Narrowband Terahertz Radiation Research Articles

Numerical modeling of a table-top tunable Smith–Purcell terahertz free-electron laser operating in the super-radiant regime

Terahertz (THz) radiation occupies a very large portion of the electromagnetic spectrum and has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as x-rays. One path for generating copious amount of tunable narrow-band THz radiation is based on the Smith–Purcell free-electron laser (SPFEL) effect. In this paper we propose a simple concept for a compact two-stage tunable SPFEL operating in the super-radiant regime capable of radiating at the fundamental bunching frequency. We demonstrate its capabilities and performances using the conformal finite-difference time-domain electromagnetic solver VORPAL.

Expected performance of a planar undulator designed for the terahertz source project at Tohoku University

A test accelerator as coherent terahertz (THz) source project (t-ACTS) has been under development at Tohoku University, in which generation of intense coherent THz radiation from the very short electron bunch less than 100 fs will progress. Such short bunch will be utilized to generate broad-band coherent radiation from bending magnets in a ring of which isochronous optics is adapted to preserve the short bunch length. In addition, narrow-band coherent THz radiation from an undulator has been considered. We describe design of the test accelerator and expected performance of undulator radiation.

Laser comb with velocity bunching: Preliminary results at SPARC

A new technique, named “laser comb”, was tested during the last SPARC run. It is able to produce electron pulse trains with a charge of some hundreds pC, a repetition rate of some terahertz, and a sub-picosecond length. This technique is based on the velocity bunching configuration of the SPARC injector. It can be useful to drive pump and probe free-electron laser experiments, to generate coherent excitation of plasma waves in plasma accelerators, and to produce narrow band terahertz radiation. In this paper, we describe the experimental results achieved so far and provide a comparison with simulations.

Enhanced tunable narrow-band THz emission from laser-modulated electron beams

We propose and analyze a scheme to generate enhanced narrow-band terahertz (THz) radiation through down-conversion of the frequency of optical lasers using laser-modulated electron beams. In the scheme the electron beam is first energy modulated by two lasers with wave numbers ${k}_{1}$ and ${k}_{2}$, respectively. After passing through a dispersion section, the energy modulation is converted to density modulation. Because of the nonlinear conversion process, the beam will have density modulation at wave number $k=n{k}_{1}+m{k}_{2}$, where $n$ and $m$ are positive or negative integers. By properly choosing the parameters for the lasers and dispersion section, one can generate density modulation at THz frequency in the beam using optical lasers. This density-modulated beam can be used to generate powerful narrow-band THz radiation. Since the THz radiation is in tight synchronization with the lasers, it should provide a high temporal resolution for the optical-pump THz-probe experiments. The central frequency of the THz radiation can be easily tuned by varying the wavelength of the two lasers and the energy chirp of the electron beam. The proposed scheme is in principle able to generate intense narrow-band THz radiation covering the whole THz range and offers a promising way towards the tunable intense narrow-band THz sources.

Terahertz Radiation-Band Engineering Through Spatial Beam-Shaping

We propose a terahertz (THz)-band engineering technique based on generation of specifically designed time-domain radiation. The desired time-domain radiation is formed by combining the time-delayed outputs of an array of photoconductive antennas. We have employed this technique to generate tunable narrowband THz radiation. By using an array of identical photoconductive antennas with identical spacings, we have demonstrated a 12.5-mW output peak-power over 0.05-THz bandwidth and a tuning range of 0.65-1.4 THz. Considering the 2.5-THz bandwidth of each individual photoconductive antenna, 50 times higher power efficiency is achieved as a result of radiation-band shaping.

Tunable narrowband terahertz emission from mastered laser–electron beam interaction

A tunable source of coherent narrowband terahertz radiation is realized by using a laser to modulate the emission characteristics of a relativistic electron beam. In the quest for sources of optical radiation in the terahertz domain, promising candidates are nonlinear optical processes occurring when an intense laser beam interacts with a material medium1,2,3,4,5. Besides conventional media (such as crystals), relativistic electrons also show striking nonlinear collective behaviours, which can lead to powerful laser-induced coherent emission6,7, revealing huge potentials of these devices as terahertz sources8. However, up to now only broadband emissions were reported, and experimental control of their radiation properties, such as their spectra9,10, remained an important challenge. Here, we demonstrate the possibility of mastering the coherent emission experimentally by producing tunable narrowband terahertz radiation. The interaction is made to occur between an electron beam and laser pulses possessing a longitudinal quasi-sinusoidal modulation, and the narrowband emission occurs in a region of quasi-uniform magnetic field. The process therefore strongly differs from classical synchrotron radiation experiments, where narrowband emission occurs inside a periodic magnetic field.

Coherent detection of pulsed narrowband terahertz radiation

We demonstrate the generation and coherent detection of narrowband terahertz radiation using a Q-switched laser pumped optical parametric oscillator as the optical source. Narrowband terahertz radiation is produced using conventional difference frequency mixing and coherently detected via a frequency domain technique that relies on coherent upconversion of the terahertz field combined with optical homodyning to suppress background noise. GaSe crystals are used for both generation and coherent detection processes. Although we provide a proof-of-principle demonstration at 2THz, the infrared radiation may be tuned from the far-infrared through the midinfrared by simply tuning the idler wavelength of the optical parametric oscillator and the orientation of the two crystals.

Generation of narrow-band terahertz radiation with preset frequency components in poled ferroelectric materials

We investigate theoretically the generation of narrow-band, multifrequency terahertz (THz) radiation via optical rectification of femtosecond laser pulses in poled ferroelectric materials. We propose a method based on simulated annealing (SA) algorithm to inversely determine the needed ferroelectric domain structure for generating the expected THz radiation. We demonstrate that multifrequency THz radiation with arbitrarily preset frequencies and relative intensities can be generated from appropriate domain structures determined by this SA-based method. The designed domain structure has a good performance stability with respect to the random fluctuation of the domain thicknesses.

