Wide Gain Bandwidth InAs/Ingaalas/InP Quantum-Dashes for Broadband Emitter Applications
Authors: Khan Mohammed Zahed , Ng Tien K , Ooi Boon S 
Published: February 2020

The reduction in the dimensionality of the active region and exploiting the quantum mechanical effects from this device design has lead to the realization of quantum well (Qwell), quantum wire and eventual quantum dot (Qdots semiconductor lasers with remarkable improvements in their performance. In particular, Qdots lasers, provides control of the optical transitions due to their delta type density of states, lead to strong reduction in the threshold current density and a high temperature stability.
In general, Qdots are grown by strain induced self assembled Stranski-Krastanow (S-K) growth technology on the matured GaAs platform. The intentional lattice mismatch between InAs Qdots and GaAs substrate facilitate the formation of these 3D islands spanning ~ 1.1 - 1.3 µm wavelength. The thrust to extend the emission wavelength in the C - L - U bands (1.4 - 2.0 µm) targeting principally the third communication window (1.55 µm) has set path to the development of InP material platform via a new class of self assembled grown quantum dash (Qdash) nano-structures. These are elongated Qdots grown as a result of low lattice mismatch between InAs and InP and the resulting complex strain distribution, with an interesting mixed type character, between Qwell and Qdot. Promising Qdash based laser demonstrations has already been reported with wide tunability spanning ~ 1.5 - 2.0 µm. The inherent inhomogeneous nature of these nano-structures resembling dot-like and wire-like physical features has also been exploited in the realization of ultra-broad gain bandwidth (> 300 nm) material, a highly promising in realization of broadband semiconductor devices, for instance, semiconductor optical amplifier, broadband laser diode, photo detector, etc, which will find novel applications in optical communications, particularly in wavelength division multiplexed system. In addition, realization of diode-based ultra-broadband laser source will offer a compact, high-efficiency, and cost-effective solution for many new applications in cross-disciplinary fields in optics and photonics, for instance, metrology, spectroscopy and sensing, imaging, etc.
In this talk, we will give a general overview and introduction to the broadband emitters. The technology for growing these nanostructures, as well as the technologies for engineering the bandgap of the InP-based Qdash system using epitaxy growth technique and postgrowth intermixing methods will be presented. Recently, we reported an ultra-broad lasing -3dB bandwidth of ~ 21 nm from the as- grown and ~ 41 nm from the intermixed Qdash laser, as shown in Fig. 1, with superior characteristics [1-3]. We will then focus our discussion on our recent progress in extending the lasing emission linewidth via employing a novel active region device design. Instead of using a fixed AlGaInAs barrier thickness in the multi-stack Qdash active region, here we utilized a varying barrier thickness (chirp design) structure that influence the vertical strain experienced by the dashes on the subsequent stacking layer thereby creating additionally broadened dash ground state optical transitions. With this as-grown device structure we achieved an enhanced lasing bandwidth of > 50 nm from a 2×830 µm ridge laser diode, as shown in Fig. 2 [4], with exhibited superior performance characteristics. This is an enhancement by a factor of ~ 50% (20%) from our previous reports on as-grown (intermixed) fixed barrier Qdash laser structure. Moreover, we also investigated the broadband emission features of our chirped Qdash structure in the form of superluminescent diodes (SLD) by reporting high performance characteristics from this device and an extra-ordinary emission bandwidth of > 700 nm, spanning the entire O - E - S - C - L - U bands [5]. All these demonstrations show that chirped device design is promising candidate in realizing broadband semiconductor devices.
[1] B. S. Ooi et al., IEEE Journal of Selected Topics in Quantum Electronics 14 (4), 1230-1238 (2008).
[2] H. S. Djie et al., Applied Physics Letters 91, 111116 (2007).
[3] C. L. Tan et al., Applied Physics Letters 93, 111101 (2008).
[4] M. Z. M. Khan et al., Applied Physics Letters 102, 091102 (2013).
[5] M. Z. M. Khan et al., Optics Letters 38 (19), 3720-3723 (2013).