News: Optoelectronics
23 February 2024
Triple-lattice photonic crystal laser
China’s Changchun Institute of Optics, Fine Mechanics and Physics, and University of Chinese Academy of Sciences have reported continuous-wave operation of a 1550nm low-threshold triple-lattice photonic-crystal surface-emitting laser (PCSEL) on indium phosphide (InP) substrate .
The team comments: “Our results present an opportunity for InP-based high-speed 1.557μm PCSELs, which are expected to play an essential role in high-speed optical communication and LiDAR applications.”
Figure 1: a PCSEL schematic. b Top-view scanning electron microscope image of triple-lattice photonic-crystal resonator. c Cross-sectional SEM image of resonator after epitaxial regrowth. d Refractive index profile and corresponding optical field distribution along crystal growth direction. e Images of laser chip bounded to thermally conductive submount with p-side down. Inset: p-side of chip.
The material for the device (Figure 1) was grown by metal-organic chemical vapor deposition (MOCVD) on n-InP substrate. The multiple quantum well (MQW) active region consisted of indium aluminium gallium arsenide (InAlGaAs) grown on an n-InP cladding layer. The first growth sequence was completed with a p-InAlAs electron-blocking layer and p-InAlGaAs. The photonic crystal consisted of a square lattice of three offset holes, created by electron-beam lithography and inductively coupled plasma etch to a 375nm depth in a square 300μm region. The holes were filled by MOCVD regrowth of low-dielectric-constant InP. This was followed by 50nm p-InAlGaAs and then 30nm grading to the p-InP cladding layer. The material structure was completed with p-InGaAs for the contact layer. The photonic crystal holes were 90nm diameter. The photonic lattice constant was 474nm. The holes were arranged with a view to maximizing the 180° coupling to the optical field of the PCSEL. The mutual interaction of the nested lattices increased the 180° coupling threefold with respect to that of a single-lattice case, according to the team. The researchers add: “The data show that the enhancement of the in-plane optical feedback significantly reduces the lasing threshold compared with the double-lattice PCSELs.” The PCSELs were fabricated with substrate thinning to 180μm, circular mesa etching, 300nm silicon dioxide electrical insulation, and a 200μm-diameter reactive-ion etched circular contact window. The p- and n-electrode metals were, respectively, titanium-platinum-gold and nickel-gold/germanium-nickel-gold. The device was mounted p-side down on copper.Figure 2: a Light–current–voltage characteristics under continuous-wave (CW) conditions at 10°C. b Emission spectra at various CW injection currents. c Magnified spectra near peak wavelength around threshold. d Light–current characteristics at various temperatures under pulsed conditions. e Emission spectra of pulsed lasers at various injection currents at 10°C. f Temperature dependence of emission spectra at 1A.
The output power of the device reached 2.1mW at 1A CW current injection (Figure 2). The threshold current was 0.52A (1.66kA/cm2 density). Spectra over the range 1350–1650nm showed emissions only around the 1551nm target wavelength. Above 0.6A injection the emissions were multi-mode. The maximum output power was 89mW with 5A pulsed current injection.
Table 1: Recent ~1.5μm PCSEL achievements. Top line is PCSEL in this report.
Year |
Wavelength (μm) | Threshold | Maximum output power (mW) | Slope efficiency (W/A) | Structure |
2023 | 1.55 | 1.66kA/cm2 | 89 | 0.02 | all-semiconductor |
2023 | 1.55 | 2.0kA/cm2 | 120 | 0.056 | void-containing |
2022 | 1.56 | 180 mA | 12.58 | 0.016 | Fabry–Pérot coupled |
2020 | 1.52 | 1.6kA/cm2 | 0.5 | 0.002 | void-containing |
The researchers also present a comparison with recent achievements in PCSEL research in this wavelength range (Table c).
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The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.