UV LEDs Suitable for Fluorescence Spectroscopy
Anne L. Fischer
LEDs that emit in the 340-nm wavelength range have been developed by scientists in the division of engineering and department of physics at Brown University in Providence, R.I., and in the department of electrical engineering at Yale University in New Haven, Conn.
Applications for a new generation of UV LEDs are beginning to emerge, but they have been limited to wavelengths of approximately 360 nm. Achieving 340-nm emission has been the goal of researchers involved with fluorescence spectroscopy of biomolecules because it’s a peak absorption wavelength of the molecule NADH, whose spectra and spectrotemporal features help identify pathogenic bioagents.
The transient fluorescence of the biomolecule NADH, measured by photoexcitation using subnanosecond pulses from an LED (blue), displays different lifetimes in different solvent environments: water (red) and propylene glycol (violet).
The work, part of the Defense Advanced Research Projects Agency’s Semiconductor Ultraviolet Optical Sources program, is based on quaternary AlGaInN quantum-well active media. Other researchers have worked on III-nitride-based UV emitters, but the quaternary AlGaInN system had not been developed because of a perceived difficulty in controlling the defects in electronic transport and the radiative processes in the layers within the quantum-well heterostructures. The scientists at Brown and Yale have optimized the structure and process to minimize the impact of these defects and have achieved output powers of up to 1 mW from small-area devices on a planar chip.
They used a typical LED structure with an AIN buffer prepared by a two-step process. First, AIN was grown at 1150 °C on a 20-nm nucleation layer deposited on sapphire at 500 °C. This template was followed by the growth of the active region, which consisted of three quantum wells, with the aluminum content separated by 4-nm-thick Al0.22GaN barriers. They processed the wafers to form cylindrical mesa-etched LEDs, with light emission extracted through the transparent sapphire substrate.
The scientists studied devices that had apertures ranging from 10 to 100 μm. A typical emission spectrum of a 50-μm-aperture device under CW room-temperature conditions displayed dominant quantum-well emission near 342 nm. Other devices operated at approximately 330 nm with comparable performance.
They demonstrated the application of these devices in transient fluorescence measurements in coumarin dye across its emission spectrum, using the UV LEDs as sources of impulsive photoexcitation. They collimated the output, measured at a pulsed current density, through a short-pass filter to eliminate the long-wavelength emission tail. They then focused it onto spectroscopic cuvettes with coumarin dye. The absorption maximum nearly coincided with the peak in the LED emission spectrum.
They also used the 340-nm pulsed LEDs to measure the fluorescence lifetime of NADH. They found a good match of the excitation source to its absorption maximum. These results are significant for the potential use of compact UV-fluorescence-based tools for identifying spectrally identifiable fingerprints, such as in airborne pathogens.
Further challenges include increasing the internal quantum efficiency to enhance the optical output power and to reduce heat generation. In addition, the group plans to focus on some effects of current crowding that can lead to spatially nonuniform UV emission, which is undesirable in applications that require a high optical flux density.
For example, it is important that the emission be uniform to image the light output pattern on a particle containing biomolecules. If this is not the case, an additional technique must be employed to homogenize the image.
Applied Physics Letters, Aug. 23, 2004, pp. 1436-1438.
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