Subpicosecond Carrier Lifetime Research Articles

Strain-free SESAMs with iron doped absorber for femtosecond fiber laser mode locking at 1560 nm.

Semiconductor saturable absorber mirrors (SESAMs) are key devices for passive mode locking of numerous laser types and have been implemented for a variety of operational wavelengths ranging from 800 nm to 2400 nm. However, for 1560 nm the fabrication of SESAMs based on the standard AlAs/GaAs material system requires highly strained InGaAs absorber layers, which reduce the device efficiency and compromise fragile long-term performance. Here, we present SESAMs for ultrashort pulse generation at 1560 nm that are grown entirely lattice-matched to InP and thus have the potential for less structural defects and a higher operational lifetime. A highly reflective InGaAlAs-InAlAs Bragg mirror is capped with a heavily iron doped InGaAs:Fe absorber layer, which facilitates an unprecedented combination of sub-picosecond carrier lifetime and high optical quality. Therefore, the presented SESAMs show ultrafast response (τA < 1 ps), low non-saturable losses and high effective modulation depth (ΔReff ≥ 5.8%). Moreover, a nearly anti-resonant SESAM design provides high saturation and roll-over fluence (Fsat ≥ 17 µJ/cm2, F2 ≥ 21 mJ/cm2). With these SESAMs, we show self-starting and stable mode locking of an erbium doped fiber laser at 80 MHz repetition rate, providing ultrashort optical pulses at 17.5 mW average power.

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers.

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with sub-picosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 µW-three orders of magnitude lower than gating powers used for conventional PCA detectors. We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.

Low‐temperature‐grown gallium arsenide photoconductors with subpicosecond carrier lifetime and photoresponse reaching 25 mA/W under 1550 nm CW excitation

The authors show in this Letter that photoconductors based on GaAs grown at low temperatures can exhibit photoresponses as high as 25 mA/W under continuous-wave 1550-nm-wavelength illumination. It is achieved by using an optical Fabry–Perot cavity in order to improve the external quantum efficiency and by decreasing the post-growth annealing temperature down-to 450°C.

Terahertz time-domain-spectroscopy system based on 1.55 μm fiber laser and photoconductive antennas from dilute bismides

We describe a terahertz time-domain-spectroscopy system that is based on photoconductive components fabricated from (GaIn)(AsBi) epitaxial layers and activated by femtosecond 1.55 μm pulses emitted by an Er-doped fiber laser. (GaIn)(AsBi) alloy grown on GaAs substrates contained 12.5%In and 8.5%Bi – a composition corresponding to a symmetrical approach of the conduction and valence band edges to each other. The layers were photosensitive to 1.55 μm wavelength radiation, had relatively large resistivities, and subpicosecond carrier lifetimes – a set of material parameters necessary for fabrication of efficient ultrafast photoconductor devices. The frequency limit of this system was 4.5 THz, its signal-to-noise ratio 65 dB. These parameters were comparable to their typical values for much bulkier solid-state laser based systems.

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection.

We report here a photoconductive material for THz detection with sub-picosecond carrier lifetime made by C(12) (Carbon) irradiation on commercially available semi-insulating (SI) GaAs. We are able to reduce the carrier lifetime of SI-GaAs down to sub-picosecond by irradiating it with various irradiation dosages of Carbon (C(12)) ions. With an increase of the irradiation dose from ~10(12) /cm(2) to ~10(15) /cm(2) the carrier lifetime of SI-GaAs monotonously decreases to 0.55 picosecond, whereas that of usual non-irradiated SI-GaAs is ~70 picosecond. This decreased carrier lifetime has resulted in a strong improvement in THz pulse detection compared with normal SI-GaAs. Improvement in signal to noise ratio as well as in detection bandwidth is observed. Carbon irradiated SI-GaAs appears to be an economical alternative to low temperature grown GaAs for fabrication of THz devices.

Transient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for low-temperature grown GaAs

We revisit pump-probe transient reflectivity (PPTR) as a probe of ultrafast carrier dynamics in photoconductive materials, using low-temperature grown GaAs (LT-GaAs) as an exemplar. The carrier dynamics in a series of annealed LT-GaAs wafers were measured by PPTR. The wafer growth and anneal conditions were tailored to produce a material system with a sub-picosecond carrier lifetime. The PPTR signals from these wafers are bipolar with time constants on the order of 100 fs and 1 ps, consistent with previous literature reports on LT-GaAs. We examined the utility of numerical simulations of the pump-probe transients described in [V. Ortiz et al., J. Appl. Phys. 102, 043515 (2007)] to model our experimental results. We discovered a discrepancy between the model's predictions and experiment with respect to the scaling of the PPTR response with injected carrier density, and show that this discrepancy is rooted in how the model accounts for the index of refraction change due to band filling (BF) and band gap renormalization (BGR). We demonstrate that any model that includes a non-negligible BGR effect is inconsistent with our experimental observations of LT-GaAs. We present a revised model to simulate PPTR signals that account for BF and incorporate optical absorption from mid-gap states. This model can reproduce the experimental results on LT-GaAs and enables comparative assessments of alternate trapping and recombination hypotheses. For LT-GaAs, we compared point defects and nanoparticles as sites for Shockley-Read-Hall recombination, with the result that nanoparticle trapping and recombination centers most accurately reproduce the PPTR probe of carrier dynamics in LT-GaAs.

