THz Source Research Articles

Four-leaf-clover-shaped antenna for a THz photomixer

To improve the output power of a photomixer used as a THz source, we propose a four-leaf-clover-shaped antenna structure composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were first investigated on a half-infinite substrate as a simplified radiation environment to reduce the computation time. Based on the antenna characteristics on that half-infinite substrate, the antenna structure was designed to have a maximum total efficiency and a maximum directivity on an extended hemispherical lens. The input resistance of this structure was six times that of a full-wavelength dipole, significantly improving the mismatch efficiency between a photomixer and the antenna. The terahertz output power from this structure is expected to be 2.7 times that of a full-wavelength dipole.

Modelling of sub-wavelength THz sources as Gaussian apertures

The THz emission point on a nonlinear electro-optical crystal for generating broadband THz radiation is modeled as a radiating Gaussian aperture. With the wavelengths of the infrared pump beam being much smaller than the wavelength components of the generated THz pulse, a THz sub-wavelength radiating aperture with Gaussian profile is effectively created. This paper comprehensively investigates Gaussian apertures in focused THz radiation generation in electro-optical crystals and illustrates the breakdown of the paraxial approximation at low THz frequencies. The findings show that the shape of the radiation pattern causes a reduction in detectable THz radiation and hence contributes significantly to low signal-to-noise ratio in THz radiation generation. Whilst we have demonstrated the findings on optical rectification in this paper, the model may apply without a loss of generality to other types of apertures sources in THz radiation generation.

Spectral characterization of porous dielectric subwavelength THz fibers fabricated using a microstructured molding technique

We report two novel fabrication techniques, as well as THz spectral transmission and propagation loss measurements of subwavelength plastic wires with highly porous (up to 86%) and non-porous transverse geometries. The two fabrication techniques we describe are based on the microstructured molding approach. In one technique the mold is made completely from silica by stacking and fusing silica capillaries to the bottom of a silica ampoule. The melted material is then poured into the silica mold to cast the microstructured preform. Another approach uses a microstructured mold made of a sacrificial plastic which is co-drawn with a cast preform. Material from the sacrificial mold is then dissolved after fi ber drawing. We also describe a novel THz-TDS setup with an easily adjustable optical path length, designed to perform cutback measurements using THz fibers of up to 50 cm in length. We fi nd that while both porous and non-porous subwavelength fibers of the same outside diameter have low propagation losses (alpha <or= 0.02 cm(-1)), the porous fi bers exhibit a much wider spectral transmission window and enable transmission at higher frequencies compared to the non-porous fibers. We then show that the typical bell-shaped transmission spectra of the subwavelengths fibers can be very well explained by the onset of material absorption loss at higher frequencies, due to strong confinement of the modal fields in the material region of the fi ber, as well as strong coupling loss at lower frequencies, due to mismatch between the modal field diameter and the size of the gaussian-like beam of a THz source.

The progress towards terahertz quantum cascade lasers on silicon substrates

AbstractA review is presented of work over the last 10 years which has been aimed at trying to produce a Si‐based THz quantum cascade laser. Potential THz applications and present THz sources will be briefly discussed before the materials issues with the Si/SiGe system is discussed. Waveguide designs and waveguide losses will be presented. Experimental measurements of the non‐radiative lifetimes for intersubband transitions in Si1‐xGex quantum wells will be presented along with theory explaining the important scattering mechanisms which determine the lifetimes. Examples of p‐type Si/SiGe quantum cascade designs with the experimental electroluminescence will be reviewed and examples of n‐type Si‐based designs will be presented. In the conclusion designs and structures will be discussed with the greatest potential to achieve an electrically pumped Si‐based THz laser.

