Marcin Gradziel

Medical applications of terahertz imaging: a review of current technology and potential applications in biomedical engineering

Terahertz (THz) imaging is in its early stages of development but already the potential clinical impact of this new imaging modality is clear. From cancer research to DNA analysis THz technology is improving or even making possible imaging of hitherto inaccessible phenomena. In this paper we present a short review of THz imaging from the point of view of biomedical engineering. We discuss the current state of the art in terms of THz imaging systems; describe current applications, future potential and our own approaches to harnessing this novel technology. We draw attention to open problems in the area with respect to the limitations of the technology before concluding with descriptions of our future work in the area.

QUBIC: The QU Bolometric Interferometer For Cosmology

The primordial B-mode polarisation of the Cosmic Microwave Background is the imprints of the gravitational wave background generated by inflation. Observing the B-mode is up to now the most direct way to constrain the physics of the primordial Universe, especially inflation. To detect these B-modes, high sensitivity is required as well as an exquisite control of systematics effects. To comply with these requirements, we propose a new instrument called QUBIC (Q and U Bolometric Interferometer for Cosmology) based on bolometric interferometry. The control of systematics is obtained with a close-packed interferometer while bolometers cooled to very low temperature allow for high sensitivity. We present the architecture of this new instrument, the status of the project and the self-calibration technique which allows accurate measurement of the instrumental systematic effects.

Direct measurement of P+ for electron impact excitation of H(2p) at 54.4 eV

We report results of direct measurements of the reduced Stokes parameters and for electron impact excitation of H(2p) at 54.4 eV over the scattering range 10–40°. These three parameters have been measured simultaneously for the first time using a VUV double-rotation polarization analyser consisting of a MgF2 retarder followed by a SiO2 reflection linear polarizer. As expected, our measurements for and are in good agreement with theoretical calculations and previous experimental data. Our data for differ significantly from previous experimental measurements and theoretical calculations. Consequently, we find that the coherence parameter P+ deviates significantly from unity at 30°. If correct, this signifies that spin-exchange scattering may be more important than has previously been thought.

Modelling of the optical performance of millimeter-wave instruments in MODAL

MODAL is an optical design and analysis package targeting the millimetre and sub-millimetre region of the electromagnetic spectrum. It is being developed at NUI Maynooth with the aim of integrating advanced modelling techniques and access to High Performance Computing into a user-friendly and yet very powerful tool for an (quasi-)optical designer. MODAL has been recently extended to allow integrated simulation of custom corrugated horns and dielectric lenses. This made it possible to model an existing instrument (QUaD), with the goal of optimising its performance. Here we present new results from analysis of the predicted performance of the QUaD telescope, with particular emphasis on polarisation information. They were obtained by using MODAL to model the whole telescope, with the distortion of the primary accounted for, for a range of component tilts and separations.

Latest Progress on the QUBIC Instrument

QUBIC is a unique instrument that crosses the barriers between classical imaging architectures and interferometry taking advantage from both high sensitivity and systematics mitigation. The scientific target is to detect primordial gravitational waves created by inflation by the polarization they imprint on the cosmic microwave background—the holy grail of modern cosmology. In this paper, we show the latest advances in the development of the architecture and the sub-systems of the first module of this instrument to be deployed at Dome Charlie Concordia base—Antarctica in 2015.

The quasi-optical performance of CMB astronomical telescopes

Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background Radiation, in particular QUaD1, the PLANCK Surveyor2 and MBI3. These telescopes contain a range of quasi-optical components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble polarisation signals. Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element optical systems. In general there is a lack of dedicated software tools for modelling the range of components and propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house software packages for this task. We describe the techniques used, their predictions and the performance of the telescopes that have been measured to-date.

Millimetre-wave and Terahertz Imaging Systems with Medical Applications

The properties of terahertz (THz) radiation make it ideal for medical imaging but the difficulty of producing laboratory sources and detectors has meant that it is the last unexplored part of the electromagnetic spectrum. In this paper we report on near-field reflection and absorption measurements of biological and non-biological samples at 0.1THz with a view to developing THz and millimetre-wave imaging schemes. In particular we have investigated the effects of standing waves on such systems and the sensitivity to water content of the sample as a means to extract medically useful information.

