Sillas Hadjiloucas

Current research projects

• Co-investigator in an International (4 year) project funded by FAPESP (Brazilian Government) Grant Title: PROJETO TEMTICO Espectroscopia Terahertz no Domnio do Tempo: Desenvolvimento de Mtodos Analticos, Tcnicas de Processamento de Sinais, Estudos Bioqumicos e Fundamentais, Coordenador: Celio Pasquini, Instituto de Qumica Universidade Estadual de Campinas, UNICAMP and Instituto Tecnolgico de Aeronutica, So Jos dos Campos (ITA), Brazil.

Research in instrumentation and measurement

Development of novel amplitude modulated reflectance fibre-optic displacement sensors and novel feedback based optoelectronic stabilization schemes for accurately measuring displacement, velocity, acceleration, force and pressure. These are suitable for a range of automotive and robotic applications, and have been also used for flow measurement and the measurement of relative humidity in the atmosphere (through the development of a feedback dew-point sensor).

Development of optical force feedback microphones (this is a continuation of work performed by L. Karatzas, D. Keating and M. Usher). The innovation in the work lies in the use of force feedback to control the motion of the diaphragm in the capsule, with the sensing performed interferometrically. By measuring the amount of effort needed to keep the capsule in a set position in the presence of a disturbance (staying at the mid-point of a single fringe in a Fabry-Perot interference pattern), a very accurate (shot noise limited) estimate of the acoustic disturbance is obtained. The work required the design of a novel capsule topology that linearizes the feedback system. Applications include non-destructive testing, biomedical applications, audio research, echo-location and photoacoustic spectrometry.

Collaboration with the Department of Electronic and Electrical Engineering (Optoelectronics Division, headed by Prof. B.Culshaw) and the Department of Pure and Applied Chemistry, the University of Strathclyde led to work towards the development of a hydrogel (hydrophilic polymer) based distributed fibre optic microbend sensor for measuring of soil moisture and the water potential (a component of the chemical potential) in solutions. This sensor was based on measurements of quasi-sinusoidal distortions of fixed periodicity on a multimode graded index optical fibre using Optical Time Domain Reflectometry.

Much of the sensing work in millimetre and sub-millimetre wave techniques and Terahertz (THz) sensing started during my PhD in 1995 and has always been performed in collaboration with J. Bowen. The work started with the use of a terahertz null-balance transmissometer (a hybrid between a Mach-Zehnder and a Martin-Puplett polarising interferometer) for continuous in situ measurements of leaf relative water content. During my post-doctoral research fellowships, additional experience was gained in the use of a wide range of THz sources such as Impatt diodes, klystrons, backward wave oscillators and optically pumped far-infrared lasers, as well as room temperature and cryogenically cooled detectors. We used a dispersive Fourier Transform Spectrometer in transmission and reflection configurations to measure the complex refractive index of a range of materials at THz frequencies. Small refinements to this well established experimental technique have been made to account for reflection measurements of extremely thin samples as well as to account for the diffractive spreading of the beam during transmission measurements. The dispersive Fourier Transform Spectrometer has also been used for characteristic impedance measurements. There has been work in the characterisation of micro-machined passive millimetre and sub-millimetre wave components, measuring E-plane and H-plane antenna patterns at 140-200 GHz and 1.6 THz for micro-fabricated devices produced by Bath, Nottingham and Leeds Universities as part of the EPSRC-funded TINTIN (Terahertz Integrated Technology Initiative) program. This is important for optimizing the coupling between free space propagating EM waves and waveguide structures. Using Jones calculus, an error analysis has been developed for quasi-optical null balance bridge circuits commonly used for transmission and reflection coefficient measurements in the frequency range 100 GHz to 1 THz. This is of relevance to de-embedding waveguide impedance and attenuation as well as for the estimation of the impedance mismatch encountered in antenna structures mounted on substrates. As visiting researcher at the Department of Electrical and Electronic Engineering, The University of Leeds I performed high-frequency (75-110 GHz) in-waveguide and free-space measurements on a WR-10 Waveguide precision short with an 8510 HP vector network analyser. The results were compared with quasi-optical null-balance reflectometer measurements after using the new de-embedding technique which was developed at Reading for validation. A theoretical treatment that accounts for errors in the measured propagation constant due to imperfect coupling of two fundamental Gaussian beams in a quasi-optical reflectometer has been developed.

