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Free-space optical communications in the mid-infrared transparency windows (4–5 and 8–14 μm wavelength regions) is emerging as a viable solution for high bitrate data transmission. Unipolar quantum optoelectronics is the technology of choice for data communication in this wavelength region, thanks to the high frequency response of detectors and modulators. In this work, it is demonstrated that the performances of these devices can be substantially enhanced by embedding them into metamaterials. It is also shown that metamaterials have to be engineered differently in detectors than in modulators, as the role of light–matter interaction must be tuned adequately in the two devices. Metamaterial-enhanced performances allow the realization of data transmission with a record rate of 68 Gbit/s, while ensuring robustness and consistency, as it should be for real-world applications. These findings underscore the promising role of metamaterial-enhanced unipolar devices in advancing free-space optical communication systems. [ABSTRACT FROM AUTHOR]

The combination of speed and resolution is a fundamental figure of merit for Fourier transform spectroscopy. Here, we show that, by combining a fast rotational delay line with a strongly modulated quantum cascade laser (QCL), it is possible to perform broadband spectroscopy over a range of 175 cm−1, with a total effective integration time of about 6 s and a resolution of 0.03 cm−1. Remarkably, even though the resolution of the measurement is limited by the source, the interleaving technique can still be applied when the QCL is not in a frequency comb regime, considerably broadening the bandwidth of the instrument. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate that in a regime of strong modulation, by generating pulses of the length of the order of a few cavity lifetimes (hundreds of ps), a broadband quantum cascade laser can be driven to lase on a bandwidth (250 cm−1) limited by the gain. In addition, the amplitude noise of the radiation was shown to be limited by the detector. A laser linewidth study has been performed under different operating conditions, finding values spanning from 20 to 800 MHz, indicating a trade-off between emission bandwidth, amplitude stability, and coherence. The pulsed intensity of the output arises from the gain switching dynamics of the laser, while the self-seeded nature of the emission is responsible for the low amplitude noise and the relatively narrow linewidth of the individual spectral lines of the multimode emission. [ABSTRACT FROM AUTHOR]

In a Quantum Cascade Detector, photocurrent is generated by the absorption of infrared and terahertz radiation in the quantum-well-based modules arranged in series. Consequently, the current responsivity is by construction inversely proportional to the number of cascading modules. Upon absorption of a photon, the electron travels through only a single period of the detector, with a mean free path corresponding to the period length. Therefore, the shot noise power density is expected to decrease by the same factor under sufficiently high illumination, reflecting the same inverse relationship with the number of cascading modules. This phenomenon leads to sub-Poissonian noise characteristics. We experimentally observe this effect in a 90-period Quantum Cascade Detector operating at 4.5 μm, confirming a reduction in the shot noise contribution by the anticipated Fano factor of 1/90. This measurement underscores the suitability of these detectors for coherent detection scenarios, particularly where shot noise dominates. [ABSTRACT FROM AUTHOR]

In quantum cascade laser frequency combs, the intensity distribution of the optical spectrum can be split into two well-separated lobes of longitudinal modes that, even when far apart, have a common phase relation and preserve equal frequency separation. The temporal dynamics of two lasers emitting at 4.4 and 8.1 µm operating in this bilobed regime are here investigated. The laser intensity shows a peculiar temporal behavior associated with the spectral features whereby, every half a round-trip, the total emitted power switches from one lobe to the other, with a perfect temporal anti-correlation. The anti-correlation between the lobes is also observed in the intensity noise figure of the emission. This coherent phenomenon arises from gain nonlinearities induced by spatial hole burning and the extremely fast gain dynamics typical of quantum cascade lasers. [ABSTRACT FROM AUTHOR]