THz-Imaging on its Way to Industrial Application

Terahertz radiation, which fills the gap between 100 GHz and 10 THz ( = 30 µm – 3 mm) in the electro-magnetic spectrum, has seldom been used outside of astronomy and other scientific research. However, in recent years there has been a significant interest in investigating THz radiation for different new applications. Especially the ability of terahertz radiation to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X-rays lead to recent interests chiefly in the fields of security technology and biomedical imaging. The attribute of many different materials to be transparent for terahertz radiation, was also the reason for many difficulties in practical applications outside of research. Using radiation that can pass through so many materials so well makes detection difficult. In addition, sources to generate light at terahertz frequencies have suffered from low output intensity and other problems. Since the 1990s, technical breakthroughs in sources and detectors have brought terahertz technology within striking distance of significant commercial markets [1]. The pressure to develop new terahertz sources arose from two dramatically different groups - ultrafast timedomain spectroscopists who wanted to work with longer wavelengths, and long wavelength radio astronomers who wanted to work with shorter wavelengths. Today there are continuous-wave (CW) sources available as well as pulsed sources [2]. The aim of this paper is to provide an overview of key scientific developments which currently represent the basics of the mentioned THz technology. Beginning with the working principle of opto-electronic THz sources and detectors, the paper explains different setups for transmitting and using THz radiation. Furthermore it shows different applications of different business branches and gives an outlook for industrial application in the fields of metrology and quality control.

Time-domain spectroscopy of molecular free-induction decay in the infrared

Time-domain spectroscopy using dual, coherent frequency combs is used to measure free-induction decay from a molecular gas sample in the near-IR with a time-domain signal-to-noise ratio of approximately 10(6) over a approximately 6 ns window at 55 fs time resolution (corresponding to the 9 THz source bandwidth) and a frequency/timing accuracy set by the frequency combs. The free-induction decay exhibits the expected periodic pulses from the rephasing of the multiply excited rovibrational levels. This demonstration represents the first high-resolution, high-accuracy, broadband measurement of optical free-induction decay, to our knowledge.

Design study of an FEL oscillator with a waveguide

We present a design study of a free electron laser (FEL) oscillator for high power THz source experiments on the basis of the Shanghai femtosecond accelerator device. A circular groove guide is used as a new interaction structure. Plane metal meshes are used as upstream and downstream mirrors of the resonator. The general design parameters are presented. We analyzed the spontaneous emission and stimulated emission in the oscillator using these parameters.

Gigantic stimulated Raman scattering in one-dimensional Mott–Hubbard insulators: A possible THz source

One-dimensional Mott–Hubbard insulators like Sr 2CuO 3, halogen-bridged Nickel chain compounds have orders-of-magnitude nonlinear optical properties compared to other one-dimensional organic or inorganic compounds. We show theoretically, that the stimulated Raman scattering susceptibility for such insulators could be order(s)-of-magnitude larger even compared to other nonlinear optical susceptibilities. The lowest two-photon state is at lower energy than the lowest one-photon state in some of these insulators. This leads to a potential for strong Stokes generation in the THz regime from these compounds. Our results and conclusions are based on exact numerical solution of finite size two-band extended Hubbard model.

THz Spectroscopy by Frequency-Tuning Monochromatic THz Source: From Single Species to Gas Mixtures

Different gases have been successfully fingerprinted on the basis of the distinct transition frequencies measured by frequency tuning a THz source developed by us. Our further analysis illustrates that one can reliably separate isotopologues by measuring and comparing their rotational constants. Based on our investigation of the N2O isotopologues, 29 rotational transition peaks were identified for the first time. For the gas mixtures, preliminary results indicate that one can identify the components of chemical species and the corresponding concentrations. Based on our spectroscopic results, the tunable THz source may offer a viable approach in the identifications and detections of chemical species.

