Spatial Terahertz-Light Modulators for Single-Pixel Cameras

Abstract

Terahertz imaging looks set to become an integral part of future applications from semiconductor quality control to medical diagnosis. This will only become a reality when the technology is sufficiently cheap and capabilities adequate to compete with others. Single-pixel cameras use a spatial light modulator and a detector with no spatial-resolution in their imaging process. The spatial-modulator is key as it imparts a series of encoding masks on the beam and the detector measures the dot product of each mask and the object, thereby allowing computers to recover an image via post-processing. They are inherently slower than parallel-pixel imaging arrays although they are more robust and cheaper, hence are highly applicable to the terahertz regime. This chapter dedicates itself to terahertz single-pixel cameras; their current implementations, future directions and how they compare to other terahertz imaging techniques. We start by outlining the competing imaging techniques, then we discuss the theory behind single-pixel imaging; the main section shows the methods of spatially modulating a terahertz beam; and finally there is a discussion about the future limits of such cameras and the concluding remarks express the authors’ vision for the future of single-pixel THz cameras.

Terahertz (THz) biophotonics technology: Instrumentation, techniques, and biomedical applications

“…This of course results in carrier diffusion limiting the THz resolution; however, it greatly reduces the system size and cost as well as improving the acquisition speed (due to higher signal to noise ratio) and can still give subwavelength resolution for low THz frequencies. The interesting part of this technology is that the carrier injection/depletion in the modulator can also be achieved by electrical biasing instead of optical pumping, 98 whereby the resolution would be limited by the photo-lithographic techniques used to manufacture the modulator. However, such electrical modulator arrays are currently in heavy development with recent work showing a 16 Â 16 graphene array with a 1 kHz switch-rate.…”

“…In this imaging modality, the fundamental acquisition speed is limited by the response time of the spatial modulator, and a recent overview of spatial THz modulators for single-pixel cameras is given in Ref. 98.…”

Very-Large-Scale Reconfigurable Intelligent Surfaces for Dynamic Control of Terahertz and Millimetre Waves

“…A metallic object placed in a paper envelope is positioned in front of the modulator. The core working principle of the camera is based on compressed sensing algorithm 13,[33][34][35] . This algorithm enables the reconstruction of the spatial information of the object (X, the vector representing the transmittance of the object) by measuring a series of structured transmission patterns generated by the modulator.…”

Real time THz imaging—opportunities and challenges for skin cancer detection

“…Such cameras can achieve rapid imaging rates, have reasonable costs, and can obtain subwavelength resolution (as the modulator can be placed in the near-field of the object) 2,31,32 . Today the most common spatial THz-light modulators work by modulating the conductivity of a semiconductor via optical excitation 1,2 , although electrical control of the conductivity is expected to be the long-term solution once such technology is further optimized 33 . The angle of incidence of the THz beam onto the surface where the conductivity is being controlled greatly affects the THz modulator efficiency.…”

“…The resolution in their approach is fundamentally limited by the photocarrier diffusion dynamics, as they photoexcite silicon with an LED to alter the conductivity. However, electrical based THz modulators, whereby an applied electric field changes the conductivity, will have their resolution set by the device manufacturing techniques as opposed to carrier diffusion dynamics as is the case with optical based modulators 33 .…”