🔬 applied physics

High resolution large working distance scanning helium microscopy

This paper reports a sixfold improvement in spatial resolution for large-working-distance scanning helium microscopy, achieving a sub-micron beamwidth of 340 nm through optimized atom optics and hardware redesigns, thereby establishing the technique as a viable platform for high-resolution imaging of delicate and insulating surfaces.

Original authors: Sam M Lambrick, Nick A von Jeinsen, Alek Radić, David J Ward, Donald MacLaren, Andrew P Jardine

Published 2026-05-20

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Sam M Lambrick, Nick A von Jeinsen, Alek Radić, David J Ward, Donald MacLaren, Andrew P Jardine

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

The Big Picture: A Gentle Giant Gets a Sharp Eye

Imagine you have a very delicate, fragile object—like a snowflake or a living bacterium. If you try to look at it with a standard electron microscope, it's like shining a high-powered laser on it; the beam is so energetic it might blast the object apart or give it an electric shock. If you use a regular optical microscope, you might not see enough detail because light waves are too "fuzzy" to resolve tiny features.

Scanning Helium Microscopy (SHeM) is the solution. It uses a beam of neutral helium atoms (which are just tiny, harmless gas particles) to "feel" the surface. Because helium atoms are neutral and move slowly, they are like a gentle breeze that can map the surface without damaging it.

However, there was a problem. To get a really sharp, high-resolution picture, you usually have to bring the microscope very close to the object. But for delicate or bumpy samples, being that close is dangerous and impractical. It's like trying to take a high-resolution photo of a mountain range with a camera lens that you have to hold just millimeters away from the ground; you'd hit a rock and break the lens.

The Goal: The researchers wanted to keep the "large working distance" (the safe space between the lens and the sample) but still get a picture sharp enough to see things smaller than a human hair (sub-micron resolution).

The Problem: The "Flashlight" Effect

Think of the helium beam like a flashlight beam.

The Old Way: In previous large-distance setups, the beam was like a wide, fuzzy flashlight beam. Even if you tried to focus it, the "fuzziness" (beamwidth) was too big to see tiny details. The resolution was stuck at about 1 micron (the width of a bacterium).
The Challenge: To make the beam narrower (sharper), you usually have to squeeze the light through a tiny hole. But if you squeeze it too much, the beam gets dim, and you can't see anything. Also, if you move the hole too far from the sample, the beam spreads out again.

The Solution: A Masterful Balancing Act

The team at Cambridge University didn't just make one change; they redesigned the whole "optical system" to find the perfect balance. They treated the microscope like a complex recipe where every ingredient had to be adjusted simultaneously.

Here are the four main "ingredients" they tweaked, using analogies:

Moving the Light Source Further Back:
Imagine trying to shine a light through a tiny keyhole. If the light bulb is right next to the keyhole, the light spreads out wildly. If you move the light bulb far away, the light rays become more parallel (straighter) before they hit the keyhole.
- What they did: They moved the helium source much further away from the pinhole. This made the incoming beam straighter, allowing for a sharper focus later.
Shrinking the Keyhole:
To get a sharp beam, you need a smaller hole.
- What they did: They manufactured a new pinhole plate with a hole only 470 nanometers wide (about 1/200th the width of a human hair). This is incredibly small.
Bringing the Sample Closer (But Not Too Close):
This is the tricky part. To get a sharp image with a small hole, the sample needs to be relatively close to the hole. But the researchers still wanted to keep a "large working distance" (around 770–850 micrometers) so they could handle bumpy samples easily.
- What they did: They redesigned the sample chamber to bring the sample closer to the pinhole than before, but still kept it far enough away to be safe and practical.
Widening the "Net" to Catch the Signal:
Because they made the hole smaller and moved the source further away, the amount of helium hitting the sample dropped significantly. It was like trying to catch rain with a thimble instead of a bucket.
- What they did: They redesigned the detector (the "net" that catches the scattered atoms) to be much larger. This allowed them to catch more of the scattered atoms, compensating for the loss caused by the tiny hole.

The Result: A New Level of Clarity

By balancing these factors, they achieved a beamwidth of 340 nanometers.

The Improvement: This is a six-fold improvement over their previous large-distance setup.
The Sweet Spot: They proved that you can have a "large working distance" (safe for samples) and "sub-micron resolution" (sharp enough to see tiny details) at the same time.

What They Showed It Can Do

The paper demonstrates this new capability on four specific examples:

Bacteria: They imaged Pseudomonas aeruginosa and Clostridioides difficile. The new microscope could clearly see the tiny rod shapes of the bacteria, which were too small to be resolved by their old setup.
Diamond: They looked at a piece of synthetic diamond that had been eroded. The old microscope could only see the biggest scratches; the new one revealed the tiny, complex facets and grooves on the surface.
Fabric: They imaged a reusable face mask. Because the fabric is insulating (doesn't conduct electricity) and has fibers going in and out of the plane, it's a nightmare for other microscopes. The helium microscope saw fibers deep inside the material and on the surface clearly, proving it has a great "depth of field" (everything stays in focus).

The Bottom Line

This paper says: "We have successfully tuned our helium microscope to be both gentle (keeping a safe distance from samples) and incredibly sharp (seeing details smaller than a micron)."

They also note that they have likely reached the limit of what this specific type of "pinhole" design can do. To get even sharper, they would need to switch to a completely different technology (like active focusing lenses), but for now, this design is the best possible version of its kind. It opens the door for studying delicate biological and insulating materials with much higher detail than before.

