This PDF file contains the front matter associated with SPIE Proceedings Volume 7763, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We present a new waveguide concept for terahertz quantum-cascade laser. The double-metal waveguide confines the active region between two metallic layers. Thereby, a modal confinement of almost 100 % is achieved. However, these metal layers are also one of the dominating loss mechanisms. Replacing the conventional metal with a superconductor helps to reduce the total losses. A surface plasmon is formed at the interface between the superconductor and the semiconductor. It can be maintained even for photon energies above the superconducting band gap. In this work we use niobium with a band gap of 2.8 meV to confine the active region of a THz-QCL emitting at 9 meV.

This paper presents a CW raster-scanning THz imaging setup, used to perform Non-Destructive Testing of KevlarTMand carbon fibre samples. The setup uses a 2.5 THz Quantum Cascade Laser as a source. Delamination defect in a Kevlar sample was detected showing a sensitivity to laser polarization orientation. Detection of a break in a carbon/epoxy sample was also performed.

We have developed and fabricated a novel surface-emitting waveguide for terahertz quantum cascade lasers. The successfully employed double metal waveguide for such devices lacks of good far field pattern and low beam divergence. We have overcome these drawbacks by combining a second-order grating for surface emission with a ring waveguide geometry. Stable single mode emission has been observed over various operating conditions. We have measured circular beam profiles with a FWHM of 15° and achieved a grating-induced tuning range of about 300 GHz. By breaking the circular symmetry with two opposite π phase shifts in the grating, the far field pattern has changed into a tight center lobe with a FWHM of 5° and a preferred polarization direction is defined.

The authors present a home made cryogenic electro-optical probe station allowing the direct modulation of quantum cascade lasers up to 40GHz. Based on a QMC cryostat, it should make the QCL bandwidth measure possible and then help answering questions about the modulation possibilities of such a kind of laser. The experimental results will be compared to simulation bandwidth prediction based on a complete set of rate equations describing the dynamic behavior of the laser. Bandwidth will be then linked to the different intrinsic and structural parameters.

We show that phohtonic technologies developed for conventional fiber-optic communications have potential for use in contemporary terahertz-wave applications, such as remote sensing and wireless communications. Advanced unitravelling photodiodes (UTC-PDs) can produce output power of 0.5 mW at 350 GHz and 10 μW at 1 THz. Using the UTC-PD and other optical devices, we demonstrate a time-continuous terahertz-wave signal generator that can tune the output signal over a wide frequency range with very narrow spectral linewidth and gas-sensing with the terahertz-wave source. We also show some preliminary results for terahertz-wave wireless communications using photonic technologies.

Doping of the lead telluride and related alloys with the group III impurities results in appearance of the unique physical features of a material, such as persistent photoresponse, enhanced responsive quantum efficiency (up to 100 photoelectrons/incident photon), radiation hardness and many others. We present the physical principles of operation of the photodetecting devices based on the group III-doped IV-VI including the possibilities of a fast quenching of the persistent photoresponse, construction of the focal-plane array, new readout technique, and others. The advantages of infrared photodetecting systems based on the group III-doped IV-VI in comparison with the modern photodetectors are summarized. The spectra of the persistent photoresponse have not been measured so far because of the difficulties with screening the background radiation. We report on the observation of strong persistent photoconductivity in Pb_0.75Sn_0.25Te(In) under the action of monochromatic submillimeter radiation at wavelengths of 176 and 241 microns. The sample temperature was 4.2 K, the background radiation was completely screened out. The sample was initially in the semiinsulating state providing dark resistance of more than 100 GOhm. The responsivity of the photodetector is by several orders of magnitude higher than in the state of the art Ge(Ga). The red cut-off wavelength exceeds the upper limit of 220 microns observed so far for the quantum photodetectors in the uniaxially stressed Ge(Ga). It is possible that the photoconductivity spectrum of Pb_1-xSn_xTe(In)covers all the submillimeter wavelength range.