Through my employment by the Space Sensor Technology and Planetary Exploration Group in DLR Berlin as an EC TMR Post-Doctoral Research Fellow in the INTERACT project, I was responsible for the further development of a frequency and amplitude stabilisation scheme of a new type (rf excited) CO2 laser from DEOS (DeMaria Electro-optics, USA). Stabilisation was achieved by frequency locking the laser output to a confocal Fabry-Perot cavity which was also locked to the wavelength of a Zeeman stabilised He-Ne laser. The stabilised CO2 laser is to be used to either optically pump a far-infrared laser which served as local oscillator (operating at discrete frequencies between 1 and 5.2 THz) for an NbN hot electron bolometer mixer aboard SOFIA (the Stratospheric Observatory for Infrared Astronomy, a jointly funded project between NASA and DLR), or as a coherent source that in conjunction with a transversely excited far infra-red cavity, formed part of a laser magnetic resonance spectrometer to be used aboard an aircraft for the detection of free radicals in the lower stratosphere and upper troposphere. The laboratories in Berlin have state-of-the-art facilities for the development of solid-state THz sources (p-Ge and Si lasers) and detectors covering the whole frequency range 1-6 THz (Ge:Ga, InSb, GaAs, Si-Bolometer) as well as complete characterisation facilities for superconductor-insulator-superconductor tunnelling junction heterodyne receivers, and hot-electron bolometer mixers (HEBs) which are used for high-resolution heterodyne spectroscopy. As member of the Berlin group, I observed the lowest ever noise temperatures in heterodyne receivers using devices developed in Chalmers (Sweden) surpassing the world record previously held by the Jet Propulsion Laboratories Group in Pasadena USA. The HEB devices developed at Moscow Pedagogical University (Russian Academy of Sciences) and characterised in Berlin may be used aboard SOFIA. As part of the Reading-DLR collaboration I used a novel quasi-optical interferometric technique (which was originally developed by Dr. Bowen at Reading) for measuring antenna phase patterns without the need of a heterodyne receiver. The technique is particularly suited for the characterisation of terahertz antennas feeding power detectors or mixers employing quasi-optical local oscillator injection. Work in Berlin concentrated in adopting this technique for further characterisation of whisker contacted Schottky barrier diode mixers mounted on cube corner reflectors as well as for further characterisation of HEBs. Provided that the technique proves sufficiently user-friendly at frequencies up to 2.5 THz, it may be adopted for the characterisation of the quasi-optical circuits aboard FIRST, a satellite built by ESA.

I worked as visiting researcher at RWTH Aachen with the fast-pulse (femtosecond) THz spectroscopy group (currently considered as one of the best group in the world in THz spectroscopy) and characterised polymers and waveguide structures at frequencies ranging from 100 GHz to 3.0 THz. This work led to a collaboration with GigaOptics GmbH, Mhlheim/Main Germany for the development of a novel dual frequency comb THz spectrometer, this is now known in the fast pulse community as the ASOPS modality. My knowledge of optics, femtosecond systems and the related research disciplines has led to my appointment to the position of 'Ultrafast Laser Lab committee member for the University until its re-deployment at Imperial College.

Research in signal processing, system identification and control

The research activities in Instrumentation and Measurement are further supported by advanced Signal Processing modalities and an innovative Chemometric framework. This work is performed in collaboration with R.K.H. Galvao of ITA, Brazil. Much of the work has focused on the signal processing (de-noising) of signals generated from THz transient spectrometers. Filtering by hard or soft thresholding is performed after transforming the signal in the Fourier, Windowed Fourier, Principal Component (PC) or wavelet domains. We have also developed a range of useful algorithms to de-embed the complex propagation constant from multi-modal excitation of micro-machined waveguides (useful for micro-spectroscopy applications where the waveguide is filled with small volumes of the dielectric to be characterised). This is achieved by adopting algorithms found in the Systems Identification literature (ARX, and N4SID subspace models for linear spectroscopy and NARX models to account for the non-linear cut-on of waveguide modes). Since a femtosecond duration pulse is capable of persistent excitation of the medium within which it propagates, all the resonances in the medium can be identified. The multimode Brownian oscillator time-domain response function represented in state-space form is a mathematically robust framework that can be used to describe the dispersive phenomena governed by Lorentzian, Debye and Drude responses. The optical properties of an arbitrary medium can be expressed as a linear combination of simple multimode Brownian oscillator functions. These algorithms are now extended to the domain of fractional order calculus (where the system dynamics are more faithfully modelled using differential equations of non-integral order). This extension should allow us to understand processes observed in broadband dielectric spectroscopy experiments of heterogeneous materials such as glasses, ceramics, amorphous semiconductors, heavily doped crystals, polymers, and composites excited at microwave, far-infrared, visible and UV frequencies which do not exhibit clear resonances and display Jonscher-like responses. The work is based on the development of a theoretical framework which utilizes descriptor representations and state space models of random RC networks. Additional routines developed include wavelet filtering of THz transients (using conventional mother wavelets and adaptive structures at each decomposition level), classification schemes in the PC domain and novel apodization structures which are needed because the time-domain background and sample interferograms are non-symmetrical. These algorithms have useful de-noising properties and can lead to a more precise estimation of the complex insertion loss function. Most of these algorithms are also applicable to a range of measurement modalities performed in the optical and infrared parts of the spectrum.

