Structural Design of a Mid-Infrared Low-Noise Waveguide Photodetector Integrated with an Ultra-Short Waveguide Taper

Optics Information Science Electronic Information Materials Science Physics Semiconductors and Information Devices
mid-infrared waveguide photodetector ultra-short waveguide taper dark current noise equivalent power

Academic Background

The mid-infrared spectrum range (2.5 to 20 µm) is widely used in gas detection, optical communication, high-quality imaging, bacterial research, and soil composition analysis due to its characteristic absorption peaks of many molecular bonds. Among these applications, waveguide photodetectors have become a key component in photonic integrated circuits (PICs) due to their high integration density, low power consumption, and ease of miniaturization. However, traditional waveguide photodetectors face limitations in sensitivity and signal-to-noise ratio, especially in terms of dark current noise control and quantum efficiency optimization.

To enhance the performance of waveguide photodetectors, researchers have proposed various improvements, such as optimizing material selection, designing novel waveguide structures, or introducing mode conversion techniques to reduce coupling losses. However, how to significantly reduce dark current noise while maintaining quantum efficiency remains an unresolved challenge. This study addresses this issue by proposing a design for a mid-infrared low-noise waveguide photodetector integrated with an ultra-short waveguide taper, aiming to compress the fiber-coupled optical field to subwavelength dimensions, effectively reducing the absorber area, thereby achieving low dark current and high signal-to-noise ratio.

Source of the Paper

This paper was co-first-authored by Yupeng Wang and Jindi Pei, with Yi Zhou and Lingfang Wang as corresponding authors. The author team is from the School of Physics and Optoelectronic Engineering at Hangzhou Institute of Advanced Study, University of Chinese Academy of Sciences, and the National Key Laboratory of Infrared Detection Technologies at Shanghai Institute of Technical Physics, Chinese Academy of Sciences. The paper was published in the journal Optical and Quantum Electronics in 2025, article number 57:157, DOI: 10.1007/s11082-025-08069-4.

Research Content and Methods

a) Research Process

This study mainly includes the following steps:

1. Design and Optimization of Ultra-Short Waveguide Taper Structure

The study first designed an ultra-short waveguide taper structure based on the principle of multi-mode interference (MMI). The core objective of this structure is to compress the fiber-sized mode field into subwavelength dimensions while minimizing transmission loss. The specific design process includes: - Input Waveguide Width: Set to 15 µm to cover the output beam of typical mid-infrared single-mode fibers. - Optimization of Linear Waveguide Taper Length and Output Port Width: A parameter scan was performed on the length (5 µm to 20 µm) and output port width (3 µm to 7 µm) of the linear waveguide taper, determining the optimal configuration as an output port width of 5 µm and taper length of 15 µm, achieving a transmission efficiency of 96.1%. - MMI Structure Optimization: Based on the self-imaging principle, the width and length of the MMI were further optimized. The final optimal configuration was determined to be a width of 4.4 µm and length of 6.4 µm, achieving a transmission efficiency of 92.4%.

2. Design and Performance Analysis of the Waveguide Photodetector Absorber

The research team conducted detailed design and simulation analysis on the absorber layer of the waveguide photodetector, including: - Impact of Absorber Thickness and Length: By adjusting the thickness (0.2 µm to 1 µm) and length (5 µm to 30 µm) of the PIN structure, the impact on quantum efficiency (QE), dark current, and noise equivalent power (NEP) was analyzed. - Dark Current Calculation: Based on the diffusion current formula, the dark current values under different thicknesses and lengths were calculated. - Quantum Efficiency Simulation: The finite-difference time-domain (FDTD) method was used to simulate the changes in quantum efficiency under different absorber thicknesses and lengths.

3. Comparative Experiment

To verify the effectiveness of the ultra-short waveguide taper structure, the research team compared the performance of two structures: - Detector with Ultra-Short Waveguide Taper: Absorber width of 1.32 µm, length of 10 µm. - Detector without Waveguide Taper: Absorber width of 15 µm, length of 30 µm.

b) Main Results

1. Performance of the Ultra-Short Waveguide Taper Structure

  • Transmission Efficiency: The optimized ultra-short waveguide taper structure has a total length of 21.4 µm and achieves a transmission efficiency of 92.4%, reducing the length by an order of magnitude compared to traditional waveguide tapers.
  • Mode Conversion Effectiveness: Efficient mode conversion from fiber size to subwavelength size was achieved through the MMI, with an output waveguide width of only 5 µm.

2. Absorber Layer Performance Analysis

  • Quantum Efficiency: When the absorber thickness is 0.5 µm and the length is 10 µm, the quantum efficiency reaches 43.4%, with a responsivity of 1.6 A/W.
  • Dark Current and Noise Equivalent Power: At a bias voltage of -0.1 V, the dark current is 2.38 × 10⁻⁶ A, and the noise equivalent power is 5.4 × 10⁻¹³ W/Hz¹/².
  • Comparison Results: Compared to the detector without a waveguide taper, the quantum efficiency decreased by 8.3%, but the noise equivalent power decreased by 68.2%.

c) Conclusion

This study shows that by introducing an ultra-short waveguide taper structure, the absorber area of the waveguide photodetector can be significantly reduced, effectively lowering dark current noise while maintaining high quantum efficiency. This design not only improves the signal-to-noise ratio of the detector but also provides new ideas for achieving highly integrated mid-infrared waveguide photodetectors.

Research Significance and Value

Scientific Value

This study proposes for the first time a design for a mid-infrared low-noise waveguide photodetector integrated with an ultra-short waveguide taper, addressing the challenge of dark current noise control in traditional waveguide photodetectors. By optimizing the waveguide taper structure and absorber parameters, efficient mode conversion and low-noise performance are achieved, providing important theoretical and technical support for related research fields.

Application Value

This design has broad potential applications, especially in mid-infrared spectral detection and molecular fingerprint recognition. Due to its high sensitivity and low noise characteristics, this detector can be widely used in environmental monitoring, biomedical diagnostics, and industrial process control scenarios.

Research Highlights

  1. Innovative Design: For the first time, a mid-infrared waveguide photodetector integrated with an ultra-short waveguide taper is proposed, significantly shortening the length of the waveguide taper structure.
  2. High Performance: The optimized detector reduces the noise equivalent power by 68.2%, providing a new paradigm for low-noise detector design.
  3. Multidisciplinary Integration: Combining advanced technologies from optics, materials science, and electronic engineering, showcasing the potential of interdisciplinary research.

Other Valuable Information

The research team received funding from multiple sources, including the National Natural Science Foundation of China (NSFC), indicating that this research has been highly valued by the academic community. Additionally, the FDTD simulation methods and MMI design principles used in the study provide important references for subsequent similar research.