Imaging arrays
Recent advances in our detector development and electronics have allowed enabled us to place and readout multiple detectors on the same wafer. This enables us to be able to produce arrays of detectors that can be used produce an image.
Imaging arrays can have multiple advantages over single pixel arrays depending on the application:
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They can be used to detect multiple frequencies simultaneously, which can be used to dramatically reduce acquisition times in traditional spectrometers (non- FTS).
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They can be coupled with optics to enable THz photography .
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They can be used to observe beam shapes and sizes.
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Can be used to simultaneously image multiple frequencies and polarisation's.
There are two main types off array that we can produce, both based off different types of detectors, Transition Edge Sensor (TES) Bolometers and Kinetic Inductance Detectors (KID's).
TES Arrays
The TES Bolometer arrays are identical to our single pixel ones, and operate around 4K at similar and frequency ranges and speeds (a few ms). We can configure these to the customers needs, for example the picture above shows a 25 pixel array used for monitoring a THz beamshape.
A TES array has all of the advantages of a single TES, with a rough spectral range (100 GHz - 30 THz) and being able to operate at ~8K. They can easily be integrated into a pulse tube cooler similar to our 403 dry systems or one of our wet systems. They are also compatible with our Winston cones, which can be used to focus the light down onto the whole array for a better collection efficiency. The bottom left picture shows a completed array with a custom designed Winston cone, mounted on a dry system coldplate. A digital render of the arrays cone is shown on the right.
AL LEKID's
Microwave Lumped Element Kinetic Inductance Detectors (LEKID's) are a type of superconducting detector that can can detect a wide range of radiation (from x-ray to sub-THz frequencies). They were first proposed in 2003, and since have been used by many astronomy groups around the world to look at the MIR-FIR spectral region.
LEKID's have several advantages over arrays of TES bolometers:
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Fast detector response (typically <100 µs)
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Can be easily multiplexed in the frequency domain to give a fast readout speed
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Arrays can be denser with more pixels per unit area.
They do however require more specialised cooling systems and electronics than a Nb-TES array.
Applications
There are many exciting uses for Al KID's, historically they have been developed for THz astronomy imaging, on projects such as NIKA, The Next Generation Blast Experiment, and MUSCAT. However, there are many more interesting THz applications that these detectors can be used for, due to the fact that many opaque materials are transparent at these frequencies. This opens up their use for many different fields where passive imaging would be useful, such as in biomedical science, non-destructive testing, object detection and security.
The picture below shows clearly how useful his technology can be, it shows a man holding a envelope, being imaged by a visible, infra-red, and LEKID cameras (left, centre and right respectively). From both the visible and the IR camera it appears the envelope is empty, wheras to the LEKID camera it clearly has an object inside. This object (mainly composed of plastic) has been made visible by the envelope being completely transparent at these frequencies.
QMC Instruments have been working closely with our partners in the Astronomy instrumentation group at Cardiff University to recently perfect a cooled LEKID array in a dry system that does not require a dilution fridge. If you have a non security related application for our cryogenic LEKID systems we would be excited to work with you to develop the idea further.
We are currently commercialising this technology for security applications through our spin out company Sequestim, as a passive terahertz scanner capable of detecting hidden objects/people. For more information on this exciting new area of our business please click the sequestim logo below.
Principle of operation
LEKID's main advantage is the fact they can be very easily arrayed and multiplexed, which arises from their basic operation. A single LEKID is essentially a superconducting LRC circuit, with a inductor, capacitor and resistor all in a closed circuit that is coupled to a "feed line." When the device is superconducting, any radiation with sufficient energy will cause cooper pairs to break, creating an excess quasi-particle population. The result of this is that the kinteic inductance of the superconducting circuit changes, which in turn changes the resonant frequency of the LRC circuit. This individual resonator can be weakly coupled and then probed with a signal along a feedline, to measure its resonant frequency, any change in the resonant frequency due to incident radiation is then easily observed.
To allow a multiplexed imaging array to be formed in a LEKID, each resonant circuit is designed to give a different resonant frequency. This is done by varying the capacitance in each individual circuit, allowing each circuit to have a unique resonance / resonance shift that can clearly be distinguished from any other.