This work analyzes transport through metal organic chemical vapour deposition grown Iron doped Indium Phosphide (InP:Fe) for use as a current blocking layer in buried heterostructure Quantum Cascade Lasers. The nature of Iron incorporation in InP and electrical transport properties of InP:Fe is investigated via simulation and compared with measurement. Through simulations, we are able to predict the threshold for the onset of current rise in test structures due to avalanche injection of carriers. In addition, the benefit of InAlAs barriers inserted in InP:Fe layers is investigated and found to reduce the leakage current at lower biases while delaying the onset of avalanche. In buried heterostructure configuration, we have determined that non ideal regrowth profiles make the structure more susceptible to high field effects such as avalanche injection and trap filling that induce leakage currents. [ABSTRACT FROM AUTHOR]

The application of high values of strain to Ge considerably improves its light emission properties and can even turn it into a direct band gap semiconductor. Raman spectroscopy is routinely used for strain measurements. Typical Raman-strain relationships that are used for Ge were defined up to ~1% strain using phonon deformation potential theory. In this work, we have studied this relationship at higher strain levels by calculating and measuring the Raman spectral shift-strain relations in several different strain configurations. Since differences were shown between the usual phonon deformation potential theory and ab-initio calculations, we highlight the need for experimental calibrations. We have then measured the strain in highly strained Ge micro-bridges and micro-crosses using Raman spectroscopy performed in tandem with synchrotron based micro-diffraction. High values of strain are reported, which enable the calibration of the Raman-strain relations up to 1.8% of in plane strain for the (001) biaxial stress, 4.8% strain along 〈100〉, and 3.8% strain along 〈110〉. For Ge micro-bridges, oriented along 〈100〉, the nonlinearity of the Raman shift-strain relation is confirmed. For the 〈110〉 orientation, we have shown that an unexpected non-linearity in the Raman-strain relationship has also to be taken into account for high stress induction. This work demonstrates an unprecedented level of strain measurement for the 〈110〉 uniaxial stress and gives a better understanding of the Raman-strain relations in Ge. [ABSTRACT FROM AUTHOR]

We report spectrally resolved gain measurements and simulations for quantum cascade lasers (QCLs) composed of multiple heterogeneous stacks designed for broadband emission in the mid-infrared. The measurement method is first demonstrated on a reference single active region QCL based on a double-phonon resonance design emitting at 7.8 lm. It is then extended to a three-stack active region based on bound-to-continuum designs with a broadband emission range from 7.5 to 10.5 lm. A tight agreement is found with simulations based on a density matrix model. The latter implements exhaustive microscopic scattering and dephasing sources with virtually no fitting parameters. The quantitative agreement is furthermore assessed by measuring gain coefficients obtained by studying the threshold current dependence with the cavity length. These results are particularly relevant to understand fundamental gain mechanisms in complex semiconductor heterostructure QCLs and to move towards efficient gain engineering. Finally, the method is extended to the measurement of the modal reflectivity of an anti-reflection coating deposited on the front facet of the broadband QCL. VC 2015 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23 ° × 19 °) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current–voltage characteristics are not affected, as well as frequency comb operation. Measurements agree well with numerical simulations, and the proposed antenna covers a broad spectral range (1.5–4.5 THz). [ABSTRACT FROM AUTHOR]

The extension of the operating frequency of Quantum Cascade Lasers (QCL) into the 20-50 μm regime is a desirable goal as it would bridge the gap between mid-infrared and THz devices. Coherent light emitters in this spectral range are also needed for spectroscopy and radio astronomy applications. Since little attention has been devoted to the subject in the past, we investigate the dominant loss mechanisms of QCLs in this spectral range. We report on an InGaAs/InAlAs QCL in an InP dielectric waveguide emitting at 23 μm wavelength whose electroluminescence spectrum shows an anomalous low-frequency cut which prevents laser action at low electric field. We also observe similar line shape in other GaAs/AlGaAs devices. The spectral features are analyzed and explained in terms of refractive index anomalies induced by phonon resonances. [ABSTRACT FROM AUTHOR]

We present a comprehensive analysis of an external-cavity (EC) quantum cascade (QC) laser system, which is based on a coupled-mode system with the length of the EC set equal to that of the laser chip and accommodating the large difference in physical cavity length by introducing an effective speed of light for light propagating in the EC. By analyzing the rate equations and discussing the cross sections and losses of the EC QC lasers, numerical results as well as suggestions for a further optimization of the spectral tuning range of the EC system are presented. Besides, the delayed onset of the EC modes with respect to that of the internal Fabry–Perot modes in pulsed operation has been simulated using the rate equations, yielding a good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature. [ABSTRACT FROM AUTHOR]

