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Terahertz (THz) quantum cascade lasers (QCLs) have been shown to emit peak powers greater than 1 W from a single facet in a single plasmon geometry. However, this is typically achieved by increasing the laser ridge width, resulting in higher-order transverse modes, limiting the achievable power density. Here, we control and fully suppress these modes through thin metallic side-absorbers, showing laser action solely on the fundamental transverse mode operation without sacrificing high THz peak powers. This leads to enhanced power densities and electric fields of up to 1.8 kW/cm2 and 1.17 kV/cm, respectively, opening up the possibility of applying THz QCLs as pump sources for investigations of nonlinear THz physical phenomena.

International audience; Mercury telluride (HgTe) nanocrystals (NCs) are very promising for THz technology as they exhibit broad THz absorption resonances and a carrier lifetime of a few picoseconds as well as being easily fabricated using solution synthesis. In this work, we show their light emission properties in the THz spectral range, up until now unexplored, and show how THz pulse generation can be used for microscopic insights into these NCs. In particular, we report on coherent THz emission from large HgTe NCs excited by linearly polarized optical pulses via second-order nonlinear effects. The peak emission frequency is tunable from 0.4 to 0.8 THz by varying incident angles of optical pulses from 0° to 45°. Our results reveal that the THz emission is induced by transient photocurrents arising from both photogalvanic and photon drag effects. By pushing the light emission of colloidal quantum dots down to the THz spectral range, our study expands the application fields of NCs, especially toward the development of easily integrable and tunable THz emitters and quantum THz devices.

We investigate the engineering of the strong light-matter coupling between a THz cavity and intersubband transitions for novel THz polaritonic devices. By varying the duty cycle of the cavity grating, we show how the light-matter interaction can be engineered over a large k-space range, measured using THz time domain spectroscopy (TDS) and fitted using a RCWA model. Secondly, we show how the reflectivity response of such cavities under optical pumping.

We report on carrier lifetimes in graphene/hBN heterostructures of ~30 ps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements. We further demonstrate the switching of carrier lifetime from ~30 ps (attributed to interband Auger) down to a few picoseconds upon ignition of hyperbolic phonon polaritons relaxation at finite bias and/or with infrared excitation power. Our study opens interesting perspectives to exploit graphene/hBN heterostructures for THz lasing and highly sensitive THz photodetection as well as for phonon polariton optics.

Comment: to appear in Advanced Optical materials (02/2022), Spin-to-charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin-locked TSS. In this work, we demonstrate sizable THz emission from a nanometric thick topological insultator (TI)/ferromagnetic junction - SnBi$_2$Te$_4$/Co - specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi$_2$Te$_3$. THz emission time domain spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by investigating the TI thickness and angular dependence of the THz emission. This work illustrates THz emission TDS as a powerful tool alongside angular resolved photoemission spectroscopy (ARPES) methods to investigate the interfacial spintronic properties of TI/ferromagnet bilayers.

International audience; We present technological building blocks and concepts under study which could ease the build-up of future THz remote sensing and communication systems. Critical issues regarding such systems mainly rely on the availability of sensitive and compact detectors together with powerful, large bandwidth and versatile sources.

Interdigitated photoconductive (iPC) switches [1] are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limits the spectroscopic resolution achievable from their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes [2]. This is realized through the growth of a buried multilayer LT-GaAs structure that retains its ultrafast properties, which after wafer bonding to a metal-coated host substrate, results in an iPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less iPC source [3], enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used. As a proof-of-principle, the 2(12)-2(21) and the 1(01)-2(12) rotational lines of water vapor have been spectrally resolved, demonstrating a spectral resolution below 10 GHz. [1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, M. Helm, Appl. Phys. Lett. 86, 121114 (2005) [2] K. Maussang, J. Palomo, J.-M. Manceau, R. Colombelli, I. Sagnes, L. H. Li, E. H. Linfield, A. G. Davies, J. Mangeney, J. Tignon, and S. S. Dhillon, Appl. Phys. Lett. 110, 141102 (2017). [3] K. Maussang, A. Brewer, J. Palomo, J.-M. Manceau, R. Colombelli, I. Sagnes, J. Mangeney, J. Tignon, S.S. Dhillon, IEEE Trans. Terahertz Sci. Technol. 6, 20 (2016)

We investigate the conductivity of epitaxial multilayer graphene in the THz range at various temperatures using time-domain THz spectroscopy. We demonstrate an increase of the THz conductivity as the temperature is increased, indicating Fermi energy of quasi-neutral layers close to the Dirac point. We also highlight phonon-assisted conduction at high temperature.

