No material can absorb 100% of light under normal conditions. However, some materials and structures can come close to total light absorption for specific wavelengths or under certain circumstances. The key factors that determine how much light something can absorb are its chemical composition, physical structure, and the properties of the light itself.
What determines how much light something absorbs?
Several factors come into play in determining the light absorption capabilities of a material:
Chemical composition: The types of atoms and molecules that make up a material determine its interactions with light. Certain elements and compounds are better than others at absorbing photons of light energy.
Electronic structure: How a material’s electrons are arranged can affect how it responds to light. The electron configurations in metals, semiconductors, and insulators lead to different light absorption properties.
Physical structure: The physical arrangement and morphology of a material influences light scattering, reflection, and transmission. Structures with high surface areas tend to absorb more light.
Wavelength: The wavelength (color) of light affects how it interacts with matter. Materials absorb and reflect different wavelengths selectively based on their electronic structure.
Thickness: Thicker materials provide more opportunities for light absorption and attenuation as photons pass through. Thin films transmit more light.
Crystallinity: The degree of molecular order and crystal structure impacts light scattering and absorption capabilities. Amorphous materials behave differently than crystalline ones.
When can nearly complete light absorption occur?
While 100% light absorption is not possible, some situations can result in extremely high, near-total absorption:
With opaque black materials: Thick blocks of carbon black, iron oxide black, and other opaque black substances can absorb up to 99% of visible light. Black hole synthetics made of carbon nanotubes absorb more than 99.9%.
With ultra-thin films: Atomically thin 2D materials like graphene can absorb up to 2.3% of incident light per monolayer. Stacking sheets increases absorption.
With internal reflection: When light strikes a high-low refractive index interface at angles greater than the critical angle, total internal reflection occurs and no light is transmitted. This principle is used in optical fibers to confine and transmit light.
With optical cavities: Micro- and nanoscale optical cavities with reflective walls can trap and absorb light at certain resonant frequencies and wavelengths. This effect enables lasers and other optics.
With plasmonic metamaterials: Metamaterials incorporating plasmonic nanostructures can achieve near-perfect light absorption by tuning geometry for improved light trapping.
With extremely dense media: Superdense materials like neutron star matter can absorb >99% of light due to high electron density and frequent photon interactions.
With astronomical objects: Extremely massive black holes and other opaque astronomical objects like super-dense stars absorb virtually all the light that hits them.
What are some examples of ultra-black materials?
Here are some examples of ultra-black materials and objects capable of absorbing up to 99.999% of light:
| Material | Description |
|---|---|
| Vantablack | A carbon nanotube coating that absorbs up to 99.965% of light. |
| Carbon black | Soot and charcoal particles that absorb up to 99% of visible light. |
| Super black paints | Paints incorporating carbon black, iron oxide black, and other pigments that reflect almost no light. |
| Iron oxide nanoparticles | Black iron oxide particles absorb across visible and infrared wavelengths. |
| Plasmonic black metals | Nanostructured metals like black gold and black aluminum that trap light. |
| Silicon nanorods | Microscale silicon pillars that form an effective black surface. |
| Black 2.0 | A carbon nanotube material that absorbs up to 99.8% of ultraviolet light. |
| Optical black hole | Carbon nanotube structure that captures >99.9% of incident light. |
What technologies rely on ultra-black materials?
Some key technologies that utilize ultra-black materials include:
Optics and sensors: Black lens coatings, infrared detectors, photoelectric devices, and other optical systems benefit from ultra-low reflectance.
Solar power: Solar panel efficiency can be improved by absorbing more light. Ultra-black materials are ideal for photovoltaic surface coatings.
Thermal devices: Radiators, collectors, emitters, and thermoelectric devices work more efficiently with highly light-absorbent black surfaces.
Camouflage: Military stealth technology uses ultra-black coatings to avoid detection across infrared, visible, and radar wavelengths.
Architecture: Black materials are used in building exteriors, interiors, and decorative structures to absorb light and create dark surfaces.
Fashion and design: Clothing, jewelry, art, and consumer products leverage ultra-black materials for aesthetic visual effects.
Calibration standards: Optical black is used to calibrate and benchmark the sensitivity of imaging devices and telescopes.
Spacecraft: Black thermal coatings on satellites and spacecraft help regulate temperature by absorbing light and emitting heat.
What limits total light absorption?
There are a few fundamental limits that prevent perfect, 100% light absorption:
Reflectivity: All materials reflect some percentage of light due to electronic transitions not being perfectly in resonance with photon energies.
Transmittance: Light can pass through sufficiently thin materials before being absorbed. Making materials thicker increases absorption but adds weight.
Scattering and diffraction: Light scattering off small structures and diffractive effects around edges mean photons take non-straight paths, reducing absorption.
Penetration depth: Light can only penetrate so far into a material before absorption. Past this depth, absorption cannot increase.
Thermodynamic limits: The second law of thermodynamics constrains how efficient light-to-heat conversion can be, limiting blackbody absorption.
Quantum effects: Photons exhibit wave-particle duality, so they cannot be fully trapped or localized by classical materials and structures.
Nanophotonic effects: Near-field interactions, surface plasmons, and nanostructure coupling effects enable light transmission past surfaces.
Can better-than-black materials be created?
While 100% light absorption is impossible, engineers are developing “better-than-black” nanophotonic materials that absorb more light than any natural material by exploiting advanced optical trapping effects. Some approaches include:
Metamaterial perfect absorbers: Resonant metamaterial geometries can achieve near-perfect absorption at target wavelengths.
Nanostructured 3D cavities: Three-dimensional periodic networks of nanoscale resonators and tunnels absorb light from all angles.
Hyperbolic metamaterials: Materials with hyperbolic dispersion geometry can absorb light across broad bandwidths and incidence angles.
Magnetic optical equivalents: Artificial media that mimic exotic astronomical objects like black holes could boost absorption.
Loss optimization: Computational techniques can help design materials and surfaces with ideal light-trapping characteristics to maximize absorption.
Optical black holes: Metamaterial structures that absorb light analogously to astronomical black holes by limiting transmission.
Surface plasmon engineering: Structured metals and dielectrics that optimize plasmonic interactions could improve over natural material limits.
Conclusion
While no natural material absorbs all light, astronomers speculate that event horizons of black holes may absorb nearly 100% of incident photons. Engineers are pushing towards this physical limit by designing nanophotonic structures, metamaterials, and other artificial media that maximize light-matter interactions and optical trapping phenomena to go beyond what is possible with ordinary materials to achieve the blackest blacks. Ongoing research aims to understand the fundamental bounds of light absorption and engineer ever-more-extreme artificial optical materials.