In this paper, we propose a design for a widely tunable solid-state optically and electrically pumped THz source based on the Smith-Purcell free-electron laser. Our design consists of a thin dielectric layer sandwiched between an upper corrugated structure and a lower layer of thin metal, semiconductor, or high electron mobility material. The lower layer is for current streaming, which replaces the electron beam in the Smith-Purcell free-electron laser design. The upper layer consists of two micro-gratings for optical pumping, and a nano-grating to couple with electrical pumping in the lower layer. The optically generated surface plasmon waves from the upper layer and the electrically induced surface plasmon waves from the lower layer are then coupled. Emission enhancement occurs when the plasmonic waves in both layers are resonantly coupled.

We have developed a new generation of optoelectronic large bandwidth terahertz sources based on TEM horn antennas monolithically integrated with several types of photodetectors: low-temperature grown GaAs (LTG-GaAs) planar photoconductors, vertically integrated LTG-GaAs photoconductors on silicon substrate and uni-travelling-carrier photodiodes. Results of pulsed (time-domain) and photomixing (CW, frequency domain) experiments are presented.

Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such interactions is found to be intervalley scattering.

The availability of frequency combs has opened new possibilities for the measurement of optical frequencies. Photomixing is an attractive solution for high resolution THz spectroscopy of gases due to the narrow spectral resolution and ability to access the 100 GHz to 3.5 THz range. One limitation of present photomixing spectrometers is the accuracy with which the THz frequency is established. Measurement of the centre frequency gas phase molecular transitions requires an accuracy better than 100 kHz in order to allow spectroscopic constants to be determined. Standard optical techniques like those employed in wavelength meters can only provide accuracies in the order of 50 MHz. We have used a turnkey fibre based frequency comb and a standard photomixing configuration to realize a THz synthesizer with an accuracy of around 50kHz. Two ECDLs used to pump the photomixer are phase locked onto the frequency comb and provide a tuning range of 10 MHz. In order to extend the tuning range an additional phase locked ECLD has been added to obtain a range in excess of 100 MHz. The absorption profiles of many Doppler limited transitions of carbonyl sulphide and formaldehyde have been measured to validate this instrument.

While the unique spectral information associated with chemical and biological molecules within the terahertz frequency regime (~ 3.0-3.0 millimeters) motivates its use for practical sensing applications, limiting factors at the macroscale (weak spectral absorption, broad line widths and masking geometrical effects introduced by the samples) provides motivation for man-engineered sensing materials that allow for the transduction of the spectral information about target molecules from the nanoscale. This brief letter will overview work being performed by our research group to define molecular-level functionality that will be useful for realizing "THz/IR-sensitive" materials. Here the goal is to define switchable molecular components that when incorporated into larger DNA-based nanoscaffolds lead to THz and/or IR regime electronic and/or photonic material properties that are dictated in a predictable manner by novel functionality paradigms. In particular, theoretical modeling and design studies are being performed to engineer organic and biological switches that can be incorporated into DNA-based architectures that enable the precise extraction of nanoscale information (e.g., composition, dynamics, conformation) through electronic/photonic transformations to the macroscale. Hence, these studies seek to define new spectral-based sensing modalities useful for characterizing bio-molecules

The terahertz region in the electromagnetic spectrum has attracted much research interest recently, because of its potential applications in many areas, such as biological and medical imaging, free-space communications, and homeland security. Here, a tunable quantum dot photodetector for terahertz detection based on intersublevel transitions is proposed and simulated. The intersublevels are formed by a lateral electric field confinement on quantum wells. The intersublevel spacing can be tuned and in hence different wavelengths in the terahertz region can be detected. Our simulation results show a tunability of peak detection wavelength from ~3.3 to ~12 THz by only changing the electrical confinement voltages and the peak absorption coefficients of the detection are in the range of 10³ cm^-1. The peak calculated detectivity of the tunable photodetector is as big as 1.7x10⁹ Jones. Compared with quantum dot terahertz photodetectors produced by self-assembled growth method, the detector presented here is easier to be tuned and the effective sizes have a much higher uniformity, because of the uniform electrical confinement.