Silicon(Si) Double-Concave Lenses

Silicon has high thermal conductivity and low density. Silicon is a widely used NIR material for applications in the 1.2 to 7 μm region due to its low cost and low density. Silicon is well suited for weight-sensitive applications, especially those in the 3 -5 μm range.

Our company offer silicon biconcave lenses in various sizes and focal lengths.

Biconcave lenses have a negative focal length and consist of two concave surfaces of equal curvature, which can disperse a parallel incident beam outward and are commonly used for beam spreading, collimation, projection and focal length expansion of optical systems. Biconcave lenses disperse a collimated beam to an imaginary focal point and are commonly used in Galilean beam expanders.

The important parameters of biconcave lenses are: size, focal length, design wavelength, finish, face accuracy, eccentricity, substrate material and other attributes. Suitable parameters of biconcave lenses can be selected according to specific applications.

Single silicon (Si) crystal is a chemically inert gas material with high hardness and insoluble in water. It has excellent light transmission properties in the 1-7μm band, and it also has excellent light transmission properties in the far infrared band 30-300μm.

Silicon has a high thermal conductivity and low density, making it suitable for making into laser mirrors. However, silicon is not suitable for CO2 laser transmission applications due to its strong absorption at 9 µm.

Coating refers to coating a transparent electrolyte film or metal film on the surface of the substrate material by physical or chemical methods. The purpose is to change the reflection and transmission characteristics of the material surface to reduce or increase the reflection, beam splitting, color separation, light filtering, polarization and other requirements.We can provide various optical coatings such as anti-reflective films, high-reflective films, spectral films, and metallic films. Broadband anti-reflective films are available for UV, visible, NIR and mid-infrared wavelengths.

Material Standards

●Material: Silicon single crystal with a chemical composition of Si

●Purity: The purity of the raw material used for crystal growth should be not

    less than 9N (99.9999999%) and the purity of the dopant used should be not

    less than 5N (99.999%).

●Crystallization quality: silicon single crystal rods should have no mosaic, no

    grain boundaries, no twin crystals.

●Dislocation density: The dislocation density should be no more than 100

    dislocations/cm2.

●Conductivity type: Silicon single crystal conductivity type of a total of two

     types: N type, P type

●The crystallographic orientation of silicon single crystals consists mainly of

     the following crystallographic directions

●Resistivity: The resistivity of silicon single crystals is divided into four classes.

●Appearance quality: no pollution, no chipping, no cracks, no holes on the

     surface of silicon single crystal 

Transmittance test

●Samples

 Monocrystalline silicon, diameter not less than 20 ~ 50mm, thickness 10 ± 0.5mm，through the polished surface finish to 80/50

●Test band

 3μm~15μm

●Qualification requirements

 T≥52.5%@3-5μm

Monocrystalline

●There are no visible grain boundaries or wicker-like stripes on the crystal surface when examined under naked eye daylight.

Sub-crystal

●When examined under naked-eye daylight, there are willow stripes on the surface of the crystal with an area < 1/6 (end diameter), and the willow stripes are not visible after polishing .

Polycrystalline

●When examined under naked-eye daylight, there are penetrating crystal boundary lines on the surface of the crystal, and the difference in the degree of light and darkness between the two sides of the crystal boundary lines is obvious. 

●N-BK7

    N-BK7 is the most commonly used optical glass for processing high quality

    optical components,, with excellent transmittance from visible to near-

    infrared wavelengths(350-2000nm), and has a wide range of applications in

    telescopes, lasers and other fields. N-BK7 is generally chosen when the 

    additional benefits of UV fused silica (very good transmittance and low 

    coefficient of thermal expansion in the UV band) are not required.

●UV fused silica

     UV fused silica has a high transmission from the UV to NIR  (185-2100nm). 

     In addition, UV fused silica has better uniformity and lower coefficient of 

     thermal expansion than H-K9L (N-BK7), making it particularly suitable for 

     high power laser and imaging applications.

●Calcium fluoride

    Due to its high transmittance and low refractive index within a wavelength

    of 180nm-8um, calcium fluoride is often used as windows and lenses in 

    spectrometers and thermal imaging systems. In addition, it has good 

    applications in excimer lasers because of its high laser damage threshold.

●Barium fluoride

    Barium fluoride have high transmittance from the 200nm-11um and they are

    resistant to stronger high-energy radiation. At the same time, barium fluoride

    has excellent scintillation properties and can be made into various infrared 

    and ultraviolet optical components. However, the disadvantage of barium

    fluoride is that it is less resistant to water. When exposed to water, the

    performance degrades significantly at 500℃, but it can be used for

    applications up to 800℃ in a dry environment. At the same time, barium 

    fluoride has excellent scintillation properties and can be made into various 

    infrared and ultraviolet optical components.It should be noted that when 

    handling barium fluoride material, gloves must be worn at all times and 

    hands must be washed thoroughly after handling.

●Magnesium fluoride 

    Magnesium fluoride is ideal for applications in the wavelength range of 200nm

    -6um. Compared to other materials, magnesium fluoride is particularly durable

    in the deep UV and far IR wavelength ranges. Magnesium fluoride is a powerful

    material for resistance to chemical corrosion, laser damage, mechanical shock

    and thermal shock. It is harder than calcium fluoride crystals, but relatively soft

    compared to fused silica, and has a slight hydrolysis. It has a Nucleus hardness

    of 415 and a refractive index of 1.38.

●Zinc selenide 

    Zinc selenide has high transmittance in the 600nm-16um and is commonly

    used in thermal imaging, infrared imaging, and medical systems. Also, due to

    its low absorption, zinc selenide is particularly suitable for use in high-power

    CO2 lasers. It should be noted that zinc selenide is a relatively soft material 

    (Nucleus hardness 120) and is easily scratched, so it is not recommended for 

    use in harsh environments. Extra care should be taken when holding, and 

    cleaning, pinching or wiping with even force, and it is best to wear gloves or

    rubber finger covers to prevent tarnishing. Cannot be held with tweezers or 

    other tools.

●Silicon 

    Silicon is suitable for use in the NIR band from 1.2-8um.Because of its low 

    density, silicon is particularly suitable in applications where weight

    requirements are sensitive, especially in the 3-5um . Silicon has a Nucleus 

    hardness of 1150, which is harder than germanium and not as fragile as 

    germanium.It is not suitable for transmission applications in CO2 lasers 

    because of its strong absorption band at 9um.

●Germanium 

    Germanium is suitable for use in the near-infrared band of 2-16um and is well 

    suited for infrared lasers. Due to its high refractive index, minimal surface 

    curvature and low chromatic aberration, germanium does not usually require 

    correction in low power imaging systems. However, germanium is more 

    severely affected by temperature, and the transmittance decreases with

    increasing temperature; therefore, it can only be applied below 100°C. The 

    density of germanium (5.33 g/cm³) is taken into account when designing 

    systems with strict weight requirements. Germanium lenses feature a

    precision diamond lathe turned surface, a feature that makes them well suited

    for a variety of infrared applications, including thermal imaging systems, 

    infrared beam splitters, telemetry, and in the forward-looking infrared (FLIR)

    field.

●CVD ZnS 

    CVD ZnS is the only infrared optical material, other than diamond, that 

    covers visible to long-wave infrared (LWIR), full wavelength and even 

    microwave wavelengths, and is currently the most important LWIR window 

    material. It can be used as windows and lenses for high-resolution thermal 

    imaging systems, as well as for advanced military applications such as 

    "tri-optical" windows and near-infrared laser/dual-color infrared composite 

    windows.