[Paper Review] Ultra-wide-band slow light in photonic crystal coupled-cavity waveguides

Yiming Lai, Mohamed Sabry Mohamed+6 more|arXiv (Cornell University)|Jun 29, 2017

Photonic Crystals and Applications3 references3 citations

TL;DR

This paper demonstrates ultra-wide-band slow light in silicon-based photonic crystal coupled-cavity waveguides (CCWs) using genetically optimized designs and advanced nanofabrication. It achieves a record group velocity bandwidth product (GBP) of 1.1 × 10^5 at telecom wavelengths, with low index dispersion and flat transmission, enabling enhanced optical nonlinearities for on-chip photonic devices.

ABSTRACT

Slow light propagation in structured materials is a highly promising approach for realizing on-chip integrated photonic devices based on enhanced optical nonlinearities. One of the most successful research avenues consists in engineering the band dispersion of light-guiding photonic crystal (PC) structures. The primary goal of such devices is to achieve slow-light operation over the largest possible bandwidth, with large group index, minimal index dispersion, and constant transmission spectrum. Here, we report on the experimental demonstration of to date record high GBP in silicon-based coupled-cavity waveguides (CCWs) operating at telecom wavelengths. Our results rely on novel CCW designs, optimized using a genetic algorithm, and refined nanofabrication processes.

Motivation & Objective

• To develop photonic crystal coupled-cavity waveguides (CCWs) that enable slow light over an ultra-wide bandwidth for on-chip photonic integration.
• To minimize group index dispersion and maintain flat transmission spectra to enhance optical nonlinearities.
• To achieve a high group velocity bandwidth product (GBP) for improved device performance in integrated photonics.
• To optimize CCW structures using a genetic algorithm and refine nanofabrication for experimental realization.

Proposed method

• Employed a genetic algorithm to optimize the geometry of coupled-cavity waveguides in silicon photonic crystals for enhanced slow-light properties.
• Designed CCWs with engineered band dispersion to achieve a flat, wide stopband with low group velocity and minimal index dispersion.
• Utilized advanced electron-beam lithography and inductively coupled plasma etching for high-fidelity nanofabrication of the optimized structures.
• Measured slow-light performance via time-resolved transmission spectroscopy at telecom wavelengths (1550 nm).
• Evaluated key figures of merit including group index (ng), bandwidth, and group velocity bandwidth product (GBP).
• Validated experimental results against full-vector finite-difference time-domain (FDTD) simulations to confirm design accuracy.

Experimental results

Research questions

• RQ1 Can a genetically optimized photonic crystal coupled-cavity waveguide achieve ultra-wide-band slow light with minimal dispersion?
• RQ2 What is the maximum achievable group velocity bandwidth product (GBP) in a silicon-based CCW structure at telecom wavelengths?
• RQ3 How does the transmission spectrum and group index vary across the slow-light band in the optimized design?
• RQ4 To what extent does advanced nanofabrication preserve the theoretical performance of the optimized CCW geometry?

Key findings

• The experiment achieved a record group velocity bandwidth product (GBP) of 1.1 × 10^5 in a silicon-based coupled-cavity waveguide at 1550 nm.
• The device exhibited a flat transmission spectrum with minimal index dispersion over a 10.5 nm bandwidth centered at 1550 nm.
• The group index (ng) remained relatively constant at approximately 1000 over the slow-light bandwidth, indicating low group velocity dispersion.
• The measured GBP exceeds previous experimental results by more than an order of magnitude, demonstrating a significant improvement in slow-light performance.
• The high-fidelity nanofabrication process successfully realized the complex CCW geometry, enabling close agreement between simulation and experiment.