Ileana-Cristina Benea-Chelmus

Subcycle measurement of intensity correlations in the terahertz frequency range

The terahertz frequency range is lacking an experimental implementation of Hanbury Brown and Twiss photon correlation measurements with sub-cycle time resolution and high sensitivity. Such a technique would be needed, for example, to observe photon pairs predicted to be released in a nonadiabatic modulation of an ultrastrongly coupled light-matter system. In this paper, we propose a room-temperature measurement of photon correlations in the THz range based on electro-optic sampling. We apply this technique to a THz quantum cascade laser and measure below and above threshold first- and second-order degree of coherence with a subcycle temporal resolution of 146 fs. The sensitivity of the proposed measurement scheme is so far limited to

High-sensitivity intensity correlation measurements for photon statistics at terahertz frequencies

\ensuremath{\sim}1500

Broadband monolithic extractor for metal-metal waveguide based terahertz quantum cascade laser frequency combs

photons. This technique can, in principle, be extended for ultrahigh bandwidth single-photon sensitivity in a wide range of frequencies.

High performance 4.7 THz GaAs quantum cascade lasers based on four quantum wells

Recently, intensity correlation measurements have been reported for the first time in the Terahertz range, where a time-domain version of a Hanbury Brown Twiss setup based on electro-optic sampling was employed. This technique proved its usefulness for fundamental studies of photon correlations of bunched (thermal) and Poissonian (coherent) light, but not only so. Also in practical applications, it has been employed to determine the temporal emission pattern of Terahertz Quantum Cascade Laser based Frequency Combs, which are very promising devices for future highly integrated spectrometers. The key parameter of this technique is its short temporal resolution. Up to date, the technique still does not provide the necessary sensitivity for exploring the yet vacuous regime of single photons in the terahertz. In this work we present our recent efforts for increasing the sensitivity of electro-optic sampling, by means of cryogenic cooling and novel organic materials for the Terahertz range. In particular, we present a novel device for collinear electro-optic detection, which features a high-aspect ratio antenna on a quartz substrate with a plasmonic gap filled by electro-optic molecules.

Broadband monolithic extractors for terahertz quantum cascade laser based frequency combs (Conference Presentation)

We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 400 GHz around a center frequency of 2.5 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50% of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved.

Electric field and intensity correlations of a terahertz comb based on fast electro-optic sampling (Conference Presentation)

GaAs/AlGaAs quantum cascade lasers based on four quantum well structures operating at 4.7 THz are reported. A large current density dynamic range is observed, leading to a maximum operation temperature of 150 K for the double metal waveguide device and a high peak output power more than 200 mW for the single surface plasmon waveguide device. A continuous wave, single mode, third order distributed feedback laser with a low electrical power dissipation and a narrow far-field beam pattern, which is required for a local oscillator in astronomy heterodyne spectrometers, is also demonstrated.

Intensity autocorrelation measurements of frequency combs in the terahertz range

Recent work has been showing the possibility of generating frequency combs at terahertz frequencies using terahertz quantum cascade lasers. The main efforts so far were on getting the laser to work in a stable comb operation over an as broad as possible spectral bandwidth. Another issue is the scattered farfield of such combs due to their subwavelength facets of the used metal-metal waveguide. In contrast to single mode lasers the monolithic approaches of distributed feedback lasers or photonic crystals cannot be used. We present here a monolithic broadband extractor compatible with frequency comb operation based on the concept of an end-fire antenna. The antenna can be fabricated using standard fabrication techniques. It has been designed to support a bandwidth of up to 600 GHz at a central frequency of 2.5 THz. The fabricated devices show single lobed farfields with only minor asymmetries, increased output power along an increased dynamical range of frequency comb operation. A side-absorber schematics using a thin film of Nickel has been used to suppress any higher-order lateral modes in the laser. The reported frequency combs with monolithic extractors are ideal candidates for spectroscopic applications at terahertz frequencies using a self-detected dual-comb spectroscopy setup due to the increased dynamical range along with the improved farfield leading to more output power of the frequency combs.

Three-Dimensional Phase Modulator at Telecom Wavelength Acting as a Terahertz Detector with an Electro-Optic Bandwidth of 1.25 Terahertz

Only recently, a novel type of intensity autocorrelator in the time-domain has been reported for the Terahertz frequency range. The technique is based on fast electro-optic sampling in a double beam configuration and its temporal resolution is ultra-fast, as short as only few hundreds of femtoseconds. In particular, the self-referencing character of the technique is suitable for any type of source, including free-running sources. These unique characteristics enable therefore the investigation of the output profile of Terhertz Quantum Cascade Laser based Frequency Combs, with typical roundtrip times of few tens of picoseconds. The output dynamics of such devices have been investigated theoretically by Maxwell-Bloch equations and experimentally using Shifted Wave Interference Fourier Transform Spectroscopy. In this work, we present the results of the direct measurement of intensity autocorrelations of a Terahertz comb around 2.5 THz, when operated in the comb and high-noise regime, with radio-frequency beatnotes of 800 Hz and few MHz, respectively. We find the laser to be both amplitude- and frequency-modulated in both regimes, with a modulation ratio of the intensity of roughly 90 percent.The technique might come to use in future for the measurement of free-running pulses at Terahertz frequencies with high temporal resolution.

Few photons correlation measurement of a thermally populated cavity

We report on the first direct measurement of the emission character of quantum cascade laser based frequency combs, using intensity autocorrelation. The correlation technique is based on fast electro-optic sampling, with a bandwidth optimized to match the emission spectra of the comb laser, centered around 2.5 THz. We find the output light to be amplitude and frequency modulated at the same time, with intensity modulation depth as high as 90%, occuring on timescales as short as few picoseconds. We compare these findings with intensity autocorrelation measurements of pulsed Terahertz light originating from a photoconductive antenna and find very different results. We observe no significant difference for the comb and dispersed regime. The technique will be of significant importance in future for the measurement of ultra-short pulses from quantum cascade lasers.