Publications Project 09 -
Coupling and Cavity Interaction in Quantum Cascade Lasers and Detectors

Project Leader: Gottfried Strasser

Substrate-emitting ring interband cascade lasers

M. Holzbauer, R. Szedlak, H. Detz, R. Weih, S. Höfling, W. Schrenk, J. Koeth, and G. Strasser

  We demonstrate interband cascade lasers fabricated into ring-shaped cavities with vertical light emission through the substrate at a wavelength of λ ≈ 3.7 μm. The out-coupling mechanism is based on a metallized second-order distributed feedback grating. At room-temperature, a pulsed threshold current-density of 0.75 kA/cm2 and a temperature-tuning rate of 0.3 nm/°C are measured. In contrast to the azimuthal polarization of ring quantum cascade lasers, we observe a radial polarization of the projected nearfield of ring interband cascade lasers. These findings underline the fundamental physical difference between light generation in interband and intersubband cascade lasers, offering new perspectives for device integration.

Published: 2017-10-26
Appl. Phys. Lett. 111, 171101 (2017)
DOI: 10.1063/1.4989514

Surface emitting ring quantum cascade lasers for chemical sensing

R. Szedlak, J. Hayden, P. Martín-Mateos, M. Holzbauer, A. Harrer, B. Schwarz, B. Hinkov, D. MacFarland, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, P. Acedo, B. Lendl, and G. Strasser

  We review recent advances in chemical sensing applications based on surface emitting ring quantum cascade lasers (QCLs). Such lasers can be implemented in monolithically integrated on-chip laser/detector devices forming compact gas sensors, which are based on direct absorption spectroscopy according to the Beer–Lambert law. Furthermore, we present experimental results on radio frequency modulation up to 150 MHz of surface emitting ring QCLs. This technique provides detailed insight into the modulation characteristics of such lasers. The gained knowledge facilitates the utilization of ring QCLs in combination with spectroscopic techniques, such as heterodyne phase-sensitive dispersion spectroscopy for gas detection and analysis.

Published: 2017-09-01
Opt. Eng. 57(1), 011005 (2017)
DOI: 10.1117/1.OE.57.1.011005

Watt-Level Continuous-Wave Emission from a Bifunctional Quantum Cascade Laser/Detector

B. Schwarz, C. A. Wang, L. Missaggia, T. S. Mansuripur, P. Chevalier, M. K. Connors, D. McNulty, J. Cederberg, G. Strasser and F. Capasso

  Bifunctional active regions, capable of light generation and detection at the same wavelength, allow a straightforward realization of the integrated mid-infrared photonics for sensing applications. Here, we present a high performance bifunctional device for 8 μm capable of 1 W single facet continuous wave emission at 15 °C. Apart from the general performance benefits, this enables sensing techniques which rely on continuous wave operation, for example, heterodyne detection, to be realized within a monolithic platform and demonstrates that bifunctional operation can be realized at longer wavelength, where wavelength matching becomes increasingly difficult and that the price to be paid in terms of performance is negligible. In laser operation, the device has the same or higher efficiency compared to the best lattice-matched QCLs without same wavelength detection capability, which is only 30% below the record achieved with strained material at this wavelength.

Published: 2017-04-18
ACS Photonics, 2017, 4 (5), pp 1225–1231 (2017)
DOI: 10.1021/acsphotonics.7b00133

High-Power Growth-Robust InGaAsInAlAs Terahertz Quantum Cascade Lasers

Christoph Deutsch, Martin Alexander Kainz, Michael Krall, Martin Brandstetter, Dominic Bachmann, Sebastian Schönhuber, Hermann Detz, Tobias Zederbauer, Donald MacFarland, Aaron Maxwell Andrews, Werner Schrenk, Mattias Beck, Keita Ohtani, Jerome Faist, Gottfried Strasser, and Karl Unterrainer

  We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction. A structure based on a three-well optical phonon depletion scheme was optimized for this bias direction. Depending on the sheet doping density, the performance of this structure shows a trade-off between high maximum operating temperature and high output power. While the highest operating temperature of 155 K is observed for a moderate sheet doping density of 2×10^10cm−2, the highest peak output power of 151 mW is found for 7.3×10^10cm−2. Furthermore, by abutting a hyperhemispherical GaAs lens to a device with the highest doping level a record output power of 587 mW is achieved for double-metal waveguide structures.