We review the most important developments in the technology of THz photoconductive antennas made from ion-bombarded semiconductors. We describe the structural, optical and electrical properties of various ion-bombarded semiconductors and discuss the nature of the defects introduced by the ion bombardment technique and their impact on the characteristics of THz photoconductive antennas. Finally, we present the performances achieved by photoconductive antennas based on ion-bombarded semiconductors for optical excitation at various wavelength.

Periodic arrays of gold wires and split ring resonators (SRR) with a minimum feature size of 50 nm are fabricated on low-doped silicon. To our knowledge, the periodic arrangement of SRRs and wires considered in this work has not been studied in the near-infrared domain yet. For normal-incidence conditions, this metamaterial structure exhibits resonances at 70 and 170 THz (i.e., at λ ≈ 4.3 and 1.75 μm), which are identified as LC- and Mie resonances, respectively. These resonances are also observed for the SRRs alone, but the amplitude of the Mie resonance is reinforced due to the coupling between the SRRs and wires. The structure is simulated using finite-element software, while transmission and reflection measurements are performed with a Fourier transform infrared spectrometer. Numerical simulations are found to be in very good agreement with experimental characterizations, thereby showing that the Drude model used in calculations is well suited to simulate gold structures at near-infrared frequencies. Theoretical calculations predict that the metamaterial has a negative permittivity and a negative permeability near each resonance.

We report on the nonlinear optical absorption of InN films deposited on GaN templates by radio-frequency (RF) sputtering. The layers are characterized through the pump-probe technique at 1.55 μm, by obtaining a nonlinear absorption coefficient of 167±30 cm/GW with a nonlinear response recovery time of 380 fs. This nonlinear behavior is attributed to a two-photon absorption process followed by a free carrier absorption by the photogenerated carriers. These results render InN films deposited by RF sputtering particularly suitable for ultrafast all-optical devices based on low-cost technology.

We show that shrinking the transverse size of metallic waveguides always leads to solutions with extreme field confinement at THz frequencies, regardless of the materials used and of the system geometry. We provide a unified framework to understand such universal behaviors, which will benefit future developments in THz science and technology.

We compare the THz pulses generated by photoconductive antennas made from two different materials: ErAs:GaAs and Br-irradiated InGaAs. The THz pulses were generated using the same 1550 nm THz time-domain spectroscopy set-up. The detection was realized with an electro-optic crystal, allowing a direct comparison of the performance from the different antennas.

Optical sidebands are generated by difference frequency mixing between a resonant bandgap near-infrared beam and a terahertz (THz) wave. This is realized within the cavity of a THz quantum cascade laser using resonantly enhanced non-linearities. Multiple order optical sidebands and conversion efficiencies up to 0.1% are shown.

We demonstrate a reduction of loss and group velocity dispersion (GVD) of THz plasmonic waveguides by using a low-loss thin flexible substrate. We present a numerical calculation of the effect of the substrate thickness on the GVD. We show experimentally low absorption and weak distortion of the propagated THz pulses along a THz plasmonic waveguide on a flexible 58μm-thick polyimide substrate.

A 2.2 ps full-width-at-half-maximum impulse response is measured for ion-irradiated InGaAs photoconductive switches triggered by ultrashort 1.55 μm laser pulses. Correspondingly, the −3 dB bandwidth is estimated to be ∼120 GHz. Measurements of the electrical signals delivered by photoconductive switches are performed using an electro-optic sampling technique. As is shown, the ion irradiation reduces the carrier lifetime to less than 1 ps. The sheet resistance is 0.6×105 Ω/square. The photoconductive switch responsivity is found to exhibit a nonlinear dependence with optical power. The results are qualitatively interpreted.

Pump-probe experiments have been used to measure the (sub-) picosecond carrier lifetimes in Au+- and proton-irradiated InGaAs samples, subsequently annealed at various temperatures. For both types of irradiation, the carrier lifetime increases with the annealing temperature. After 600 °C annealing, the defects are totally recovered in proton-irradiated samples, whereas they are still present in Au+-irradiated samples. The defect annealing kinetics observed in proton-irradiated samples is described well by a Frenkel pair recombination model, thereby indicating the dominance of isolated point defects. In contrast, the model is not adapted to describe the thermal behavior of Au+-irradiation-induced defects that are clusters of point defects as observed via transmission electronic microscopy. It is clearly shown that heavy-ion irradiated InGaAs exhibits a much higher thermal stability than proton-irradiated InGaAs.

