We report the room-temperature frequency response, in the range 0-220GHz, of GaAs-based QWIP photodetectors operating at 10um. Detectors rely on 2D arrays of patch-antennas, connected to an integrated 50-Ohm coplanar line allowing on-wafer characterization. By difference frequency mixing of two QCLs, we obtain a RF 3dB bandwidth of ~90GHz. The frequency response of devices based on 4 antennas is compatible with a carrier capture-time of ~2.5ps. By replacing the coplanar line with a log-spiral antenna we also demonstrate devices radiating directly in free-space. The perspective of exploiting the latter as QCL-pumped photomixers for the generation of microwave/sub-mm radiation is discussed.
The room temperature detection of terahertz (THz) waves remains a challenging task, especially for detectors operating at room temperature. Among the solutions that have been proposed, thermal detectors based on bilayer membranes combined with metamaterial absorbers provide competitive sensitivity and detection speed in the kHz range. We have recently proposed a new device concept, where the photo-thermal interactions take place in a single meta-atom resonator with subwavelength dimensions. Thanks to the compactness of our device, the thermal diffusion time is reduced down to the µs range and the detector speed is increased beyond 10MHz, with 1GHz at reach. I will discuss experimental measurements and dynamical modelling of the detector performance.
F. Simoens, J. Meilhan, B. Delplanque, S. Gidon, G. Lasfargues, J. Lalanne Dera, D. T. Nguyen, J. L. Ouvrier-Buffet, S. Pocas, T. Maillou, O. Cathabard, S. Barbieri
Terahertz uncooled antenna-coupled microbolometer focal plane arrays are being developed at CEA-LETI for THz
imaging and sensing. This detector technology relies on amorphous silicon bolometer know-how and aims at opening the
way to real-time video rate 2D imaging, with potential low cost either in fabrication and in operation. First prototypes of
320x240 pixel arrays have been designed for 1-3 THz sensing. Sensors have been fabricated monolithically above
CMOS Integrated Circuits while applying only full Si standard silicon processes. We present the results of extensive
work of characterization and simulations made to estimate the sensitivity and spectral absorption of these prototypes.
Tests of latest real-time imaging with active illumination by QCLs are then reported while explosives samples were
placed in an optical set-up in reflection configuration.
Spectral signatures of solid materials in the THz range can provide spectroscopic information for chemical identification.
Previously we have demonstrated the absorption coefficient extraction by scanned imaging of QCL THz beams
attenuated through explosive samples. The detection was achieved by a unique pixel addressed within an uncooled
antenna-coupled microbolometer 160x120 array specifically designed for the 1-5 THz range. This detector technology
developed at CEA-LETI relies on amorphous silicon bolometer know-how and aims at opening the way to real-time
video rate, with potential low cost.
We report complementary tests of imaging in reflection configuration and the first tests of a second prototype where
320x240 bolometers are monolithically processed above a CMOS read-out circuit.
Terahertz (THz) time domain spectroscopy (TDS) is widely used in a broad range of applications where knowledge of
both the amplitude and phase of a THz wave can reveal useful information about a sample. However, a means of
amplifying THz pulses which would be of great benefit for improving the applicability of TDS is lacking. While THz
quantum cascade lasers (QCL) are promising devices for THz amplification, gain clamping limits the attainable
amplification. Here we circumvent gain clamping and demonstrate amplification of THz pulses by ultrafast gain
switching of a QCL via the use of an integrated Auston switch. This unclamps the gain by placing the laser in a non-equilibrium
state that allows large amplification of the electromagnetic field within the cavity. This technique offers the
potential to produce high field THz pulses that approach the QCL saturation field.
Integrated terahertz (THz) pulse generation and amplification in a THz quantum cascade laser (QCL) is demonstrated.
Intra-cavity THz pulses are generated by exciting the facet of the quantum cascade laser with an ultrafast Ti:Sapphire
laser (~100fs) and detected using electro-optic sampling. Maximum THz field emission is found with an interband
transition of 1.535eV (809nm) and by narrowing the excitation laser bandwidth to ~3THz. These resonance conditions
correspond to the narrowband excitation of the quantum cascade miniband, indicating that the THz pulse is generated by
the photo-excited carriers that are accelerated by the applied field. The generated pulse is subsequently amplified by the
narrowband gain of the laser as it propagates through the QCL cavity. As an integrated THz generator-amplifier, the
technique avoids the issues associated with the coupling of external THz pulses into sub-wavelength dimensioned
cavities.
