Publications Project 02 -
THz Synthesis and Interaction

Project Leader: Karl Unterrainer

Dispersion in a broadband terahertz quantum cascade laser

D. Bachmann, M. Rösch, G. Scalari, M. Beck, J. Faist, K. Unterrainer and J. Darmo

  We present dispersion data of a broadband terahertz quantum cascade laser with a heterogeneous active region. The experimental method to extract the group velocity dispersion of the entire laser cavity, including the contributions of the active region, the semiconductor material, and the waveguide relies on a time-domain spectroscopy system. The obtained group velocity dispersion curves exhibit oscillations with amplitudes up to 1 × 105 fs2/mm between 2.0 and 3.0 THz and strongly depend on the driving conditions of the laser. This indicates that the group velocity dispersion is mainly determined by the intersubband gain in the active region. The obtained dispersion data are compared to a dispersion model based on multiple Drude-Lorentz gain media yielding a significant correlation.

Published: 2016-12-01
Applied Physics Letters 109, 221107 (2016)
DOI: 10.1063/1.4969065

Spectrally resolved far-fields of terahertz quantum cascade lasers

M. Brandstetter, S. Schönhuber, M. Krall, M. A. Kainz, H. Detz, T. Zederbauer, A. M. Andrews, G. Strasser, and K. Unterrainer

  We demonstrate a convenient and fast method to measure the spectrally resolved far-fields of multimode terahertz quantum cascade lasers by combining a microbolometer focal plane array with an FTIR spectrometer. Far-fields of fundamental TM0 and higher lateral order TM1 modes of multimode Fabry-Pérot type lasers have been distinguished, which very well fit to the results obtained by a 3D finite-element simulation. Furthermore, multimode random laser cavities have been investigated, analyzing the contribution of each single laser mode to the total far-field. The presented method is thus an important tool to gain in-depth knowledge of the emission properties of multimode laser cavities at terahertz frequencies, which become increasingly important for future sensing applications.

Published: 2016-10-21
Optics Express, Vol. 24, No. 22, 25462-25470 (2016)
DOI: 10.1364/OE.24.025462
(--> also listed in Project 09)

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

D. Bachmann, M. Rösch, M. J. Süess, M. Beck, K. Unterrainer, J. Darmo, J. Faist, and G. Scalari

  Ultra-short pulses are an attractive way of expanding today’s terahertz time-domain systems toward frequencies above 2 THz, and moreover mode control enables reliable generation of terahertz frequency combs based on quantum cascade lasers. We report on a waveguide engineering technique that enables the generation of a bandwidth up to ∼1  THz and an 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 metallization setback 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 flat 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.

Published: 2016-09-22
Optica Vol. 3, Issue 10 (2016)
DOI: 10.1364/OPTICA.3.001087

Press release TU Wien, 2016-09-26 [65/2016]

Random lasers for broadband directional emission

S. Schönhuber, M. Brandstetter, T. Hisch, C. Deutsch, M. Krall, H. Detz, A. M. Andrews, G. Strasser, S. Rotter, and K. Unterrainer

  Broadband coherent light sources are becoming increasingly important for sensing and spectroscopic applications, especially in the mid-infrared and terahertz (THz) spectral regions, where the unique absorption characteristics of a whole host of molecules are located. The desire to miniaturize such light emitters has recently led to spectacular advances, with compact on-chip lasers that cover both of these spectral regions. The long wavelength and small size of the sources result in a strongly diverging laser beam that is difficult to focus on the target that one aims to perform spectroscopy with. Here, we introduce an unconventional solution to this vexing problem, relying on a random laser to produce coherent broadband THz radiation as well as an almost diffraction-limited far-field emission profile. Our random lasers do not require any fine tuning and thus constitute a promising example of practical device applications for random lasing.

Published: 2016-09-21
Optica 1035, Vol. 3, No. 10 (2016)
DOI: 10.1364/OPTICA.3.001035
(--> also listed in Project 09 & 10)

InAs based terahertz quantum cascade lasers

M. Brandstetter, M. Kainz, T. Zederbauer, M. Krall, S. Schönhuber, H. Detz, W. Schrenk, A.M. Andrews, G. Strasser, K. Unterrainer

  We demonstrate terahertz lasing emission from a quantum cascade structure, realized with InAs/AlAsSbheterostructures. Due to the lower effective electron mass, InAs based active regions are expected to provide a higher optical gain compared to structures consisting of GaAs or InGaAs. The growth by molecular beam epitaxy enabled the fabrication of monolayer-thick barriers, required for the active region, which is based on a 3-well resonantphonon depletion design. Devices were processed in a double-metal waveguide geometry to ensure high mode confinement and low optical losses. Lasing emission at 3.8 THz was observed at liquid helium temperatures by applying a magnetic field perpendicular to the layered structure in order to suppress parasitic scattering channels. These results demonstrate the feasibility of InAs based active regions for terahertz quantum cascade lasers, potentially enabling higher operating temperatures.