Picosecond Joule heating in photoconductive switch electrodes

We present experimental observations of picosecond Joule heating inside the gold metallization of ErAs:GaAs photoconductive switches. Femtosecond laser time-domain thermoreflectance is employed to resolve the fast thermal dynamics in the central switch electrode during generation and transmission of rf electrical pulses. Sharp features in the thermoreflectance signal scaling quadratically with the bias/pulse voltage reveal Joule heating with durations $\ensuremath{\le}$5 ps inside the gold metal. The temporal shape and rise time of the signals is in excellent agreement with the theoretical pulse wave form and subpicosecond carrier lifetime of the active medium. Probing different locations on the electrode shows the propagation, attenuation, and dispersion of the electrical pulses along the coplanar waveguide structure. Overall, the presented photothermal experiments demonstrate great potential to reveal the internal dynamics of photoconductive switches, characterize transmission lines, and study ultrafast heating in metals.

A metal-metal Fabry–Pérot cavity photoconductor for efficient GaAs terahertz photomixers

The low responsivity of the low-temperature-grown GaAs based planar photoconductors used in the photomixing experiments can be improved by using a metal-metal Fabry–Pérot cavity. This resonant cavity photoconductor exhibits a dc-responsivity above 0.1 A/W and current density higher than 50 kA/cm2 with a low-temperature-grown-GaAs epitaxial layer presenting a subpicosecond carrier lifetime. Based on these results, up to 100 μW output power at 1 THz could be expected if this photoconductor is used in a photomixing experiment with a resonant antenna.

Femtosecond carrier dynamics in native and high resistivity iron-doped GaxIn1−xAs

We report femtosecond transient reflectivity measurements of as-grown and iron-doped GaxIn1−xAs. A hybrid vertical Bridgman and gradient freezing directional solidification process was employed for the growth of high quality Ga0.69In0.31As:Fe crystals with the uniform impurity doping concentration necessary for high resistivity (1.6×107 Ω cm) and high mobility [(2–3)×103 cm2/V s] material. Carrier lifetimes range from ∼62 fs for as-grown Ga0.09In0.91As to ∼306 fs for Ga0.69In0.31As:Fe. The high carrier mobility along with high resistivity and subpicosecond carrier lifetimes make Ga0.69In0.31As:Fe an excellent candidate for photoconducting antenna based terahertz emitters.

GaAs photodetectors prepared by high-energy and high-dose nitrogen implantation

The authors report on the fabrication and characterization of photodetectors based on nitrogen-ion-implanted GaAs and the annealing dynamics in these devices. An energy of 400keV was used to implant N ions in a GaAs substrate at an ion concentration of ∼1×1016cm−2. Dark current measurements as well as measurements under illumination show that the material properties rapidly change during the annealing process. Photodetectors based on nitrogen-implanted GaAs materials with annealing temperatures up to 400°C exhibit a subpicosecond carrier lifetime up to 0.6ps. These properties make nitrogen-ion-implanted GaAs an ideal material for ultrafast photodetectors, as alternative to low-temperature-grown GaAs.

Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200kV∕cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720fs in our LT GaN material.

Electrical properties of 1.55 µm sensitive ion-irradiated InGaAs with subpicosecond carrier lifetime

Carrier lifetimes as short as 270 fs with carrier mobility of 200 cm2/V/s and good performances in terms of layer resistivity have been obtained from ion-irradiated InGaAs. The residual carrier concentration measured in Hall effect experiments was found to be weakly modified in spite of the high defect concentration created by the ion bombardment. Ion-irradiated InGaAs appears to be specially adapted to fast photoconductive devices operating at optical communication wavelengths.

Characterization of annealed high-resistivity InP grown by He-plasma-assisted epitaxy

In this work we report on the effect of annealing high-resistivity InP grown by gas source molecular beam epitaxy in the presence of an electron cyclotron resonance generated He plasma. Previous work has shown that InP grown using this technique exhibits resistivity greater than 105 Ω cm and Be-doped InGaAsP (lattice matched to InP, band-gap wavelength 1.5 μm) exhibits subpicosecond carrier lifetime. This behavior is due to the presence of defects caused by the plasma particles during growth. To gain a better understanding of the nature of these defects, samples were annealed over the temperature range 500–700 °C and evaluated by variable energy positron annihilation measurements, transient ellipsometric surface photoreflectance, sheet resistance, and n–i–n device resistivity studies. For all samples, the resistivity increased with annealing, as did the carrier lifetime from the optical measurements which suggests the presence of more than one defect type in the material. Positron annihilation studies suggest that the open volume defects, present in the as-grown material as single vacancies and vacancy clusters, become larger upon annealing although this effect may be obscured in the Be-doped samples at higher annealing temperatures. The Be-doped samples were found to be more resistive and optically faster than the undoped material. Also, the sheet resistance measurements may indicate that the Be in the doped samples is activated at annealing temperatures above 600 °C, since the material changes from weakly n-type to p-type.

