Ultraviolet Light

definition:
Invisible light with a wavelength less than about 400nm.

Ultraviolet light is light with a wavelength less than about 400 nm (the lower limit of visible light wavelength).  
There are several different definitions for distinguishing different spectral regions:

1. The near UV spectral range is from 400nm to 300nm. The medium UV light ranges from 300nm-200nm, while 200nm-10nm belongs to the far UV region. Shorter wavelengths belong to extreme ultraviolet light (EUV).  
2. Vacuum UV (less than 200nm) refers to the wavelength range commonly used in vacuum devices, as the light energy at this wavelength is strongly absorbed by air. Vacuum UV includes far UV and extreme ultraviolet light.  
3. UVA represents a wavelength range of 320-400nm, UVB is 280-320nm, and UVC is 200-280nm

UV light has many uses, such as disinfecting water and tools, curing adhesives, controlling the quality of many materials, and exciting fluorescence.  

catalogue

1. The main properties of ultraviolet light
2. Generate ultraviolet light
3. UV Optics
4. potential safety hazard

The main properties of ultraviolet light
Ultraviolet light differs from visible light in two ways:

1. Its short wavelength can accurately focus and produce very fine structures (if a highly spatially coherent light source is used). This can be applied to UV lithography technology for the fabrication of microelectronic devices, such as microprocessors and chips. Future microprocessors require finer structures and photolithography technology in the EUV region. We are currently developing EUV light sources and their corresponding photoresist.  
2. Its photon energy is higher than the bandgap energy of many objects. Therefore, ultraviolet light can be absorbed by many substances, and the resulting excitation process can cause changes in the chemical structure of the substance (such as chemical bond breakage). This can be used in laser material processing (such as laser etching, pulsed laser deposition, preparation of fiber Bragg gratings) to disinfect and sterilize water or medical devices. UV light can damage human skin, especially UVC light has bactericidal effects. When ultraviolet light interacts with trace hydrocarbons in the air, organic thin layers can be deposited on nearby surfaces; This type of light pollution can reduce the quality of nonlinear crystals in UV laser sources.  

Generate ultraviolet light
Laser generation of ultraviolet light faces many problems, but there are still some ultraviolet lasers that can directly generate UV light: some bulk lasers (such as using cerium doped crystals, Ce: LiCAF), fiber lasers, laser diodes (mostly using GaN), dye lasers, quasi molecular lasers, and free electron lasers.
Another way to generate ultraviolet light is to perform nonlinear frequency conversion on the output light of near-infrared lasers. Refer to UV laser for more details.  
Especially in the EUV region, gas discharge (such as xenon or tin vapor) or laser-induced plasma is commonly used to generate high-power UV radiation of several watts or even tens of watts. However, this type of light source is not coherent.  
Sometimes ultraviolet light is not generated by lasers. Of particular importance are gas discharge lamps (such as mercury tubes), and light emitting diodes (UV LEDs) are also widely used.  

UV Optics
When dealing with UV light, special UV optical theory is required. The important material parameters in UV applications are low foam and inclusion content, good uniformity of refractive index, low birefringence, and smooth surface. Especially when using strong UV lasers, long-term UV resistance is also important.
UV optics is required in pure calcium fluoride, which has low UV absorption, high uniformity, low birefringence, high hardness (compared to other fluoride materials), high stability, and high damage threshold. It can be used below 160nm, therefore it can be used for argon fluoride excimer lasers.
But it is fragile, non isotropic, and hygroscopic. Its substitute is UV grade fused silica, which can be used for wavelengths below 200nm, while cheap standard fused silica has significant losses below 260nm. Another available material is diamond, which is transparent below 230nm but very expensive.  
Some optical fibers can be used in the near ultraviolet spectral region, but the propagation loss is relatively high. Transmitting ultraviolet light through optical fibers is not feasible in situations where the wavelength is shorter or the power is higher.  
In the EUV region, almost all solid materials have strong absorption, and strong attenuation occurs even in air below 200nm. Therefore, vacuum UV or EUV is required for lithography under vacuum conditions.
The Bragg reflector can be prepared using a molybdenum/silicon (Mo/Si) structure in the EUV region, achieving a reflectivity of approximately 70% at 12nm. Due to its limited reflectivity, it is necessary to modify the EUV optical structure design to achieve the minimum number of reflective surfaces.  

Safety hazards
Ultraviolet light is harmful to both the eyes (especially at 250-300nm) and the skin (especially at 280-315nm), as it can cause cataracts or keratitis. In addition to causing pigment deposition and erythema, it can also lead to skin cancer.
And small doses that are not sufficient to cause acute reactions can also accelerate skin aging. Therefore, if UV light is used for operation, especially UV lasers, special laser protection measures are required. For example, UV beams in open optical devices need to be enclosed with some metal tubes.