Published: 2017-02-27
ACS Photonics 2017, 4, 957 - 962 (2017)
DOI: 10.1021/acsphotonics.7b00009
(--> also listed in Project 02)

Incorporation of Sb and As in MBE grown GaAsxSb1−x layers

T. Zederbauer, A. M. Andrews, D. McFarland, H. Detz, W. Schrenk and G. Strasser

  With the increasing interest in low effective mass materials for intersubband devices, mixed As-Sb compounds, like GaAsxSb1-x or AlxIn1-xAsySb1-y, gain more and more attention. The growth of these materials, however, still provides significant challenges due to the complex interaction between As and Sb. In this work, we provide an in-depth study on the incorporation of Sb into the GaAsxSb1-x layers and compare our findings to the present literature on this topic. It is found that both the composition and the crystal quality of GaAsxSb1-x layers are strongly influenced by the growth rate due to the As-for-Sb exchange reaction which takes place at the growing surface, and that high crystal quality can be achieved when the growth is performed under Sb limited conditions.

Published: 2017-01-13
APL Materials 5, 035501 (2017)
DOI: 10.1063/1.4973216

Far-Infrared Quantum Cascade Lasers Operating in the AlAs Phonon Reststrahlen Band

K. Ohtani, M. Beck, M. J. Süess, J. Faist, A. M. Andrews, T. Zederbauer, H. Detz, W. Schrenk and G. Strasser

  We report on the operation of a double metal waveguide far-infrared quantum cascade laser emitting at 28 μm, corresponding to the AlAs-like phonon reststrahlen band. To avoid absorption by AlAs-like optical phonons, the Al-free group V alloy GaAs0.51Sb0.49 is used as a barrier layer in the bound-to-continuum-based active region. Lasing occurs at a wavelength of 28.3 μm, which is the longest wavelength among the quantum cascade lasers operating from mid-infrared to far-infrared. The threshold current density at 50 K is 5.5 kA/cm2, and maximum operation temperature is 175 K. We also discuss the feasibility that the operation wavelength can cover the whole spectral range bridging between mid-infrared and terahertz by choosing properly suited group III–V materials.

Published: 2016-12-12
ACS Photonics, 2016, 3 (12), pp 2280–2284 (2016)
DOI: 10.1021/acsphotonics.6b00750

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 02)

Remote Sensing with Commutable Monolithic Laser and Detector

R. Szedlak, A. Harrer, M. Holzbauer, B. Schwarz, J. P. Waclawek, D. MacFarland, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, B. Lendl, and G. Strasser

  The ubiquitous trend toward miniaturized sensing systems demands novel concepts for compact and versatile spectroscopic tools. Conventional optical sensing setups include a light source, an analyte interaction region, and a separate external detector. We present a compact sensor providing room-temperature operation of monolithic surface-active lasers and detectors integrated on the same chip. The differentiation between emitter and detector is eliminated, which enables mutual commutation. Proof-of-principle gas measurements with a limit of detection below 400 ppm are demonstrated. This concept enables a crucial miniaturization of sensing devices.

Published: 2016-10-05
ACS Photonics 3 (10) (2016)
DOI: 10.1021/acsphotonics.6b00603

Press release TU Wien, 2016-10-20 [69/2016]

Measurement of bound states in the continuum by a detector embedded in a photonic crystal

Gansch, R., S. Kalchmair, P. Genevet, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, F. Capasso, M. Loncar, and G. Strasser

  We directly measure optical bound states in the continuum (BICs) by embedding a photodetector into a photonic crystal slab. The BICs observed in our experiment are the result of accidental phase matching between incident, reflected and in-plane waves at seemingly random wave vectors in the photonic band structure. Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory. Polarization mixing between photonic crystal slab modes was observed and described using a plane wave expansion simulation. The ability to probe the field intensity inside the photonic crystal and thereby to directly measure BICs represents a milestone in the development of integrated opto-electronic devices based on BICs.

