🔬 applied physics

Direct laser writing of high aspect ratio nanochannels for nanofluidics

This paper presents a direct laser writing technique that fabricates high aspect ratio, optically accessible nanochannels between diamond films and glass substrates, demonstrating their ability to spontaneously fill with water via capillary action while maintaining mechanical stability and clog resistance for advanced nanofluidic applications.

Original authors: Stoffel D. Janssens, Meissha Ayu Ardini, David Vázquez-Cortés, Cathal Cassidy, Eliot Fried

Published 2026-02-09

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CC BY 4.0

Original authors: Stoffel D. Janssens, Meissha Ayu Ardini, David Vázquez-Cortés, Cathal Cassidy, Eliot Fried

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a very thin, transparent sheet of diamond sitting on top of a piece of glass. Now, imagine you want to carve a tiny, invisible tunnel between them to let water flow through. This is the challenge of nanofluidics: making microscopic pipes so small that water behaves differently than it does in a glass.

The problem is that making these tunnels is usually like trying to sculpt a statue with a sledgehammer: it's expensive, slow, and requires a sterile "cleanroom" environment.

This paper introduces a new way to do it using a laser pen that acts like a magical sculpting tool. Here is how they did it, explained simply:

1. The "Magic Strip" Trick

Think of the diamond film as a stiff piece of plastic wrap glued to a table (the glass).

The Old Method: The researchers previously found that if you zap the diamond with a laser, it turns a tiny strip of that diamond into a different kind of carbon (like turning a diamond ring into soft graphite). This new material takes up more space, like a balloon inflating. Because it expands, it pushes the surrounding diamond film up, peeling it off the glass. This creates a tiny, triangular tunnel on either side of the strip.
The New Method: In this paper, they didn't just draw one line. They drew two parallel lines with the laser.
- Imagine drawing two lines of expanding glue on a piece of paper. The paper between the two lines gets pushed up by both sides at once.
- Instead of a triangular wedge, the space between the two lines lifts up to form a flat, rectangular tunnel.
- These tunnels are incredibly flat and wide compared to their height (like a very wide, shallow river), with a width-to-height ratio of over 50 to 1.

2. What's Inside the Tunnel?

The team looked at these tunnels under a super-powerful microscope (electron microscopy). They found that the "glue" holding the tunnel open is a layer of amorphous carbon (a messy, non-diamond form of carbon).

This layer sits right between the diamond film and the glass.
It acts like a structural beam. Without this carbon layer, the diamond film would just snap back down onto the glass. The carbon holds the roof up, keeping the tunnel open.
They also noticed that the laser seems to "know" where the weak spots are (defects near the glass), turning the diamond into this supportive carbon exactly where it's needed.

3. Seeing the Invisible

Since these tunnels are so small, you can't see them with normal eyes. However, because the tunnels are flat and wide, the researchers could shine light through them and measure how much light bounced back (reflectance).

The Analogy: Think of the tunnel like a thin layer of oil on water. The thickness of the oil changes how the light reflects.
They found that as the tunnel gets taller (the roof lifts higher), the way it reflects light changes in a predictable way. They could even use a computer model to guess the height of the tunnel just by looking at the color of the light bouncing off it.

4. The Water Test

To prove these tunnels actually work, they built a tiny device where the tunnels connected to little reservoirs (like tiny lakes).

Capillary Action: They put water in the reservoirs. Just like a paper towel soaking up a spill, the water naturally sucked itself into the tiny tunnels without any pumps.
The Proof: When the tunnel was empty (filled with air), it reflected light brightly. When it was full of water, it looked darker. This change confirmed the water was inside.
Durability: They filled and drained the tunnel with water over 100 times, heating it up to speed things up. The tunnel didn't break, didn't clog, and didn't collapse. It remained sturdy, proving the "carbon beam" is strong enough to hold up against the pressure of the water.

Why This Matters

The paper concludes that this method is a versatile, cleanroom-free platform.

You don't need expensive factories to make these.
You can make tunnels that are optically clear (you can look through them with light).
They are strong enough to handle fluids.

