Four-wave mixing in quantum dot lasers epitaxially grown on silicon

Photonic integrated circuits (PICs) on silicon can significantly advance the level of component integration and performance for both conventional and quantum information processing. The advantages of silicon-based PICs are the availability of manufacturing approaches using modern nanofabrication techniques as well as the potential for miniaturization and integration of optoelectronic components with complementary functionalities.

 

In this situation, quantum dot (QD) nanostructures are a highly promising semiconductor material that can be integrated either monolithically or heterogeneously on a compact and scalable platform. In particular, QD lasers epitaxially grown on silicon have recently exhibited record performance such as ultra-low threshold currents, high temperature continuous-wave operation, very long device lifetimes, as well as high yield.

 

Within a nonlinear gain medium that has third-order nonlinear susceptibility, the beating between two co-polarized fields at different frequencies results in the occurrence of four-wave mixing (FWM) and the generation of two new fields. FWM is an important approach for optical frequency comb generation, which are key components in coherent communication technologies and wavelength division multiplexing (WDM) systems. FWM is also responsible for the phase and mode locking properties observed in comb QD lasers.

 

In the case of WDM systems, a single-section mode-locked QD laser that is self-mode-locked produces a high-bandwidth frequency comb, which can potentially replace the large number of lasers presently necessary for the task. However, serious challenges must be addressed before the self-mode-locking technology is commercially available.

 

For instance, the spontaneous emission along with the group velocity dispersion of the gain medium can largely affect the FWM efficiency. In this context, a deeper understanding than we have presently of the intricate interplay of physics associated with mode competition and FWM is required to control the self-mode-locking in QD lasers. For QD systems, the basic understanding of FWM is limited by the conventional investigation method, which concentrates on the FWM coefficient measured with semiconductor optical amplifiers (SOAs).

 

To addresses this weakness, the research group from Institut Polytechnique de Paris (France) together with Sandia National Laboratories (USA) and University of California Santa Barbara (USA) have proposed and demonstrated the QD laser experiments to account for all optical nonlinearities contributing to the FWM signal. The research results are published in Photonics Research, Volume 10, No. 5, 2022 (Jianan Duan, Bozhang Dong, Weng W. Chow, Heming Huang, Shihao Ding, Songtao Liu, Justin C. Norman, John E. Bowers, Frédéric Grillot. Four-wave mixing in 1.3 μm epitaxial quantum dot lasers directly grown on silicon[J]. Photonics Research, 2022, 10(5): 05001264).

 

This work compared undoped and p-doped epitaxial QD lasers on silicon as well as a GaAs-based QW laser. A comparison with first-principles based multimode laser theory indicates measured FWM conversion efficiencies that are close to the theoretical limit. As shown in the figure, results reported reveal that the FWM efficiency is higher for lasers with p-doped active regions than those with undoped active regions, despite the same value of FWM coefficient.

 

Figure. Four-wave mixing conversion efficiency and coefficient for p-doped quantum dot laser, undoped quantum dot laser, and quantum well laser

 

Furthermore, owing to the near-zero linewidth enhancement factor, the measured FWM coefficient and conversion efficiency of the QD laser are more than one order of magnitude higher than those of the QW laser. This leads to stable self-mode locking in QD lasers. The net FWM bandwidth of the QD laser is also twice larger than that of its QW counterpart. It is worth stressing that the FWM efficiency of QD laser has not attained its theoretical limit yet, which can be further improved.

 

"We believe that there have been many papers on FWM measurements in SOAs for the purpose of investigating the semiconductor active medium for production frequency combs and mode-locked pulse trains. The point of our investigation is that eventually the devices will be lasers and therefore the FWM gain should be measured in lasers to get the true picture of mode-locking in the presence of other optical nonlinearities occurring during lasing. As SOAs operate in the amplified spontaneous emission regime, they can only give a partial answer. As such, our work focuses for the first time on the epitaxial QD lasers on silicon and refers to the earlier papers for connection to SOA measurements." said by Dr. Jianan Duan.

 

Overall, these findings highlight the strong potential of light emitters made with QDs for mode-locked pulses and optical frequency comb generation, which is crucial for integrated WDM technologies on silicon in future photonic integrated systems. Further analysis could possibly extend this work to semiconductor-based quantum information systems. For instance, the high FWM coefficient in QDs can be used for light squeezing to reduce noise below the standard quantum limit.