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Terahertz (THz) science will profoundly impact biotechnology. It has tremendous potential for applications in imaging, medical diagnosis, health monitoring, environmental control and chemical and biological identification. THz research will become one of the most promising research areas in the 21st century for transformational advances in imaging, as well as in other interdisciplinary fields. However, terahertz wave (T-ray) imaging is still in its infancy. This paper discusses the uniqueness and limitations of T-ray imaging, identifies the major challenges impeding T-ray imaging and proposes solutions and opportunities in this field. It also concentrates on the generation, propagation and detection of T-rays by the use of femtosecond optics.

The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular, coherent) radiation sources and detectors. Recent demonstrations, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interest; yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals, and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave, THz sources which operate at room temperature.

The methods for generating few-cycle THz radiation from semiconductors without external applied fields are reviewed. Their spectral characteristics, efficiency and prospects for imaging and tomography at terahertz frequencies are discussed.

We have built a relatively simple, highly efficient, terahertz (THz) emission and detection system centred around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused on a 120 μm diameter spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1 mm thick (110) oriented ZnTe crystal. The maximum THz-induced differential signal that we observe is ΔP/P = 7 × 10⁻³, corresponding to a THz peak amplitude of 95 V cm⁻¹. The THz average power was measured to be about 40 μW, to our knowledge the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

We have studied the sensitivity and noise of optically gated dipole receivers made from ion implanted Si and GaAs in an optimized time domain THz spectrometer. The spectrometer uses a room temperature, dc biased, semi-insulating GaAs stripline source capable of generating up to 30 µW average power. The 10% amplitude system bandwidth for 10 µm (50 µm) dipole receivers is 3 THz (1.5 THz). A dynamic range of 4 × 10⁵ Hz^−1/2 is achieved using a 10 µm dipole GaAs receiver and 2 × 10⁶ Hz^−1/2 using a 50 µm dipole for a total laser power of 110 mW and THz beam power of 20 µW. The dynamic range achieved with comparable silicon receivers is a factor of 2 smaller.

We have developed a signal amplifier for the detection of weak repetitive pulsed signals superimposed on a large background. It is based on a double gated integrator principle with two channels locked to a sequence of alternate signal and background pulses. The circuit is compact and cost effective, and thus suitable for use in large numbers with detector arrays, for example in terahertz imaging. The performance was verified in terahertz time-domain spectroscopy based on an amplified femtosecond laser and found to be better than that of a commercial lock-in amplifier.

A novel THz near-field spectrometer is presented which allows the performance of biological and medical studies with high spectral resolution combined with a spatial resolution down to λ/100. In the setup an aperture much smaller than the used wavelength is placed in the beam very close to the sample. The sample is probed by the evanescent wave behind the aperture. The distance is measured extremely accurately by a confocal microscope. We use monochromatic sources which provide powerful coherent cw radiation tuneable from 50 GHz up to 1.5 THz. Transmission and reflection experiments can be performed which enable us to study solids and molecules in aqueous solution. Examples for spectroscopic investigations on biological tissues are presented.

A near-field probe is described that enables high spatial resolution imaging with terahertz (THz) pulses. The spatial resolution capabilities of the system lie in the range of few microns and we demonstrate a resolution of 7 µm using broad-banded THz pulses with an intensity maximum near 0.5 THz. We present a study of the performance of the near-field probes in the collection mode configuration and discuss some image properties.

We review the recent development of T-ray computed tomography, a terahertz imaging technique that allows the reconstruction of the three-dimensional refractive index profile of weakly scattering objects. Terahertz pulse imaging is used to obtain images of the target at multiple projection angles and the filtered backprojection algorithm enables the reconstruction of the object's frequency-dependent refractive index. The application of this technique to a biological bone sample and a plastic test structure is demonstrated. The structure of each target is accurately resolved and the frequency-dependent refractive index is determined. The frequency-dependent information may potentially be used to extract functional information from the target, to uniquely identify different materials or to diagnose medical conditions.

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

We have developed a real-time THz imaging system based on the two-dimensional (2D) electro-optic (EO) sampling technique. Employing the 2D EO-sampling technique, we can obtain THz images using a CCD camera at a video rate of up to 30 frames per second. A spatial resolution of 1.4 mm was achieved. This resolution was reasonably close to the theoretical limit determined by diffraction. We observed not only static objects but also moving ones. To acquire spectroscopic information, time-domain images were collected. By processing these images on a computer, we can obtain spectroscopic images. Spectroscopy for silicon wafers was demonstrated.