In short, the researchers figured out how to use a laser to peel up a diamond film in a very controlled way, creating a sturdy, flat, rectangular highway for water to travel through, all while being able to "see" the water flow using light.

Technical Summary: Direct Laser Writing of High Aspect Ratio Nanochannels for Nanofluidics

Problem Statement
Nanochannels with high width-to-height aspect ratios are critical for applications requiring optical access and fluidic transport, such as water desalination, biomedical analysis, and energy harvesting. However, fabricating these structures remains challenging. Conventional methods like electron beam lithography, focused ion beam milling, and reactive ion etching are complex, expensive, and time-consuming. While laser writing has emerged as an alternative for creating nanostructures, previous iterations of this technique in polycrystalline diamond (PCD) films produced nanochannels with triangular cross-sections flanking a single nanostrip. These triangular geometries limited the achievable aspect ratios and optical interrogation capabilities.

Methodology
The authors introduce a direct laser writing approach to fabricate rectangular nanochannels between pairs of laser-written nanostrips on PCD films grown on glass substrates. The process utilizes a femtosecond laser system (1033 nm wavelength) to locally transform a portion of the diamond film into a nanostrip of non-diamond carbon. This transformation induces volume expansion, causing the surrounding film to delaminate from the substrate.

By writing two parallel nanostrips, the authors demonstrate that the delamination processes initiated at each strip interact. When the spacing between strips is sufficiently small, the delaminated regions merge before forming independent wedge-like cavities, resulting in a central region with a rectangular cross-section. The study employs:

Atomic Force Microscopy (AFM): To measure channel profile heights and determine the maximum strip spacing ( $d_M$ ) that permits rectangular channel formation.
Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS): To characterize the microstructure and chemical composition of the nanostrips and the interface.
Microspectrophotometry: To measure spatially resolved reflectance spectra of the channels, a capability not previously available for the triangular slit-like channels.
Nanofluidic Device Fabrication: Creating devices where channels connect to open microchannels and reservoirs to test capillary filling and mechanical stability.

Key Results

Geometry and Aspect Ratio: The method successfully produces nanochannels with rectangular cross-sections. By varying the laser pulse energy ( $E$ ), the authors control the channel height. They established a design rule linking nanostrip spacing to channel height, achieving width-to-height aspect ratios exceeding 50.
Microstructural Characterization: TEM and EELS analysis revealed that the nanostrips consist predominantly of amorphous carbon formed via allotropic transformation of the diamond. This amorphous carbon layer, located between the PCD film and the glass substrate, provides the mechanical support necessary to maintain the delaminated channel structure. The transformation is attributed to defect-mediated light absorption near the film-substrate interface.
Optical Properties: Microspectrophotometry demonstrated that channel reflectance ( $R$ ) increases with channel height ( $h$ ). Experimental reflectance trends aligned closely with simulations based on the transfer matrix method, though experimental values were systematically lower due to surface roughness, non-flat channel profiles, and light scattering by nanoparticles.
Fluidic Functionality and Stability: The fabricated channels filled with water via capillary action. Reflectance measurements confirmed the presence of water, showing a distinct decrease in reflectance compared to air-filled states, consistent with simulations. The channels remained mechanically stable and resistant to clogging after 101 fill-drain cycles conducted at 333 K.

Significance
The paper presents a versatile, cleanroom-free platform for producing high-aspect-ratio nanochannels that are both optically accessible and fluidically functional. By enabling the fabrication of rectangular channels with aspect ratios >50, this work expands the design space for nanofluidic devices. The ability to integrate these channels with optical probing techniques (microspectrophotometry) and their demonstrated mechanical stability against repeated fluidic cycling suggest their suitability for advanced applications in lab-on-a-chip systems and microspectrophotometric studies. The authors conclude that direct laser writing in PCD-glass systems offers a practical route for creating these structures using abundant materials (diamond and glass) without the need for complex lithographic infrastructure.

1. The "Magic Strip" Trick

2. What's Inside the Tunnel?

3. Seeing the Invisible

4. The Water Test

Why This Matters

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