A LIVING cell has been turned into a laser for the first time. The human kidney cell involved survived the experience, and though we are a long way from the laser eyes of Cyclops from the X-Men franchise, the achievement suggests that 鈥渓iving lasers鈥� might be created inside live animals. This could allow internal tissues to be imaged in unprecedented detail.
Typically, a laser consists of two mirrors on either side of a gain medium 鈥� a material whose structural properties allow it to amplify light. A source of energy excites the atoms in the gain medium, releasing photons. As these bounce back and forth between the mirrors, repeatedly passing through the gain medium, they stimulate other atoms to release photons of exactly the same wavelength, phase and direction. Eventually, a concentrated single-frequency beam of light erupts through one of the mirrors as laser light.
Hundreds of different gain media have been used, including various dyes and gases, and the 鈥渓iving laser鈥� is not the first to use an unexpected material in this role (see 鈥淢eet the edible, nuclear and anti-lasers鈥�). But no one has previously used living tissue. Mostly out of curiosity, Malte Gather and Seok-Hyun Yun of Harvard University decided to investigate using a single mammalian cell.
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They injected a human kidney cell with a loop of DNA that codes for an enhanced form of green fluorescent protein. Originally isolated from jellyfish, GFP glows when exposed to blue light and has been invaluable as a biological beacon, tracking molecules inside cells and lighting up when certain genes are expressed.
After placing the cell between two mirrors, the researchers bombarded it with pulses of blue light until it began to glow. As the green light bounced between the mirrors, certain wavelengths were preferentially amplified until they burst through the semi-transparent mirrors as laser light. Even after a few minutes of lasing, the cell was still alive and well (Nature Photonics, ).
Christopher Fang-Yen of the University of Pennsylvania in Philadelphia, who has worked on single-atom lasers but was not involved in the recent study, is impressed. 鈥淕FP is similar to dyes used to make commercial dye lasers, so it鈥檚 not surprising that if you put it in a little bag like a cell, and pump it optically, you should be able to get a laser,鈥� he says. 鈥淏ut the fact that they show it really works is very cool.鈥�
Yun has been mulling over a few possible applications. 鈥淲e would like to have a laser inside the body of the animal, to generate laser light directly,鈥� he says. This could provide higher-resolution images than current techniques, which either fire lasers from outside the body or rely on the fainter glow from living cells doped with GFP. The mirrors in Yun鈥檚 laser would have to be replaced with nanoscale slivers of metal that act as antennas to collect the light.
鈥淲e would like to have a laser inside the body of the animal, to generate laser light directly鈥�
鈥淧reviously the laser was considered an engineering material, and now we are showing the concept of the laser can be integrated into biological systems,鈥� Yun says.
Meet the Edible, nuclear and anti-lasers
The living laser is a first, but other strange lasers have been made since Theodore Maiman built the first such device from a fingertip-sized ruby rod. On 16 May 1960, Maiman blasted the ruby with a brilliant burst of light from a photographic flash lamp, generating a bright red beam.
About a decade later, two future Nobel laureates created the first edible laser 鈥� well, almost. Theodor H盲nsch and Arthur Schawlow tried 12 flavours of Jell-O dessert before settling on an 鈥渁lmost non-toxic鈥� fluorescent dye. When added to unflavoured gelatin, this yielded a bright laser beam when illuminated with UV light. Schawlow, who had snacked on the failures, gave the successful one a miss.
Around the same time, NASA wanted powerful lasers for beaming energy into space, and proposed powering these by exciting molecules with fragments from nuclear fission inside a small reactor. Pulses of up to 1 kilowatt were achieved before NASA abandoned the programme. The 鈥淪tar Wars鈥� missile-defence programme of the Reagan era later funded a project to develop reactor-powered laser weapons, but they never got off the ground.
In 2009, the world鈥檚 smallest laser was made at the University of California, Berkeley. It generated green laser light in strands of cadmium sulphide only 50 nanometres across, one-tenth of the wavelength of the light it emitted.
And don鈥檛 forget the anti-laser, from Hui Cao鈥檚 lab at Yale University, which soaks up light. Strange as it sounds, it may even have a practical use: converting optical signals into electrical form. Jeff Hecht