We report a heterodyne receiver using a superconducting NbN hot electron bolometer (HEB) integrated with a tight winding spiral antenna as mixer and a distributed feedback (DFB) terahertz quantum cascade laser (QCL) operating at 3.42 THz as local oscillator. The aim is to demonstrate the readiness of both devices for the detection of OH lines at 3.5 THz in a real instrument. We show that the improved single-spot beam of the terahertz QCL can easily pump the HEB mixer. We measured a double sideband receiver noise temperature of 2100 K at the optimum local oscillator power of 290 nW. This noise temperature can be further reduced to 1100 K if we correct the loss due to the use of an uncoated lens, and the losses of the window and the air. Therefore, the combination of a HEB and such a DFB QCL can in principle be used to detect an OH line at 3.5 THz. However, a high input power of several watts, which is needed to operate the QCL in a liquid-helium cryostat, poses a big challenge to the receiver stability. [ABSTRACT FROM AUTHOR]

We present a terahertz quantum cascade laser operating on a thermoelectric cooler up to a record-high temperature of 210.5 K. The active region design is based on only two quantum wells and achieves high temperature operation thanks to a systematic optimization by means of a nonequilibrium Green's function model. Laser spectra were measured with a room temperature detector, making the whole setup cryogenic free. At low temperatures (∼40 K), a maximum output power of 200 mW was measured. [ABSTRACT FROM AUTHOR]

Deals with a systematic study of phase modulation in GaAs/AlGaAs double-heterostructure waveguides with different doping profiles. Electric field effect; Computation of the phase modulation; Discussion and conclusion.

Deals with a study which reported a systematic and accurate measurement of phase modulation in double heterostructures with different doping profiles. Experimental details; Phase measurements and comparison with theory; Measurements of electro-optic coefficients.

Presents a study which characterized gallium arsenide/(gallium arsenide)[subn](aluminum arsenide)[subm] surface-emitting laser structures through reflectivity and high-resolution electron microscopy measurements. Sample growth; Discussion of electron microscopy; Reflectivity measurements.

We design and fabricate large-area (1.1 mm × 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength ∼ 8.5 µm) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is <1∘ and with no side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field pattern is realized. The photonic band structure is measured with high spectral (0.72 cm−1) and angular (0.1°) resolution by using the photonic crystal quantum cascade laser itself as a detector. [ABSTRACT FROM AUTHOR]

The gain recovery time of a heterogeneous active region terahertz quantum cascade laser is studied by terahertz-pump-terahertz-probe spectroscopy. The investigated active region, which is based on a bound-to-continuum optical transition with an optical phonon assisted extraction, exhibits a gain recovery time in the range of 34–50 ps dependent on the operation condition of the laser. The recovery time gets shorter for stronger pumping of the laser while the recovery dynamics slows down with increasing operation temperature. These results indicate the important role of the intracavity light intensity for the fast gain recovery. [ABSTRACT FROM AUTHOR]

We study the intersubband absorption in Si doped AlAsSb/InGaAs quantum wells (QWs) grown on InP substrates by molecular beam epitaxy. The investigated multiple QW structures contain strained In0.78Ga0.22As layers and additional AlAs layers for strain compensation. By varying the nominal QW width from 3 to 9 ML (monolayer) a minimum central absorption wavelength of 1.76 μm is found for a 5 ML thick QW. Simulations of these experimental results using a self-consistent Schrödinger-Poisson solver show a deviation from the perfect squarelike potential caused by intermixing effects at the interfaces. These blurred interfaces are also revealed by transmission electron microscopy measurements. [ABSTRACT FROM AUTHOR]

