The characteristics of limited carrier-filling levels and radiation bandwidths in traditional quantum well structures result in some drawbacks in the applications of tunable lasers.

The band-filling effect of electrons and holes is an important physical mechanism to reveal the luminescence performance of semiconductor lasers, which is significant for evaluating the wavelength tuning ability. The band-filling level of non-equilibrium carriers is closely related to the energy band structure and material properties. To improve the wavelength tuning ability of semiconductor lasers, it is urgent to explore a new type of quantum confinement structure. Recently, the indium-rich cluster (IRC) effect in InGaAs/GaAs materials is investigated, which leads to a well-cluster composite (WCC) nanostructure containing a large number of active regions with different band gaps. The migration of indium atoms in WCC nanostructures produces a special asymmetric band feature and very interesting emission characteristics. The quasi-Fermi energy level and carried-injected band-filling effect are greatly improved to bring about ultra-wide radiative energy levels and spectral bandwidths. However, the research on band-filling patterns of semiconductor lasers mainly focuses on traditional quantum well structures rather than WCC structures. To further reveal the improved wavelength tuning ability, we investigate the carrier-filling level in the novel WCC structure, which is of significance for the development of new types of tunable lasers.

The approach replaces conventional quantum wells or quantum dots with an InGaAs-based WCC quantum-confined structure as a gain medium. Firstly, the epitaxial structure of the InGaAs-based WCC sample is grown on the GaAs (001) substrate using the metal organic chemical vapor deposition (MOCVD) technique, where the In_0.17Ga_0.83As/GaAs/GaAs_0.92P_0.08 material system is employed as the active region. To generate the necessary lattice mismatch and strain accumulation for the migration of indium atoms, we design the indium composition and layer thickness of InGaAs material as 0.17 and 10 nm respectively. Secondly, the experimental sample is processed into an in-plane configuration of 1.5 mm×0.5 mm in size. One end is coated to give a transmittance of 99.99%, with the other end uncoated. The photoluminescence (PL) spectra are collected from the dual facets of WCC nanostructures vertically pumped by 808 nm fiber-coupled lasers. Thirdly, the material gain with different carrier densities is calculated by the PL spectra. The quasi-Fermi energy of electrons and holes is obtained according to the photon energy at which the material gain is zero. Finally, the band-filling level is studied by comparing with traditional InGaAs/GaAs quantum well structures. The greatly improved carrier-filling level and spectral bandwidths are revealed based on the special asymmetric band characteristics.

To study the band-filling pattern in WCC nanostructures and reveal the application advantages in tunable lasers, we obtain the PL spectrum curves with multi-peak structures emitted from the multi-component active regions, which are caused by the migration of indium atoms in the three-dimensional (3D) growth mode. According to the model-solid theories and Gaussian fitting of the PL spectra, the indium content in In_xGa_1-xAs material can be evaluated as x=0.12, 0.15, and 0.17 respectively (Fig. 3). The material gain curves of the special WCC structure and conventional InGaAs quantum wells are measured and compared to reveal the advantages of WCC nanostructure in carrier-filling capacity (Fig. 4). The gain bandwidth (96.5 nm) is broadened to three-fold broader than that (32.8 nm) from a classic InGaAs quantum well. According to the photon energy at which the material gain is zero, the quasi-Fermi separation of electrons and holes is 1.358, 1.365, 1.381, and 1.399 eV, while the quasi-Fermi spacing in traditional InGaAs quantum well structures is only 1.2787, 1.2795, 1.2803, and 1.2811 eV under the carrier injection of 9×10¹⁷, 9.2×10¹⁷, 9.4×10¹⁷, and 9.6×10¹⁷ cm^-3. The Fermi level represents the boundary between quantum states that are basically occupied or empty. The quasi-Fermi separation of the WCC structure is 1.1 times broader than that of the traditional structure, which indicates that carriers in WCC structures are easier to occupy high energy levels. Therefore, the quasi-Fermi separation and carried-injected band-filling effect are greatly improved, which leads to an ultra-wide radiative energy level and spectral bandwidth, and enormously enhances the wavelength tuning ability of semiconductor lasers.

The band-filling effect of electrons and holes is an important physical mechanism to reveal the luminescence performance of semiconductor lasers. We calculate the material gain and quasi-Fermi separation of electrons and holes according to the PL spectra collected from the dual facets of the InGaAs/GaAs WCC structure under different carrier densities. Compared with the traditional InGaAs quantum well structure, the gain bandwidth and quasi-Fermi separation of the WCC structure can reach up to 3 and 1.1 times respectively. It is demonstrated that the WCC structure exhibits higher performance in carrier-filling level and effective radiative energy spacing. According to the formation mechanism of the WCC structure in the 3D growth mode, an asymmetric step-like band structure is obtained. The special band makes it easier for the photo-generated carriers to occupy higher energy levels, which can improve the non-equilibrium carrier-filling capacity, and directly lead to higher effective radiative levels and superwide spectral bandwidth. The excellent characteristics of higher carrier-filling levels and ultra-wide spectral bandwidths are revealed to provide a novel design concept and application potential for semiconductor lasers with ultra-wide wavelength tuning range.