A Microscopic Approach for THz Intersubband Challenges

The main candidate to be a practical and low cost high power THz source is the intersubband-based quantum cascade laser, which can have a tremendous impact in many practical applications, including last mile and indoor telecommunication systems. In this review we discuss current challenges for THz intersubband device development from a microscopic point of view. Next summarize the search for new mechanisms and structure designs that can lead to intersubband gain without population inversion. This is a very important topic of current research, since is both an extremely elegant phenomenon from the basic physics of view and crucial for effective lasing in the THz range. The reason is that scattering phenomena can lead to level broadenings of the same order of magnitude of the lasing transitions, making population inversion by carrier injection in upper lasing subbands extremely difficult. Previous work in the literature is compared and contrasted with a new scheme that may lead to high temperature lasing by engineering the nonequilibrium population inversion with a combination of band structure and many body effects mediated by a k-space filter.

높은 출력의 THz 포토믹서를 위한 네잎클로버 형태의 안테나

테라헤르츠 광원인 포토믹서의 출력을 향상시키기 위하여 강한 공진의 복사부와 안정적인 DC 급전부로 구성된 네잎클로버 형태의 새로운 안테나구조를 제안하였다. 먼저 복사환경을 단순화시킨 무한기판 위에서 제안한 안테나구조의 공진특성을 살펴보았고, 이를 바탕으로 제안한 구조가 확장형 반구렌즈 위에서 최대의 전체효율과 지향성을 지니도록 설계하였다. 기존의 전파장다이폴안테나와 비교한 결과, 제안한 구조는 공진주파수에서 6배 이상의 높은 입력임피던스 특성을 가짐으로써 포토믹서와의 부정합효율을 크게 개선시켰으며, 이로 인해 테라헤르츠 출력이 전파장 다이폴의 경우보다 2.7배 높게 예상되었다. To improve the output power of a photomixer as a THz source, we propose a four-leaf clover-shaped antenna structure which is composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were investigated on a half-infinite substrate first as a simplified radiation environment in order to save the computation time. Based on the antenna characteristics on a half-infinite substrate, the antenna structure was designed to have a maximum total efficiency and a maximum directivity on an extended hemispherical lens. In comparison with a full-wavelength dipole, an input resistance of this structure increased six fold and this characteristic significantly improved the mismatch efficiency between a photomixer and an antenna. THz output power from this structure is expected to increase by 2.7 times as compared to a full-wavelength dipole case.

A tunable continuous wave (CW) and short-pulse optical source for THz brain imaging applications

We demonstrate recent advances toward the development of a novel 2D THz imaging system for brain imaging applications both at the macroscopic and at the bimolecular level. A frequency-synthesized THz source based on difference frequency generation between optical wavelengths is presented, utilizing supercontinuum generation in a highly nonlinear optical fiber with subsequent spectral carving by means of a fiber Fabry–Perot filter. Experimental results confirm the successful generation of THz radiation in the range of 0.2–2 THz, verifying the enhanced frequency tunability properties of the proposed system. Finally, the roadmap toward capturing functional brain information by exploiting THz imaging technologies is discussed, outlining the unique advantages offered by THz frequencies and their complementarity with existing brain imaging techniques.

Adaptive shaping of THz-pulses generated in ⟨110⟩ ZnTe crystals

Next generation THz applications require specially tailored THz pulses. Systems that use optical rectification in crystals as their THz source are susceptible to changes in the pulse envelope. Our feedback-loop system designed around a pulse shaper with a liquid crystal mask can shape the phase and amplitude of the excitation pulse and optimize the shape of the emitted THz waveform. This is demonstrated using as an example the creation of single-cycle THz pulses.

RF-Current Effects on Intrinsic Josephson Junctions

We have fabricated intrinsic Josephson junction arrays using Tl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> thin films. The junctions have dimensions of order 1 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and therefore high slope resistance in the range 1 kOmega to 10 kOmega per junction on the quasiparticle branches. We have irradiated the junctions with microwaves at a frequency of 6.37 GHz. We observe both microwave-induced vortex-flow branches and strong suppression of the switching current but only when no junctions are in the voltage state. We attribute this observation to the shunting of the high-resistance junctions by the environmental impedance, and discuss the significance of this result for the application of intrinsic junctions as THz sources and qubits.