Published: 2016-09-23
Light Sci. Appl. 5, e16147 (2016)
DOI: 10.1038/lsa.2016.147


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 02 & 10)


Advanced gas sensors based on substrate-integrated hollow waveguides and dual-color ring quantum cascade lasers

E. Tütüncü, V. Kokoric, R. Szedlak, D. MacFarland, T. Zederbauer, H. Detz, A. M. Andrews, W. Schrenk, G. Strasser and B. Mizaikoff

  This study shows the first combination of a ring-shaped vertically emitting quantum cascade laser (riQCL) providing two distinct emission wavelengths combined with a substrate-integrated hollow waveguide (iHWG). This ultra-compact riQCL-iHWG gas sensing device enables the simultaneous detection of two vapor phase species – here, furan and 2-methoxyethanol – providing distinctive absorption features at the emission wavelengths of the riQCL (i.e., 1144 and 1170 cm−1). Hence, multianalyte gas sensing via a unique mid-infrared (MIR) sensor concept is demonstrated.

Published: 2016-08-07
Analyst, Advance Article (2016)
DOI: 10.1039/C6AN01130F


4.3 μm quantum cascade detector in pixel configuration

A. Harrer, B. Schwarz, S. Schuler, P. Reininger, A. Wirthmüller, H. Detz, D. MacFarland, T. Zederbauer, A. M. Andrews, M. Rothermund, H. Oppermann, W. Schrenk, and G. Strasser

  We present the design simulation and characterization of a quantum cascade detector operating at 4.3μm wavelength. Array integration and packaging processes were investigated. The device operates in the 4.3μm CO2 absorption region and consists of 64 pixels. The detector is designed fully compatible to standard processing and material growth methods for scalability to large pixel counts. The detector design is optimized for a high device resistance at elevated temperatures. A QCD simulation model was enhanced for resistance and responsivity optimization. The substrate illuminated pixels utilize a two dimensional Au diffraction grating to couple the light to the active region. A single pixel responsivity of 16mA/W at room temperature with a specific detectivity D* of 5⋅10⁷ cm √Hz/W was measured.

Published: 2016-07-19
Optics Express Vol. 24, Nr. 15 (2016)
DOI: 10.1364/OE.24.017041


Enhanced Crystal Quality of AlxIn1-xAsySb1-y for Terahertz Quantum Cascade Lasers

Zederbauer, T., A.M. Andrews, D. MacFarland, H. Detz, W. Schrenk, and G. Strasser

  This work provides a detailed study on the growth of AlxIn1-xAsySb1-y lattice-matched to InAs by Molecular Beam Epitaxy. In order to find the conditions which lead to high crystal quality deep within the miscibility gap, AlxIn1-xAsySb1-y with x = 0.462 was grown at different growth temperatures as well as As2 and Sb2 beam equivalent pressures. The crystal quality of the grown layers was examined by high-resolution X-ray diffraction and atomic force microscopy. It was found that the incorporation of Sb into Al0.462In0.538AsySb1-y is strongly temperature-dependent and reduced growth temperatures are necessary in order to achieve significant Sb mole fractions in the grown layers. At 480 ∘C lattice matching to InAs could not be achieved. At 410 ∘C lattice matching was possible and high quality films of Al0.462In0.538AsySb1-y were obtained.

Published: 2016-04-20
Photonics 3, 20 (2016)
DOI: 10.3390/photonics3020020


Application of a ring cavity surface emitting quantum cascade laser (RCSE-QCL) on the measurement of H2S in a CH4 matrix for process analytics

Moser, H., A. Genner, J. Ofner, C. Schwarzer, G. Strasser, and B. Lendl

  The present work reports on the first application of a ring-cavity-surface-emitting quantum-cascade laser (RCSE-QCL) for sensitive gas measurements. RCSE-QCLs are promising candidates for optical gas-sensing due to their single-mode, mode-hop-free and narrow-band emission characteristics along with their broad spectral coverage. The time resolved down-chirp of the RCSE-QCL in the 1227-1236 cm−1 (8.15-8.09 µm) spectral range was investigated using a step-scan FT-IR spectrometer (Bruker Vertex 80v) with 2 ns time and 0.1 cm−1 spectral resolution. The pulse repetition rate was set between 20 and 200 kHz and the laser device was cooled to 15-17°C. Employing 300 ns pulses a spectrum of ~1.5 cm−1 could be recorded. Under these laser operation conditions and a gas pressure of 1000 mbar a limit of detection (3σ) of 1.5 ppmv for hydrogen sulfide (H2S) in nitrogen was achieved using a 100 m Herriott cell and a thermoelectric cooled MCT detector for absorption measurements. Using 3 µs long pulses enabled to further extend the spectral bandwidth to 8.5 cm−1. Based on this increased spectral coverage and employing reduced pressure conditions (50 mbar) multiple peaks of the target analyte H2S as well as methane (CH4) could be examined within one single pulse.