We compare the photocarrier lifetime measured in Br-irradiated InGaAs and cold Fe-implanted InGaAsP. We also demonstrate the possibility of a two-photon absorption (TPA) process in ErAs:GaAs. The lifetime and the TPA were measured with a fiber-based 1550 nm time-resolved differential transmission (?T) set-up. The InGaAs-based materials show a positive ?T with sub-picosecond lifetime, whereas ErAs:GaAs shows a negative ?T consistent with a two-photon absorption process.

The absorption saturation relaxation time in light- and heavy-ion-irradiated GaAs saturable absorbers has been measured as a function of the temperature in the range from 7 to 300 K. For both types of samples, the relaxation time is shorter than 4 ps at 7 K. A regular increase of this time with temperature is observed for light-ion-irradiated samples, a value of 9.5 ps being reached at room temperature. In contrast, an almost temperature-independent relaxation time is found for heavy-ion-irradiated samples. The results are interpreted on the basis of a simplified relaxation model accounting for capture and emission from defect levels. We suggest that light-ion irradiation creates shallow centers whereas heavy-ion irradiation creates deep centers.

We have compared light- and heavy-ion irradiation of InGaAs/InAlAs multiple-quantum wells for ultrafast saturable absorption applications. Under heavy-ion impacts, defect clusters were produced, as observed via transmission electronic microscopy. By contrast, in proton-irradiated samples, only point defects were formed. Nonlinear absorption measurements were performed with excitonic resonance pumping. The relaxation time of absorption saturation (minimum value 2 ps) did not depend on the irradiating ion, and was practically independent of the pulse repetition rate (up to 10 GHz) and optical excitation fluence (0.1 mJ/cm2). We conclude that irradiating multiple-quantum wells with light ions is as effective as using heavy ions, when fabricating ultrafast saturable absorber devices operating at high bit rate and near bandedge wavelength.

We report terahertz time-domain spectroscopy system based on Er:fiber laser at 1.55 µm wavelength. Ion-irradiated In 0.53 Ga 0.47 As photoconductive antenna is used as emitter. The detection is based on a phase modulation detection scheme in DAST electro-optic sensor.

International audience; We report continuous wave generation at frequencies up to 2 THz using ion-irradiated In0.53Ga0.47As photomixers coupled to transverse-electromagnetic-horn antennae driven at 1.55 m wavelength. Output powers up to 0.1 W at 700 GHz have been achieved. The dependence of the output power on incident optical power and the bias voltage is analyzed in the both regimes of Ohmic transport and recombination-limited transport. The fundamental limitations of the performance of the photomixer devices based on photoconductive phenomenon in recombination-limited transport are analyzed

Room temperature homodyne and heterodyne THz detections are reported using an 1.55 µm-based THz source. The first experimental demonstration is a continuous wave terahertz homodyne detection, where THz emission is performed with an InGaAs/InP uni-travelling carrier photodiode monolithically integrated with a horn antenna. The coherent homodyne detection is achieved with an ion-irradiated InGaAs photoconductor integrated with a spiral antenna. Experimental results has been obtained up to 700 GHz with a good signal to noise ratio. The second experimental setup is a room temperature heterodyne detection with a schottky mixer.

