Juraj Darmo

Phase-resolved measurements of stimulated emission in a laser

Lasers are usually described by their output frequency and intensity. However, laser operation is an inherently nonlinear process. Knowledge about the dynamic behaviour of lasers is thus of great importance for detailed understanding of laser operation and for improvement in performance for applications. Of particular interest is the time domain within the coherence time of the optical transition. This time is determined by the oscillation period of the laser radiation and thus is very short. Rigorous quantum mechanical models predict interesting effects like quantum beats, lasing without inversion, and photon echo processes. As these models are based on quantum coherence and interference, knowledge of the phase within the optical cycle is of particular interest. Laser radiation has so far been measured using intensity detectors, which are sensitive to the square of the electric field. Therefore information about the sign and phase of the laser radiation is lost. Here we use an electro-optic detection scheme to measure the amplitude and phase of stimulated radiation, and correlate this radiation directly with an input probing pulse. We have applied this technique to semiconductor quantum cascade lasers, which are coherent sources operating at frequencies between the optical (>100 THz) and electronic (<0.5 THz) ranges. In addition to the phase information, we can also determine the spectral gain, the bias dependence of this gain, and obtain an insight into the evolution of the laser field.

Imaging with a Terahertz quantum cascade laser.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.

Metallic wave-impedance matching layers for broadband terahertz optical systems

We examine the potential of ultra-thin metallic layers for broadband wave-impedance matching in the terahertz frequency range. The metallic layer is modeled using Fresnel formulae for stratified optical medium. Experimental data for chromium and indium-tin-oxide layers, measured using time-domain terahertz spectroscopy over the frequency range 0.4 - 4.5 THz, are compared with theoretical results.

Terahertz meta-atoms coupled to a quantum well intersubband transition

We present a method of coupling free-space terahertz radiation to intersubband transitions in semiconductor quantum wells using an array of meta-atoms. Owing to the resonant nature of the interaction between metamaterial and incident light and the field enhancement in the vicinity of the metal structure, the coupling efficiency of this method is very high and the energy conversion ratio from in-plane to z field reaches values on the order of 50%. To identify the role of different aspects of this coupling, we have used a custom-made finite-difference time-domain code. The simulation results are supplemented by transmission measurements on modulation-doped GaAs/AlGaAs parabolic quantum wells which demonstrate efficient strong light-matter coupling between meta-atoms and intersubband transitions for normal incident electromagnetic waves.

Gain and losses in THz quantum cascade laser with metal-metal waveguide.

Coupling of broadband terahertz pulses into metal-metal terahertz quantum cascade lasers is presented. Mode matched terahertz transients are generated on the quantum cascade laser facet of subwavelength dimension. This method provides a full overlap of optical mode and active laser medium. A longitudinal optical-phonon depletion based active region design is investigated in a coupled cavity configuration. Modulation experiments reveal spectral gain and (broadband) losses. The observed gain shows high dynamic behavior when switching from loss to gain around threshold and is clamped at total laser losses.

Ultrafast probing of light-matter interaction in a midinfrared quantum cascade laser

In this work, we study the interaction of ultrashort midinfrared pulses with the active medium of an InGaAs∕InAlAs∕InP quantum cascade laser emitting at an 11.7μm wavelength. Applying an electro-optic sampling technique allowed us to measure the complete phase resolved transmission spectra at operating conditions below and above lasing threshold in a spectral range much broader than the gain band width. Far below threshold, we locate broadband resonant absorption, which spectrally overlaps with the electrically induced gain, forming areas of net absorption and net gain. Above threshold, gain clamping is seen, and it is found that echoes delayed by the round trip time experience spectral pulse shaping converging toward the emission spectrum.

Longitudinal spatial hole burning in terahertz quantum cascade lasers

The propagation of a quasi-single-cycle terahertz pulse through an active terahertz quantum cascade laser is studied. The terahertz pulse is found to be diffracted on the optically induced longitudinal modulation of the refractive index of the laser active region. This modulation is a result of longitudinal spatial hole burning due to the formation of a standing wave in the laser cavity.

Polarization of terahertz radiation from laser generated plasma filaments

An analysis of the polarization of terahertz (THz) radiation from a laser-induced plasma source is presented. THz emission is achieved by mixing a laser pulse with its second harmonic after focusing through a β-BaB2O4 (β-BBO) crystal. Numerical calculations, based on the nonlinear four-wave mixing model and the microscopic polarization model, are compared with experimental results. The main focus lies on the study of the dependence of THz polarization on the polarization and relative phase of the incident fundamental and second-harmonic pulses. We show that the modulation of the fundamental pulse by the BBO crystal has to be taken into account in order to describe experimental observations. By including the finite extension of the plasma and considering cross- and self-phase modulation of the two-color pump pulse, we are able to explain the observed ellipticity of the THz pulse as well as the orientation of the polarization axis.

Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser

The dynamics of optical gain in an InGaAs/AlInAs quantum cascade laser is studied in midinfrared pump-probe measurements. Pump and probe pulses of identical wavelength, polarization, and propagation direction through the laser waveguide are detected separately by electro-optic sampling. For injection currents below the lasing threshold, we observe an absorption decrease in the laser transition with a ≈3 ps recovery time. Above threshold, the gain strongly saturates and shows a fast recovery time decreasing with current. Such kinetics is superimposed by oscillations with a frequency of 0.8 THz originating from coherent electron tunneling through the injection barrier.

Short pulse generation and mode control of broadband terahertz quantum cascade lasers

We report on a waveguide engineering technique that enables the generation of a bandwidth up to 1 THz and record ultra-short pulse length of 2.5 ps in injection seeded terahertz quantum cascade lasers. The reported technique is able to control and fully suppress higher order lateral modes in broadband terahertz quantum cascade lasers by introducing side-absorbers to metal-metal waveguides. The side-absorbers consist of a top metalization set-back with respect to the laser ridge and an additional lossy metal layer. In continuous wave operation the side-absorbers lead to octave spanning laser emission, ranging from 1.63 to 3.37 THz, exhibiting a 725 GHz wide at top within a 10 dB intensity range as well as frequency comb operation with a bandwidth of 442 GHz. Numerical and experimental studies have been performed to optimize the impact of the side-absorbers on the emission properties and to determine the required increase of waveguide losses. Furthermore, these studies have led to a better understanding of the pulse formation dynamics of injection-seeded quantum cascade lasers.