Fluorescence and Phosphorescence

Fluorescence and Phosphorescence are utilized by the smartphone for 3D information encryption with dual-light-emitting materials.

About Fluorescence:

• Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
• It is a form of luminescence.
  • Luminescence: The emission of light by a substance that has not been heated.
• In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation.
• A notable example of fluorescence is when absorbed radiation falls within the ultraviolet range of the electromagnetic spectrum (invisible to the human eye), while the emitted light is in the visible spectrum.
  • This imparts a distinct color to the fluorescent substance, which becomes visible only when the substance is exposed to ultraviolet (UV) light.
• Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

Mechanism:

• Fluorescence occurs when an excited molecule,atom, or nanostructure, relaxes to a lower energy state (usually the ground state) through emission of a photon without a change in electron spin.
  • When the initial and final states have different multiplicity (spin), the phenomenon is termed phosphorescence.

Application:

• It finds practical applications in various fields, such as mineralogy, gemology, medicine, chemical sensors (fluorescence spectroscopy), fluorescent labeling, dyes, biological detectors, cosmic-ray detection, vacuum fluorescent displays, and cathode-ray tubes.
• The most common everyday application of fluorescence is found in (gas-discharge) fluorescent lamps and LED lamps.
  • In these fluorescent coatings transform UV or blue light into longer wavelengths, producing white light that can closely resemble the traditional but less energy-efficient incandescent lamp’s light, making it a widely used and more energy-efficient alternative.

About Phosphorescence:

• Phosphorescence is a type of photoluminescence related to fluorescence.
  • Any luminescence that is induced by the absorption of photons is called photoluminescence.
• When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength.
• Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs.
• Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.

Mechanism:

• Phosphorescence can be generated by two distinct mechanisms: triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or persistent luminescence).
• Triplet phosphorescence occurs when an atom absorbs a high-energy photon, and the energy becomes locked in the spin multiplicity of the electrons, generally changing from a fluorescent “singlet state” to a slower emitting “triplet state“.
• Persistent phosphorescence occurs when a high-energy photon is absorbed by an atom and its electron becomes trapped in a defect in the lattice of the crystalline or amorphous material.
  • A defect such as a missing atom (vacancy defect) can trap an electron like a pitfall, storing that electron’s energy until released by a random spike of thermal (vibrational) energy.
  • Such a substance will then emit light of gradually decreasing intensity, ranging from a few seconds to up to several hours after the original excitation.

Application:

• Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint and clock dials that glow after being charged with a bright light such as in any normal reading or room light.

Ref: Source