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  • Molecules Made Mini Lasers
Oct 2009
CAMBRIDGE, Mass., Oct. 21, 2009 -- A new optical microscopy technique squeezes photons out of nonfluorescent molecules to provide 3-D images of living cells and tissues for applications in medical imaging and biological research.

The method, developed by Xiaoliang Sunney Xie and colleagues including Wei Min at Harvard University, exploits stimulated emission, during which a photon hitting an atom causes an electron to drop an energy level and results in the creation of another photon. Stimulated emission can be applied to all molecules, which gives the technique a big advantage over other microscopy methods that either use fluorescent molecular tags (fluorophores) or use nonfluorescent techniques that are much less sensitive.

Ex vivo stimulated emission image of blood vessels of a mouse ear based on natural hemoglobin contrast (in red color), surrounding sebaceous glands (green overlay based on confocal reflectance) (Images: W. Min/S. Lu, Harvard University)

Using stimulated emission could give researchers the opportunity to study many nonfluorescent colorful molecules that have not, until now, been seen in superresolution microscopy.

In a paper appearing in the Oct. 22 issue of Nature, Sunney Xie and his group irradiated molecules in a biological specimen with ultrashort pulses of light. Molecules that absorb a photon enter an excited state, and are then irradiated by another light pulse of slightly lower energy. When a photon from the second pulse interacts with an excited molecule, the molecule relaxes back into its ground state by emitting a duplicate photon. The excitation pulse photon is absorbed and lost, while the second-pulse photon adds a twin to the transmitted light, and the gain in photons is measured to quantify the number of molecules in the sample.

Ex vivo stimulated emission image of blood vessels of a mouse ear based on natural hemoglobin contrast (in red color), showing individual blood cells in the vessel network (inset) surrounding sebaceous glands (green overlay based on confocal reflectance)

The researchers show that the technique can be used to monitor the delivery of a particular nonfluorescent drug across the skin, highlighting distribution of the blue dye at both cellular and tissue levels. They also demonstrate detailed imaging of the blood vessels in a nude mouse ear, tracking the position of single red blood cells within individual capillaries and raising the prospect of 3-D mapping of blood oxygenation levels.

"...Min and colleagues’ method is a bold step towards unveiling details of live cells and tissues that would otherwise be left uncharted," said Stefan W. Hell and Eva Rittweger in a "News and Views" article appearing in the same issue.

"An intriguing possibility for the future would be to design a set of laser pulses that fulfill both roles of stimulated emission — switching off molecular signals and stimulating photon emission — to provide images of unlabelled, nonfluorescent molecules at subdiffraction (nanoscale) resolution for the first time," added Hell and Rittweger, who work in the department of nanobiophotonics, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.

Earlier this year in another Nature paper, Sunney Xie and colleagues described a microscopy technique based on stimulated Raman scattering that achieves label-free imaging with very high sensitivity.

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1. A single unit in a device for changing radiant energy to electrical energy or for controlling current flow in a circuit. 2. A single unit in a device whose resistance varies with radiant energy. 3. A single unit of a battery, primary or secondary, for converting chemical energy into electrical energy. 4. A simple unit of storage in a computer. 5. A limited region of space. 6. Part of a lens barrel holding one or more lenses.
A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
1. The process by which an atom acquires energy sufficient to raise it to a quantum state higher than its ground state. 2. More specifically with respect to lasers, the process by which the material in the laser cavity is stimulated by light or other means, so that atoms are converted to a semistable state, initiating the lasing process.
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
Pertaining to optics and the phenomena of light.
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
stimulated emission
Radiation similar in origin to spontaneous emission but determined by the presence of other radiation having the same frequency. Because the phase and amplitude of the stimulated wave depend on the stimulating wave, this radiation is coherent with the stimulating wave. The rate of stimulated emission is proportional to the intensity of the stimulating radiation.
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