Coherent THz Radiation Research Articles

Characterization of coherent THz radiation bursting regime at Elettra

Coherent Terahertz radiation has been observed in the third generation synchrotron light source Elettra. The radiation emitted in bursts shows a wavelength distribution shorter than the nominal electronic bunch length suggesting that the microscopic emission mechanism is due to the beam instabilities. In this paper we discuss the effects of the instabilities and present preliminary spectroscopic measurements.

On the enhancement of coherent emission by energy-phase correlation in a bunched electron beam

The coherence effect [1,2] describes emission of radiation from a group of sources whose emission phases are such that the fields of individual sources may be added constructively. While the power of usual incoherent radiation grows in proportion to the number of sources, that of the coherent radiation grows in proportion to its square. A collection of free electrons in a short bunch is an example of such a group of sources. Recent advances in rf accelerator technology have provided us with intense electron bunches that are short enough for inducing coherent effects in the THz frequency range [3, 4]. Coherent THz radiation utilizing those bunches has been demonstrated through various emission processes, such as synchrotron radiation [5], transition radiation [6], and undulator radiation [7]. Recently, Doria et al. proposed that an appropriate energy-phase correlation may enhance the output power of coherent undulator radiation significantly [8,9]. Their claim is that the proposed correlation induces constructive interference of the individual radiation fields emitted by each electron when the electron distribution satisfies the phase-matching condition. This condition is derived from the mode amplitude at the undulator exit [8]:

Generation and spectral manipulation of coherent terahertz radiation with two-stage optical rectification

We propose and experimentally demonstrate the generation of single-cycle terahertz radiation with two-stage optical rectification in GaSe crystals. By adjusting the time delay between the pump pulses employed to excite the two stages, the terahertz radiation from the second GaSe crystal can constructively superpose with the terahertz field injected from the first stage. The high mutual coherence between the two terahertz radiation fields is ensured with the coherent optical rectification process and can be further used to synthesize a desired spectral profile of coherent THz radiation. The technique is also potentially useful for generating high-power single-cycle terahertz pulses, usually limited by the pulse walk-off effect of the nonlinear optical crystal used.

RF gun for an intense THz radiation source**Supported by Exploration Project of Knowledge Innovation Program of Chinese Academy of Sciences, and Shanghai Science and Technology Commission (02QF14059)

A new facility is under construction at the Shanghai Institute of Applied Physics, to generate femto-second electron bunches and intense coherent THz radiation pulses. A thermionic RF-gun is used to be the electron source of the linac, which is 1.6 cell, π/2, side coupled in design. In the following of this paper, the design, manufacture and beam operation of this gun are presented.

Study of the optical gap in novel superconductors by coherent THz radiation

We show how coherent synchrotron radiation (CSR) allows one to measure the slight increase in the reflectivity of a BCS superconductor as it is cooled below Tc. In fact, by CSR coupled to a conventional interferometric apparatus, one can obtain a signal-to-noise ratio ∼103 in the sub-THz range. We apply this technique to the measurement of the optical gap in boron-doped diamond and in CaAlSi, a superconductor isostructural to MgB2. In the latter compound we are also able to determine a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis.

Coherent and incoherent radial THz radiation emission from femtosecond filaments in air

We show that the THz radiation emitted in the radial direction by a femtosecond filament created in air is linearly polarized and coherent. By applying an electric field along the filament axis this THz radiation is strongly enhanced and becomes incoherent and not polarized.

Double-pulse THz radiation bursts from laser-plasma acceleration

A model is presented for coherent THz radiation produced when an electron bunch undergoes laser-plasma acceleration and then exits axially from a plasma column. Radiation produced when the bunch is accelerated is superimposed with transition radiation from the bunch exiting the plasma. Computations give a double-pulse burst of radiation comparable to recent observations. The duration of each pulse very nearly equals the electron bunch length, while the time separation between pulses is proportional to the distance between the points where the bunch is accelerated and where it exits the plasma. The relative magnitude of the two pulses depends upon by the radius of the plasma column. Thus, the radiation bursts may be useful in diagnosing the electron bunch length, the location of the bunch’s acceleration, and the plasma radius.

Far Infrared Coherent Synchrotron Edge Radiation at ANKA

The region of the electromagnetic spectrum between 0.3 and 20 THz is a “frontier region” of spectroscopy in physics, chemistry, biology, material sciences, and medicine [1]. Radiation in the THz range allows the investigation and excitation of phenomena with time constants of about 1 ps. Examples on this time scale are atomic electron orbits in highly excited Rydberg states, rotations of small molecules and modes of collective oscillations of proteins and polar liquids like water. Resonances of electrons in semiconductor nanostructures and band gaps of superconductors can equally be studied with THz radiation. The THz region lies above the frequencies of traditional electronics but below the range of optical and infrared generators. Until now, possibilities to generate sufficiently intense and brilliant radiation at the wavelengths in question were scarce and the region was therefore named the “THz gap” (Figure 1). In accelerators such highly intense coherent THz radiation can be generated under special conditions when the electron bunch length is comparable to the wavelength of the emitted radiation [2,3]. Since the emission of long wavelengths is suppressed by shielding effects of the vacuum chamber, the bunch length needs to be sufficiently short in order to yield observable intensities.