Published: 2016-01-05
Applied Physics Letters 108, 011109 (2016)
DOI: 10.1063/1.4939551
(--> also listed in Project 09)


CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts

T. Balčiūnas, D. Lorenc, M. Ivanov, O. Smirnova, A.M. Zheltikov, D. Dietze, K. Unterrainer, R. Rathje, G.G. Paulus, A. Baltuška, S. Haessler

  We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Published: 2015-06-02
Optics Express 15278, Vol. 23, No. 12 (2015)
DOI: 10.1364/OE.23.015278
(--> also listed in Project 03)


From Photonic Crystal to Subwavelength Micropillar Array Terahertz Lasers

M. Krall, M. Brandstetter, C. Deutsch, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser, K. Unterrainer

  We investigate terahertz quantum cascade lasers with a dense array of active micropillars forming the gain medium. Depending on the size of these pillars relative to the emission wavelength different optical regimes are identified. For pillar dimensions on the order of the emission wavelength, a photonic crystal resonator is created. Singlemode emission is observed at high symmetry points of the photonic band structure. The selection mechanism of the favored laser mode is studied by analyzing the gain enhancement effect for eigenmodes with a low group velocity and a large mode confinement of the electric field energy in the micropillars. Subwavelengthmicropillar arrays constitute a photonic metamaterial, which can be described using an effective medium approximation. Similar to a bulk laser ridge, the array forms a Fabry–Pérot resonator that is defined by the boundaries of the array. From the longitudinal mode spacing, we derive an effective group index of the pillarmedium. Limitations in terms of minimum
filling factors and waveguide losses for the realization of subwavelength micropillar and nanowire array terahertz quantum cascade lasers are investigated.

Published: 2015-04-02
IEEE Journal of Selected Topics in Quantum Electronics, Vol. 21, No.6, Nov/Dec 2015
DOI: 10.1109/JSTQE.2015.2419216
(--> also listed in Project 09)


Coupled cavity terahertz quantum cascade lasers with integrated emission monitoring

M. Krall, M. Martl, D. Bachmann, C. Deutsch, A.M. Andrews, W. Schrenk, G. Strasser, K. Unterrainer

  We demonstrate the on-chip generation and detection of terahertz radiation in coupled cavity systems using a single semiconductor heterostructure. Multiple sections of a terahertz quantum cascade laser structure in a double-metal waveguide are optically coupled and operate either as a laser or an integrated emission monitor. A detailed analysis of the photon-assisted carrier transport in the active region below threshold reveals the detection mechanism for photons emitted by the very same structure above threshold. Configurations with a single laser cavity and two coupled laser cavities are studied. It is shown that the integrated detector can be used for spatial sensing of the light intensity within a coupled cavity.

Published: 2015-02-05
Opics Express 3581, Vol. 23, No. 3 (2015)
DOI: 10.1364/OE.23.003581
(--> also listed in Project 09)


Broadband terahertz amplification in a heterogeneous quantum cascade laser

D. Bachmann, N. Leder, M. Rösch, G. Scalari, M. Beck, H. Arthaber, J. Faist, K. Unterrainer, J. Darmo

  We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging subnanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.

Published: 2015-02-02
Optics Express, Vol. 23, No. 3, pp. 3117-3125 (2015)
DOI: 10.1364/OE.23.003117


Spectral gain profile of a multi-stack terahertz quantum cascade laser

D. Bachmann, M. Rösch, C. Deutsch, M. Krall, G. Scalari, M. Beck, J. Faist, K. Unterrainer, J. Darmo

  The spectral gain of a multi-stack terahertz quantum cascade laser, composed of three active regions with emission frequencies centered at 2.3, 2.7, and 3.0 THz, is studied as a function of driving current and temperature using terahertz time-domain spectroscopy. The optical gain associated with the particular quantum cascade stacks clamps at different driving currents and saturates to different values. We attribute these observations to varying pumping efficiencies of the respective upper laser states and to frequency dependent optical losses. The multi-stack active region exhibits a spectral gain full width at half-maximum of 1.1 THz. Bandwidth and spectral position of the measured gain match with the broadband laser emission. As the laser action ceases with increasing operating temperature, the gain at the dominant lasing frequency of 2.65 THz degrades sharply.

Published: 2014-11-07
Appl. Phys. Lett. 105, 181118 (2014)
DOI: 10.1063/1.4901316


Reversing the pump dependence of a laser at an exceptional point

M. Brandstetter, M. Liertzer, C. Deutsch, P. Klang, J. Schöberl, H.E. Türeci, G. Strasser, K. Unterrainer, S. Rotter

  When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of nontrivial physics in a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.

Published: 2014-06-03
Nature Communications 5, Article number: 4034
DOI: 10.1038/ncomms5034
(--> also listed in Project 09 and 10)

Press release TU Wien, 2014-06-17 [63/2014]







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