Terahertz waveform synthesis via optical rectification of shaped ultrafast laser pulses

We demonstrate the use of optical pulse-shaping technique in conjunction with difference frequency generation in a non-linear optoelectronic crystal for generating synthesized waveforms at terahertz frequencies. Spectral phase modulations, programmed using Gerchberg-Saxton algorithm and prepared in a spatial light Fourier filter, produce tailored terahertz pulses, including chirped pulses, zero-area pulses, and trains of multiple pulses for tunable narrow-band terahertz radiation up to 2.0 THz.

Generation of coherent terahertz radiation with multifrequency modes in a Fibonacci optical superlattice

We investigate theoretically the generation of coherent narrow-band terahertz (THz) radiation with multifrequency modes in an optical superlattice produced by quasiperiodically poled ferroelectric crystal. The superlattice consists of two building blocks A and B, in which each containing a pair of antiparallel 180° ferroelectric domains, arranged as a Fibonacci sequence. Second-order nonlinear optical rectification of femtosecond pulses gives rise to a THz wave form whose multifrequency modes are determined by the quasiperiodic structure properties. The relationship between multifrequency modes terahertz generation and Fibonacci structure parameters is also analyzed and studied.

High-power terahertz radiation from relativistic electrons.

Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers and fast diodes. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources and by short electron bunches in accelerators, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors; the present source would allow full-field, real-time capture of such images. High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump-probe studies of dynamical properties of materials.

Tunable narrow-band terahertz generation from periodically poled lithium niobate

We describe a technique for generating tunable narrow-band terahertz radiation via optical rectification in periodically-poled lithium niobate. Frequency tuning is accomplished by spatially chirping the domain width laterally to the beam propagation direction, and adjusting the temperature of the sample. We demonstrate tuning over a continuous range from 0.8 to 2.5 THz. The bandwidth of the terahertz waveforms is as narrow as 0.02 THz.

Generation of tunable narrow-band surface-emitted terahertz radiation in periodically poled lithium niobate

Generation of tunable narrow-band terahertz (THz) radiation perpendicular to the surface of periodically poled lithium niobate by optical rectification of femtosecond pulses is reported. The generated THz radiation can be tuned by use of different poling periods and different observation angles, limited only by the available bandwidth of the pump pulse. Typical bandwidths were 50-100 GHz, depending on the collection angle and the number of periods involved.

Enhancement in the spectral irradiance of photoconducting terahertz emitters by chirped-pulse mixing

We describe the use of mixing linearly chirped optical pulses in biased photoconductors to generate tunable narrow-band terahertz (THz) radiation with enhanced spectral brightness. The increase in conversion efficiency from optical to THz radiation at a given THz frequency arises from the improved saturation characteristics of the photoconductor for chirped-pulse mixing compared with the usual case of excitation by an ultrafast optical pulse. In the weak saturation limit, the enhancement in the saturation fluence scales with the ratio of the duration of the chirped optical pulse to the photocurrent relaxation time in the emitter and is essentially independent of the beat frequency generated by the chirped-pulse mixing technique. This dependence allows for substantial enhancements in the saturation fluence and, hence, in the THz spectral brightness. We demonstrate enhanced saturation fluences experimentally for dipole emitters fabricated on radiation-damaged Si on sapphire.

High-power narrow-band terahertz generation using large-aperture photoconductors

Large-aperture biased photoconductive emitters which can generate high-power narrow-band terahertz (THz) radiation are developed. These emitters avoid saturation at high fluence excitation and achieve enhanced peak power spectral density by employing a thick layer of short-lifetime low-temperature-grown GaAs (LT-GaAs) photoconductor and multiple-pulse excitation. THz waveforms are calculated from the saturation theory of large aperture photoconductors, and a comparison is made between theory and measurement. A direct comparison of the multiple-pulse saturation properties of THz emission from semi-insulating GaAs and LT GaAs emitters reveals a strong dependence on the carrier lifetime. In particular, the data demonstrate that saturation is avoided only when the interpulse spacing is longer than the carrier lifetime.

Novel sources and detectors for coherent tunable narrow-band terahertz radiation in free space

A novel scheme for the optical generation and coherent detection of tunable narrow-band far-infrared radiation in free space is described. This technique involves the optical heterodyning of two linearly chirped broadband pulses to produce a quasi-sinusoidal intensity modulation at tunable terahertz frequencies. The frequency content of the narrow-band terahertz radiation produced by mixing chirped optical pulses in a nonlinear optoelectronic device such as a photoconducting dipole antenna can be tailored simply by controlling the phase modulation of the optical pulses and the delay between them. An optoelectronic terahertz beam system composed of a tunable narrow-band Hertzian dipole emitter and a synchronously gated tunable dipole detector is presented. The performance of this system as a potentially powerful tool for far-infrared pump–probe spectroscopy and the limitations produced by cubic phase modulation in the chirped optical pulses are discussed.

Enhancement of narrow-band terahertz radiation from photoconducting antennas by optical pulse shaping

We report the use of optical pulse shaping to convert single femtosecond pulses into lower-intensity pulse sequences for excitation of photoconductive dipole antennas, which results in the generation of bursts of tunable narrow-band free-space terahertz radiation. Our experiments demonstrate that the use of such pulse sequences can significantly enhance the spectral amplitude of the narrow-band terahertz radiation by avoiding the saturation effects that occur with single-pulse excitation. Our technique also provides the capability for terahertz wave-form shaping.