Published: 2016-03-16
Optics Express 24(6), 6572-6585 (2016)
DOI: 10.1364/OE.24.006572


Ring quantum cascade lasers with grating phase shifts and a light collimating dielectric metamaterial for enhanced infrared spectroscopy

R. Szedlak, M. Holzbauer, P. Reininger, D. MacFarland, T. Zederbauer, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser

  Brighter and more efficient light sources are desired to further increase the sensitivity of spectroscopic measurements. Simultaneously, a strong trend towards miniaturization and compactness is ubiquitous.
In the last years quantum cascade lasers have become powerful and reliable tools for infrared spectroscopy. We report on ring quantum cascade lasers with two distinct features which increase the efficiency of these devices and provide enhanced performance for spectroscopic applications. First, the distributed feedback grating exhibits two π-phase shifts. This provides far fields with a central intensity maximum. Second, a gradient index metamaterial is fabricated into the substrate. This collimates the emitted light and can replace external optics.

Published: 2016-03-09
Elsevier, "Vibrational Spectroscopy", ISSN: 0924-2031 (2016)
DOI: 10.1016/j.vibspec.2016.03.009


Mid-infrared surface transmitting and detecting quantum cascade device for gas-sensing

A. Harrer, R. Szedlak, B. Schwarz, H. Moser, T. Zederbauer, D. MacFarland, H. Detz, A.M. Andrews, W. Schrenk, B. Lendl, G. Strasser

  We present a bi-functional surface emitting and surface detecting mid-infrared device applicable for gas-sensing. A distributed feedback ring quantum cascade laser is monolithically integrated with a detector structured from a bi-functional material for same frequency lasing and detection. The emitted single mode radiation is collimated, back reflected by a flat mirror and detected by the detector element of the sensor. The surface operation mode combined with the low divergence emission of the ring quantum cascade laser enables for long analyte interaction regions spatially separated from the sample surface. The device enables for sensing of gaseous analytes which requires a relatively long interaction region. Our design is suitable for 2D array integration with multiple emission and detection frequencies. Proof of principle measurements with isobutane (2-methylpropane) and propane as gaseous analytes were conducted. Detectable concentration values of 0–70% for propane and 0–90% for isobutane were reached at a laser operation wavelength of 6.5 μm utilizing a 10 cm gas cell in double pass configuration.

Published: 2016-02-18
Nature, Scientific Reports 6, 21795 (2016)
DOI: 10.1038/srep21795


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 02)


The influence of whispering gallery modes on the far field of ring lasers

R. Szedlak, M. Holzbauer, D. MacFarland, T. Zederbauer, H. Detz, A.M. Andrews, C. Schwarzer, W. Schrenk, G. Strasser

  We introduce ring lasers with continuous π-phase shifts in the second order distributed feedback grating. This configuration facilitates insights into the nature of the modal outcoupling in an optical cavity. The grating exploits the asymmetry of whispering gallery modes and induces a rotation of the far field pattern. We find that this rotation can be connected to the location of the mode relative to the grating. Furthermore, the direction of rotation depends on the radial order of the whispering gallery mode. This enables a distinct identification and characterization of the mode by simple analysis of the emission beam.

Published: 2015-11-17
Nature, Scientific Reports 5, 16668 (2015)
DOI: 10.1038/srep16668


High performance bi-functional quantum cascade laser and detector

B. Schwarz, D. Ristanic, P. Reininger, T. Zederbauer, D. MacFarland, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser

  An improved bi-functional quantum cascade laser and detector emitting and detecting around 6:8 μm is demonstrated. The design allows a significantly higher laser performance, showing that bi-functional designs can achieve a comparable pulsed performance to conventional quantum cascade lasers. In particular, the device has a threshold current density of 3 kA=cm2, an output power of 0:47W, and a total wall-plug efficiency of 4.5% in pulsed mode. Optimized electron extraction and the prevention of thermal backfilling allow higher duty cycles, operation up to 10%, with 15mW average output power at room temperature without optimization of the laser cavity or coatings. At zero bias, the device has a responsivity of around 40mA=W and a noise equivalent power of 80pW/Hz0,5 at room temperature, which in on-chip configuration outperforms conventional uncooled discrete detectors. VC 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Published: 2015-08-17
Applied Physics Letters 107, 071104 (2015)
DOI: 10.1063/1.4927851