We report on the intraband emission at room-temperature from GaN/AlN quantum dots grown by plasma-assisted molecular beam epitaxy as well as on the measurements of GaN quantum dot intraband relaxation using femtosecond spectroscopy. The quantum dots exhibit s-pz intraband absorption peaked at 1.55 μm. The pz-s intraband luminescence at λ = 1.48 μm is observed under optical excitation at λ = 1.34 μm. The population of the pz state arises from Raman scattering by GaN A1 longitudinal optical phonons. Based on the emission spectral shape, we estimate that the homogeneous linewidth of the s-pz intraband transition is less than 4 meV. The carrier lifetime of the pz state (T1) is measured by femtosecond pump-probe spectroscopy. T1 = 165 fs while the thermalization of electrons in the ground state proceeds in 1.5 ps. The saturation intensity of the s-pz intraband absorption is estimated to be in the 15 to 137 MW/cm2 range taking into account the uncertainty on the pump beam waist. These results demonstrate the promising capabilities of GaN/AlN QDs for application to telecom intraband lasers as well as multi-Tbit/s all-optical switching devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast dynamics of charge carriers in ${\text{Br}}^{+}$-bombarded InP were studied using time-resolved terahertz spectroscopy. Carrier lifetimes and mobilities in various samples prepared with irradiation doses spanning from ${10}^{9}$ up to ${10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$ were determined. The lifetime of photoexcited carriers appears to be determined primarily by the density of defects resulting from host-atom displacements while it is not significantly influenced by the Br-atom implantation. In the most irradiated sample, a carrier lifetime as short as 300 fs was found. All samples exhibit a high mobility $(3000\ensuremath{-}900\text{ }{\text{cm}}^{2}\text{ }{\text{V}}^{\ensuremath{-}1}\text{ }{\text{s}}^{\ensuremath{-}1})$; the lower values correspond to a smaller irradiation dose. In selected samples, the density of traps along with electron and hole lifetimes was determined.

We report the generation of continuous terahertz waves from microwave frequencies up to 2 THz obtained by photomixing two optical waves at 1.55-mum wavelengths in an optimized ion-irradiated In0.53Ga0.47As interdigitated photomixers. Output powers greater than 40 nW at 0.5 THz and 10 nW at 1 THz have been achieved. The optimum excitation conditions regarding optical power and bias voltage are discussed.

We present a fiber-based 2-port THz electro-optic sampling system at 1.55 mum wavelength, including an ultrafast In0.53Ga0.47As photoconductive switch and a freely positionable prismatic electro-optic probe. Frequency components are extended up to 2 THz and the dynamic range is larger than 40 dB regardless of the direction of the electromagnetic wave propagating in the waveguide thanks to the 2-port system. The symmetrical 2-port pigtailed electro-optic probe allows to determine the direction of propagation of the guided picosecond electromagnetic pulses.

Here, we report the gigahertz-rate modulation of a tunable terahertz carrier. Terahertz radiation, tunable from 300 GHz to 1.2 THz, is generated by mixing two telecom lasers with an offset frequency in an ultrafast In0.53Ga0.47As photoconductive device coupled to a broadband antenna. The microwave modulation applied to the telecom lasers is directly transferred to the terahertz carrier. A maximum modulation rate of 20 GHz has been achieved, and the bandwidth is independent of the carrier frequency.

Design, fabrication and characterization of germanium on silicon photodetector integrated in SOI waveguide are reported. A responsivity of 1 A/W and a -3 dB bandwidth of 25 GHz under 6 V bias have been obtained at lambda=1.55 mum.

We report the generation of continuous terahertz waves from microwave frequencies of up to 2THz obtained by photomixing two optical waves at 1.55μm wavelengths in ion-irradiated In0.53Ga0.47As interdigitated photomixers. A 200nm thick silicon nitride coating is used for antireflection and passivation layer, improving the reliability and the heat tolerance of the photomixer. In such devices, output powers greater than 40nW at 0.5THz and 10nW at 1THz have been achieved. Considering the observed saturation of the output power with the increase of bias voltage, the optimum excitation conditions regarding optical power and bias voltage are discussed.

International audience; We present a detailed study of the effect of the carrier lifetime on the terahertz signal characteristics emitted by Br+-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength femtosecond optical pulses. The temporal waveforms and the average radiated powers for various carrier lifetimes are experimentally analyzed and compared to predictions of analytical models of charge transport. Improvements in bandwidth and in average power of the emitted terahertz radiation are observed with the decrease of the carrier lifetime on the emitter. The power radiated by ion-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength optical pulses is measured to be 0.8 μW. This value is comparable with or greater than that emitted by similar low temperature grown GaAs photoconductive antennas excited by 780 nm wavelength optical pulses.

We report the first demonstration of a terahertz photomixer made of ion-irradiated In0.53Ga0.47As lattice-matched to InP and fiber-optic coupled with the 1.55 μm wavelength drive lasers. The detected signal is at the most 15 dB lower than the one obtained from similar photomixers fabricated from low-temperature-grown GaAs.

