Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

A broadband, compact, all-electrically driven mid-infrared frequency comb based on a quantum cascade laser widens the scope of application of combs in this frequency range beyond that of sources which depend on a chain of optical components. Optical frequency combs are light sources that produce a comb-like spectrum, with sharp equidistant frequency modes, and have many uses in metrology and spectroscopy applications. The mid-infrared regime is particularly important for molecular fingerprinting, but so far the comb sources in this wavelength regime are bulky and rely on a chain of optical components. For wide practical applications, an electrically injected, compact scheme is desired. Andreas Hugi et al. now demonstrate a mid-infrared frequency comb generator based on a semiconductor device, a continuous-wave quantum cascade laser. Optical frequency combs1 act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational–vibrational bands of most light molecules are found2. Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals3. Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity4,5,6,7. Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser8,9,10. However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser11 are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7 micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10 hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200 hertz. The intermode beat can be varied over a frequency range of 65 kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers.

/pdf/mid-infrared-frequency-comb-based-on-a-quantum-cascade-laser-1a0a174joi.pdf

Mid-infrared frequency comb based on a quantum cascade laser

In this paper techniques for the generation and measurement of ultrashort pulses in the frequency range from about 0.1 to 10 THz are reviewed. The methods for generation are restricted to table-top systems based on short-pulse lasers in the visible or in the near-infrared. Three techniques are dealt with in detail: photoconductive switches, difference frequency generation and plasma sources. Definitions and methods to measure the pulse width are given, among them cross-correlation and measurements of the electric field of these pulses as a function of time by photoconductive switches and electro-optic sampling.

https://iopscience.iop.org/article/10.1088/0034-4885/70/10/R02/pdf

Table-top sources of ultrashort THz pulses

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

/pdf/mode-locked-pulses-from-mid-infrared-quantum-cascade-lasers-4j3y9tvk5q.pdf

Mode-locked pulses from mid-infrared quantum cascade lasers.

Free electron laser in Poland ? POLFEL is a coherent light source of the IV th generation, characterized by very short pulses in the range of 10-100fs, of big power 0,2GW and UV wavelength of 27nm of average power 1W, with effective high power third harmonic of 9nm. The laser consists of a linear superconducting accelerator 100m in length, undulator and experimental lines. It generates a monochromatic and coherent radiation and can be tuned from THz range to UV. The linac works in quasi-CW or real-CW mode. It is planned by XFEL-Poland consortium as a part of the ESFRI priority EuroFEL infrastructure collaboration network, part of the ERA

/pdf/polfel-free-electron-laser-in-poland-13km0no156.pdf

POLFEL - Free Electron Laser in Poland

Active mode locking in broadband quantum cascade (QC) lasers with a repetition rate of about 14.3 GHz has been achieved through the modulation of the laser bias current. At low driving currents, the active mode locking in broadband QC lasers resembles the active mode locking in single-wavelength QC lasers, while at high driving currents, the mode locking properties are governed by the broad spectral gain of these lasers. At high bias currents, the active modulation excites Fabry-Perot modes across the entire gain spectrum from 6.7 to 7.4 /spl mu/m, with clear evidence of mode locking. The spectral width of the optical gain in the broadband QC lasers exceeds 2 THz and indicates the potential for generating subpicosecond pulses.

Active mode locking of broadband quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

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Papers

Active mode locking of broadband quantum cascade lasers

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers