What is Luminescence?

What is Luminescence? What are the different types?

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- GoPhotonics

Jan 17, 2023

Luminescence is the process of absorbing light at a particular wavelength and emitting light at a wavelength greater than the absorbed wavelength. In this phenomenon, no heating of the substance takes place like incandescence in which light is emitted from a heated source. Light emission from the substance is spontaneous in this method. This light can be called “cold light” because the emitted light is not from a heated substance. The light is emitted due to different causes like chemical reactions, electrical energy, subatomic motions, or stress on a crystal. Luminescence is of different types such as bioluminescence, chemiluminescence, electroluminescence, photoluminescence, and thermoluminescence.

What are the types of Luminescence?

  • Photoluminescence 

Figure 1: Photoluminescence process

The emission of light with a photoexcitation by photon absorption is called Photoluminescence. It is a form of luminescence that occurs when electromagnetic radiation is absorbed by a substance and the radiation is re-emitted. Photoexcitation initiates this process. That is, when the substance absorbs a photon, the electrons of the substance get excited to higher energy states from the ground state followed by relaxation of these electrons with the emission of a photon. Figure 1 shows the process of photoluminescence. Depending on the substance, the time period between the absorption and emission of photons may vary. Photoluminescence includes fluorescence and phosphorescence. Fluorescence is a light-emitting phenomenon in which a substance immediately re-emits the absorbed radiation, while in phosphorescence, the substance does not re-emit the radiation soon after the absorption.

Fluorescence is defined as the process of light emission from a substance that has absorbed energy or electromagnetic radiation before. This light emission occurs spontaneously and hence it is a type of luminescence. The light emitted usually has a longer wavelength than the light absorbed. This also means that the energy of emitted light is lower than the energy of the absorbed light. This difference in wavelength is called stoke’s shift. In fluorescence, light emission occurs as a result of the excitation of atoms in the substance when the substance absorbs light in the form of photons. But since the higher energy state is unstable, electrons release back to the ground state by emitting photons which can be seen as a glow on the substance. The energy absorbed is often released as luminescence in a short time period, about 10-8 seconds which means that we can observe fluorescence as soon as we remove the source of radiation that causes excitation. Fluorescence has applications in different fields, such as mineralogy, gemology, chemical sensors, biochemical research, medicine, dyes, biological detectors, fluorescent lamp production, etc. We can also see fluorescence as a natural process in minerals. These fluorescing substances that occur in nature have broad excitation and emission spectrum. 

Phosphorescence is a light emitting phenomenon like fluorescence in which a substance is exposed to the light of a short wavelength causing the substance to glow. In this process, the substance absorbs light and re-emits it at a longer wavelength. The emission of light takes place for a long time even after the radiation source is removed. Phosphorescence can occur due to two different mechanisms: triplet phosphorescence and persistent phosphorescence. Triplet phosphorescence occurs when a high-energy photon is absorbed by the atom. Persistent phosphorescence occurs when a high-energy photon is absorbed by an atom and this causes the electrons of the atom to get trapped in a defect in the lattice of the crystalline or amorphous material. Phosphorescence is a very slow process when compared to fluorescence since the electron decay from the excited state slows down to the ground state. "Glow-in-the-dark" toys that can be "charged" with a regular light bulb or natural light and then emit light for several minutes or even hours are common examples of phosphorescent materials. 

Figure 2: Jablonski Diagram

Figure 2 is named as Jablonski diagram and it is used to illustrate fluorescence and phosphorescence. Radiative and non-radiative transitions of excited molecules are demonstrated in this diagram. Here S0, S1, S2, and S3 on the left side represent singlet energy states and the energy states on the right side T1 represent triplet energy states. When a photon at a particular wavelength is absorbed by a molecule or electron in the ground state S0, it gets excited to higher singlet energy states S1, S2, and S3. Then they will make a vibrational relaxation through internal conversion and reaches energy state S1. They further release energy and returns to the ground state S0 emitting photons that causes Fluorescence. The released photon will have energy lower than the energy of the absorbed photon. There are also molecules that move from S1 to T1 which is the first triplet state by making a non-radiative relaxation. This process is called intersystem crossing. The triplet state has lower energy compared to the singlet state as the energy corresponding to the triplet state is lower and they are long-lived. The electrons then make a radiative transition from T1 to the ground state S0 causing Phosphorescence.

  • Chemiluminescence

Chemiluminescence is the phenomenon in which light is emitted as a result of a chemical reaction. This spontaneously emitted light is called luminescence. In chemiluminescence, there is the chance to form heat if the chemical reaction that occurs is exothermic. Here, the excitation of electrons is not due to the absorption of photons but due to the chemical reaction. During the chemical reaction, the electrons get excited to a higher energy state and then relax to the ground state by emitting photons. Luminol is a very good chemiluminescent substance that is used in criminalistics to find blood traces. Here, the Fe2+ ions in hemoglobin serve as a catalyst to convert Luminol into its light-emitting configuration.

Figure 3: Reaction of luminol with hydrogen peroxide

Blue light is released when luminol reacts with hydrogen peroxide in the presence of a catalyst. Figure 3 shows this reaction. The reaction formula is given below:

C8H7N3O2 (luminol) + H2O2 (hydrogen peroxide) → 3-APA (vibronic excited state) → 3-APA (decayed to a lower energy level) + light

where 3-APA is 3-Aminopthalalate

Chemiluminescence has applications in glow sticks. This luminescence results from a fluorescent dye (a fluorophore), which absorbs the light from chemiluminescence and releases it as another color. It is important to note that Chemiluminescence does not only occur in liquids. For example, the gas-phase reaction between vaporized phosphorus and oxygen results in the green glow of white phosphorous.

  • Bioluminescence 

The light emission by living organisms is called bioluminescence. It mainly occurs in marine vertebrates and invertebrates. 

Figure 4: Chemical reaction involved in bioluminescence

A form of chemiluminescence where light energy is released by a chemical reaction is bioluminescence. This reaction involves a light-emitting pigment, luciferin, and luciferase, the enzyme component. Figure 4 shows the reaction in which the luciferase reacts with atmospheric oxygen to form luciferin or oxyluciferin and light. E.g., Rotting meat and fish are bioluminescent just before putrefaction, the process of decay or rotting in a body or other organic matter. It is not the meat itself that glows, but bioluminescent bacteria. Bioluminescence can be observed in some fungi species, microorganisms such as bioluminescent bacteria, terrestrial arthropods (fireflies), etc. Firefly uses a chemiluminescent process to produce light. There are a number of marine organisms like corals, algae, crustacea, or even squids that emit light, mostly in the blue or green spectrum. Jellyfish is also a bioluminating organism that generates blue light by a chemical reaction with the help of the protein Aequorin. 

  • Electroluminescence 

Electroluminescence is a chemical phenomenon in which a material emits light when an electric current is applied to it. This is both an optical and electrical phenomenon that can occur in the presence of an electric current or a strong electric field. This feature is different from black body light emission that occurs due to heat, a chemical reaction, sound, and other mechanical action. Phosphor-based (powder) electroluminescent panels are generally used as backlights for liquid crystal displays.

Figure 5: Electroluminescence process

The electrons and holes are separated by doping to form a p-n junction (in semiconductors) or by excitation of electrons using a strong electric field (as with the phosphors in electroluminescent displays). When an electric current is applied, radiative recombination of electrons and holes takes place. The electrons excited release energy in the form of photons. This phenomenon is called electroluminescence. Figure 5 shows a schematic of electroluminescence process.

  • Mechanoluminescence

Mechanoluminescence, also known as triboluminescence, is a phenomenon of light emission when an external mechanical stimulus such as grinding, rubbing, crushing, or pressing is applied to a substance.  That is, mechanical stress in solids causes light emission. In this method, mechanical energy is converted into light. This has potential applications in pressure sensors, damage detection, bioimaging, lighting, and display devices. It is an interesting and least understood luminescence phenomenon. There are different types of mechanoluminescence depending on the type of mechanical stimuli such as piezoluminescence, triboluminescence, crystalloluminescence, and sonoluminescence. 

In piezoluminescence, a small pressure on the crystal surface without any fracture on them leads to the appearance of light. Elastico mechanoluminescence (ML) is a piezoluminescence and is detected in host materials with piezoelectricity (the process of using crystals to convert mechanical energy into electrical energy, or vice versa) and doped ions such as manganese and lanthanide. Upon the application of mechanical stress, the piezoelectric hosts attain electric fields leading to electronic conversions between the dopant energy levels, ultimately emitting light from the dopant ions. Plastico mechanoluminescence is a radiative mechanoluminescence category where mechanoluminescence strain and stress play a major role. All the elastic mechanoluminescent materials exhibit plastico ML. The plastic deformation causes dislocation movement. The electric field causes the bending of the valance band and conduction band as well as dislocation bands (bands created due to crystallographic defect). Thus, the electrons from the electron trap tunnel to the conduction band. The energy released during the recombination of electrons with the holes gives rise to the light emission characteristic of the activator centers. 

In the case of Mn-doped II-VI semiconductors, the energy released during the electron-hole recombination excites Mn2+ ions and the subsequent de-excitation gives rise to the light emission characteristic of Mn2+ ions. Polymers, alkaline earth oxides, certain non-colored alkali halide crystals, a certain variety of rubbers, and certain metals also exhibit plastic ML. Applications of plastico ML include non-destructive testing of materials, writing of secret messages in photography, and giving information about the stress (pressure or tension exerted on a material object) and strain (amount of deformation experienced by the body in the direction of force applied, divided by the initial dimensions of the body) of crystals.

Triboluminescence is a phenomenon in which light is generated when a material is mechanically pulled apart, ripped, scratched, crushed, or rubbed. It is also called fractoluminescence. In this, light is emitted from the fracture of a material occurred by any mechanical means. All elastic mechanoluminescent and plastic mechanoluminescent materials exhibit fracto ML. Applications of fracto mechanoluminescent include ML damage sensors, fracture sensors, impact sensors, fuse systems for army-warhead, evaluation of the design of milling machine, online monitoring of the grinding process, fragmentation studies of solids, the potential for earthquake indicator, determination of several parameters of solids, triboluminescence X-ray unit, micrometeoroid, and ultra-high speed (hypervelocity) debris impacts pose a significant threat to a spacecraft. 

For e.g., When a crack moves in a piezoelectric crystal (like glass or quartz), one of the newly created surfaces gets positively charged and the other surface gets negatively charged in which a strong electric field is generated between the two walls of a crack as shown in the figure below. Thus, an electric field may be generated between the newly created oppositely charged surfaces. This field may cause the dielectric breakdown of the surrounding gases and in turn may give rise to the gaseous discharge ML. The field may also cause the dielectric breakdown of the crystals, and the recombination of free charge carriers may give rise to recombination luminescence. Figure 6 shows the piezoelectrification model of fracto mechanoluminescence.

Figure 6: Piezoelectrification model of fracto mechanoluminescence

If there is a total transfer of energy from the excited gas molecules to the luminescence centers or the light produced due to gas discharge is absorbed completely by the crystals, then only the solid-state Fracto ML will be produced. Moreover, if the electric field is not sufficient to cause the gas discharge, then the gas discharge Fracto ML will not be observed. Furthermore, if the surface charges relax before the penetration of gases between the walls of the cracks, then also the gas discharge ML will not be observed. On the other hand, if there is a partial transfer of energy from the excited gas molecules to the luminescence centers or partial absorption of the light produced due to the gas discharge by the crystals, then the combination of both the solid-state Fracto ML and gas discharge Fracto ML will be observed.

In the case of crystalloluminescence, light is emitted due to the growth of crystals from saturated solution causing mechanical stress on the crystal surfaces. There are some crystalline salts that give crystalloluminescence from an aqueous solution like barium or strontium bromates etc. In this phenomenon, the emergence of fragments with large opposite electric charges occurs due to the fragmentation of crystals. These electric discharges can excite the molecules at the crystal surface. 

Light bursts occur as a result of sudden pressure changes applied to a gas-saturated liquid and this phenomenon is called sonoluminescence. The implosion of gas bubbles during the cavitation in liquid produce extreme temperature inside the bubbles as a result of adiabatic compression. A continuous emission spectrum that corresponds to the emission of a black body is obtained when the gas is heated to such a high temperature. Sonoluminescence of seawater gives additional yellow emission because of the strong sodium excitation. Mechanoluminescence has very few practical applications. Mechanoluminescent materials can be used to make new damage sensors that find application in spaceships or crash sensors for car airbags.

Applications of Luminescence

Luminescence is used in scientific research and day-to-day life. Some of the examples are given below:

  • Luminescence is used in laser and lamp technology.
  • Bioluminescence is used for non-invasive monitoring of cellular processes. This can be used to observe the activity of the internal organs of a living organism.
  • Light-emitting diodes are used in flat-screen TVs, computer monitors, street lights, modular lighting, outdoor area lighting, and task lighting.
  • Determination of surface temperature using a phosphor thermometer based on luminescence emitted from a phosphor material.
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