Technical Summary: High Resolution Large Working Distance Scanning Helium Microscopy

Problem Statement
Scanning Helium Microscopy (SHeM) offers a unique capability for imaging delicate, insulating, and topographically complex surfaces by utilizing a non-destructive, neutral-atom probe with strong surface sensitivity. However, a significant limitation has persisted in "Cambridge-type" SHeM instruments, which utilize pinhole collimation and large working distances. While these instruments provide practical advantages such as easier sample handling, tolerance of significant sample topography, and an extended depth of field, they have historically been restricted to spatial resolutions greater than 1 µm. In contrast, "Portland-type" instruments achieve sub-micron resolution (e.g., 350 nm) but only at very short working distances (<100 µm), sacrificing the practical flexibility that makes SHeM attractive for broader applications. The challenge addressed in this work is to achieve sub-micron resolution in a large-working-distance configuration without compromising the sample access and depth of field that define the Cambridge-type geometry.

Methodology
The authors approached this challenge through a combination of constrained atom-optical optimization and significant hardware redesign.

Optical Optimization:
The team utilized an optimization framework (building on models by Bergin et al. and Salvador-Palau et al.) to balance the three primary contributors to beamwidth: geometric projection, source-size demagnification, and diffraction. Unlike previous configurations where geometric broadening dominated, the goal was to reach a regime where these terms contribute comparably.
- Target: An intrinsic beamwidth (Full Width at Half Maximum, FWHM) of approximately 300 nm.
- Constraints: The working distance ( $f$ ) was constrained to be >650 µm to maintain a useful depth of field and sample manipulation space.
- Parameters: The optimization determined an optimal pinhole diameter ( $d \approx 440$ nm) and an optimal angular source size ( $\beta \approx 2.2 \times 10^{-4}$ rad). To achieve the required angular source size, the distance between the source (skimmer) and the pinhole ( $L$ ) was increased to ~385 mm.
Experimental Implementation:
To realize these theoretical parameters, the authors redesigned the instrument's sample chamber and optical elements:
- New Sample Chamber: A custom rectangular chamber was installed to allow for a variable source-to-pinhole distance (up to ~400 mm) and to improve background reduction via differential pumping.
- Miniaturized Pinhole-Plate: A critical innovation was the redesign of the pinhole-plate assembly. To maintain a large detector solid angle despite the reduced working distance ( $f \approx 0.71$ mm), the authors employed a custom, miniaturized silicon nitride membrane (1 mm scaffold) and a 3D-printed pinhole-plate. This allowed the detector aperture to be enlarged significantly (solid angle increased from 0.094 sr to 1.44 sr) to recover signal lost by the smaller pinhole and increased source distance.
- Fabrication: The final pinhole (470 nm diameter) was milled using a Focused Ion Beam (FIB) in a custom membrane.

Key Results

Beamwidth Achievement: The instrument achieved an intrinsic beamwidth of 340 nm at a working distance of 850 µm. This represents a six-fold improvement over the authors' previous large-working-distance configuration.
Validation: Measured beamwidths across a range of working distances (550–1100 µm) agreed well with the modified optimization model. The data confirms that the instrument now operates in a near-optimized regime where geometric, source-size, and diffraction terms contribute on a similar footing (68%, 64%, and 36% contributions respectively, relative to the overall beamwidth).
Signal Trade-off: The optimization incurred a raw signal penalty of approximately one order of magnitude due to the smaller pinhole and increased source distance. However, the enlarged detector aperture recovered much of this loss, resulting in a net signal reduction of only ~2.25x compared to the previous configuration, which was deemed acceptable with moderate increases in measurement time.
Imaging Demonstrations: The improved resolution was demonstrated on four distinct samples:
- Biological: High-fidelity imaging of Pseudomonas aeruginosa biofilms and isolated Clostridioides difficile bacteria (<1 µm diameter), which were previously unresolvable with large-working-distance SHeM.
- Materials: Detailed topographic imaging of mechanically eroded synthetic diamond, revealing faceted surfaces and grooves previously invisible.
- Complex Topography: Imaging of insulating PPE face mask fabric, demonstrating the retention of a large depth of field even at sub-micron resolution.

Significance and Claims
The paper establishes large-working-distance pinhole SHeM as a viable sub-micron imaging platform. The authors claim that this work bridges the performance gap between the practical, flexible Cambridge-type geometry and the high-resolution, short-distance Portland-type instruments.

Key claims regarding the significance of this work include:

Optimization Frontier: The work clarifies the technical frontier for passive, pinhole-collimated SHeM. The authors note that further gains within the current design paradigm are likely limited, as the system is now balanced between geometric, source, and diffraction terms.
Expanded Utility: The sub-micron regime extends the utility of SHeM for topographic imaging of delicate and insulating specimens with features below the micron scale.
Micro-diffraction: The improved resolution makes it more realistic to probe smaller individual domains and heterogeneous regions for atom-diffraction and spot-profile measurements, particularly in 2D systems where micron-scale beams were previously a limitation.
Future Direction: The authors modestly suggest that while this represents the limit of the current passive pinhole architecture, future substantial gains beyond this performance envelope will likely require active focusing approaches, such as Fresnel zone plates.

In summary, the work demonstrates that the practical advantages of large working distance need not be confined to low-resolution operation, positioning helium microscopy as a flexible platform for challenging real-world specimens.