment of tattoos. The prinicple of Selective Photothermolysis transformed the paradigm of the medical laser from an expensive and complicated method of coagulating tissue to a precision tool for selective "microsurgery", with little or no collateral damage to normal structures. Treatment of vascular lesions was revolutionized by the advent of pulsed vascular lasers, and for the first time, effective, scar-free treatment of tattoos became a reality.

Scanning devices became generally available in the early 1990s, enabling precision computerized control of laser beams. Scanned and pulsed lasers revolutionized the practice of plastic and cosmetic surgery by making safe, consistent laser resurfacing possible, as well as increasing public awareness of laser medicine and surgery. Laser hair removal was introduced in the mid 90s, and quickly became one of the most popular applications of medical lasers. By the turn of the century, new wavelengths and delivery devices had been introduced and skin cooling methods became more sophisticated, as researchers, engineers, and physicians gained a better understanding of the interaction of light and biologic tissues.

Medical lasers and their applications are just a very small part of the infant field of photonics, but this new and valuable tool in the physican's armamentarium has made it possible to treat conditions which only a few years ago were difficult or impossible to treat. Patients benefit by improved results and less cost. In the last few years, the main focus of research and development of medical lasers has been on laser hair removal, multiwavelength treatment of vascular malformations, cosmetic applications including fractional skin resurfacing, laser-assisted lipolysis ("laser liposuction"), photodynamic therapy and vision correction..