We present a two-quantum well THz intersubband laser operating up to 192 K. The structure has been optimized using non-equilibrium Green's function model. The result of this optimization was confirmed experimentally by growing, processing, and measuring a number of proposed designs. At high temperature (T>200 K), the simulations indicate that lasing fails due to a combination of electron-electron scattering, thermal backfilling, and, most importantly, re-absorption coming from broadened states. [ABSTRACT FROM AUTHOR]

We study terahertz (THz) superconducting split-ring resonators showing that the resonance of the metasurface is fully switchable via the superconducting transition, exhibiting a resonance only below the critical temperature and a critical magnetic field. Combining this metamaterial with a two-dimensional electron gas (2DEG), which is forming Landau levels in magnetic field, we aim for an ultra-strongly coupled system. We report on experiments showing strong coupling between the excited cavity mode and the 2DEG and we discuss possible scenarios for observing ultra-strong coupling with the fundamental LC mode. [ABSTRACT FROM AUTHOR]

In this work, we report the characterization of a quantum cascade laser frequency comb with an optical power of 1.05W at λ~8.2 μm. A 4.5mm long device has a high reflectivity coating on the back facet as well as a top cladding designed to lower the group velocity dispersion and is operated at 258 K. Very strong (more than 60 dB) narrow beatnotes are shown, and frequency comb operation is obtained on a bandwidth of 85 cm-1 in a very large range of light-versus current characteristics. A bandwidth of 82 cm-1 has a power per mode of more than 1 mW and an average power per mode of 4.1 mW. Finally, a multi-heterodyne spectrum with 215 lines covering an optical bandwidth of more than 70 cm-1 measured with lasers showing similar performances is presented with very good line separation. [ABSTRACT FROM AUTHOR]

We introduce a double metal terahertz quantum cascade laser meant for astrophysical heterodyne measurements. The laser ridge is embedded in benzocyclobutene, and the device exhibits single mode, continuous wave operation around 4.745 THz with a peak power of almost 1.8 mW at 10K and a power consumption of≈1.6 W. Moreover, thanks to the integration of a top metal contact with a patch array antenna for light out-coupling the beam of the emitted light has a lowdivergence single-lobe profile and an FWHM of≈30°. [ABSTRACT FROM AUTHOR]

Ge under high strain is predicted to become a direct bandgap semiconductor. Very large deformations can be introduced using microbridge devices. However, at the microscale, strain values are commonly deduced from Raman spectroscopy using empirical linear models only established up to ε100= 1.2% for uniaxial stress. In this work, we calibrate the Raman-strain relation at higher strain using synchrotron based microdiffraction. The Ge microbridges show unprecedented high tensile strain up to 4.9% corresponding to an unexpected Δω = 9.9 cm-1 Raman shift. We demonstrate experimentally and theoretically that the Raman strain relation is not linear and we provide a more accurate expression. [ABSTRACT FROM AUTHOR]

We present an array of single-mode quantum cascade lasers operating in the range 8– 9.5~\mu \textm . The resonator features a pair of distributed Bragg reflectors for the mode selection, together with a second-order extraction grating. The radiation of these substrate emitting devices was combined in a single collector ridge. A proof of principle featured seven laser devices with a coverage of 170 cm ^-1 . The functionality of our approach is corroborated with a set of polarization and near-field measurements confirming the collection of multiple frequencies in a unique ridge. [ABSTRACT FROM PUBLISHER]

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorptionassisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm-1's at 200 K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime. [ABSTRACT FROM AUTHOR]

Dual-comb spectroscopy is emerging as an appealing application of mid-infrared frequency combs for high-resolution molecular spectroscopy, as it leverages on the unique coherence properties of frequency combs. Here, we present an on-chip dual-comb source based on mid-infrared quantum cascade laserfrequency combs. Control of the combs repetition and offset frequencies is obtained by integrating micro-heaters next to each laser. We show that a full control of the dual-comb system is possible, by measuring a multi-heterodyne beating corresponding to an optical bandwidth of 32cm-1 centered at 1330cm-1 (7.52μm), demonstrating that this device represents a critical step towards compact dual-comb systems. [ABSTRACT FROM AUTHOR]