Progress in semiconductor quantum dots-based continuous-wave laser

REVIEWS . . . . . . . . . . . . . . . . . . . . . . . . . . SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 26 May 2020 | https://doi.org/10.1007/s40843-020-1336-6 Sci China Mater 2020, 63(8): 1382–1397 SPECIAL ISSUE: Optical Gain Materials towards Enhanced Light-Matter Interactions Progress in semiconductor quantum dots-based continuous-wave laser 1* 2 2 1* Lei Wang , Guang Dai , Luogen Deng and Haizheng Zhong ABSTRACT Continuous-wave (CW) operated laser is a key component for photonic chips. It has been highly desired to develop low-threshold, high-efficiency, long-term operational, low-cost and easily integratable CW laser. As excellent optical gain materials, quantum dots (QDs) have been intensively investigated for on-chip CW lasers owing to their high photoluminescence quantum yields, tunable emission wavelengths and easy integration. Base on the difference of preparation processes, QDs can be classified into epitaxial QDs and solution-processed colloidal QDs (CQDs). In this mini-review, we summarize the research progresses of epitaxial III–V semiconductor QD and solution-processed CQD-based CW lasers. The challenges associated with the realization of CQD CW lasers are discussed in detail. In particular, the emerging perovskite-based CW lasers are highlighted. Finally, a short perspective of QD-based CW lasers is presented. Keywords: quantum dots, continuous-wave laser, perovskite, photonics INTRODUCTION In response to the explosive growth of global information, the high-speed, high-density and low-cost information processing technique is intensively needed [1]. Photonic chips hold great promise to be the next generation data processing technology due to their fast responsibility. As described by Paul [2], a photonic chip at least includes the components with functionalities of light generation, transportation, and detections. Over the past decades, great success has been made in the on-chip light sources, high-bandwidth detectors, fast modulators and waveguides [3–8]. In comparison, the development of on-chip light source lags behind the other counterparts, which has become a great challenge for the field of integrated photonics. Continuous-wave (CW) lasers have been considered a remarkable breakthrough to achieve on-chip integrated light source. The epitaxial growth of III-V semiconductor quantum wells (QWs) on silicon (Si) substrates has enabled CW laser for chip-to-chip optical communications. However, the performance is limited due to the lattice mismatch and the thermal expansion coefficient difference between III–V materials and Si substrates [3]. The III-V quantum dots (QDs) have ability to overcome these hindrances [9–14]. The performance of III-V QD CW laser has been greatly improved in the past few years, which approaches the requirements of on-chip light source in near infrared (NIR) region. Along with that, the demand of CW laser also promotes the research of colloidal QDs (CQDs) as economical and color-tunable alternatives for CW laser applications [15]. The gain thresholds of CQD-based CW lasers have been greatly reduced by means of materials engineering. However, the thresholds are still far from the practical application. Recently, the halide perovskites are emerging as alternative gain media to reduce the threshold to achieve CW lasers [16–23]. The primary results have encouraged the entire community to pursue perovskite-based roomtemperature CW lasing. In this mini-review, we summarize the recent developments of CW lasers based on epitaxial QDs and solution-processed CQDs. In the first part, we describe the progress of III–V semiconductor QD-based CW lasers on different substrates, including germanium (Ge), Ge-on-Si and Si substrates with a following discussion about micro-disk (MD) CW lasers. In the second part, we review 1 MIIT Key Laboratory for Low-dimensional Quantum Structure and Devices, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China 2 School of Physics, Beijing Institute of Technology, Beijing 100081, China * Corresponding authors (emails: (Zhong H); (Wang L)) 1382 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 August 2020 | Vol. 63 No. 8 SCIENCE CHINA Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REVIEWS the advancement in the field of solution-processed CQDbased CW lasers and discuss the current challenges for room-temperature-operated CQD CW lasers. In the third part, the recent perovskite-based CW lasers are presented and discussed. Finally, we give a short conclusion with our perspectives for the future investigation. Instead of a comprehensive description, we try to give the milestones and future trends of QD-based CW lasers. III–V SEMICONDUCTOR QD CW LASERS The room-temperature-operated semiconductor CW lasing was firstly achieved using AlAs-GaAs heterostructure QW by Alferov et al. [24] in 1971. Since then, CW lasers based on QWs have been extensively explored. However, their thresholds are temperature sensitive. To solve this problem, Arakawa et al. [25] theoretically proposed the use of QDs as gain media to decrease the temperature sensitive threshold in 1982. Twelve years later, Kirstaedter et al. [26] fabricated the first QD-based CW laser using the Stranski–Krastanow growth method. They observed room-temperature lasing from (InGa)As −2 QDs with a threshold current density of 950 A cm . From then on, the development of QD-based CW lasers has drawn a great number of attentions from both of scientific and industrial fields. The zero-dimensional (0D) QDs can more easily Table 1 achieve population inversion compared with 2D or 3D materials. Semiconductor QDs represent particle-in-abox-like quantum-confined structures, exhibiting atomor molecule-like properties [27]. At quantum length scale, the electronic spectra of QDs show discrete density of states due to the quantum confinement effect, tending to delta-function-like energy levels. Similar to atoms, the ground state of QDs is doubly degenerated, which has a maximum occupancy of two electrons. Therefore, the 0D QDs have the lowest achievable gain threshold. Because of the high energy level spacing between the discrete energy levels, the carrier population is restrained to broaden into higher states [27]. Besides, the thermal spreading of carriers in QDs is decreased due to the deltafunction like state density. These characters make the threshold of QD lasers be far less temperature sensitive than that of conventional QW lasers [25]. With the efforts from all over the world, the performance of III-V QD-based CW laser has been greatly improved. Table 1 summarizes the milestones in the developments. In the following, we will describe the details in the material innovations on the growth. III–V semiconductor QD CW lasers on Ge or Ge-on-Si substrates In 2000, Kazi et al. [28] fabricated the self-formed InGaAs Represe (...truncated)