Free electron lasers (FELs) allow the generation of electromagnetic radiation (EM) in a wide field of frequencies (respectively wavelengths) through the proper adjustment of the energy of an electron beam and the field configuration of a magnetic undulator passed by this beam. Terahertz (THz) radiation covers the region of the electromagnetic spectrum between approximately 0.3 and 30 THz and thus can be considered a continuation of the optical spectrum beyond the far infrared (IR). The very interesting results obtained from various studies of the interaction between IR radiation and biomolecules or tissue have stimulated increasing interest in the study of biological systems using THz radiation. This paper points out what role modern FELs can play in this research.

We report the production of high power (20 W average, ∼1 MW peak) broadband THz light based on coherent emission from relativistic electrons. We describe the source, presenting theoretical calculations and their experimental verification. For clarity we compare this source with that based on ultrafast laser techniques, and in fact the radiation has qualities closely analogous to those produced by such sources, namely that it is spatially coherent, and comprises short duration pulses with transform-limited spectral content. In contrast to conventional THz radiation, however, the intensity is many orders of magnitude greater due to the relativistic enhancement.

Guided-wave single-mode propagation of sub-ps terahertz (THz) pulses in a plastic photonic crystal fibre has been experimentally demonstrated. The plastic photonic crystal fibre (PPCF) is fabricated from high-density polyethylene tubes and filaments. The fibre exhibits low loss and relatively low dispersive propagation of THz pulses within the experimental bandwidth of 0.1–3 THz. Such PPCFs have the promise of low loss, mechanically flexible interconnect channels for compact THz devices and systems.

A new instrument for terahertz time-domain spectroscopy (THz-TDS) has been developed. It consists of a composite THz-TDS system and a high throughput (Martin–Puplett) interferometer. The instrument is for use in the qualitative study of optoelectronic constants of materials. The spectral transmission intensity and phase shift related to phonon–polariton dispersion have been measured between 100 cm⁻¹ and 3 cm⁻¹ on ferroelectric crystals of industrial interest. These include bismuth titanate Bi₄Ti₃O₁₂ (a key material for FeRAM), lithium niobate LiNbO₃ (a typical nonlinear crystal for parametric oscillator applications) and lithium heptagermanate Li₂Ge₇O₁₅ for surface elastic wave filter applications. The complex dielectric constants are well reproduced by the phonon–polariton dispersion relation based on the Kurosawa formula. The instrument details and phonon–polariton dispersion results are described.

By illuminating the sample with a broadband 10–300 GHz stimulus and coherently detecting the response, we obtain reflection and transmission spectra of common powdered substances, and compare them as a starting point for distinguishing concealed threats in envelopes and on personnel. Because these samples are irregular and their dielectric properties cannot be modulated, however, the spectral information we obtain is largely qualitative. To show how to gain quantitative information on biological species at micro- and millimetre-wave frequencies, we introduce thermal modulation of a globular protein in solution, and show that changes in single-wavelength microwave reflections coincide with accepted visible absorption spectra, pointing the way towards gaining quantitative chemical and biological spectra from broadband terahertz systems.

We report the first use of differential terahertz time-domain spectroscopy for bioaffinity sensing. Binding is observed by measuring the transmission of a thin layer of biotin bound to the sensor protein avidin. We demonstrate the THz wave transmission of a sub-micron-thick film and sensitivity to 0.1 µg cm⁻² of biotin. These results point the way for a host of biosensor applications using T-rays, or pulsed far-infrared (FIR) radiation.

We discuss the use of terahertz time domain spectroscopy for studies of conformational flexibility and conformational change in biomolecules. Protein structural dynamics are vital to biological function with protein flexibility affecting enzymatic reaction rates and sensory transduction cycling times. Conformational mode dynamics occur on the picosecond timescale and with the collective vibrational modes associated with these large scale structural motions in the 1–100 cm⁻¹ range. We have performed THz time domain spectroscopy (TTDS) of several biomolecular systems to explore the sensitivity of TTDS to distinguish different molecular species, different mutations within a single species and different conformations of a given biomolecule. We compare the measured absorbances to normal mode calculations and find that the TTDS absorbance reflects the density of normal modes determined by molecular mechanics calculations, and is sensitive to both conformation and mutation. These early studies demonstrate some of the advantages and limitations of using TTDS for the study of biomolecules.