Uncooled antenna-coupled microbolometer focal plane arrays have been specifically tailored for optimum performance
in the 1-5 Terahertz range. A prototyping batch of 160 × 120 pixel chips has been designed and then processed above 8"
silicon substrates. An actively illuminated system has been experimentally tested where Quantum Cascade Lasers
(QCLs) are associated with the room-temperature operating 2D sensor. Whereas explosives samples were introduced in
the THz beam optical path, the profile of the modified beam has been sensed by a unique pixel translated via an X-Y
stage. These represent the first demonstration essays of explosive identification using our system.
The authors present a home made cryogenic electro-optical probe station allowing the direct modulation of
quantum cascade lasers up to 40GHz. Based on a QMC cryostat, it should make the QCL bandwidth measure
possible and then help answering questions about the modulation possibilities of such a kind of laser. The
experimental results will be compared to simulation bandwidth prediction based on a complete set of rate
equations describing the dynamic behavior of the laser. Bandwidth will be then linked to the different intrinsic
and structural parameters.
This paper presents a CW raster-scanning THz imaging setup, used to perform Non-Destructive Testing of KevlarTMand carbon fibre samples. The setup uses a 2.5 THz Quantum Cascade Laser as a source. Delamination defect in a Kevlar sample was detected showing a sensitivity to laser polarization orientation. Detection of a break in a carbon/epoxy sample was also performed.
At Terahertz (THz) frequencies metals are still excellent materials to guide and confine electromagnetic
radiation with relatively low losses. Therefore the concepts developed in the microwave range to design efficient
waveguides and resonators can be successfully transferred up to this frequency region. A successful example of such
"technology transfer" is the so-called metal-metal resonator, effectively used as a waveguide for THz Quantum Cascade
Lasers (QCLs). This type of resonator is essentially a downscaled version of a microstrip waveguide, widely used at
microwave frequencies. In this work we report on microwave impedance measurements of metal-metal ridge-waveguide
THz QCLs. Experimental data, recorded at 4K in the 100MHz-55GHz range, are well reproduced by distributed-parameter
transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that
increase for higher microwave frequencies, yielding a 3dB modulation bandwidth of ~70GHz for a 1mm-long ridge. By
using a shunt-stub matching we demonstrate amplitude modulation of a 2.3THz QCL up to 24GHz. In the last part of this
work we discuss the experimental evidence of a feedback-coupling between the intracavity THz field and the microwave
field generated by the beating of the Fabry-Perot longitudinal modes above the lasing threshold.
Portability, low cost and fast acquisition rates are key features that a THz imaging system should satisfy for extended
commercialized applications. With regards to these features, the source - detector association of a THz Quantum
Cascade Laser (QCL) with an un-cooled micro-bolometer two-dimensional array looks promising for THz active
imaging. QCLs performance is rapidly improving, with higher operating temperatures and output powers recently
demonstrated. On the detector side, un-cooled micro-bolometer array opens the way to real-time video rate, with no
raster scanning and potential low cost.
In parallel to the development of room temperature micro-bolometer sensors specifically designed for the THz range, the
authors have characterized experimentally the sensitivity of CEA-LETI standard amorphous Silicon infrared microbolometers
illuminated by a 3THz QCL. The sensitivity of these existing sensors is then compared to the expected
sensitivity of the CEA-LETI upcoming THz sensors.
We show how metallic waveguides offer the opportunity of implementing interesting functionalities for semiconductor
lasers within a simple technological approach. In the THz, we show that the active region thickness of quantum cascade
lasers can be reduced by a factor of 2 without effects on the threshold current density and maximum operating
temperature of the laser. Pulsed and continuous-wave operation - with a low threshold Jth= 71 A/cm2 - are obtained for a 5.86-μm-thick THz QC laser. The emission is peaked at λ≈115 μm and the waveguide resonator is based on a metal-metal geometry. In the mid-infrared, we demonstrate surface-plasmon distributed-feedback quantum cascade lasers with
a first-order grating realised by the sole patterning the top metallic contact. The devices have a single mode emission
with a side-mode suppression ratio greater than 20dB. The emission wavelength at 78K is centred at λ = 7.3 μm and has
tuning rate as a function of the temperature of ≈0.4 nm/K.
We report on the fabrication of THz quantum cascade lasers (QCLs) based on different optical waveguide
configurations and compare the thermal properties of THz devices fabricated with metal-metal optical waveguides based
on Au/Au or In/Au wafer bonding. In particular, we show how the careful choice of the metal sequence used for the
reactive bonding may lead to a considerable improvement of the device thermal performance. This information was
obtained from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in
continuous wave (cw). The experimental normalized thermal resistances (RL*), show that the use of Au/Au wafer
bonding optimizes the heat dissipation. An extensive comparison with a set of surface-plasmon based THz QCLs,
demonstrate that the use of metal-metal wafer bonding can allow cw operation at progressively higher temperatures.
