Photonics
An overview of Photonics
What is Photonics?
Photonics is a field of physics and a sub-discipline of the broader branch of optics, both of which have ties within engineering.
The two areas differ in their specificity, photonics refers to the interaction that light has with matter, whereas optics focuses more generally on light and how it behaves. Examples of the interactions within photonics include how matter manipulates, transmits and reflects light; as well as how matter emits and detects light.
This webpage will explore these interactions and how they tie into Torr Scientific’s technological research and products.
Photonics: Manipulation
This area of photonics refers to the controlling or changing of directions of light, including refraction, reflection and transmission. All three are highly relevant to areas in optics such as the vacuum viewports sold here at Torr, as well as mirrors and prisms. An example of this at Torr Scientific is our anti-reflective coatings.
Anti-reflective coatings
A photo showing a calcium fluoride viewport with an anti-reflective coating
The purpose of anti-reflective (AR) coatings is to improve the transmission and reduce the reflection of light through a material. This often relates to altering how light passes through an optic.
At Torr Scientific, our coatings are focused on improving the transmission of a specific wavelength or a range of wavelengths of light through a vacuum viewport, or a glass cell. These work by providing a stepping-stone between the different refractive indices of the optic and the air. Each coating is custom-made, constructed based on the wavelength of the laser being used and the material of the optic. The thickness of the coating is calculated for it to cancel out reflections from both faces of the optic, by a process called destructive interference.
Learn more about coatings and how they work on our AR coatings webpage.
Birefringence
Diagrams demonstrating how linear polarisation considers rays of light, showing s-polarised and p-polarised light and their direction of oscillation.
Birefringence is a property that changes the refraction of light. A material with this quality has two different behaviours, depending on the incidence angle of the light. This means that changing the angle of entering light, will determine if the beam will pass through, or if it will split into two light rays.
To explain this: assuming the idea of linear polarisation, light can be visualised as two directional components.
Diagram demonstrating how light behaves as it interacts with a birefringent material.
To avoid the effects of the birefringence, light beams can be fired along the optical axis. At this angle, the two refractive indices combine and the beam is impacted wholly by this new refractive index.
Torr Scientific offers an optic material with the quality of birefringence, natural z-cut quartz. Due to this property, quartz is transmissible from between 250nm and 3µm, as well as from 50µm to 1000µm+.
These dual transmission capabilities mean it can be used with both IR and UV applications.
Maintaining polarisation
A diagram demonstrating the effect of the Brewster window
Brewster windows can be used to partially polarise light and can be an alternative to requiring optics with anti-reflective coatings. Any material transmissive of light has a specific refractive index (how the medium impacts the speed of light compared to light within a vacuum) and using this you can calculate the “Brewster angle”. Light can be visualised as two directionally oscillating polarised waves and when firing a beam of light from the Brewster angle, one direction of polarisation is partially reflected and the other is transmitted.
You can enquire about Brewster windows with our sales team and the window can be designed to suit your needs.
Find out more about Brewster Windows in this blog post or visit our Brewster windows page.
Photonics: Detection
Photonic detectors are used to recognise single or multiple incoming photons, they can measure interesting properties such as energy, incidence direction, wavelength(s), chromaticity and polarisation. Detecting photons can have applications across many different industries. Some main examples link to environmental and medical analysis, or within defence and surveillance systems.
An example of a photon detection device is the MCP detector.
Microchannel plate detectors
A photo showing a fully constructed MCP detector
Microchannel plate (MCP) detectors measure incoming particles, photons, or electromagnetic radiation within or beyond the ultraviolet (UV) spectrum. They convert these particles/photons into a measurable electrical pulse.
Here is how they work:
- The particle enters the channel and hits the MCP.
- This causes a cascade of electrons to bounce along the internal walls, each strike generating secondary electrons.
- These electrons are accelerated along the MCP due to a potential voltage difference between the faces.
- After numerous strikes along the walls, this becomes a beam of electrons, which reaches the end and collides with the output screen.
This output could be an anode, or a phosphor screen, depending on the data you want from the experiment:
- Anodes can detect this collision as an electron pulse, providing data about the intensity but losing spatial information.
- Phosphor screens convert the collision into light to provide information about the shape and size of the electron beam.
A single electron entering the MCP can lead to the generation of up to 10,000 electrons.
Find out more on our synchrotron webpage.
Photonics – Emission
The emission of photons has two main processes; these are stimulated, or spontaneous emission.
Stimulated emission has a predictable quantifiable probability for when a material will emit light. Stimulating an atom has the chance of releasing a photon based on the intensity of the stimulation and this can be calculated mathematically. The technique led to the creation of the laser, hinted at by the acronym “light amplification by stimulated emission of radiation.”
Spontaneous emission factors around the chance of a high energy atom releasing a photon, which moves it to a lower level where it regains some of its stability. This occurs with properties like luminescence.
Phosphor screens
A picture of an activated phosphor screen which glows bright green.
Torr Scientific offers a range of phosphor screens that emit light with different properties based on the chemical composition of the screen.
These are the properties we measure:
- Typical peak wavelength
This is the wavelength that has the highest emission for the phosphor type - Chromaticity coordinates
Coordinates referencing a graphical representation of the hue and saturation of light. - Decay time
How quickly the image fades after excitation (e.g., activation from an MCP detector)
The differences in these properties and compositions lead to different types of phosphor screens.
Typically, our phosphor screens are used within MCP detectors or for RHEED (reflection high-energy electron diffraction).
*Please note: phosphor screens do not contain phosphorus.
Laser emission
Vacuum viewport with an incoming laser beam
Lasers are commonly used for experiments involving vacuum viewports and atom trap chambers.
Viewports and anti-reflective coatings can be damaged by high-power lasers and there are many specifications of the laser to be taken into consideration. These include power, spot size, wavelength and pulse width. These specifications can be virtually modelled to determine if they will cause damage to the optic.
The durability of the optics and coatings is important to be able to confirm the usability of our products and includes ratings for the different types of lasers.
The rating system is based on the laser-induced damage threshold (LIDT).
- Our coated optics are rated to:
- 500 W / cm2 at 530 nm for continuous wave (CW) lasers
- 2 J / cm2 at 530 nm with a 10 ns pulse width for pulsed lasers
Magneto-optical traps
A typical miniature atom trap chamber
Torr Scientific manufactures magneto-optical traps (MOT), also known as atom trap chambers (ATC) with custom optics allowing lasers of different frequencies to enter the experimental zone. Inside the experimental zone, lasers of particular wavelengths are used to interreact with the target atoms. The laser wavelengths are chosen to match the frequency at which the atoms vibrate at their resonance frequency. Matching this frequency can allow the absorption of energy.
This is how it works:
- As atoms move towards the lasers, the wavelengths appear compressed (due to the Doppler effect)
- The lasers are deliberately set slightly below the resonance frequency for the targeted atom.
- This means that when the wavelength is compressed, it now appears at the resonance frequency.
- The atom then absorbs the light and is slowed.
Learn more about atom trapping on our Magneto-optical traps webpage.
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