Optimal position of an emitter in a wavelength-scale parabolic reflector (article)

This is the final version. Available on open access from the Optical Society of America via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.1883, with the title 'On the optimal position of an emitter in a wavelength-scale parabolic reflector'We investigate the optimum emitter position within reflecting parabolic antennas whose size is comparable to the emission wavelength. Using finite-element modeling we calculate the dependence of the amount of power directed into a 20° half-angle cone on the emitter’s position and compare with results obtained using geometrical optics. The spatially varying density of states within the wavelength-scale reflector is mapped and its impact discussed. In addition, it is demonstrated that changing the characteristic size of the reflector within the range from 0.5 to 1.5 times the emission wavelength has a strong bearing on the optimum emitter position, a position that does not in general coincide with the parabola’s focus. We calculate that the optimal antenna size and emitter position allow for the maximum directed power to exceed that obtained in the geometrical optics regime.DysonEngineering and Physical Sciences Research Council (EPSRC

Imaging through scattering media by exploiting the optical memory effect: a tutorial

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this recordData availability: The raw data, taken with the setup shown in fig.3, used to generate the figures in this tutorial is available at https://zenodo.org/doi/10.5281/zenodo.13642167, together with a code in Mathematica and one in Matlab that perform all the data analysis described in the sections above/Scattering, especially multiple scattering, is a well known problem in imaging, ranging from astronomy to medicine. In particular it is often desirable to be able to perform non-invasive imaging through turbid and/or opaque media. Many different approaches have been proposed and tested through the years, each with their own advantages, disadvantages, and specific situations in which they work. In this tutorial we will show how knowledge of the correlations arising from the multiple scattering of light allows for non-invasive imaging through a strongly scattering layer, with particular attention on the practicalities of how to make such an experiment work.Leverhulme Trus

Implicit image processing with ghost imaging

This is the final version. Available on open access from Optica Publishing Group via the DOI in this recordData availability: Data created during this research are openly available from https://doi.org/10.5281/zenodo.5779444In computational ghost imaging, the object is illuminated with a sequence of known patterns and the scattered light is collected using a detector that has no spatial resolution. Using those patterns and the total intensity measurement from the detector, one can reconstruct the desired image. Here we study how the reconstructed image is modified if the patterns used for the illumination are not the same as the reconstruction patterns and show that one can choose how to illuminate the object, such that the reconstruction process behaves like a spatial filtering operation on the image. The ability to directly measure a processed image allows one to bypass the post-processing steps and thus avoid any noise amplification they imply. As a simple example we show the case of an edge-detection filter.Leverhulme TrustEngineering and Physical Sciences Research Council (EPSRC

Rapid terahertz beam profiling and antenna characterization with phase-shifting holography

This is the final version. Available from Nature Research via the DOI in this record. Data availability: The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.In this paper we investigate the application of phase-shifting digital holography for the real-time characterization of electromagnetic sources in the THz frequency range. We use an of-the-shelf terahertz detector array composed of 64 × 64 power-sensitive pixels, over an area of 96 mm × 96 mm, to record intensity interferograms cast between the coherent radiation emitted from a reference source and an unknown antenna under test. This approach parallelizes the acquisition process with respect to conventional near-feld point scanning methods, reducing the measurement time by orders of magnitude. In our system, the measurement time is limited only by the refresh rate of the detector array and the speed of a delay line stage that is used to phase-shift the reference wave. As a proof-ofprinciple demonstration, we map the 2D near-feld distribution and estimate the far-feld radiation pattern emitted by a plano-convex PTFE spherical lens antenna illuminated by a diagonal horn at 290GHz frequency with ∼ 1 Hz refresh rate.Engineering and Physical Sciences Research Council (EPSRC)Engineering and Physical Sciences Research Council (EPSRC) / QinetiQ Ltd.Royal Academy of Engineering and the European Research Counci

