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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.

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 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)

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.

For the processing of high bit rate optical data transmitted in broadband fiber optic networks, the development of efficient all-optical active devices is of crucial importance, as in many instances one would like to avoid the complexity of using high speed electronics.

The authors demonstrate, using pump-probe experiments, that defects created by a 200 MeV Au/sup +/ ion beam in bulk GaAs reduce the relaxation time of the saturable absorption of the material to as little as 200 fs. The sample absorption is modulated over a spectral width of 50 nm. The integrated value of the absorption modulation thus shows the high absorption efficiency of the irradiated material. A very small variation in relaxation time is found when the density of photocreated carriers is increased to /spl sim/5/spl times/10/sup 18/cm/sup -3/. This material appears to be very promising for applications in optical processing and ultrafast measurement at high pulse repetition rates.

International audience; We present results of infrared pump--terahertz probe experiments applied to a set of In0.53Ga0.47As films irradiated with heavy ions (Br+) at doses from 109 to 1012 cm−2. Photoexcitation at 1400 nm (0.89 eV) allowed us to characterize the response close to telecommunications' wavelengths whilst avoiding the intervalley carrier scattering observed when a shorter wavelength excitation is used. The excitation fluence was varied in our experiments in order to characterize the dynamics in detail: the lifetimes and mobilities of both electrons and holes were retrieved, and the trap filling and carrier diffusion were clearly observed. The In0.53Ga0.47As film irradiated by the dose of 1012 cm−2 exhibits simultaneously ultrashort electron lifetime (∼300 fs) and very high electron mobility (2800 cm2V−1s−1). These findings are particularly important for the design of terahertz emitters controlled by lasers operating at standard telecommunication wavelengths.

We study the carrier relaxation dynamics in Ga1−yIny N0.03As0.97 layers grown on InP substrate by molecular beam epitaxy. This dilute nitride semiconductor is a potential candidate for ultrafast components such as photoconductive terahertz devices driven at 1.55 μm wavelength. Carrier lifetimes are measured by pump-probe experiments at 1550 nm wavelength and the shortest carrier lifetime is 2.6 ps under photoexcitation with ∼4.2 μJ/cm2. We observe two contributions in the differential transmittance decay. These contributions are discussed in terms of photocarrier recombination processes.

We study GaInAs/GaInNAs superlattice structures grown on InP substrate as potential candidates for photoconductive terahertz devices or saturable absorbers working at 1.55 μm wavelength. The N-rich GaInNAs layers are flat, with no three-dimensional islanding, and act as trapping layers where carriers can recombine rapidly. The carrier lifetime in GaInAs/GaInNAs superlattice was measured for various growth parameters using time-resolved differential transmission experiments at 1.55 μm wavelength. The carrier lifetime is found to depend strongly on N content and can be reduced down to 3.8 ps for samples with 14% of N. The mechanisms involved in the capture process of photocarriers are discussed.

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

We report the experimental demonstration of a germanium metal-semiconductor-metal (MSM) photodetector integrated in a SOI rib waveguide. Femtosecond pulse and frequency experiments have been used to characterize those MSM Ge photodetectors. The measured bandwidth under 6V bias is about 25 GHz at 1.55 microm wavelength with a responsivity as high as 1 A/W. The used technological processes are compatible with complementary-metal-oxide-semiconductor (CMOS) technology.

We report the fabrication and the characterization of interdigited metal-germanium on silicon metal photodetectors (metal-semiconductor-metal or MSM) for operation at both optical telecommunication wavelengths: 1.31 and 1.55μm. Femtosecond impulse and frequency experiments have been carried out to characterize those MSM Ge photodetectors. For both wavelengths, the measured 3dB bandwidth under 2V bias are close to 10, 18, 20, and 35GHz for electrode spacings equal to 2000, 1000, 700, and 500nm, respectively.

The electrical and optical properties of H+- and Au+-irradiated InGaAs layers were studied using Hall-effect, van der Pauw, and relaxation-time measurements. Comparing the different results allows us to obtain information on the nature of the defects created by these two irradiations. Proton irradiation introduces donor-acceptor paired defects. Gold-ion irradiation creates neutral defect clusters and ionized point defects. The carrier mobilities in all of the irradiated materials are degraded, decreasing with increasing irradiation dose. A scattering model taking into account the paired defects is developed and the mobility evolution calculated from this model agrees with the experimental data of both annealed and unannealed samples. The photocurrent spectra reveal a metallic conduction in the band gap in the case of light-ion irradiation, while such type of conduction does not appear for heavy-ion irradiation. This metallic conduction is a consequence of band tailing induced by shallow defects and vanish...

A semiconductor saturable absorber based on a heavy-ion-irradiated quantum well vertical cavity is implemented In a 10 Gbit/s RZ loop transmission. Induced reduction of amplified spontaneous emission accumulation is experimentally demonstrated for the first time to improve the error-free transmission distance by a factor of 2.5.

Improved robustness to signal-to-noise ratio impairment is shown for the first time at 20 Gbit/s in an RZ transmission loop using a 1.5 µm heavy-ion-irradiated quantum well saturable absorber. Error-free transmission is achieved over 7800 km.

Measurements of absorption saturation in heavy-ion-irradiated InGaAs/InAlAs multiplequantum-well reflection-mode vertical-cavity devices have been performed with short pulses at 1.55 μm and repetition rates up to 10 GHz. The relaxation time was essentially independent of the pulse repetition rate and optical excitation fluence, with a lower value of 2.4 ps for an ion dose of 1012 cm−2. Efficient optical switching was obtained, with a saturation energy smaller than 12 pJ, a contrast ratio up to 3.5:1, and a switching amplitude up to 20% of the incident signal. A relaxation model accounting for capture and recombination on defect levels indicates an upper limit of 2 ps of the defect level recombination time.

Optical communications (2): Transmission systems and networks; We review recent advances in the investigation of vertical cavity saturable absorber devices and their use for the all-optical regeneration of telecommunication signals in ultralong- haul transmission systems. Such devices are polarization-insensitive and operate as fully passive nonlinear optical elements. Two approaches for obtaining fast recovery absorber materials are described, relying upon ion irradiation or upon iron doping. The vertical microcavity devices are designed so as to optimize the switching contrast and the operating power. Their functional behaviour as extinction ratio amplifiers has been characterized and their optimal operating conditions have been determined. The potential application of these devices to all-optical regeneration has been investigated through numerical simulations and fully demonstrated in several long-distance transmission loop experiments, with results obtained at 10, 20 and 40 Gbit/s, showing significant improvements in system haul or operational margins. A four-channel fibered module has also been fabricated, as a perspective towards the development of wavelength division multiplexing (WDM) saturable absorber modules."

This letter reports the results of a comparative study on heavy- and light-ion-irradiated semiconductors for fast saturable absorbers. The linear absorption of bulk GaAs irradiated either by Au+ ions or protons was measured over a wide range of wavelengths below the gap. Good correspondence was found between the absorption measurements and the calculated elementary defect concentrations. Defect clustering is evidenced in the heavy-ion case. Pump–probe experiments were used to measure the time-resolved absorption variations for weakly irradiated GaAs samples under intense illumination. Much shorter carrier recombination times are estimated for the heavy-ion case.