Power measurement of frequency-locked Smith-Purcell radiation

Frequency-locked Smith-Purcell radiation (FL-SPR), generated by a train of electron bunches traveling above a grating, is characterized by a broad range of frequencies which are locked to the train frequency in a discrete comb and are spatially dispersed in space. We report absolute-scale power measurement of FL-SPR in the millimeter wave range. A 50 ns long train of $170\text{ }\ensuremath{\mu}\mathrm{m}$ electron bunches was produced by a 15 MeV, 17 GHz accelerator with 80 mA of average current. The grating had 20 periods spaced by 2.54 mm. The experimental results were compared, on an absolute scale, with the electric-field integral equation model which takes into consideration the finite length of the grating. Very good agreement was obtained. The present results should be useful in planning SPR applications such as diagnostics of electron bunch length on the femtosecond scale and coherent THz radiation sources.

On Photo-Induced Phenomena in Complex Materials: Probing Quasiparticle Dynamics using Infrared and Far-Infrared Pulses

It is now possible to routinely generate and detect subpicosecond pulses in the mid and far-infrared portions of the electromagnetic spectrum enabling novel time-resolved spectroscopic investigations. In the context of complex materials, spectral selectivity from approximately 0.001-1.0eV is especially important since many relevant quasiparticle excitations lie in this range. This includes, as examples, gapped excitations related to superconductivity, charge ordering, and hybridization phenomena, phonon and polaron dynamics, and the coherent Drude response so intimately related to metal-insulator transitions. Temporally resolving spectral changes associated with such excitations in pump-probe-like experiments is proving to be a powerful tool in materials where multiple degrees of freedom (charge, lattice, spin, and orbital) conspire to determine functionality. Quite generally, important insights into ground state properties, coupling parameters, degrees of freedom influencing quasiparticle transport, the nature of phase transitions, and nonequilibrium dynamics can be obtained. Following a brief review of illustrative examples highlighting such possibilities we will present, in some detail, recent experiments on two different ferromagnetic materials. First, we describe terahertz emission experiments on single crystal iron thin films. Following photoexcitation, a burst of coherent THz radiation is emitted which is shown to be consistent with partial demagnetization of the Fe film occurring on a picosecond timescale. As a second example, we describe recent optical-pump infrared-probe measurements on the low carrier density magnetoresistive pyrochlore Tl 2 Mn 2 O 7 . In the ferromagnetic and paramagnetic phases, the recombination of photoexcited carriers is strongly influenced by spin fluctuations.

Complex Phonon-Polariton Dispersion of Congruent Lithium Niobate Studied by THz Time-Domain Spectroscopy

Complex polariton dispersion relations were determined in the congruent lithium niobate crystal using the latest terahertz time domain spectroscopy. We measured the power and phase transmission spectra by the coherent THz radiation. The complex dispersion relations of phonon polariton frequency and damping have been investigated simply by rotating the polarization plane of an incident beam. These dispersive properties have been consistently reproduced by the damped harmonic oscillator model related to the lowest frequency infrared active modes.

Time and Frequency Resolved THz Spectroscopy of Micro- and Nano-Systems

1. IntroductionTerahertz technology [1], at present a dynamically developing fleld, was en-abled by advances in ultra-short pulse lasers and semiconductor coherent sourcesin the frequency range of 1{50 THz. The extensive activities are focused on THzoptics, spectrally bright sources, and detectors. The terahertz time-domain spec-troscopy and terahertz imaging are main flelds, where terahertz technology flndsapplication. In this contribution we present our recent results in the fleld of tera-hertz time-domain spectroscopy (THz-TDS) applied to micro- and nano- systemsand in the development of THz optics.First, we report on a broadband absorptive anti-re°ection coating that sup-presses the re°ection of terahertz pulses within an electro-optic detector. Theanti-re°ection coating is made of a thin chromium layer. We prove functionalityof the layer in the THz time-domain spectroscopy. We estimate for the flrst timethe complex index of refraction of such layers in the spectral range of 0.1{4 THz.Second, in the contribution we also report on a complex permittivity of nanoscalecomposite material | the single-wall carbon nanotube (SWCNT) mats measuredby THz-TDS at frequencies from 0.1 THz up to 4 THz.2. THz time-domain spectroscopic set-upThe samples were evaluated by THz time-domain spectroscopy [2]. In ourlaboratory we have built such a system. It uses a Ti:sapphire laser with 80 fem-tosecond long near infrared (NIR) pulses to generate coherent THz radiation from(92)

Terahertz time-domain spectroscopy of low-energy excitations in glasses

The low-energy excitations in glasses were studied by the terahertz time-domain spectroscopy (THz-TDS). The coherent THz radiation by a femto-second pulse laser enables the determination of the real and the imaginary parts of complex dielectric constants in the frequency range between 0.3 and 2.4 THz. The complex dielectric spectra of glasses clearly show the existence of broad low-energy excitations called boson peaks.

