New laser discovery shines unexpected light on laser research

Posted by Rick C. Hodgin

Princeton (NJ) - A research team led by Princeton University has discovered a new type of double-beam laser that is not explained by existing theories. The findings were most unexpected and have now proven the second laser beam is actually more powerful and more efficient than the primary. Research is continuing at a feverish pace to bring this quantum cascade laser into reality, as it would require less power, be far more powerful while operating in a way less susceptible to temperature changes (compared to conventional lasers).





Conventional laser



The team was working with something called a quantum cascade laser. This involves a series of atomic layer silicon deposited on top of each other. The device is quite tiny, at just 1/10th the size of a hair in diameter and 3mm long.






The conventional portion of the laser operates like those found in video and CD players. Electrical current is run through gallium arsenide and, when enough electricity is passed through, electrons enter a "quasi-equilibrium state" where they have almost zero quantum momentum. In that state, they start lasing (emitting laser light) in the mid- to far-infrared range.



In 2007 while researching this device (built of silicon, not gallium arsenide), a second laser beam of a slightly smaller wavelength was discovered.



No existing quantum cascade theory of laser operations explained that there should be a second beam. The team researched it and found that when the primary beam was more powerful, the secondary beam was less, and vice-versa. They also observed that the secondary beam was less sensitive to temperature change and actually increased in power as temperatures rose (to a point).





New laser understanding



The second, unexpected beam has proven more efficient and powerful than the first when considering the amount of power applied. In fact, it's demonstrating that this type of laser doesn't have to be in the "quasi-equilibrium state" to begin lasing. The new laser, while not in that state, only re-absorbs only 10% of the emitted lasing photons (compared to the conventional laser's reabsorbtion rate), making it far more efficient.



Intense study is continuing as this class of laser, mid- to far-infrared, is useful in detecting minute traces of water vapor, ammonia, nitrogen oxides and other gases that absorb infrared light.



The team believes future applications will include air monitoring, medical diagnostics, homeland security and other areas that "require extremely sensitive detection of different chemicals."





Funding



The research was sponsored in part by the Mid-Infrared Technologies for Health and the Environment (MIRTHE) center and National Science Foundation, as directed by Claire Gmacl who led the study. Additional support came from European Union's Marie Curie Research Training Network and its Physics of Intersubband Semiconductor Emitters (POISE) program, which brought Stefan Menzel to Princeton University from the University of Sheffield, UK, who physically built the laser.