Amplitude–only spatial light modulation using phase-change meta-films

This is the final version. Available from META Conference via the link in this record. Current spatial light modulator (SLM) technology offers off-the-shelf spatial phase control of light, but amplitude control is much more limited. The development of amplitude-only modulators would enable devices to perform full-wavefront control. Here we present an approach to the realization of such modulators, using a phase-change material based approach. Fabricated devices allow for the control of the amplitude, with near zero effect on the phase of the reflected wave, offering a potential route to ultra-fast, solid-state wavefront control.Engineering and Physical Sciences Research Council (EPSRC

Phase-change spatial light modulators with amplitude-only control

This is the final version.Current spatial light modulator (SLM) technology allows for the spatial control of the phase of light, but amplitude control is more limited. The realisation of a SLM that can efficiently modulate amplitude without altering the phase of the light would enable the development of devices that can perform full wavefront control. Here we discuss the design, fabrication and characterisation of such modulators using a phase-change material layer as the active component. These devices allow for the modulation of the amplitude of a reflected wave with near-zero change to its phase., providing a potential route to ultra-fast, solid-state wavefront control.Engineering and Physical Sciences Research Council (EPSRC

A Route to Ultra‐Fast Amplitude‐Only Spatial Light Modulation using Phase‐Change Materials

This is the final version. Available on open access from Wiley via the DOI in this recordData Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.A phase-change material based, thin-film, amplitude-only spatial light modulator is presented. The modulator operates in reflection and modulates the amplitude of light incident on its surface with no effect on optical phase when the phase-change material is switched between its amorphous and crystalline states. This is achieved using a thin-film device with an embedded, switchable, GeTe phase-change layer. Test modulation patterns are written to the device using laser scans, and the amplitude and phase response measured, using optical spectroscopy and off-axis digital holography. Experimental results reveal reflected intensity to be modulated by up to 38%, with an averaged phase difference of less than ≈π/50. Since phase-change materials such as GeTe can be switched on sub-microsecond timescales, this approach maps out a route for ultra-fast amplitude spatial light modulators with widespread applications in fields such as wavefront shaping, communications, sensing, and imaging.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Real-time millimeter wave holography with an arrayed detector (article)

This is the final version. Available on open access from Optica Publishing Group via the DOI in this recordData Availability: The research data supporting this publication are openly available in ORE at https://doi.org/10.24378/exe.4986Millimeter and terahertz wave imaging has emerged as a powerful tool for applications such as security screening, biomedical imaging, and material analysis. However, intensity images alone are often insufficient for detecting variations in the dielectric constant of a sample, and extraction of material properties without additional phase information requires extensive prior knowledge of the sample. Digital holography provides a means for intensity-only detectors to reconstruct both amplitude and phase images. Here we utilize a commercially available source and detector array, both operating at room temperature, to perform digital holography in real-time for the first time in the mm-wave band (at 290 GHz). We compare the off-axis and phase-shifting approaches to digital holography and discuss their trade-offs and practical challenges in this regime. Owing to the low pixel count, we find phase-shifting holography to be the most practical and high fidelity approach for such commercial mm-wave cameras even under real-time operational requirements.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC)Royal Academy of Engineering (RAE)Qineti

Terahertz imaging through emissivity control

This is the final version. Available on open access from Optica Publishing Group via the DOI in this recordData availability: Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.Electromagnetic radiation in the terahertz (THz) frequency band has unique potential for future communication and imaging applications.However, the adoption of THz technologies is hindered by the lack of cost-effective THz sources. Here we demonstrate a way to generate and control THz radiation, via spatio-temporal emissivity modulation. By patterning the optical photoexcitation of a surface-passivated silicon wafer, we locally control the free-electron density, and thereby pattern the wafer's emissivity in the THz part of the electromagnetic spectrum.We show how this unconventional source of controllable THz radiation enables a form of incoherent computational THz imaging.We use it to image various concealed objects, demonstrating that this scheme has the penetrating capability of other THz imaging approaches, without the requirement of femtosecond pulsed laser sources. Furthermore, the incoherent nature of thermal radiation also ensures the obtained images are free of interference artifacts. Our spatio-temporal emissivity control could enable a family of long-wavelength structured illumination, imaging, and spectroscopy systems.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC)Royal Academy of Engineering (RAE