The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrations involving a substantial fraction of the atoms forming the molecule and motion associated with intermolecular hydrogen bond vibrations. Due to their collective nature such modes are highly sensitive to the intra- and intermolecular structure and thus provide a unique fingerprint of the conformational state of the molecule and effects of its environment. We demonstrate the use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared (0.5–4.0 THz) dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA). We observe numerous distinct spectral features with large differences between the molecules in both frequency-dependent absorption coefficient and index of refraction. Assisted by results from density-functional calculations we interpret the origin of the observed resonances as vibrations of hydrogen bonds between the molecules.

A label-free sensing approach for the label-free characterization of genetic material with terahertz (THz) electromagnetic waves is presented. Time-resolved THz analysis of polynucleotides demonstrates a strong dependence of the complex refractive index of DNA molecules in the THz frequency range on their hybridization state. By monitoring THz signals one can thus infer the binding state (hybridized or denatured) of oligo- and polynucleotides, enabling the label-free determination the genetic composition of unknown DNA sequences. A broadband experimental proof-of-principle in a free-space analytic configuration, as well as a higher-sensitivity approach using integrated THz sensors reaching femtomol detection levels and demonstrating the capability to detect single-base mutations, are presented. The potential application for next generation high-throughput label-free genetic analytic systems is discussed.

We describe a new class of experiments involving applications of terahertz radiation to problems in biomedical imaging and diagnosis. These involve scale model measurements, in which information can be gained about pulse propagation in scattering media. Because of the scale invariance of Maxwell's equations, these experiments can provide insight for researchers working on similar problems at shorter wavelengths. As a first demonstration, we measure the propagation constants for pulses in a dense collection of spherical scatterers, and compare with the predictions of the quasi-crystalline approximation. Even though the fractional volume in our measurements exceeds the limit of validity of this model, we find that it still predicts certain features of the propagation with reasonable accuracy.

We report preliminary results from studies of biological effects induced by non-thermal levels of non-ionizing electromagnetic radiation. Exponentially growing Saccharomyces cerevisiae yeast cells grown on dry media were exposed to electromagnetic fields in the 200–350 GHz frequency range at low power density to observe possible non-thermal effects on the microcolony growth. Exposure to the electromagnetic field was conducted over 2.5 h. The data from exposure and control experiments were grouped into either large-, medium- or small-sized microcolonies to assist in the accurate assessment of growth. The three groups showed significant differences in growth between exposed and control microcolonies. A statistically significant enhanced growth rate was observed at 341 GHz. Growth rate was assessed every 30 min via time-lapse photography. Possible interaction mechanisms are discussed, taking into account Frohlich's hypothesis.

Terahertz spectroscopy represents a frontier in the field of biomedical imaging. It is possible to image complex objects that are opaque to visible and infrared light. In this paper, we have used THz imaging to reveal the structure inside a sunflower seed. We compare images based on time- and frequency-domain representations of the THz scans, and conclude that for this type of specimen the time-domain THz scans provide more detailed information than their frequency-domain counterparts.

'Visualization' in imaging is the process of extracting useful information from raw data in such a way that meaningful physical contrasts are developed. 'Classification' is the subsequent process of defining parameter ranges which allow us to identify elements of images such as different tissues or different objects. In this paper, we explore techniques for visualization and classification in terahertz pulsed imaging (TPI) for biomedical applications. For archived (formalin-fixed, alcohol-dehydrated and paraffin-mounted) test samples, we investigate both time- and frequency-domain methods based on bright- and dark-field TPI. Successful tissue classification is demonstrated.

We demonstrate the application of terahertz pulse imaging (TPI) in reflection geometry for the study of skin tissue and related cancers both in vitro and in vivo. The sensitivity of terahertz radiation to polar molecules, such as water, makes TPI suitable for studying the hydration levels in the skin and the determination of the lateral spread of skin cancer pre-operatively. By studying the terahertz pulse shape in the time domain we have been able to differentiate between diseased and normal tissue for the study of basal cell carcinoma (BCC). Basal cell carcinoma has shown a positive terahertz contrast, and inflammation and scar tissue a negative terahertz contrast compared to normal tissue. In vivo measurements on the stratum corneum have enabled visualization of the stratum corneum–epidermis interface and the study of skin hydration levels. These results demonstrate the potential of terahertz pulse imaging for the study of skin tissue and its related disorders, both in vitro and in vivo.

