Imagine a future where doctors can non-invasively treat deep brain regions with pinpoint accuracy, all thanks to a beam of light that never spreads out.
Have you ever tried using a laser pointer and noticed how the dot quickly grows into a large, blurry spot as you move it further away? This phenomenon, known as diffraction, is a fundamental law of physics that affects all light beams. For neuroscientists and doctors, this same principle poses a massive challenge when trying to focus therapeutic energy deep within the brain without damaging surrounding healthy tissue. But what if we could defy this law? Enter the Bessel beam—a special kind of light beam that resists diffraction and maintains its focus over astonishing distances. Through the power of advanced computer simulations, researchers are now unraveling how these unique beams interact with the complex environment of the brain, paving the way for a new era of non-invasive, high-precision medical treatments.
In 1987, scientist J. Durnin first theorized and demonstrated a beam that could resist the spreading that affects normal light 1 . Unlike the common Gaussian beam from a standard laser pointer, which forms a narrow waist that rapidly diverges, a perfect Bessel beam is a "non-diffracting" beam that propagates without spreading. Its secret lies in its unique wavefront, which is shaped like a cone rather than a series of flat planes 1 .
When this conical wavefront converges, it interacts with itself to create a distinctive pattern: a very intense, narrow central core surrounded by a series of concentric rings 1 . While a mathematically perfect Bessel beam would require infinite energy, scientists create excellent approximations in the lab using optical elements called axicons—conically shaped lenses that transform a standard laser beam into a Bessel beam 1 .
Bessel Beam Wavefront Visualization
The properties of Bessel beams make them exceptionally suited for interacting with biological tissue, particularly the brain.
A Bessel beam can remain in focus for a much longer distance than a Gaussian beam. For example, one microscopy study used a Bessel beam to image a volume of brain tissue over 360 micrometers deep while maintaining capillary-level resolution, a seven-fold increase compared to a standard Gaussian beam system 3 .
Perhaps their most magical property is the ability to "self-reconstruct." If a Bessel beam's central core is partially blocked by an obstacle, it will regenerate after a short propagation distance 1 . This makes it remarkably robust for penetrating through scattered, heterogeneous environments like brain tissue.
The intense, narrow core of a Bessel beam acts like an elongated "light needle," capable of delivering energy with high precision at great depths 6 .
Bessel beams enable unprecedented precision in targeting specific brain regions, minimizing damage to surrounding healthy tissue during therapeutic procedures.
The following table contrasts the key characteristics of standard Gaussian beams and Bessel beams.
| Characteristic | Gaussian Beam | Bessel Beam |
|---|---|---|
| Diffraction | Diverges over distance | Resists diffraction over a long range |
| Intensity Profile | Single, spreading central spot | Narrow core surrounded by concentric rings |
| Depth-of-Field | Limited, short | Extended, long |
| Self-Healing | No | Yes |
| Typical Generation | Standard lens | Axicon |
Interactive Beam Propagation Visualization
Gaussian beam (red) vs Bessel beam (blue) propagation over distance
Before building expensive lab equipment, researchers can use sophisticated simulation software to test theories and optimize designs. COMSOL Multiphysics is one such powerful tool that uses the finite element method to solve complex physics problems, including the propagation of light.
A compelling 2018 study, "Generation of Divergence-Free Bessel-Gauss Beam from an Axicon Doublet for km-Long Collimated Laser," showcases exactly how COMSOL is used to design better Bessel beams 4 . While this particular study aimed for long-range applications like LIDAR, its methodology and findings are directly applicable to the challenge of delivering focused energy through the brain.
Advanced simulation platform for modeling physics-based systems
The researchers faced a common problem: a Bessel beam generated by a single axicon has a limited non-diffracting range. Their goal was to see if a more complex optical system—a cemented axicon doublet—could generate a "divergence-free" Bessel-Gauss beam with a dramatically extended range, all within a compact, solid-state device 4 .
In the COMSOL environment, they built a virtual model of this axicon doublet. They then used the Electromagnetic Waves, Beam Envelopes interface, which is specially designed for simulating wave propagation in very large domains without excessive computational cost 4 . They defined an input laser beam with a 25 mm waist and set the software to calculate how the beam would evolve over a propagation distance of several kilometers.
The simulation yielded impressive results. It verified that the axicon doublet could indeed generate a Bessel-Gauss beam with a divergence-free range of at least 2 kilometers 4 . This represents a massive improvement over what a single axicon can achieve.
