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A schematic image of DFB Si laser; Inset: photograph of a fabricated DFB device. (b) Emission spectra of the Si laser as a function of pump power; Background: a cross-sectional SEM image of the DFB structure.

Integrated Si photonics incorporates the essence of the two pillar industries of "microelectronics" and "optoelectronics", which is expected to bring new technological revolution in a variety of fields such as communication, sensing, lighting, dispalay, imaging, detection, etc. Si lasers are the key to achieve integrated Si photonics. However, the optical gains of Si are lower than those of III-V compound semiconductors by one order of magnitude or two, due to its indirect bandgap feature. Although the fabrication of matured III-V compound lasers on Si substrates has been proposed to circumvent this problem, the development of all-Si laser is still in high demand for integrated Si photonics, due to its better compatibility with modern Si techniques.

Recently, a joint research team led by Prof. X. Wu, Prof. M. Lu and associate Prof. S.-Y. Zhang from Fudan University developed the world's first all-Si laser using Si nanocrystals with high optical gains. First, they enhanced the Si emission intensity greatly by developing a film growth technique for high-density silicon nanocrystals (Physica E, 89, 57-60(2017)). Then, they developed a high-pressure low-temperature passivation approach, which contributed to a full saturation of dangling bonds, leading to increased optical gains that were comparable to those achieved by gallium arsenide (GaAs) and indium phosphide (InP). On this basis, they designed and fabricated distributed feedback (DFB) resonance cavities and successfully achieved optically pumped all-Si DFB lasers. The optically pumped all-Si laser also paves the way towards the realization of electrically pumped all-Si laser.

It was found that the optical gain of Si nanocrystals was constantly enhanced as the passivation proceeded and eventually reached the value comparable to those of GaAs and InP. Lasing characteristics - the threshold effect, the polarization dependence, the significant spectral narrowing and small spread of divergence angle of stimulated emission - were fulfilled, suggesting the realization of an optically pumped all-Si laser. The lasers also showed reliable repeatability. The lasing peaks of the four additional samples made under the similar fabrication conditions were within the spectral range of 760 nm to 770 nm. The variation in the lasing peak was due to the slight difference in effective refractive indices. The full-width-at-half-maximum (FWHM) of the emission peak was narrowed from ~120 nm to 7 nm when the laser was pumped above threshold. This program is supported by the National Natural Science Foundation of China (51472051, 61275178, 61378080, 61705042) and Shanghai Sailing Program (16YF1400700).

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