A new technique for cellular analysis incorporates biological cells in a semiconductor laser cavity to allow re searchers to rapidly identify cell types and shapes. Led by Paul Gourley, researchers at Sandia National Laboratories (Albuquerque, NM) have designed an optically pumped semiconductor laser structure in which cells are placed on an aluminum gallium arsenide/gallium arsenide (AlGaAs/ GaAs) vertical cavity surface-emitting laser (VCSEL) and topped with a dielectric mirror to form a laser resonator.
Biological cells are largely transparent at the 850-nm emission wavelength of the laser, so the cells act as intracavity waveguides and alter the distribution of the transverse cavity modes, which changes the emission spectrum of the laser. The length of the cavity is on the order of a few microns, short enough to support only one longitudinal mode. This is a different mechanism than that of fluorescence or absorption spectroscopy, in which a sample absorbs or emits at certain wavelengths when illuminated. In the biological microcavity laser, the dielectric properties of the cells are a critical part of the emission process itself.
Each cell type imparts a unique spectral signature to the laser output, so the technique can be used to rapidly identify cells based on one-dimensional spectra without need for actual images. It is, therefore, significantly faster than conventional systems—using a simple emission intensity (pulse-height) technique, the system can identify as many as 20,000 cells per second. To obtain more detailed information, a user can acquire spectra for individual cells. "We are doing this manually at this time," says Gourley. "With an automated system you could identify thousands of cells in a second."
Cell identification
The VCSEL consists of a multiple-quantum-well gain region atop a semiconductor mirror structure on a GaAs substrate. The biological microcavity laser thus formed is optically pumped with powers on the order of a few milliwatts by an indium gallium aluminum phosphide diode laser operating at 670 nm. Output powers from the microcavity laser vary with sample and quantum-well structure, but are typically several hundred micro watts to a milliwatt. A fiberoptic/photodiode array acquires spectral data, while a CCD camera captures coherent light images of the lasing modes imparted by the dielectric properties of the cells.
Initial experiments have included probing the human immune system to obtain nucleus dimensions of lymphocytes, quantifying normal and sickled red blood cell shapes, and distinguishing cancerous cells from normal cells. The technique could also be useful for pharmaceutical drug testing during development, allowing researchers to view cells in real time and watch their response to drug stimulation.
Research is now focusing on the development of an electrically pumped system, with the microcavity fabricated in the structure. The group is currently working on the problem of fluid-transport into the cavity—depending on the application, fluid could flow through the cell or be static. Using surface-tension transport, the group has obtained cavity filling times on the order of one second, demonstrating feasibility.
The researchers do not currently have commercial partners for the project. "We're still in the research stage developing this laser and finding out how to make it more useful," says Gourley, noting the reliability, compactness, and portability of the system. "I'd expect the technique to be used widely but it will take a lot of work to implement—we're talking years down the road. However, I think this is going to happen, and I think we`ll see development of new industries surrounding these and similar micro-optical and mechanical technologies for biomedical and environmental applications."