The success of this design lies in its ability to create a beam with an inward radial wave component that precisely compensates for the natural diffraction of a Gaussian mode 4 . The table below summarizes the parameters and results of this key simulation.
| Parameter | Value / Description | Significance |
|---|---|---|
| Input Beam Waist | 25 mm | A reasonably sized, practical laser source |
| Optical Element | Cemented Axicon Doublet | All solid-state, robust design for real-world use |
| Simulated Propagation Distance | Several kilometers | Tests the ultimate limits of the beam's range |
| Achieved Non-Diffracting Range | > 2 km | Demonstrates a dramatic extension of the beam's focus |
This study is a perfect example of how COMSOL acts as a virtual lab. It allows scientists to prototype a complex optical system, test its performance under ideal conditions, and confirm its viability before ever manufacturing a physical component. For medical applications, this means researchers can simulate how a Bessel beam device would propagate through layers of skin, skull, and brain tissue to optimize it for both safety and efficacy.
Limited non-diffracting range
Extended non-diffracting range (>2 km)
While simulations are crucial, experimental data from biological studies confirms the immense potential of Bessel beams. Recent groundbreaking research has moved from theory to tangible results.
In light-sheet microscopy, a technique for imaging biological samples, traditional Gaussian light sheets are plagued by "streaking artifacts"—dark shadows caused when the light is blocked by microscopic obstacles in the brain . This corrupts data and obscures important features.
Scientists have demonstrated that switching to Bessel beam illumination solves this problem. The self-healing property allows the beam to bypass these obstacles, resulting in clearer, more uniform images . This translates to more accurate data; one study found that Bessel beams provided a fivefold increase in sensitivity for detecting calcium transients (indicators of neural activity) and a 20-fold increase in accurately identifying correlated activity between neurons .
Perhaps the most exciting application is in ultrasound neuromodulation—using sound waves to stimulate or inhibit neural activity without surgery. A 2025 preprint report introduced a miniaturized device called OBUS (Optically-generated Bessel beam Ultrasound) 6 .
This device generates a column-shaped ultrasound field using a Bessel beam, achieving a lateral resolution of just 152 micrometers 6 . It successfully stimulated cells in a mouse brain at a depth of 2.2 mm, and crucially, it outperformed conventional Gaussian ultrasound in maintaining its beam shape and efficiency while passing through the skull 6 . This proves that the Bessel beam's properties are not just theoretical but can be engineered into a compact tool for precise, volumetric brain stimulation.
Target specific brain regions with minimal collateral damage
Reach deep brain structures without invasive procedures
Self-healing property protects against scattering obstacles
Maintain beam quality over longer distances than Gaussian beams
Bringing this technology from a simulation to a real-world application requires a specific set of tools. Here are some of the essential "research reagents" and their functions in the study of Bessel beams for brain interaction.
| Tool / Solution | Function in Research |
|---|---|
| COMSOL Multiphysics Software | Creates virtual models to simulate beam propagation and optimize optical designs before physical prototyping 4 . |
| Axicon | A conical lens that is the primary component for transforming a standard Gaussian laser beam into an approximate Bessel beam 1 . |
| High-Sensitivity Detectors | Essential for capturing the detailed intensity profile and propagation characteristics of the beam in experiments. |
| Wavefront Sensors | Measures the phase and intensity of the Bessel beam, allowing scientists to verify its non-diffracting and self-reconstructing properties. |
| Tissue Phantoms | Synthetic materials that mimic the optical scattering properties of real brain tissue, used for benchtop testing of beam penetration and focus. |
J. Durnin first theorizes and demonstrates non-diffracting Bessel beams 1 .
Advancements in axicon technology enable more practical generation of Bessel beams for laboratory use.
Bessel beams applied to microscopy, demonstrating superior imaging capabilities in biological tissues 3 .
COMSOL simulations demonstrate extended-range Bessel beams using axicon doublets 4 .
Medical applications emerge, including ultrasound neuromodulation with Bessel beams 6 .
The journey of the Bessel beam from a mathematical curiosity to a potential medical marvel is a powerful example of how fundamental physics can drive technological revolution. Through the predictive power of COMSOL simulations, researchers can design and refine these beams, creating devices that are more effective and safer for patients. The experimental success in high-resolution imaging and precise neuromodulation confirms that we are on the cusp of a new paradigm.
As this technology matures, we can envision a future where treatments for neurological disorders like Parkinson's, epilepsy, or brain tumors are administered not with a scalpel, but with a perfectly focused, non-invasive beam of energy—a beam that knows how to navigate the delicate landscape of the human brain without getting lost.
Precise ablation of pathological tissue without opening the skull
Targeted stimulation of specific neural circuits for therapeutic effects
High-resolution imaging during procedures for immediate feedback