Science Knowledge： The Arduous Journey of Humanity Breaking the Optical Diff Limit

Diffraction Limit

Diffraction spots

Diffraction occurs when a point light source passes through a circular aperture, creating a diffraction pattern behind the aperture. This pattern consists of a series of concentric bright and dark rings known as Airy discs. When the Airy discs of two point sources overlap, interference occurs, making it impossible to distinguish between the two sources. The distance between the centers of the Airy discs, which is equal to the radius of the Airy disc, determines the diffraction limit.

The diffraction limit imposes a limitation on the resolution of optical microscopes, preventing the resolvable distinction of objects or details that are too close together. The shorter the wavelength of light, the smaller the diffraction limit and the higher the resolution. Moreover, optical systems with a larger numerical aperture (NA) have a smaller diffraction limit and thus higher resolution.

Airy discs

The formula for calculating resolution,  NA represents the numerical aperture:

Resolution（r） = 0.16λ /  NA

Throughout history, scientists have embarked on a long and challenging journey to surpass the diffraction limit in optical microscopes. From early optical microscopes to modern super-resolution microscopy techniques, researchers have continuously explored and innovated. They have attempted various methods, such as using shorter wavelength light sources, improving the design of objectives, and employing specialized imaging techniques.

Some important breakthroughs include:

1. Near-field scanning optical microscopy (NSOM): NSOM uses a probe placed close to the sample surface to take advantage of the near-field effect and achieve high-resolution imaging.

2. Stimulated emission depletion microscopy (STED): STED utilizes the stimulated emission depletion effect of fluorescent molecules to achieve super-resolution imaging.

3. Structured illumination microscopy (SIM): SIM enhances imaging resolution through specific illumination patterns and image processing algorithms.

4. Single-molecule localization microscopy (SMLM): SMLM achieves super-resolution imaging by precisely localizing and tracking individual fluorescent molecules.

5. Oil immersion microscopy: Immersing the objective lens in a transparent oil increases the numerical aperture in the object space, resulting in improved resolution.

6. Electron microscope: By substituting electron beams for light beams, electron microscopy takes advantage of the wave nature of matter according to the de Broglie principle. Electrons, having mass compared to photons, possess a smaller wavelength and exhibit less diffraction, enabling higher imaging resolution.

Inverted fluorescence microscope

CIQTEK 120kV Field Emission Transmission Electron Microscope TH-F120

These developments have allowed us to observe the microscopic world at a higher level, gaining a deeper understanding of the structure and function of biological molecules and the microscopic properties of materials.

Furthermore, scientists are currently researching the following techniques to push the boundaries of optical diffraction limit:

1. Surface plasmon resonance microscopy: This technique utilizes surface plasmon resonances on metal surfaces to enhance localized electromagnetic fields and improve imaging resolution.

2. Nanophotonic microscopy: Nanophotonic microscopy manipulates photon structures at the nanoscale to achieve super-resolution imaging.

3. Acoustic microscopy: Acoustic waves are used for imaging, with shorter wavelengths compared to light, enabling higher resolution.

Additionally, other research directions are waiting to be explored to surpass the diffraction limit, including:

1. Linear optics microscopy: Nonlinear optical effects, such as second harmonic generation and third harmonic generation, are utilized to improve imaging resolution.

2. Interference microscopy: Interference phenomena are used to acquire phase information of the sample, thereby enhancing imaging contrast and resolution.

3. Light field microscopy: Light field microscopy simultaneously records both intensity and phase information of the light field, enabling three-dimensional imaging and super-resolution imaging.

4. Metamaterial microscopy: Metamaterials with special properties, such as negative refractive index and superlenses, are utilized to overcome the diffraction limit.

5. Deep learning-assisted microscopy: The combination of deep learning algorithms and microscopy techniques enhances image resolution and analysis capabilities.

Surpassing the diffraction limit remains an ongoing challenge. In the future, we may witness the emergence of even more advanced microscopy techniques, opening up new possibilities for scientific research applications.