We demonstrated that coherently driven atomic or molecular media potentially yield strong controllable short pulses of THz radiation. The method is based on excitation of maximal coherence in an atomic or molecular gas by optical pulses and coherent scattering of infrared radiation to produce pulses of THz radiation. The pulses can reach the energies ranging from seeral nJ to μJ and time durations from several cycles to ns at room temperature.
We demonstrate an efficient room temperature source of narrow-bandwidth terahertz (THz) radiation using femtosecond pump pulses and periodic GaAs structure as a nonlinear material. In the past, several THz generation schemes exploited optical rectification in nonlinear crystals using femtosecond laser technology. Most of them generated single-cycle THz-pulses with broad bandwidth, using nonlinear crystals shorter than the phase-matching coherence length. Recently a novel technique to generate multi-cycle THz-pulses in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals has been demonstrated. Quasi-phase matching (QPM) structures such as PPLN consist of a periodic system of domains of inverted crystal orientation. The sign of second order nonlinear polarization generated by femtosecond pulses is inverted at domain boundaries. If domain length is comparable with coherence length, QPM between THz-wave and nonlinear polarization extends the
interaction length between THz and optical pulses. In the present
work, using periodic GaAs structures we have achieved exceptionally high photon as well as energy conversion efficiency: 3% and 0.07% respectively. We have examined two different types of periodic QPM GaAs samples: diffusion-bonded GaAs wafers and all-epitaxially-grown orientation-patterned GaAs crystals with 3-10 mm thicknesses. The incident optical pulse energy was in the micro-Joule range and pulse duration was ~100 fsec. We measured spectral properties of THz radiation using Michelson interferometer and a bolometer. Narrow-bandwidth (~100GHz) THz output, tunable between 1 and 3 THz, was achieved. THz frequency was tuned either by tuning the light source wavelength between 2 and 4.4 microns, or by selecting GaAs samples with different QPM periods. Our theoretical analysis, based on known GaAs dispersion properties, shows good agreement between the measured and predicted THz frequencies.
We propose to use photonic band gap (PBG) structures for constructing traveling wave tubes (TWTs) at 100 GHz, a completely novel approach. Using a PBG fiber allows us to create an all-dielectric slow-wave structure with very large band width and low losses in the mm-wave regime, compared to TWTs made out of metals. Additional capabilities such as mode selectivity are also achievable. We designed two 100 GHz pencil beam PBG TWTs using Ansoft's HFSS, 3D electromagnetic simulation software for high frequency applications. The first design is a periodic array of vacuum rods in a dielectric matrix, with a smaller vacuum rod forming the line defect. A fiber drawing procedure is being utilized to construct this design out of fused silica. The second structure is a periodic array of dielectric rods in a vacuum matrix, surrounding a thick hollow dielectric tube that accommodates the electron beam. This model is being fabricated out of silicon by means of high-pressure laser chemical vapor deposition (HP-LCVD), a versatile approach to synthesize fibers from the vapor phase. Additionally, a scaled 10 GHz cold test made from alumina rods is being produced for design confirmation purposes, and a 100 GHz sheet beam PBG TWT is being investigated for even greater power generation.
It is possible to use wave-front reconstruction for imaging at millimetre wavelengths employing off-axis holography (a frequently used technique at visible wavelengths). We report on how the technique can also be used for imaging the phase centre of non-standard feed antennas at millimetre wavelengths such as planar lens antennas for example. Holography provides a method for recording a lens-less image of an object reducing loss of spatial frequency information important for maximum resolution. An experimental arrangement at 100 GHz based on a simple form of near-field off-axis holography was developed, with the object and reference beams derived from two radiating horn antennas fed by a single coherent source via a 3dB cross-guide coupler. The reference beam derived from a well understood and characterised horn was collimated using a large off-axis mirror, while the object beam was derived directly from the horn antenna whose pattern is to be measured. The hologram (or intensity pattern) resulting from the interference of the two beams was recorded over an area of 150 × 150 mm with a spatial resolution of 1 mm by a scanning detector and the object wave-front recovered by simulating the reconstruction through near-field diffraction of the reference beam. It is possible to model the propagation of the recovered object beam back towards the horn and recover the object horn fields in the vicinity of the waist (the effective phase centre of the horn). This is a useful inexpensive experimental method for recovering the phase centre position of non-standard feeds.