Fast CAD software for the optical design of long wavelength systems

We report on efficient CAD software (MODAL) that we are developing to specifically model long wavelength systems. Beam mode analysis which includes both aberration and truncation scattering using fast SVD methods allows sufficiently accurate and computationally highly efficient representations of a propagating beam for real time design feedback purposes.

Developments in quasi-optical design for THz

Optical design in the terahertz (THz) waveband suffers from a lack of dedicated software tools for modelling the range of electromagnetic and quasi-optical propagation conditions encountered in typical systems. Optical engineers are forced to use packages written for very different wavelength systems and there is often a lack of confidence in the results because of possible inappropriate underlying physical models. In this paper we describe the analytical techniques and dedicated CAD software tools (MODAL) that we are developing for long-wavelength design and analysis in the THz waveband. Our basic approach to modelling long-wavelength propagation is the application of modal analysis appropriate to the problem under investigation. We have extended this to include the efficient description of common off-axis (tilted) components such as simple curved reflectors. In earlier research we have investigated the conditions under which approximate methods (ray tracing, paraxial modes) can provide extremely efficient and accurate solutions and situations where a more rigorous approach is required. As a rigorous model of electromagnetic wave propagation, physical optics can be used to characterize complete systems to high accuracy. However, the straightforward approach is computationally intensive and, therefore, not suitable for the initial design or preliminary analysis of large multi-element optical systems. In order to improve the computational efficiency of the usual PO approach we have developed fast physical optics software, initially for the analysis of the ESA PLANCK system. The MODAL code is modular and multi-platform, and different propagation models can be used within the same framework. Distributed parallel computing enables significant reduction of the time needed to perform the calculations. We present the new software and analyses of the QuaD and Herschel (HIFI) telescope systems.

Efficient algorithms for optimising the optical performance of profiled smooth walled horns for future CMB and Far-IR missions

Astronomical observations in the far-infrared are critical for investigation of cosmic microwave background (CMB) radiation and the formation and evolution of planets, stars and galaxies. In the case of space telescope receivers, a strong heritage exists for corrugated horn antenna feeds to couple the far-infrared signals to the detectors mounted in a waveguide or cavity structure. Such antenna feeds have been utilized, for example, in the Planck satellite in both single-mode channels for the observation of the CMB and the multi-mode channels optimized for the detection of foreground sources. Looking to the demands of the future space missions, it is clear that the development of new technology solutions for the optimization and simplification of horn antenna structures will be required for large arrays. Horn antennas will continue to offer excellent control of beam and polarization properties for CMB polarisation experiments satisfying stringent requirements on low sidelobe levels, symmetry, and low cross polarization in large arrays. Similarly for far infrared systems, multi-mode horn and waveguide cavity structures are proposed to enhance optical coupling of weak signals for cavity coupled bolometers. In this paper we present a computationally efficient approach for modelling and optimising horn character-istics. We investigate smooth-walled horns that have an equivalent optical performance to that of corrugated horns traditionally used for CMB measurements. We discuss the horn optimisation process and the algorithms available to maximise performance of a merit parameter such as low cross polarisation or high Gaussicity. A single moded horn resulting from this design process has been constructed and experimentally verified in the W band. The results of the measurement campaign are presented in this paper and compared to the simulated results, showing a high level of agreement in co and cross polarisation radiation patterns, with low levels of integrated cross polar power. For future Far IR receivers using waveguide bounded bolometers and absorbers, an optimisation of the waveg-uide structures and absorber location within the integrating cavity is critical to maximise coupling performance particularly for multimoded systems. We outline the benefit of using multi-moded horns in focal plane arrays and illustrate the increased optical sensitivity associated with a many-moded approach, which may be optimized for coupling to particular incident beams.