We measured the terahertz (THz) emission from heavy-ion irradiated In/sub 0.35/Ga/sub 0.47/As photoconductive (PC) antennas excited at 1550 nm. The In/sub 0.35/Ga/sub 0.47/As layer irradiated at 10/sup 12/ cm/sup -2/ shows a carrier lifetime shorter than 200 fs, a steady-state mobility of 490 cm/sup 2/V/sup -1/s/sup -1/, and a dark resistivity of 3.1 /spl Omega/cm. The spectrum of the electric field radiated from the heavy-ion irradiated In/sub 0.35/Ga/sub 0.47/As antenna extends beyond 2 THz. Comparative measurements performed on In/sub 0.35/Ga/sub 0.47/As PC antennas irradiated at 10/sup 11/ cm/sup -2/ and 10/sup 12/ cm/sup -2/ demonstrate the impact of the defect center scattering on the photo-excited carrier mobility.

International audience; The authors investigate the generation and the detection of terahertz radiation by Br+-irradiated In0.53Ga0.47As photoconductive antennas using an optical excitation at 800nm wavelength. The detected temporal wave form presents a full width at half maximum of 650fs. The signal-to-noise ratio and frequency range of the emitted terahertz radiations are similar to those emitted by low-temperature-grown GaAs photoconductive antennas. The dependences of terahertz wave form amplitude on optical pump power for both Br+-irradiated In0.53Ga0.47As and low-temperature-grown GaAs photoconductive antennas are investigated and compared.

International audience; The performance from a high-energy Br+-irradiated InP photoconductive antenna excited at 800 nm has been investigated. The ionic irradiation into InP reduces significantly the lifetime of photo-carriers in the substrate material down to 1.4 ps without the requirement of an epitaxial layer growth. THz emitter and detector efficiency are strongly dependent on irradiation doses. It was found that equivalent bandwidth and amplitude compared with LTG-GaAs switches excited at 800 nm under the same experimental conditions.

We propose a new version of electro-optic sampler at 1.5 mum, which uses an ion-irradiated InP-based photoconductor for the optical-to-electrical conversion. A measurement bandwidth larger than 300 GHz is obtained for a signal-to-noise ratio higher than 15 dB and excitation pulses of peak amplitude as high as 0.81 V. The performances of our system are validated from test measurements performed on coplanar waveguide transmission lines made on semi-insulating InP substrates

Ga/sub 0.47/In/sub 0.53/As/InP single-quantum wells with thin top barrier layers of InP and InAlAs have been studied by pump-probe experiments and photoluminescence spectroscopy. The excitonic lifetime is reduced by a fast trapping mechanism of carriers into surface states.

A 3 dB bandwidth of 35 GHz at 1.31 /spl mu/m and 1.55 /spl mu/m wavelengths is reported for inter-digited germanium on silicon-on-insulator metal-semiconductor-metal photodetector with finger spacing of 500 nm.

THz waves have recently attracted considerable interest mainly because of their various applications. Efficient and compact CW sources are currently under development using either electronic or optical components. On the electronic side, an important innovation of last years has been to excite THz plasma oscillations in short gate length HEMT. On the optoelectronic side, high speed photodetectors and photodiodes are currently under development with aim of a most efficient coupling with already existing 1.55 /spl mu/m semiconductor laser technology. Besides active device review, studies devoted to controllable THz photonic crystals and specific antennas for the THz domain will also be discussed.

We investigate terahertz (THz) emission from heavy-ion-irradiated In0.53Ga0.47As photoconductive antennas excited at 1550 nm. The carrier lifetime in the highly irradiated In0.53Ga0.47As layer is less than 200 fs, the steady-state mobility is 490 cm2 V–1 s–1, and the dark resistivity is 3 Omega cm. The spectrum of the electric field radiating from the Br+-irradiated In0.53Ga0.47As antenna extends beyond 2 THz. The THz electric field magnitude is shown to saturate at high optical pump fluence, and the saturation fluence level increases with the irradiation dose, indicating that defect center scattering has a significant contribution to the transient mobility.

Summary form only. All-optical communication technologies require nonlinear optical devices with ultrafast response times, compatible with WDM. Heavy-ion irradiation is an interesting technique for reducing the carrier recombination time in saturable absorbers, because in quantum wells it preserves the excitonic absorption lines up to relatively high irradiation doses and the relaxation time does not saturate at high average carrier generation rates. The device is a reflection-mode vertical-cavity structure based on MBE-grown InGaAs/InAlAs MQW groups. The results show that heavy-ion irradiated quantum wells can provide the basis for efficient compact optical switching devices, operating at optical communication wavelengths. The devices can be made fast enough to extend the bit rate capability beyond 100 GHz.