As with other imaging modalities, the performance of terahertz (THz) imaging systems is limited by factors of spatial resolution, contrast and noise. The purpose of this paper is to introduce test objects and image analysis methods to evaluate and compare THz image quality in a quantitative and objective way, so that alternative terahertz imaging system configurations and acquisition techniques can be compared, and the range of image parameters can be assessed. Two test objects were designed and manufactured, one to determine the modulation transfer functions (MTF) and the other to derive image signal to noise ratio (SNR) at a range of contrasts. As expected the higher THz frequencies had larger MTFs, and better spatial resolution as determined by the spatial frequency at which the MTF dropped below the 20% threshold. Image SNR was compared for time domain and frequency domain image parameters and time delay based images consistently demonstrated higher SNR than intensity based parameters such as relative transmittance because the latter are more strongly affected by the sources of noise in the THz system such as laser fluctuations and detector shot noise.

We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems.

This paper concerns the robustness of discrete wavelet transform (DWT) compression in terahertz pulsed imaging (TPI). TPI datasets consist of terahertz time-domain series which are sampled at each 'pixel' of the image, leading to file sizes which are typically of the order of several megabytes (MB) per image. This makes efficient compression highly desirable for both transmission and storage. However, since the data may be required for diagnostic purposes it is essential that no relevant information is lost or artefacts introduced. We show that for a nylon step wedge the estimates of refractive index and absorption coefficients are not significantly altered when the terahertz data are reconstructed from only 20% of DWT coefficients.

It is now just over one-hundred years since Roentgen's first x-ray image was obtained and approximately twenty-five years since the first MRI picture was recorded. Between the frequencies of the electromagnetic spectrum that are deployed in these medical imaging modalities lies the terahertz (THz) region, here defined as 300 GHz - 10 THz. This special issue of Physics in Medicine and Biology is devoted to papers reporting applications of this radiation in both medicine and biology. The papers in this issue are an edited selection from the First International Conference on Biomedical Imaging and Sensing Applications of THz Technology (BISAT) held in Leeds, UK, towards the end of 2001.

Until recently, the THz range had not been exploited fully due to the severely limited number of sources available. Such sources as were available were bulky, expensive, inefficient and, usually, incoherent. A step-change was brought about by the development, some five or more years ago, of optically generated THz sources, which enable both the production and detection of coherent radiation. Spectroscopic systems, usually with imaging capability, were then quickly realized and these have been shown to operate with extraordinary sensitivity. THz radiation passes easily through most non-metals, and interacts with molecular rotations and vibrations in matter. In consequence, applications of this new technology in areas of biological and medical interest were soon identified.

THz radiation, unlike x-rays, is non-ionizing. Since THz radiation has a longer wavelength than near infrared radiation, it will suffer less Rayleigh scattering, and so images (especially of biological materials) should be sharper. Moreover, THz images have better spatial resolution than those that can be obtained by millimetre waves. THz radiation is, of course, strongly attenuated by the presence of water but can, nevertheless, pass through almost one kilometre of mist and through several millimetres of tissue using state-of-the-art technology. Terahertz signals and images provide full spectroscopic information about the real and imaginary parts of the refractive index of the tissue under examination. This capability, coupled with the resonant effects in tissue, suggests that THz techniques will be a rich source of information about tissue both in vivo and in vitro. At the time of writing this Editorial in vivo studies of skin cancers, particularly melanoma and basal cell carcinoma, have been reported, as have in vitro measurements of teeth for the investigation of dental disease. In addition there appears to be every possibility that methods will soon be devised to deliver THz radiation via endoscopes or catheters and analyse reflections from internal body surfaces.

The resolution of THz radiation enables it to be used for molecular sensing with the exciting prospect of studying biomolecules. Biomolecules will exhibit resonances in the THz region of the spectrum, and these may be used to probe both intra- and inter-molecular motion. Although this field is still at a very early stage, the characteristic spectra of the basic building blocks of DNA are now being assembled. As an extension of this work, the ability of THz radiation to determine the hybridization state of DNA has been shown, and this has led to the development of a genetic diagnostics system with femtomol sensitivity.