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 2016
DOI: 10.1109/JSTQE.2015.2419216
(--> also listed in Project 02)


Quantum cascade detector utilizing the diagonal-transition scheme for high cavities

P. Reininger, B. Schwarz, R. Gansch, H. Detz, D. MacFarland, T. Zederbauer, A.M. Andrews, W. Schrenk, G. Strasser

  A diagonal optically active transition in a quantum cascade detector is introduced as optimization parameter to obtain quality factor matching between a photodetector and a cavity. A more diagonal transition
yields both higher extraction efficiency and lower noise, while the reduction of the absorption strength is compensated by the resonant cavity. The theoretical limits of such a scheme are obtained, and the impact of losses and cavity processing variations are evaluated. By optimizing the quantum design for a high quality cavity, a specific detectivity of 109 Jones can be calculated for λ = 8μm and T = 300K.

Published: 2015-03-02
Optics Express, Vol. 23, issue 5, pp. 6283-6291 (2015)
DOI: 10.1364/OE.23.006283


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 02)


Monolithically integrated mid-infrared sensor using narrow mode operation and temperature feedback

D. Ristanic, B. Schwarz, P. Reininger, H. Detz, T. Zederbauer, A.M. Andrews, W. Schrenk, G. Strasser

  A method to improve the sensitivity and selectivity of a monolithically integrated mid-infrared sensor using a distributed feedback laser (DFB) is presented in this paper. The sensor is based on a quantum cascade laser/detector system built from the same epitaxial structure and with the same fabrication approach. The devices are connected via a dielectric-loaded surface plasmon polariton waveguide with a twofold function: it provides high light coupling efficiency and a strong interaction of the light with the environment (e.g., a surrounding fluid). The weakly coupled DFB quantum cascade laser emits narrow mode light with a FWHM of 2 cm1 at 1586 cm1. The room temperature laser threshold current density is 3 kA=cm2 and a pulsed output power of around 200 mW was measured. With the superior laser noise performance, due to narrow mode emission and the compensation of thermal fluctuations, the lower limit of detection was expanded by one order of magnitude to the 10 ppm range.

Published: 2015-01-26
Applied Physics Letters 106, 041101 (2015)
DOI: 10.1063/1.4906802


Plasmonic lens enhanced mid-infrared quantum cascade detector

A. Harrer, B. Schwarz, R. Gansch, P. Reininger, H. Detz, T. Zederbauer, A.M. Andrews, W. Schrenk, G. Strasser

  We demonstrate monolithic integrated quantum cascade detectors enhanced by plasmonic lenses. Surface normal incident mid-infrared radiation is coupled to surface plasmon polaritons guided to and detected by the active region of the detector. The lens extends the optical effective active area of the device up to a 5 times larger area than for standard mesa detectors or pixel devices while the electrical active region stays the same. The extended optical area increases the absorption efficiency of the presented device as well as the room temperature performance while it offers a flexible platform for various detector geometries. A photocurrent response increase at room temperature up to a factor of 6 was observed.

Published: 2014-10-30
Applied Physics Letters 105, 171112 (2014)
DOI: 10.1063/1.4901043


Diagonal-transition quantum cascade detector

P. Reininger, B. Schwarz, H. Detz, D. MacFarland, T. Zederbauer, A.M. Andrews, W. Schrenk, O. Baumgartner, H. Kosina, G. Strasser

  We demonstrate the concept of diagonal transitions for quantum cascade detectors (QCD). Different to standard, vertical QCDs, here the active transition takes place between two energy levels in adjacent wells. Such a scheme has versatile advantages. Diagonal transitions generally yield a higher extraction efficiency and a higher resistance than vertical transitions. This leads to an improved overall performance, although the absorption strength of the active transition is smaller. Since the extraction is not based on resonant tunneling, the design is more robust, with respect to deviations from the nominal structure. In a first approach, a peak responsivity of 16.9mA/W could be achieved, which is an improvement to the highest shown responsivity of a QCD for a wavelength of 8 lm at room-temperature by almost an order of magnitude.

Published: 2014-09-04
Applied Physics Letters 105, 091108 (2014)
DOI: 10.1063/1.4894767


Grating-based far field modifications of ring quantum cascade lasers

R. Szedlak, C. Schwarzer, T. Zederbauer, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser

  We present methods for beam modifications of ring quantum cascade lasers emitting around λ = 9 μm, which are based on novel distributed feedback grating designs. This includes the creation of a rotationally symmetric far field with a central intensity maximum using an off-center grating as well as the generation of partial radially polarized emission beams induced by a rotation of the grating slits.