Subpicosecond carrier lifetime in beryllium-doped InGaAsP grown by He-plasma-assisted molecular beam epitaxy

The carrier dynamics and absorption edges of InGaAsP samples grown by He-plasma-assisted molecular beam epitaxy and doped with various concentrations of beryllium are investigated via pump-probe experiments and Fourier transform infrared (FTIR) absorption spectroscopy. Carrier lifetimes from 10 to &amp;lt;1 ps are obtained for samples of increasing doping concentrations. The reduced in carrier lifetimes are attributed to Be compensation of the deep donor levels introduced by the He plasma. The carrier lifetime increases with photogenerated carrier density due to trap saturation. The FTIR results reveal sharp absorption edges in this material for doping concentrations up to 1×1018 cm−3. The fast carrier dynamics and the steep absorption edge make this material very attractive for ultrafast optical switching devices for use in high-speed time-division-multiplexing fiber communication systems.

Subpicosecond carrier lifetimes in arsenic-ion-implanted GaAs

We have investigated photoexcited carrier lifetimes in arsenic-ion-implanted semi-insulating GaAs by time-resolved reflectivity measurements. Subpicosecond carrier lifetimes (220 to 550 fs) which do not exhibit apparent dosage dependence for samples bombarded with 200 keV arsenic ions at increasing dosages in the range of 1012 and 1016 ions/cm2 are reported. The shortest carrier lifetime was observed for the sample irradiated at 1013 ions/cm−2. These are the shortest lifetimes ever observed for ion-damaged GaAs and comparable to those of low-temperature molecular beam epitaxially grown GaAs, which is also nonstoichiometric with excess-arsenic-related, deep-level defects.

Subpicosecond carrier lifetimes in GaAs grown by molecular beam epitaxy at low substrate temperature

Time-resolved photoluminescence transients of low-temperature molecular beam epitaxially grown GaAs layers have been measured with femtosecond temporal resolution and compared with numerical Monte Carlo calculations. It has been shown that the shape of these transients measured at different emission energies is determined not only by the carrier lifetime but also by the electron redistribution in the conduction band. Analysis of the experimental results yields the carrier lifetime in the investigated samples of 400 fs.

Relation Between Structure and Carrier Lifetime in As-Implanted GaAs

AbstractThe structure of As implanted GaAs layers before and after annealing are described and the relation between the structural quality and carrier lifetime was determined. Subpicosecond carrier lifetimes were found already for as-implanted layers, and this value changes only slightly after annealing in the temperature range up to 600°C. Annealing of As-implanted layers leads to the growth of As precipitates with a similar orientation relationship as those observed in low-temperature MBE-grown GaAs layers. However, it is still not clear whether point defects created by implantation or the As precipitates are responsible for the short carrier lifetime.

Subpicosecond carrier lifetime in InP

Femtosecond time-resolved reflectivity investigations reveal a significant reduction in carrier lifetime from 100ps in nonintentionally doped InP to less than 1ps in S, Fe, and Zn doped InP. This reduction is because S, Fe, and Zn in InP serve as recombination and trapping centers. Photoluminescence and Raman measurements were used to confirm these results.

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

Structure and carrier lifetime in LT-GaAs

The relationship between the structural quality of low-temperature GaAs layers and the photoexcited carrier lifetime has been studied. Transmission electron microscopy, x-ray rocking curves, time-resolved reflectance methods, and photoconductive-switch-response measurements were used for this study. For a variety of samples grown at temperatures in the vicinity of 200°C, subpicosecond carrier lifetimes were observed both in as-grown layers, as well as in the same layers after post-annealing and formation of As precipitates. These results suggest that the carrier lifetime, which was found to be shorter in the as-grown layers than in the annealed ones, might be related to the density of AsGa antisite defects present in the layers. The annealed layers which contained structural defects before annealing appeared to exhibit the longest carrier lifetime due to gettering of As on these defects (and formation of relatively large As precipitates) and depletion of extra As (AsGa) defects from the layer. It was found as well that the responsivity of detectors fabricated on these layers depended strongly on the structural quality of the layers, with the greatest response obtained not for the layers with the fewest defects, but for the layers with 107–108/cm2 of pyramidal defects.