Magnetic-Field-Modulated Tuning of Light-Emitting Properties

by | Nov 23, 2017

Researchers from Hong Kong Polytechnic University and the Hong Kong Polytechnic University Shenzhen Research Institute achieve remote and temporal tuning of luminescence intensity and wavelength in green- and blue-emissive piezophosphors by modulating the magnetic field. This novel method is promising for applications in magnetic optical sensing, piezophotonics, energy harvesting, nondestructive environmental surveillance, novel light sources, and displays.

Luminescence is a grand display in nature; it can be seen in the sky above or in insects to help them attract a mate. But it also plays a role in our daily lives—in solid-state lighting, sensors, displays, and bioimaging, for example. The ability to control the color and intensity of luminescence is a desirable feature, especially if it can be achieved in a fixed single-phase phosphor.

In a communication in Advanced Materials, Man-Chung Wong, Jianhua Hao, and co-workers from the Hong Kong Polytechnic University and the Hong Kong Polytechnic University Shenzhen Research Institute demonstrate remote and temporal tuning of luminescence intensity and wavelength in green- and blue-emissive piezophosphors through modulating magnetic field, achieving proof-of-concept devices including tunable white-light and red-green-blue full-color displays.

The laminate phosphor composite is composed of a layer of metal-ion-doped (Al, Cu) ZnS piezophosphors embedded into polydimethylsiloxane (PDMS) and an adjacent layer of soft ferromagnetic particles mixed into PDMS. When the frequency of a low magnetic field is remotely modulated, the ferromagnetic-particles layer generates a magnetostrictive strain that induces a piezophotonic effect—and thus light emission—in the neighboring piezophosphor layer. Increasing the excitation frequency of the magnetic field from 50 to 470 Hz causes a hypsochromic shift of the luminescence: the blue-emission intensity increases, while the green-emission is suppressed. Therefore, the emission spectrum can be reversibly tuned simply by alternating the magnetic field frequency. Additionally, increasing the root-mean-square magnetic field strength generally increases the emission intensity.

The researchers then demonstrate a tunable white-light and multicolor display by modulating the frequency of the magnetic field.  White-light emission is achieved by combining a blue-emissive magnetic-induced-luminescence (MIL) composite with a yellow fluorescence phosphor: cerium-doped yttrium aluminum garnet (Y3Al5O12). Under a different modulation frequency of the magnetic field, tunable warm- to cold-white light is achieved. Similarly, a full-color display was shown using green- or blue-emissive MIL composites with a europium-doped, red-emitting phosphor ((Ca1-xSrx)S: Eu).

To find out more about the magnetic-field tuning of piezophotonic luminescence, please visit the Advanced Materials homepage.

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