Kirchhoff’s law of thermal radiation states that the thermal emission spectrum of a real object at a temperature T is: Ireal(ω,T) = A(ω) x IBB(ω,T), where A(ω) is the absorptivity spectrum of the object and IBB(ω,T) the thermal emission intensity spectrum at a frequency, ω. The thermal emission intensity of the object can be modulated by changing its temperature, but this is difficult to do at frequencies higher than 100 Hz because the rate at which temperature can be increased or decreased is limited by heat transport between the object and its surroundings. But, Kirchhoff’s equation implies that we can much more quickly control the thermal emission intensity if we rapidly change the absorptivity of the object at a constant temperature.

Susumu Noda and colleagues have now made a device, made of doped quantum wells (QWs) incorporated into a p-i-n diode and a 2D photonic crystal slab, in which they can do just this.

QWs are tiny pieces of semiconducting material, and photonic crystals are nanostructures in which periodic variations of the refractive index on the visible length scale produces a photonic "band gap". This gap affects how photons propagate through the structure and is similar to the way in which a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In the case of photonic crystals, light of certain wavelength ranges can pass through the photonic band gap while light in other ranges is reflected.

“Optical transitions induced by electrons in the QWs and an optical resonance in the photonic crystal enhance light-matter interactions at a specific wavelength, which leads to a very narrowband thermal emission peak,” explains Noda. “We found that as we increase the number of electrons in the QWs, the light-matter interaction (or absorptivity) becomes even stronger, so increasing the thermal emission intensity even more. The opposite is true if we decrease the electron density in the QWs: the light-matter interaction weakens, which decreases the thermal emission intensity.”

The researchers control the electron density in the QWs by applying an electric field to the p-i-n diode. They first tested their device by imaging it using a thermal camera and a Fourier-transform infrared spectrometer. They then showed that they could modulate the thermal emission intensity of the device at high speeds (600 kHz) by applying an AC voltage signal from a pulse generator to it. They measured the emission power using HgCdTe detectors.

“The high-speed narrowband thermal emitter we have developed could mean the end of expensive and cumbersome infrared optical components, such a mechanical choppers, bandpass filters and monochrometers,” Noda tells nanotechweb.org. “The device might help us make more compact and highly-efficient systems for a number of applications, including chemical analysis, biosensing, thermal imaging and environmental monitoring.”

The Kyoto team says that it is now busy trying to make high-speed narrowband thermal emitters that work in a wider spectral range – from the near infrared to the far infrared. “Such different colour devices will allow for ultrafast wavelength scanning in the entire infrared range,” says Noda.

The researchers report their work in Nature Materials.