Published: 2014-06-20
OSA - Optics Express, Vol. 22, Issue 13, pp. 15829-15836
DOI: 10.1364/OE.22.015829


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 02 and 10)

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


Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures

B. Schwarz, P. Reininger, D. Ristanic, H. Detz, A.M. Maxwell, W. Schrenk, G. Strasser

  The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. So far, all miniaturization concepts have been demonstrated with discrete optical components. Here we present a monolithically integrated sensor based on mid-infrared absorption spectroscopy. A bi-functional quantum cascade laser/detector is used, where, by changing the applied bias, the device switches between laser and detector operation. The interaction with chemicals in a liquid is resolved via a dielectric-loaded surface plasmon polariton waveguide. The thin dielectric layer enhances the confinement and enables efficient end-fire coupling from and to the laser and detector. The unamplified detector signal shows a slope of 1.8–7 μV per p.p.m., which demonstrates the capability to reach p.p.m. accuracy over a wide range of concentrations (0–60%). Without any hybrid integration or subwavelength patterning, our approach allows a straightforward and cost-saving fabrication.

Published: 2014-06-06
Nature Communications, Article number: 4085
DOI: 10.1038/ncomms5085

Press release TU Wien, 2014-06-11 (61/2014)


On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers

R. Szedlak, C. Schwarzer, T. Zederbauer, H. Detz, A.M. Andrews, W. Schrenk, G. Strasser

  We report on collimated emission beams from substrate emitting ring quantum cascade lasers with an on-chip focusing element fabricated into the bottom side of the device. It is formed by a gradient index metamaterial layer, realized by etching subwavelength holes into the substrate. The generated optical path length difference for rays emitted under different angles from the ring waveguide flattens the wavefront and focuses the light. Our far field measurements show an increased peak intensity corresponding to 617% of the initial value without the focusing element. Far field calculations, based on a Fourier transformation of the metamaterial area, are in good agreement with our experimental data.

Published: 2014-04-15
AIP Publishing - Applied Physics Letters 104, 151105
DOI: 10.1063/1.4871520


Time-resolved spectral characterization of ring cavity surface emitting and ridge-type distributed feedback quantum cascade lasers by step-scan FT-IR spectroscopy

M. Brandstetter, A. Genner, C. Schwarzer, E. Mujagic, G. Strasser, B. Lendl

  We present the time-resolved comparison of pulsed 2nd order ring cavity surface emitting (RCSE) quantum cascade lasers (QCLs) and pulsed 1st order ridge-type distributed feedback (DFB) QCLs using a stepscan Fourier transform infrared (FT-IR) spectrometer. Laser devices were part of QCL arrays and fabricated from the same laser material. Required grating periods were adjusted to account for the grating order. The stepscan technique provided a spectral resolution of 0.1 cm<sup>−1</sup> and a time resolution of 2&nbsp;ns. As a result, it was possible to gain information about the tuning behavior and potential mode-hops of the investigated lasers. Different cavity-lengths were compared, including 0.9&nbsp;mm and 3.2&nbsp;mm long ridge-type and 0.97&nbsp;mm (circumference) ring-type cavities. RCSE QCLs were found to have improved emission properties in terms of linestability, tuning rate and maximum emission time compared to ridge-type lasers.

Published: 2014-01-30
OSA - Optics Express, Vol. 22, Issue 3, pp. 2656-2664
DOI: 10.1364/OE.22.002656


Linearly polarized light from substrate emitting ring cavity quantum cascade lasers

C. Schwarzer, R. Szedlak, S. Ahn, T. Zederbauer, H. Detz

  The authors report on substrate emitting ring cavity quantum cascade lasers that feature linearly polarized emission beams at an emission wavelength of 8.8 μm. A central lobed far field is achieved by a modification of the distributed feedback grating, at which two p phase-shifts at an angular distance of 180º are applied. In this central lobe, 80% linear polarization is measured. In order to extend this polarization property to the whole far field, an on-chip wire grid polarizer is used. These devices show linearly polarized substrate emission with an extinction ratio higher than 1:16.

Published: 2013-08-19
AIP Publishing - Applied Physics Letters 103, 081101
DOI: 10.1063/1.4819034







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