Under development are novel system identification tools in the wavelet domain for the further processing of spectroscopic transients. Defining the background and sample interferograms as the input and output signals, the frequency response of an identified model provides an estimate of the complex insertion loss (CIL). A wavelet-packet formulation is adopted and sub-band models Mi,j(z) are identified from the sample and background interferograms by following a least-squares procedure.

The wavelet-packet model structure consists of a decomposition tree, which defines frequency sub-bands. H(z), G(z) denote low-pass and high-pass decomposition filters, respectively, with reconstruction counterparts represented by Hr(z), Gr(z). The sub-band models are represented by the transfer functions M0,2(z), M1,2(z), M2,2(z), M3,2(z).

Model identification of a sample interferogram for a given frequency sub-band.

The signal processing work has also been extended to Wiener-Hammerstein systems to provide an identification window related to receiver motion dynamics using information from the non-linear addition of electric field amplitudes associated with multiple propagation channels (direct line-of-sight, ground/wall reflections, diffractive components) from multiple emitters. This is of interest from a mobile communications perspective. We have also performed analysis and classification of ECG signals using Linear Discriminant Analysis Classifiers as well as non-linear neural network based classifiers. A novelty in the work is the representation of the input signals to the classifiers in compact support wavelet bases. Wavelets are used to de-noise the signal and minimize the number of inputs to the neural network. The approach facilitates network training and provides a better framework for feature extraction. Furthermore, the generalization ability of the classifier is improved.

The work in Control theory (performed in collaboration with V. Becerra) concentrates on the development of a robust control theory framework for the analysis of biochemical processes encountered in cellular networks (e.g., the apoptotic process). Using mass action kinetics, a state space representation of the system dynamics is made. The network is then decomposed into several subsystems, each one associated with linear and non-linear processes. Well established tools from linear control theory such as Mu-synthesis and the D-K iteration are adopted to perform sensitivity analysis in the linear subsystems. Loop shaping design procedures (LSDP), co-prime factorization and the method of inequalities are adopted for controller design. Control is theoretically possible by adding new compounds antagonizing existing reactions, this may become possible in the future through advances in Synthetic Biology. The purpose of the simulations is to identify the sensitivity points of the system (rate sensitivities) and then postulate target sites where the Biologists should concentrate their efforts to maintain the optimal conditions that would keep a cell at a preferred state. We use a genetic algorithms multi-objective optimization (MOGA) framework for designing controllers of arbitrary order that fulfil simultaneously desirable time and frequency domain criteria in a robust manner. These algorithms represent extensions of common H-infinity analysis and synthesis approaches. A non-linear control framework based on the state-dependent Riccati equation (SDRE) is currently developed extending the linear framework. This work is of fundamental importance to Systems Biology and of relevance to Metabolic Control Analysis in general. The developed control framework has also important applications in Metrology (optimal controller design for electrical feedback substitution schemes).

Research in genetic algorithms for femtosecond pulse shaping experiments

Another line of work at Reading involves the development of a series of evolutionary meta-algorithms in MATLAB, running within the LabVIEW environment to perform pulse shaping of femtosecond duration laser pulses. Proof of principle experiments were performed where the signal from a LabVIEW controlled function generator was sent to an RF-excited acousto-optic modulator crystal which controls the spectral content and time domain characteristics of 120 femtosecond duration pulses generated from an optically pumped Ti:sapphire Kerr lens mode-locked laser oscillator. The pulses have a useful Fourier transform limited bandwidth of 50-80 nm around a centre frequency of 800 nm. The genetic algorithm (GA) population in the developed software consists of a collection of waveforms, each made from two concatenated vectors, one specifying phases over a range of frequencies and the other specifying corresponding magnitudes. Amplitude and phase genes are used to search the evolutionary landscape for desired pulse shapes with specific characteristics. With appropriate pulse shaping schemes realised in pump-probe experiments, it should in principle allow us to control bulk relaxation processes at picosecond timescales.

