Color center lasers are an attractive tunable laser source owing to their very broad emission spectrum. However, typically their operation is restricted to near-infrared wavelengths and cooling is often required to prevent damage. A significant step towards room-temperature color center lasers in the visible has now been taken by chemists from Kobe University in Japan1, who demonstrate laser emission from MgO microcrystals.

Color centers are defects in crystals, where the formation of a vacancy leads to trapped electrons in the defect. The electrons absorb light of a broad range of wavelengths as they are excited into higher energy states, often leading to colorful crystals. The broad absorption and emission spectrum of these electrons enables their use as crystals in tunable laser sources.

To bring color center lasers towards visible wavelengths, oxides are considered the material of choice. There, the color centers are characterized by a much deeper potential well and are thermally more stable, suggesting shorter operation wavelengths and higher operation temperatures. However, the problem is the difficulty of fabricating suitable oxide crystals containing a large density of color centers.

Colored MgO are grown by a surprisingly simple method based on the reaction between magnesium and SiO. Small crystals of SiO react with Mg, resulting in sublimates consisting mostly of colored MgO as well as Si waste. The sublimated crystals look grey in visible light but show a strong blue-green emission under UV light, ascribed to the color centers (Fig. 1).

Fig. 1: The photograph shows MgO crystals containing color centers. In visible light, no emission occurs. However, under UV irradiation the electrons in the color centers are excited into higher states and emit blue-green light.

Room-temperature lasing is easily achieved by optical pumping, which contrasts with attempts to synthesize colored MgO, where no stable color centers could be produced by conventional thermochemical methods.

In future research, the aim will be to translate this approach to other oxides. As the emission wavelength is strongly depending on the host crystal, the spectrum could be significantly expanded. “Applying the present strategy to other relevant oxides we could in principle create color center lasers that cover a spectral range from UV to the near-infrared,” says Takashi Uchino, who led the project. Further developments in this field may therefore lead to a whole range of novel tunable lasers in the visible.