Design of a new compact THz source based on Smith-Purcell radiation

In recent years, people are dedicated to the research work of finding compact THz sources with high emission power. Smith–Purcell radiation is qualified for the possibility of coherent enhancement due to the effect of FEL mechanism. The compact experiment device is expected to produce hundreds mW level THz ray. The electron beam with good quality is provided under the optimized design of the electron gun. Besides, the grating is designed as an oscillator without any external feedbacks. While the beam passes through the grating surface, the beam bunching will be strong and the second harmonics enhancement will be evident, as is seen from the simulation results.

New lasers based on c-cut vanadat crystals

Spectroscopic and lasing properties of c-cut Nd-doped Nd:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 crystals were investigated. Spectral tuning from 1062 to 1067 nm was demonstrated. CW, Q-switching and mode-locking regimes for two-color laser operations were realized. A novel THz source based on Q-switch two-color diode-pumped solid state c-cut Nd:GdVO4 laser with Filter Lio as selective element and the GaSe nonlinear optical crystals as convertor was demonstrated. Terahertz radiation with wavelength 436 mm (0.56 THz) was detected. One picosecond laser pulses in mode-locking diode pumped c-cut vanadat lasers with a Kerr-lens and PbS-doped glasses as saturable absorbers are observed.

Investigation of THz Emission by p-GaAsSb

With a suitable electric field imposed and under illumination by ultrashort pulses of near-infrared radiation, GaAsSb emits THz-frequency electromagnetic radiation. We conclude that high-temperature-grown GaAsSb is a new class of THz source.

Long Lifetime State of Shallow Donor Centres for Silicon Based THz Sources

Impurity centres in high purity silicon with ionization energies 38.32 and 40.09meV are observed by photothermal ionization spectroscopy measurements. Their typical shallow donor characteristics are approved by good agreement with theoretic evaluation under varying magnetic fields. The 2p0 state lifetime of one donor centres are more than 3 times longer than that of phosphorus in the same silicon sample. Compared to phosphorus which has been already successfully exploited to produce silicon-based THz radiations, this shallow donor centre has a pronounced enhancement on lifetime and quality factor of population inversion level. Hence, silicon with this donor centre would be a potential excellent candidate to develop improved silicon-based THz sources.

Semiconductor laser based THz generation and detection

AbstractWe present different concepts for compact and cost effective THz technology based on semiconductor diode lasers. First, we discuss THz sources starting with modelocked diode lasers for time domain THz sampling, and two‐colour diode lasers for continuous wave THz generation. Then we show how THz radiation can be generated directly out of two‐colour diode lasers and discuss possible schemes for an optimisation of this effect towards application relevant power levels. Finally, we demonstrate that diode lasers can also serve as detectors for THz radiation. We couple the emission of a THz gas laser into the active region of a diode laser and demonstrate that this leads to a variation of the voltage over the p‐n‐junction which is almost proportional to the THz power coupled into the device. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The soft x-ray free-electron laser FLASH at DESY: beamlines, diagnostics and end-stations

FLASH, the Free-electron LASer in Hamburg, is a worldwide unique source for extremely bright ultra-short laser-like pulses tunable in a wide spectral range in the extreme ultraviolet and soft x-ray region (Ackermann et al 2007 Nat. Photonics 1 336–42). To fully exploit the features of this new generation of light sources, a user facility with efficient radiation transport to the experimental area and novel online photon diagnostics capable of characterizing the unique parameters of the FLASH radiation has been built. It serves a broad user community active in many scientific fields ranging from atomic and molecular physics to plasma and solid state physics as well as chemistry and biology. A special focus is placed on the exploitation of the ultra-short FLASH pulses using pump–probe techniques. Thus, the facility is equipped with optical and THz sources synchronized to FLASH. This paper gives a detailed overview of the FLASH user facility.