Finally, we present the experimental results obtained on a bound-to-continuum QCLs (2.84 THz) emitting 77 mW peak
power at 4K, fabricated from an MBE wafers acquired by a commercial provider.
We report the operation of a 2 THz quantum cascade laser based on a GaAs/Al0.1Ga0.9As heterostructure. Lasing action takes place between an isolated subband and the upper state of a 14 meV wide miniband. In pulsed mode, with a 3.16mm long device, we report a threshold current density of 115 A/cm2 at T = 4K, with a maximum measured peak power of 50 mW. The device shows lasing action in continuous wave up to 47K, with a maximum power in excess of 15 mW at T = 4K.
In this work we demonstrate for the first time that terahertz (THz) quantum cascade lasers can be realised in a buried-waveguide geometry. In our prototype devices the optical mode is a surface plasmon bound at the interface between the top contact and the semiconductor, providing for both vertical and lateral confinement without the need to define a cavity ridge. Proton-implanted high-resistivity sections are used to define the current channel where electrons can be injected into the active region. This way the electrical and optical confinement can be controlled independently: the former is defined by the non-implanted regions and the latter by the width of the top contact metal strip. Compared to standard ridge waveguides this technique allowed for a narrowing of the device effective cross section without introducing additional losses and improving the thermal conductivity, resulting in an increase of the maximum operating temperature up to 77K in continuous wave at 2.9 THz. In addition, we present preliminary results from buried-waveguide THz quantum cascade lasers obtained by combining a double-metal waveguide geometry with proton implantation. Initial results are promising, yielding record low threshold currents of 19mA at 4.2K in both pulsed and continuous wave operation.
In this work we have investigated the operation of a recently demonstrated bound-to-continuum quantum cascade laser emitting at 2.9 THz under different active region doping densities. In addition, we have studied the injection efficiency as a function of the thickness of the Al0.15Ga0.85As barrier controlling the tunnel coupling between the superlattice miniband and the upper state of the laser transition. By tuning these two parameters threshold current densities as low as 52 A/cm2 and 83 A/cm2 were obtained at 5 K, corresponding to a reduction over the base design of 55% and 25% respectively. In both cases we attribute the improved threshold performance to a reduction of the parasitic leakage current. The decrease in threshold was always at the expense of a smaller laser dynamic operating range, which ultimately limited maximum operating temperatures in pulsed and continuous wave mode to 95 K and 80 K respectively.
We present an experimental study of electron transport in electrically driven quantum cascade laser structures. Ultrafast quantum transport from the injector into the upper laser subband is investigated by mid-infrared pump-probe experiments directly monitoring the femtosecond saturation and subsequent recovery of electrically induced optical gain. For low current densities, low lattice temperatures, and low pump pulse intensities the charge transport is dominantly coherent, i.e., we observe pronounced gain oscillations upon excitation giving evidence for a coherent electron motion between the injector and the upper laser subband. Increasing either the current density, the lattice temperature, or the pump pulse intensity the gain recovery shows an additional slow incoherent component which essentially follows the pump-initiated heating and subsequent cooling of the carrier gas in the injector.
We demonstrate the operation of a superlattice GaAs/AlGaAs quantum cascade laser emitting at λ = 103 μm. The maximum operating temperature is 95K in pulsed mode and 70K in continuous wave. At 4K, we measured a peak output power in the tens of mW range and a threshold current density of 110 A/cm2 (300 A/cm2 at 90K). We attribute this excellent performance to a low ratio between the lower and upper state lifetimes, as well as to a low leakage current. These characteristics are highlighted by a pronounced decrease of the differential resistance at threshold and by the fact that the slope efficiency remains constant up to approximately 70K. At any temperature, we observe an abrupt decrease of the output power at the breaking of miniband alignment, corresponding to a strong negative differential resistance feature in the current/voltage characteristics. Ultimately, this effect limits the operation of the device at high temperatures. By comparing this laser with a previous design, we will outline the importance of (i) having a diagonal rather than vertical laser transition in real space, and (ii) avoiding possible intersubband re-absorption of the emitted radiation.
Quantum cascade lasers are coherent light sources in the mid-IR spectral region. They are based on resonant tunneling and optical transitions between discrete energy levels in the conduction band arising form size quantization in semiconductor heterostructures. QCLs have been demonstrated on GaInAs/AlInAs/InP and GaAs/AlGaAs outperforming existing semiconductor laser technologies in the mid-IR spectral range. The present paper reports the realization of a QCL based on GaAs/AlGaAs material designed with an emission wavelength of 9.3 micrometers . Specific properties inherent to this material system and their influence on laser operation are discussed in detail. The paper concludes with the presentation of a new waveguide concept, which offers considerable performance improvements.
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