Terahertz Signal Generation in 1-D Photonic Crystals

Theoretical and numerical results are presented to assure that a tunable, narrow-band, coherent THz radiation source can be based on parametric down-conversion in aphotonic crystal. Our proposal is based on down-conversion mixing and a local-field enhancement mechanism that is available by tuning each of the two driving laserfields either to band-edge or to a defect mode in the band gap. The frequency of the down-converted signal can be tuned by intersecting two non co-linear laser sources. The polarizations are degenerate at normal incidence and have sub-THz down-conversion maximum. For aspecific sample geometry we show that by changing the angle of incidence of one tunable laser to 30 degrees the THz frequency is about11.5 THz for p-polarization and 3.5THz for s-polarization, since the angle-dependent transmission spectrum is different for p- and spolarizations.The peak conversion efficiency for both polarizations is enhanced by over two orders of magnitude. Finally we also introduce some preliminary experimental results which agree with the numerical results we present here.

Far-infrared cw difference-frequency generation using vertically integrated and planar low temperature grown GaAs photomixers: application to H2S rotational spectrum up to 3�THz

The generation of continuous coherent THz radiation by mixing two cw Ti:Sa laser beams with a well-controlled frequency separation for a new scheme of vertically integrated low temperature grown GaAs (LTG-GaAs) spiral photomixer is reported. For this new photomixer device used in THz emission, the LTG-GaAs active layer is sandwiched between the two parallel metal plates of a high-speed photodetector loaded by a broadband spiral antenna. We have exploited the advantage of a higher delivered power in the low part of the spectrum (<2000 GHz), while a low RC time constant planar interdigitated detector was used at the upper frequency. The performances of the spectroscopic setup in terms of spectral resolution (5 MHz), tunability and frequency capability are assessed by measurements of the pure rotational spectra of hydrogen sulfide (H2S) up to 3000 GHz.

Far-Infrared Ferroelectric Soft Phonon-Polariton Probed by THz Time Domain Spectroscopy

This paper reports the pioneering application of the terahertz time domain spectroscopy to ferroelectric bismuth titanate and lithium niobate crystals. By the use of the THz coherent radiation both the intensity and phase of transmittance have been measured accurately. The complex dielectic constant is determined by Fourier transformation of time domain transmission amplitude and phase in the frequency region from 3 cm m 1 to 60 cm m 1 . The polariton-dispersion curves were determined by the phase delay as a function of THz radiation frequency. The obtained lowest polariton limit of 3 cm m 1 is far below the limit of the Raman scattering technique.

Terahertz Time Domain Spectroscopy of Phonon-Polaritons in Ferroelectric Lithium Niobate Crystals

We report the observation of far-infrared A1(z) and E(y) symmetry phonon-polaritons of ferroelectric lithium niobate crystals by THz time domain spectroscopy. The obtained lowest polariton limit of 3 cm-1 is far below the limit of the Raman scattering technique. The intensity and phase of transmittance for propagation along the x-axis have been accurately measured using THz coherent radiation. With an incident polarization plane parallel to the z-axis (E//z), the polariton dispersion curve of A1(z) symmetry has been observed in detail. The dispersion shows no “avoided crossing” predicted by strong anharmonicity. With an incident polarization plane parallel to the y-axis (E//y), the lowest polariton branch of E(x,y) symmetry has been clearly observed for the first time. The complex dielectic constants of εz(ω) and εy(ω) from 0.1 THz to 2 THz have been also determined.

THz-wave emission by coherent optical phonons

Coherent optical phonons are excited in single-crystal Tellurium by above-band-gap excitation with femtosecond laser pulses. Coherent Raman-active modes are resolved in amplitude and phase by monitoring the time-resolved reflectivity changes in a pump-probe experiment. By detecting the far-infrared radiation emitted from the crystal, we observe for the first time coherent THz radiation from infrared-active lattice modes. This technique opens a new way for the spectroscopy of lattice dynamics. We discuss details of the generation and detection mechanism of the coherent lattice modes. We show that the driving force for the coherent infrared-active modes is a diffusive process, i.e. the ultrafast build-up of a photo-Dember field.