[Paper Review] Water-Walled Microfluidics Makes an Ultimate Optical Finesse

Shai Maayani, Leopoldo L. Martín+1 more|arXiv (Cornell University)|Aug 17, 2015

Photonic and Optical Devices31 references3 citations

TL;DR

This paper presents a water-walled microfluidic platform that achieves an optical finesse exceeding 1,000,000 by using a nano-water-bridge to sustain a water-air interface microcavity for over 16 hours. The device leverages surface tension for self-stabilization and enables high-Q optical resonances with 98% of its walls composed of tunable water-air interfaces, demonstrating a new class of arbitrary-shape optofluidic resonators.

ABSTRACT

Liquids serve microcavity research ever since Ashkins studies on optical resonances in levitating droplets to recent optofluidic resonators. Droplets can provide optical quality factor (Q) in proximity to the limit restricted by water absorption and radiation loss. However, water micro-drops vaporize quickly due to their large area to volume ratio. Here we fabricate a water-air interface that almost entirely surrounds our device, allowing for more than 1,000,000 recirculations of light (finesse). We sustain the droplets for longer than 16 hours using a nano-water-bridge that extends from the droplet to a practically-unlimited distant-reservoir that compensates for evaporation. Our device exhibits surface tension 8000-times stronger than gravity that self-stabilizes its shape to a degree sufficient to maintain critical coupling as well as to resolve split modes. Our device has 98 percents of their surrounding walls made strictly of water-air interfaces with concave, convex or saddle geometries, suggesting an arbitrary-shape microfluidic technology with water-walls almost all-over.

Motivation & Objective

• To overcome the rapid evaporation of water microdroplets in optofluidic resonators, which limits their practical use.
• To achieve ultra-high optical finesse in a microcavity by minimizing material losses and radiation leakage.
• To develop a self-stabilizing, shape-tunable microfluidic platform using water-air interfaces as structural walls.
• To demonstrate long-term stability of water-based optical cavities through continuous evaporation compensation via a nano-water-bridge.
• To enable arbitrary cavity geometries (concave, convex, saddle) using only water-air interfaces for tunable optical properties.

Proposed method

• The device uses a nano-water-bridge to connect a suspended water droplet to a distant reservoir, continuously replenishing evaporated water.
• Surface tension provides self-stabilization, maintaining the droplet's shape with minimal deformation over time.
• The microcavity is formed entirely by water-air interfaces, with 98% of the wall surface composed of such interfaces.
• Optical modes are excited via evanescent coupling, and the system achieves critical coupling due to shape stability.
• The cavity supports more than 1,000,000 round-trip light recirculations, indicating ultra-high finesse.
• Theoretical and experimental analysis confirms the ability to resolve split modes, indicating high Q-factor and stability.

Experimental results

Research questions

• RQ1 Can a water-based microcavity sustain long-term optical resonance by compensating for evaporation?
• RQ2 To what extent can surface tension stabilize a water-air interface to maintain high optical quality factor?
• RQ3 Can a microfluidic device be constructed with nearly all walls made of water-air interfaces in arbitrary geometries?
• RQ4 What is the maximum achievable optical finesse in a water-walled microcavity?
• RQ5 Can such a system resolve fine spectral features like split modes due to high stability and Q-factor?

Key findings

• The device sustains a water-based optical cavity for over 16 hours, overcoming rapid evaporation through a nano-water-bridge.
• The optical finesse exceeds 1,000,000, indicating more than 1 million round-trip light recirculations.
• Surface tension provides self-stabilization, enabling shape stability sufficient to maintain critical coupling and resolve split modes.
• 98% of the cavity's wall surface is composed of water-air interfaces, with controllable geometries including concave, convex, and saddle shapes.
• The system demonstrates high Q-factor due to minimal material absorption and radiation loss, approaching the theoretical limit set by water absorption.
• The device enables arbitrary-shape microfluidic cavities using only water-air interfaces, opening a new class of tunable optofluidic resonators.