June 13, 2011
In a new study published today in Nature Photonics, scientists from Wellman Center for Photomedicine and Harvard Medical school have developed a “living laser” by using biological materials- human cell and jellyfish fluorescent protein. In order to get coherent beam of light from a lasing device, three things are required- a pump source (typically flash lamp, electric current or other laser source), a ‘gain media’ for amplifying the source (optical gain) and an ‘optical cavity’ for concentration and alignment of the laser beam. Typically, crystals, dyes, gas mixtures and even alcohol have been used as gain media to amplify the light. Pumping source produces population inversion in the gain media wherein majority of the atoms and molecules are in excited state. When a photon of appropriate wavelength interacts with such a system of atoms/molecules, stimulated emission occurs. In a very novel approach, researcher Malte Gather has used Green Fluorescent Protein (GFP) and inserted the protein in living human embryonic kidney cell. Bioluminiscent GFP was first isolated from jellyfish in 1962 which ultimately resulted in Noble prizes for it’s discoverers. The kidney cell-GFP combo (gain medium) was then kept in between an optical cavity made of mirrors kept 20 microns apart. The dimension of single cell gain media was also about the same. When researchers hit the cell with low energy pulses of blue light using a microscope, typical diffused ordinary fluorescence was observed. But after reaching a certain threshold of input energy (~0.9 nJ), the light output from the cell changed drastically and resulted in bright, directional and narrowband emission of green light, which are the characteristics of a laser beam. Certain regions of cell showed intense lasing action (as shown in picture above) which occurred at different but close range of wavelengths (~514-519 nm). Researchers also reported that even after prolonged lasing action, the cell was still alive. The lasing action lasted for few nanonseconds and was easily detectable. The cell was able to lase about 100 pulses at excitation pulse energy of 50 nJ after which photobleaching occurred and depleted the GFP. But an interesting aspect of GFP infused cell is that the cell is able to heal itself and replenish GFP with time.
Here is an output emission spectra of the laser filled with purified GFP solution when pumped using different wavelengths of light. As can be seen that the output spectra is independent of the pump wavelength. This spectra tells two things i) independence of excitation wavelength on the emission rules out any stimulated scattering process as an explanation for lasing action by GFP, ii) FWHM ( Full Width at Half Maximum) of about 12 nm signifies the presence of simultaneous oscillations of various longitudinal modes.
Next figure shows that replacing the GFP solution with GFP-transfused cell resulted in much narrow output spectra. At energy threshold of 0.9 nJ, single emission peak was observed at 516 nm (FWHM <0.04 nm). As the energy was increased, multiple emission peaks were observed which can be attributed to multiple longitudinal oscillation modes. The spectral spacing between these emission line was in the range of 5 nm.
Researchers speculate that the resulting light could be used to study various intercellular processes. Before producing output light, the light travels several times through the cell placed inside the optical cavity and the resulting lasing light should contain information regarding the intercellular processes. Another possible use could be to produce such lasing beams inside the body itself to kill certain cancerous cells .
Creators of living laser, Yun and Gather, have some broad and speculative ideas about how the technology might be used.
They suggest that biologists could turn cells of interest into lasers to study them. The light produced has a unique emission spectrum related to both the structure of the cell and the proteins inside it. “By analysing the pattern you can get some idea of what is happening inside the cell,” says Yun.
The researchers also suggest possible medical applications. Doctors today shine lasers into the body to gather images or to treat disease by attacking cells. Yun thinks that lasers could instead be generated or amplified inside the body, where they could penetrate the relevant tissues more deeply. [Nature News]
Image credit: 1) Malte Gather | Nature Photonics | Wired 2) From the supplement files of the article provided on Nature website 3) Snapshot of the plots as seen at Nature website