3College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 101408, China
Email: jqliu@semi.ac.cn; biewang2001@126.com; and zpweicust@126.com
We demonstrate a high-speed 8.5 μm quantum cascade laser with room temperature continuous wave operation. The maximum output power of 141 mW is obtained at 20 ℃. The parasitic capacitance of the device is decreased from 36.6 pF to 7.1 pF by monolithic integrating a π-shape metal contact electrode. This results in an increase in the -3 dB RF modulation bandwidth from 870 MHz to 4.5 GHz compared with the conventional electrode configuration.
Introduction: High-speed quantum cascade lasers (QCLs) are essential for a plethora of applications in free-space optical communication (FSOC) [1], high-sensitivity spectroscopy [2], mode-locking [3], etc. Due to the particularity of the transition between subbands, the theoretical modulation bandwidth of QCLs can reach the order of 100 GHz [4]. However, higher electrical parasitics limit the modulation performance of QCLs, various approaches have been proposed to reduce electrical parasitics. Using thicker chalcogenide glass (Ge0.25Se0.75) as insulating material to achieve 7 GHz modulation response at 20 K. Single-mode buried grating QCL using thickened SiO2 as insulating layer achieves a flat frequency response of 3 GHz at room temperature [5]. A frequency response of 15 GHz at 77 K is achieved through the integration of microstrip lines with dielectric laser waveguide [6]. A single-mode distributed feedback (DFB) QCL integrated with a three-terminal microwave coplanar waveguide structure displayed 3-dB bandwidth of up to 6.6 GHz [7]. High-frequency modulation of 3-dB bandwidth at 5.6 GHz achieved by buried heterostructure ultrashort resonator [8]. The RC constant is reduced by inserting a matching printed circuit, and the 3-dB bandwidth is increased from about 0.9 to 2.2 GHz [9]. Although high-speed QCLs have been studied a lot, most of these devices can only operate below room temperature.
In this letter, we propose a room temperature high-speed QCL emitting at 8.5 μ m with the maximum continuous wave (CW) output power of 141 mW at 20 ℃. After optimization of the monolithically integrated π-shape metal contact electrode, a parasitic capacitance is reduced to a very low value of 7.1 pF resulting in a high -3 dB RF-modulation bandwidth of 4.5 GHz.
Design and fabrication: The QCL structure was grown on an n-doped InP substrate by metal-organic chemical vapor deposition (MOCVD) with an active-region (AR) structure similar to Ref. [10]. The layer thicknesses in nanometers, starting from the first injection barrier, are as follows:4.0 /1.3/1.0 /5.2/0.9 /5.1/1.0 /4.7/1.6 /3.6/2.2 /2.9/1.8 /2.7/1.9 /2.6/2.0/2.4/2.5/ 2.5/3.1 /2.3, where InAlAs barriers are in bold, InGaAs wells in roman, and underlined numbers correspond to the doped layers (Si, 2 × 1017cm-3). The epitaxial layer sequence starting from the InP substrate is as follows: 0.5 μ m InP buffer layer (Si, 1 × 1017 cm−3), 3 μ m lowdoped InP cladding layer (Si, 4 × 1016 cm−3), 300 nm InGaAs confinement layer (Si, 5 × 1016cm−3), 35 periods InGaAs/InAlAs active structure, 300 nm InGaAs confinement layer (Si, 5 × 1016cm−3), 3.3 μ m low-doped InP cladding layer (Si, 4 × 1016 cm−3), 0.4 μ m graded-doped InP layer (Si, 1 × 1017cm−3 – 1 × 1018cm−3), and 0.3 μ m InP contact layer (Si, 5 × 1018 cm−3).