This special issue is organized as follows. Following this Editorial, there is an Overview of the field from X-C Zhang, the Honorary Chair of the BISAT meeting. There then follow three sections: Sources and Systems Development; Applications of Terahertz Radiation in Biomolecular Sensing and Spectroscopy; and Applications of Terahertz Imaging in Medicine.

The first section provides a coherent picture of the present state of development of THz pulsed imaging systems, with especial reference to applications in both biology and medicine. Of particular note are the advances that have been made to improve signal to noise ratios and to speed-up the rate of image acquisition; attowatt sensitivity now appears to be achievable, and real-time imaging is a very close possibility. Developments in electronics that will lead to the eventual realization of robust, compact equipment suitable in all respects for a hospital environment are also reported. The deployment of near-field imaging techniques to break the Abbé limit is a significant achievement, and will in due course provide a new tool to establish the origin of contrast in THz imaging of medical material, and to probe cellular vibrations and related effects. Although impulsive optically generated THz pulses are used in the majority of imaging systems at present, a new class of CW imaging systems, that have particular advantages in cost and sensitivity, are now in the course of development and these are also reported in this special issue. Finally, the potential of free electron laser approaches to biomedical measurements is reviewed.

The second section deals with biomolecular sensing and spectroscopy. A significant instrumentation development reported here is the use of all-electronic systems in the range of frequencies up to 1 THz. This equipment might, for example, be used to determine rapidly the presence of unusual (chemical or biological) substances concealed within envelopes or under clothing. THz time-domain spectroscopy provides an ideal tool for probing the response of biomolecules over a wide frequency range, and gaining an understanding of the form of the dielectric function for the four nucleobases (and corresponding nucleosides) that form the building blocks of DNA is an essential step for both pure and applied research in this area. The development of biosensors that rely on THz frequency interactions is clearly a major goal for applicable science at THz frequencies. This section details advances in THz sensor technology for bioaffinity detection and the label-free probing of genes. Finally, a wholly new approach to the question of modelling the propagation of THz radiation through biological media is presented.

The third section describes recent advances in THz medical imaging. This field is now growing rapidly and several significant advances are reported here. The first comparisons of the results of THz imaging with histopathological studies provide an ideal example, as does the development of dark-field imaging techniques. Visualization and classification techniques, which are widely used elsewhere in imaging science, have many possibilities in the THz field, especially in view of the many parameters that may be used to form a THz image. Finally, the use of other types of THz imaging equipment using, for example, CW sources and bolometric detection, may be another fruitful route for eventual exploitation.

This meeting provided the first opportunity for the terahertz medical and biological community to identify itself as a distinct grouping. During the course of the meeting, as is reflected in the present selection of papers, a number of very significant questions emerged. For example, what is the origin of contrast in a THz image of a biomedical sample: is it simply absorption by increased water concentration in tissue, or are there important scattering effects that should be considered? Other debatable issues are: What are the critical design choices that must now be used for future THz imaging systems (in terms of frequency choice, pulsed or CW operation, electronic or optical generation, etc)? What are the engineering challenges that must be addressed for remote delivery and analysis, and to achieve practical imaging times for moving subjects? Will these challenges be met with the present generation of large-scale pulsed systems, or will quantum cascade laser sources be deployed more effectively in future smaller scale systems that might incorporate, for example, tomographic capability? Finally, and perhaps most crucially, how can THz imaging be accepted alongside other established clinically useful modalities and what could it image that cannot be imaged by other techniques?

For applications of THz radiation in bio-sensing and spectroscopy, some significant questions might include: What will be the crucial developments that will enable THz devices to compete effectively with established technologies such as fluorescence marking of genes? Can THz measurements be accomplished at the cellular level, perhaps to shed light on mechanisms of protein folding? Will an effective theoretical framework ever be established to interpret both spectroscopic data and the passage of THz radiation through biomedical materials?

The Editors offer the following papers to the readers of Physics in Medicine and Biology, in the hope that they will provide a useful record of the first meeting of this community, and stimulate further scientific, technological and medical advances in the years to come.