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Optical thermometer feels the heat

01 Oct 2002

Researchers reveal how a luminescent organic semiconductor acts as a versatile thermometer.

Scientists in Germany have created a molecular thermometer from a common organic semiconductor. By studying the emission spectrum of the material, the team claims the thermometer will allow thermal imaging of organic LEDs and laser diodes as well as nanometer-sized devices. (Applied Physics Letters 81 2478)

John Lupton and colleagues from the Max Planck Institute for Polymer Research in Mainz base their device on platinum octaethly porphyrin (PtOEP), a key ingredient of organic light-emitting diodes. When excited by ultraviolet light, PtOEP emits in two narrow bands centered on 540 nm and 650 nm.

"The intensity ratio of these peaks provides an absolute measure of the ambient temperature and increases by an order of magnitude as the temperature is raised 40 K above room temperature," explains Lupton.

To demonstrate the thermometer's potential, the researchers doped a 100 nm thick film of polystyrene with PtOEP. A 80 MHz mode-locked, Ti:sapphire operating at 380 nm excites the sample. A filtered CCD camera, gated to the excitation pulses, gathers the optical emission spectra.

"Using a standard spectrometer, the sensitivity is roughly 0.25 K," Lupton told Optics.org. "It should be possible to get this down to a few 10s of mK using phase sensitive detection." Using spin-coating, Lupton says a PtOEP layer could be spun onto a microchip and removed after any temperature measurement.

He now intends to apply the thermometer to a range of organic optoelectronic systems, such as polymer lasers. "My main aim is to miniaturize the collection volume required for thermal imaging and work out how many thermometer molecules I need to get a signal," he told Optics.org.

To date, the researchers have used their device to look at the non-radiative decay process present in a highly fluorescent conjugated polymer called PFO. "By blending PtOEP into PFO, we can look at the amount of energy that is absorbed but not dissipated in the form of photons," explains Lupton. "This energy has to be lost as heat, which can be measured by the thermometer."

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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