By: Yvonne Carts-Powell
An efficient and compact light-emitting device can be tuned across 56 nm -- changing color output from red to green, or green to blue -- and the researchers expect that further development could yield a source that tunes across the entire visible spectrum. The device could be used as a wavelength converter or as a tunable light source.
Richard H. Friend, H. Becker, and T. D. Wilkinson of the Cavendish Laboratory and Cambridge University (both in Cambridge, England) created a microcavity in which an optically-pumped photoluminescent polymer created wide-spectrum output, while an AC voltage applied across the cavity controlled the output wavelength.1
The tunable microcavity is a thin glass sandwich containing a luminescent polymer thin film and nematic liquid crystal (see figure). The inner side of each glass substrate is coated with a transparent electrical contact and mirror. When blue or ultraviolet light excites the poly(p-phenylenevinylene) polymer, the material emits a broad visible spectrum of light that peaks at 550 nm.
The cavity length for the device is only a few times longer than the emission wavelength. Such microcavities emit at resonant wavelengths. The spectral widths for the cavity modes are fairly narrow -- about 6nm (FWHM). Output from a broadband emitter in the cavity is enhanced for resonant wavelengths by as much as three times the free-space radiative rate, and suppressed for other wavelengths. Thus, microcavity emitters offer a more efficient alternative to generating light from a broadband source and filtering out all of the unwanted wavelengths. A blue microcavity emitter, for example, would be more energy efficient than a blue bulk light-emitting diode (LED).
In addition to the increased efficiency, the effective optical length of the microcavity cavity can be electrically controlled, allowing users to tune the resonant wavelengths. The voltage aligns the liquid crystal molecules, which changes the effective refractive index, and thus changes the effective cavity length. When an AC bias voltage from 0 to 16 V was applied to the Cavendish devices, the twelfth cavity mode shifted spectrally from 616 nm to 560 nm.
A 90-nm tuning range would allow the device to tune from blue to red wavelengths -- effectively across the visible spectral range. To build microcavities that are tunable over the entire visible spectrum, researchers must develop materials with larger proportionate changes in refractive index. In addition, cell thicknesses must shrink from the current 3.5 microns to 1.23 microns.
1. R. Friend et. al., Applied Physics Letters, 72, p. 1266, (1998)