Selecting a breeding subset from the N best fit members (amplitudes and phases) is a very greedy heuristic that often leads to convergence around local optima instead of the global solution. One attempt to circumvent this problem is to use a probabilistic selection of breeding candidates. For instance, with roulette wheel selection (RWS), each member of the population is assigned a slice with an area proportional to fitness. Then, using the proportional probabilities from the roulette wheel, one or two candidates are selected out of the entire population and passed to the operators for production of children. The process is repeated again and again until the desired number of children are created. Because members are never removed from the roulette wheel, members with high fitness are virile and, consequently, they are chosen as parents more frequently than those with low fitness. The process can easily be coded into the algorithm by firstly creating a vector F of corresponding fitnesses for the population and then creating a cumulative summed vector for each of all indices in F. As the population starts to converge to a solution, the average fitness of the population will also start to converge to the best fitness. In RWS, the slice size for each population member will be roughly the same size, leaving little differentiation between worst and best fit members. This in turn has a detrimental effect on the selection process, and slows down the convergence of the algorithm considerably. To keep the selection pressure high, we employ fitness scaling and use the scaled fitnesses in RWS instead. After measuring and then linearly scaling the fitness, an operator is selected by RWS from a vector of operator weights. Depending on the operator chosen, either one or two parents are selected and then passed on to the operator, thereby creating the child. Current work concentrates in developing a differential evolution scheme and operators in the wavelet domain to perform high resolution waveform matching with a much higher convergence rate. An important applications of the algorithms is in the control of chemical reactions. The light pulses can be used to catalyse a reaction. Other interesting applications include Opto-Genetics (where the light pulses will trigger/silence the expression of specific genes).

Research skills map

A summary of research skills mapped to a range of application areas is provided below.

Past project support and donations in kind

• EPSRC GR/R36213/01 award for 'Cavity Ringdown Far-Infrared Absorption Spectroscopy' Individual Grant Review Overall Assessment:'Good' Communication of Research Outputs 'Tending to Outstanding' (PI).
• EPSRC CASE AWARD 'Future terahertz mesh network architectures for mobile telecommunications' (project co-sponsored by VODAFONE (PI).
• CCLRC AWARD 'Heterodyne cavity ringdown absorption spectroscopy for atmospheric chemistry studies at THz frequencies,' (PI).
• HEFCE funded: 'Establishment of the Ultrafast Laser Laboratory at Reading University' (with John Bowen in Cybernetics, Dr. L,. Frasinski and Dr. S. O'Leary in Physics), (Co-I).
• GREEK GOVERNMENT: PhD studentship on 'Terahertz measurement techniques for plant water relations' (PI).
• NATIONAL INSTRUMENTS donation in kind: Hardware and LABVIEW software from National Instruments towards the 'Flying Brains' student project (PI).
• MAX-PLANCK Institute for Biochemistry, Munich (Prof. Keilmann) donation in kind: a custom-made close cycle temperature control incubator which is integrated with microwave components to perform in vivo studies of the biological effects of THz radiation on yeast cell cultures (PI).
• UoR, Department of Physics donations in kind: A laser (Nd:YAG, 1 Joule 10 ns pulse), with harmonic generator crystal, polarization optics and dedicated heat exchanger, an Argon ion laser (14W) with close cycle heat exchanger, an ultrasonic scanner for biomedical applications and various optoelectronic and microwave (10 GHz) equipment and lasers (PI).
• UoR RETF three year PhD Studentship for 'Biological Investigations Using Terahertz Radiation' (PI).
• Travel and subsistence visiting Scholarships in 2006 and 2010 to Instituto Tecnolgico de Aeronutica, So Jos dos Campos Brazil for projects titled 'Control of Dynamic systems' sponsored by PRONEX/CNPq (Brazilian government) and 'Spectro-analytical Instrumentation and Methodologies' sponsored by PROCAP/CAPES (Brazilian government), (PI for UK).
• Travel and subsistence funds for Xiaoxia (Sunny) Yin to visit Reading University as Pysden Memorial Fellow to conduct research in the summer term of 2007. Collaboration with Prof. Abbott's group (Department of Electrical and Electronic Engineering, University of Adelaide) in the area of signal processing of THz transients.
• Travel and subsistence grant from RIKEN Institute to Japan for collaborations on 'Investigations on the effects of non-thermal levels of terahertz radiation in yeast cells'.
• INTAS travel and subsistence grant for collaboration with Russian Academy of Sciences, Nizhny Novgorod, Russia on 'Prospects for Cavity Ringdown Spectroscopy at THz frequencies'.
• Collaborative agreement with GigaOptics GmbH, Mhlheim/Main Germany for the development of a novel dual frequency comb THz spectrometer based around two Ti:Sapphire oscillators producing 30 femtosecond duration pulses and operated at 1 GHz repetition rate. Prototype delivered at cost price.