A split-grating-gate detector design has been implemented in an effort to combine the tunability of the basic grating-gate detector with the high responsivity observed in these detectors when approaching the pinchoff regime. The redesign of the gates by itself offers several orders of magnitude improvement in resonant responsivity. Further improvements are gained by placing the detector element on a thermally isolating membrane in order to increase the effects of lattice heating on the device response.
Recent efforts in our group towards the fabrication of sensors capable of detecting passive levels of millimeter-wave radiation have led to the development of an optically-based detector with sub-picowatt noise equivalent powers. This sensor is based on upconverting the received radiation into sidebands on an optical carrier using electro-optic modulation techniques and, subsequently, suppressing the remaining carrier energy. The noise equivalent power of such detectors is critically dependent on the ability of the electro-optic modulator to efficiently convert frequencies up to and exceeding 95 GHz onto the optical carrier while suppressing potential noise sources. In this paper, we discuss the specific device requirements generated by this unique potential application of high-frequency optical modulators. The effects of various modulator properties, such as half-wave voltage, frequency response, and maximum optical power density are discussed in the context of millimeter-wave detection capability. In addition, we present experimental efforts towards fabricating a passive millimeter-wave detector based on this approach, including efforts to develop an optimized optical modulator technology.
A model of fast Semiconductor Hot Electron Bolometer (SHEB) is developed. In this bolometer radiation heats
only electrons in bipolar semiconductor without inertial lattice heating. For conditions proposed, such heating
changes both generation and recombination processes, that leads to the electron and hole concentration decreases.
This and the electron mobility decrease, because of their heating, cause the semiconductor resistance rise, which is
used for the output signal creation. Semiconductors with the high conductivity, mobility and electron energy
relaxation time are important for the SHEB manufacturing. Narrow-gap semiconductors have such properties, and
therefore the bolometer model is constructed for them. According to this model the SHEB on base of Hg0.8Cd 0.2Te
at temperature of 77 K can have detectivity of (0.3-2)107 cmHz1/2/W for radiation frequency (0.01-1.5) THz.
In recent times, Terahertz (1 THz = 1012 cycles/sec and 300 μm in wavelength) spectroscopy has become a promising technique for spectroscopic identification of different materials having contemporary interest. In this study we report a direct measurement of reflection spectra of the explosive C-4, which shows significant absorption around 0.8 THz, using THz time domain spectroscopic techniques. A contrast in reflection of around 8% has also been observed between the neighboring frequencies of 0.7 THz and 0.9 THz. The spectral data have been used to create realistic synthetic images for use in simulations of interferometric detection in a stand-off THz imaging system. The results obtained are analyzed using Artificial Neural Networks for positive identification of the agents with an interferometric array of few linear detectors in near field mode.
In recent times, terahertz (THz) or the far-infrared region of the electromagnetic spectrum has gained critical significance due to many potential applications including medical diagnostics, nondestructive evaluation of material parameters, chemical sensing, remote sensing and security screening. However with the development of various applications, the need of guided systems for the transmission of THz radiation have posed a challenge, as a flexible waveguide could simplify the propagation and detection of THz waves in remote locations without atmospheric absorption. Different structures, such as, rigid hollow metallic waveguides, solid wires, or short lengths of solid-core transparent dielectrics such as sapphire and plastic have already been explored for THz guiding to characterize their individual loss and dispersion profile. Recently, it has been reported that copper coated flexible, hollow polycarbonate waveguide has low loss of less than 4 dB/m with single mode operation at 1.89 THz. In the present study, using a broadband THz source of photoconductive antennae, we characterize the loss and dispersion profile of hollow core polycarbonate metal waveguides in the frequency range of 0.2 to 1.2 THz.
A tunable terahertz source (TTS) based on a Smith-Purcell emitter will be described. The tunable THz source is analogous to low frequency electron beam devices such as magnetrons, backward wave oscillators and traveling wave tubes. The device offers continuous tunability, compactness, and robust operation. Examples of THz spectroscopy will be given.
