Bow-Tie Cavity for Terahertz Radiation

We report on the development, testing, and performance analysis of a bow-tie resonant cavity for terahertz (THz) radiation, injected with a continuous-wave 2.55 THz quantum cascade laser. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F=123, and a quality factor Q = 5.1x10^5. The resonator performances and the relevant parameters are theoretically predicted and discussed, and a comparison among simulated and experimental spectra is given

Spettroscopia e metrologia delle frequenze con la radiazione terahertz

A spectroscopic technique in the terahertz range with a very high frequency resolution has been developed that makes it possible to measure the frequency of molecular transitions with an accuracy of 10-9. This system can also be employed to perform spectroscopic measurements in saturation regime, with a further gain of two orders of magnitude in terms of accuracy.È stata sviluppata una tecnica di spettroscopia nella gamma terahertz con un’altissima risoluzione in frequenza, che ha permesso di misurare la frequenza di transizioni molecolari con un’accuratezza di 10-9. Tale sistema si presta inoltre alla realizzazione di misure spettroscopiche in regime di saturazione, con un ulteriore guadagno di due ordini di grandezza in termini accuratezza

Real-time terahertz digital holography with a quantum cascade laser

Coherent imaging in the THz range promises to exploit the peculiar capabilities of these wavelengths to penetrate common materials like plastics, ceramics, paper or clothes with potential breakthroughs in non-destructive inspection and quality control, homeland security and biomedical applications. Up to now, however, THz coherent imaging has been limited by time-consuming raster scanning, point-like detection schemes and by the lack of adequate coherent sources. Here, we demonstrate real-time digital holography (DH) at THz frequencies exploiting the high spectral purity and the mW output power of a quantum cascade laser combined with the high sensitivity and resolution of a microbolometric array. We show that, in a one-shot exposure, phase and amplitude information of whole samples, either in reflection or in transmission, can be recorded. Furthermore, a 200 times reduced sensitivity to mechanical vibrations and a significantly enlarged field of view are observed, as compared to DH in the visible range. These properties of THz DH enable unprecedented holographic recording of real world dynamic scenes

Retrieval of phase relation and emission profile of quantum cascade laser frequency combs

The major development recently undergone by quantum cascade lasers has effectively extended frequency comb emission to longer-wavelength spectral regions, i.e. the mid and far infrared. Unlike classical pulsed frequency combs, their mode-locking mechanism relies on four-wave mixing nonlinear processes, with a temporal intensity profile different from conventional short-pulses trains. Measuring the absolute phase pattern of the modes in these combs enables a thorough characterization of the onset of mode-locking in absence of short-pulses emission, as well as of the coherence properties. Here, by combining dual-comb multi-heterodyne detection with Fourier-transform analysis, we show how to simultaneously acquire and monitor over a wide range of timescales the phase pattern of a generic frequency comb. The technique is applied to characterize a mid-infrared and a terahertz quantum cascade laser frequency comb, conclusively proving the high degree of coherence and the remarkable long-term stability of these sources. Moreover, the technique allows also the reconstruction of electric field, intensity profile and instantaneous frequency of the emission.Comment: 20 pages. Submitted to Nature Photonic

Waveguided Approach for Difference Frequency Generation of Broadly-Tunable Continuous-Wave Terahertz Radiation

The 1–10 terahertz (THz) spectral window is emerging as a key region for plenty of applications, requiring not yet available continuous-wave room-temperature THz spectrometers with high spectral purity and ultra-broad tunability. In this regard, the spectral features of stabilized telecom sources can actually be transferred to the THz range by difference frequency generation, considering that the width of the accessible THz spectrum generally scales with the area involved in the nonlinear interaction. For this reason, in this paper we extensively discuss the role of Lithium Niobate (LN) channel-waveguides in the experimental accomplishment of a room-temperature continuous wave (CW) spectrometer, with μW-range power levels and a spectral coverage of up to 7.5 THz. To this purpose, and looking for further improvements, a thought characterization of specially-designed LN waveguides is presented, whilst discussing its nonlinear efficiency and its unprecedented capability to handle high optical power (107 W/cm2), on the basis of a three-wave-mixing theoretical model

THz QCL-Based Cryogen-Free Spectrometer for in Situ Trace Gas Sensing

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