We report on the first successful generalized Mueller-matrix ellipsometry measurements in the THz-frequency domain using the high-brilliance THz synchrotron radiation source IRIS at the electron storage ring BESSY, Germany. Generalized Ellipsometry, which is known as a powerful tool for measurement of optical constants including anisotropy and which was previously used in the FIR to VUV spectral range, is now employed for the first time to investigate condensed matter samples in the frequency range from 0.9 to 8 THz (30 to 650 cm-1). Exemplarily, results obtained from bound and unbound charge-carrier investigations in low-dimensional semi- and superconducting systems are presented. Future applications of this technique for investigation of charge-carrier dynamics in magnetic fields are envisioned.
Compelling applications for terahertz technology include medical diagnostics and imaging, extremely wideband communication (XWB), pollution monitoring and security threat detection. One impediment is a lack of practical sources in the 1-10THz frequency range. Room-temperature CW operation, high spectral purity, tunability, and power levels above the milliwatt range are additional requirements that compound the challenge. Continued scaling of feature sizes will enable traditional semiconductor devices to provide good small-signal performance at THz frequencies, but practically obtaining the desired output power levels is much less certain. Traditional coherent optical sources encounter downward frequency scaling difficulties as the energy state differences approach 25meV (corresponding to approximately 6THz, or 50μm free-space wavelength). Electronic devices dependent on velocity modulation obey rather different scaling laws and are strong candidates for bridging the "terahertz gap." As relatively few contemporary engineers are familiar with vacuum electronic devices (VEDs), and fewer still with velocity modulation, this paper reviews the history of velocity-modulated VEDs, starting with the observations of Barkhausen and Kurz in 1919, and proceeding to practical devices, such as the klystron, magnetron, traveling-wave tube (TWT) and backward-wave oscillator (BWO). We additionally consider how modern process technology enables a reconceiving of classical VEDs to produce devices capable of practical operation in the 1-10THz region.
In this paper, we report on a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in submillimetre optical systems. Standing waves or multiple reflections have been traditionally difficult to model but this analytical method proves to be very versatile in first order predictions. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between two coupled corrugated horns. This technique can in addition be applied to reflections between components such as lenses and apertures. As our method uses a full multi-moded scattering matrix description of the feed horn (typically a corrugated horn), which is then transformed to equivalent free space Gaussian modes, multiple reflections between the source/detector device, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. An in-depth overview of the technique is presented including analysis of the eigenmodes or most natural mode set that describes the standing wave itself that can exist within a quasioptical system, which we hope will give new insights into optical cavity phenomena. We investigate mechanisms to reduce standing wave ripples often present in submillimeter optics and try to understand more deeply the form and structure of the reflected power component.
In recent times, the far infrared or the terahertz (1 THz = 1012 cycles/sec and 300μm in wavelength) region of electromagnetic spectrum has become a promising radiation for spectroscopic identification of different types of biomaterials. The present work investigates the effect of grain size on the THz spectra of chalk, salt, sugar and flour using THz time-domain spectroscopy. It has been observed that at lower frequencies, solids of small grain sizes of nonabsorbing materials show rising trends in their extinction spectra. Here, we obtain extinction spectra of granular salt, chalk, sugar and flour between 0.2 to 1.2 THz and show that the experimentally obtained extinction can be predicted on the basis of the Mie Scattering model for small grain sizes. The current study is an attempt to understand the absorption spectrum of a few such materials having no significant intrinsic absorption in the THz region by separating the independent contributions of true absorption of the material and scattering losses due to its morphology in the extinction of the material. This would help in distinguishing these materials based on their rising trend of the extinction spectra at lower frequencies.
In the presentation we report on novel applications of Gaussian beam mode (GBM) analysis, including in image
deconvolution and Fourier grating design.
GBMs are the natural modes with which to describe propagation of quasi-collimated long-wavelength beams, with only
a small number of modes required to reach adequate accuracy for many practical applications. GBMs provide a more
efficient and natural basis set with which to describe propagation than for example plane wave decomposition,
especially because of the limited spatial frequency content (only a few degrees of freedom are necessary to describe
such beams and the degrees of freedom can be associated with component GBMs).
We discuss how GBM analysis provides a useful alternative scheme to FFT approaches for performing deconvolutions
and image retrieval in long-wavelength quasi-collimated systems. The convolving beam is usually described very
efficiently in terms of beam modes and an SVD approach can be used to extract the mode coefficients of the
deconvolved image. We discuss in particular the novel application to mapping in astronomical telescope observations.
Another useful area of application is in the design of Fourier phase gratings. Fourier gratings can be used for beam
multiplexing of local oscillator power in array imaging systems. In this case phase retrieval is often driven by an
iterative approach to the solution based on FFTs and thus by implication plane waves. A GBM approach leads to a more
efficient and physically more meaningful approach, especially again because of the limited spatial frequencies possible
in long wavelength systems.
We argue that nanostructure based THz lasers of standard design have a principal limitations of gain value. These limitations rise from the obvious necessity to engineer both THz gap and population inversion simultaneously. Typical approach to the gap engineering inherited from midIR lasers utilizes intersubband transitions. However, contrary to midIR range, for THz lasing selective depopulation is problematic. The problem is that the selectivity of
both depopulation mechanisms, LO phonon emission and electron - electron scattering, in THz region is substantially weaker than in midIR region. We suggest to use InAs/GaSb coupled quantum wells as a way to overcome this fundamental limitation. This is the only heterostructure where THz lasing can be based not on intersubband but on interband transitions. A proper design of this structure leads to a hybridization gap coming from anti-crossing of the GaSb valence band and InAs conduction band naturally appearing in the THz range. Two more advantages of this design are (i) a large value of the interband dipole matrix element and (ii) W-shaped spectrum leading to a singular density of states. These advantages lead to a gain much higher than for intersubband THz lasing.
The water content in polyglycol oils is investigated by Terahertz Time-Domain Spectroscopy (THz-TDS). These oils are able to dissolve a certain percentage of water. Changes in the absorption coefficient and refractive index are observed related to the amount of water added to the pure oils. Comparison of the experimental data with predictions based on Beer-Lambert and Lorentz-Lorenz-theory, respectively, exhibits an excellent agreement. Analyses with Fourier Transform Infrared (FTIR) Spectroscopy reveal sensitivity similar to the THz-TDS experiment. THz-TDS may offer powerful tools to quantitatively determine the water concentration in petroleum products.
We demonstrate that, through coherent measurement of the transmitted terahertz electric fields, broadband (0.3-8THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of illegal drugs and high explosives that are of interest to the forensic and security services. Our results show that these absorption features are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low frequency inter- and intra-molecular vibrational modes of the molecules.
We report the operation of a 2 THz quantum cascade laser based on a GaAs/Al0.1Ga0.9As heterostructure. Lasing action takes place between an isolated subband and the upper state of a 14 meV wide miniband. In pulsed mode, with a 3.16mm long device, we report a threshold current density of 115 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device shows lasing action in continuous wave up to 47K, with a maximum power in excess of 15 mW at T = 4K.
The on-chip detection of nanolitre volumes of dielectric material is demonstrated using terahertz (THz) pulses. Simultaneous analysis at different frequencies on separate, lithographically defined locations is shown to be possible using THz band-stop filter elements connected by a microstrip line. Integrated thin film low-temperature-grown GaAs photoconductive switches are used for THz pulse generation and detection within the microstrip interconnect. This technique is expected to be useful in sensing organic films such as DNA, and other biomolecular materials, in an on-chip environment.
We have developed a detector which records the full polarization state of a terahertz (THz) pulse propagating
in free space. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic
pulse in two orthogonal directions as a function of time. A prototype device fabricated on Fe+ ion
implanted InP exhibited a cross polarized extinction ratio better than 390:1. The design and optimization of
this device are discussed along with its significance for the development of new forms of polarization sensitive
time domain spectroscopy, including THz circular dichroism spectroscopy.
At the present time the interaction of Terahertz (THz) radiation with random structures is not well understood. Scattering effects are particularly relevant in this spectral regime, where the wavelength, and the size and separation of scattering centres are often commensurable. This phenomenon can both be used to advantage in imaging and sensing, but conversely can have adverse effects on the interpretation of a "fingerprint" spectrum. A new mathematical method, the Phase Distribution Model, is reported here for the calculation of attenuation and scattering of THz radiation in random materials. This uses a Phase Distribution Function to describe the effect of the non-absorbing scatterers within the media. Experimental measurements undertaken using previously published results, data obtained from specially constructed phantoms and from everyday textiles have been compared with the theory. These experimental results encompass both cylindrical and spherical scattering situations. The model has also been compared with exact calculations using the Pendry code.