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.

Ultrabroadband terahertz time-domain spectroscopy using III-V photoconductive membranes on silicon.

Electromagnetic waves in the terahertz (THz) frequency range are widely used in spectroscopy, imaging and sensing. However, commercial, table-top systems covering the entire frequency range from 100 GHz to 10 THz are not available today. Fiber-coupled spectrometers, which employ photoconductive antennas as emitters and receivers, show a bandwidth limited to 6.5 THz and some suffer from spectral artifacts above 4 THz. For these systems, we identify THz absorption in the polar substrate of the photoconductive antenna as the main reason for these limitations. To overcome them, we developed photoconductive membrane (PCM) antennas, which consist of a 1.2 µm-thin InGaAs layer bonded on a Si substrate. These antennas combine efficient THz generation and detection in InGaAs with absorption-free THz transmission through a Si substrate. With these devices, we demonstrate a fiber-coupled THz spectrometer with a total bandwidth of 10 THz and an artifact-free spectrum up to 6 THz. The PCM antennas present a promising path toward fiber-coupled, ultrabroadband THz spectrometers.

Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth.

Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

Fiber Coupled Transceiver with 6.5 THz Bandwidth for Terahertz Time-Domain Spectroscopy in Reflection Geometry.

We present a fiber coupled transceiver head for terahertz (THz) time-domain reflection measurements. The monolithically integrated transceiver chip is based on iron (Fe) doped In0.53Ga0.47As (InGaAs:Fe) grown by molecular beam epitaxy. Due to its ultrashort electron lifetime and high mobility, InGaAs:Fe is very well suited as both THz emitter and receiver. A record THz bandwidth of 6.5 THz and a peak dynamic range of up to 75 dB are achieved. In addition, we present THz imaging in reflection geometry with a spatial resolution as good as 130 µm. Hence, this THz transceiver is a promising device for industrial THz sensing applications.

Improving the dynamic range of InGaAs-based THz detectors by localized beryllium doping: up to 70 dB at 3 THz.

In this Letter, we report on photoconductive terahertz (THz) detectors for 1550 nm excitation based on a low-temperature-grown InGaAs/InAlAs superlattice with a localized beryllium doping profile. With this approach, we address the inherent lifetime-mobility trade-off that arises, since trapping centers also act as scattering sites for photo-excited electrons. The localized doping of the InAlAs barrier only leads to faster electron trapping for a given mobility. As a result, we obtain THz detectors with more than 6 THz bandwidths and 70 dB dynamic ranges (DNRs) at 3 THz and 55 dB DNR at 4 THz. To the best of our knowledge, this is the highest DNR for photoconductive THz time-domain spectroscopy systems published so far.

Twinning superlattice formation in GaAs nanowires.

Semiconductor nanowires have proven a versatile platform for the realization of novel structures unachievable by traditional planar epitaxy techniques. Among these, the periodic arrangement of twin planes to form twinning superlattice structures has generated particular interest. Here we demonstrate twinning superlattice formation in GaAs nanowires and investigate the diameter dependence of both morphology and twin plane spacing. An approximately linear relationship is found between plane spacing and nanowire diameter, which contrasts with previous results reported for both InP and GaP. Through modeling, we relate this to both the higher twin plane surface energy of GaAs coupled with the lower supersaturation relevant to Au seeded GaAs nanowire growth. Understanding and modeling the mechanism of twinning superlattice formation in III-V nanowires not only provides fundamental insight into the growth process, but also opens the door to the possibility of tailoring twin spacing for various electronic and mechanical applications.

Three-dimensional in situ photocurrent mapping for nanowire photovoltaics.

Devices based upon semiconductor nanowires provide many well-known advantages for next-generation photovoltaics, however, limited experimental techniques exist to determine essential electrical parameters within these devices. We present a novel application of a technique based upon two-photon induced photocurrent that provides a submicrometer resolution, three-dimensional reconstruction of photovoltaic parameters. This tool is used to characterize two GaAs nanowire-based devices, revealing the detail of current generation and collection, providing a path toward achieving the promise of nanowire-based photovoltaic devices.

Polarized recombination of acoustically transported carriers in GaAs nanowires.

: The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport.

Lattice parameter accommodation between GaAs(111) nanowires and Si(111) substrate after growth via Au-assisted molecular beam epitaxy.

Using out-of-plane and in-plane X-ray diffraction techniques, we have investigated the structure at the interface between GaAs nanowires [NWs] grown by Au-assisted molecular beam epitaxy and the underlying Si(111) substrate. Comparing the diffraction pattern measured at samples grown for 5, 60, and 1,800 s, we find a plastic strain release of about 75% close to the NW-to-substrate interface even at the initial state of growth, probably caused by the formation of a dislocation network at the Si-to-GaAs interface. In detail, we deduce that during the initial stage, zinc-blende structure GaAs islands grow with a gradually increasing lattice parameter over a transition region of several 10 nm in the growth direction. In contrast, accommodation of the in-plane lattice parameter takes place within a thickness of about 10 nm. As a consequence, the ratio between out-of-plane and in-plane lattice parameters is smaller than the unity in the initial state of growth. Finally the wurtzite-type NWs grow on top of the islands and are free of strain.

Suitability of Au- and self-assisted GaAs nanowires for optoelectronic applications.

The incorporation of Au during vapor-liquid-solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free grown GaAs/(Al,Ga)As core-shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures.

Effect of the solvent on the conformation of isolated MEH-PPV chains intercalated into SnS2.

Photophysical processes in conjugated polymers are influenced by two competing effects: the extent of excited state delocalization along a chain, and the electronic interaction between chains. Experimentally, it is often difficult to separate the two because both are controlled by chain conformation. Here we demonstrate that it is possible to modify intra-chain delocalization without inducing inter-chain interactions by intercalating polymer monolayers between the sheets of an inorganic layered matrix. The red-emitting conjugated polymer, MEH-PPV, is confined to the interlayer space of layered SnS(2). The formation of isolated polymer monolayers between the SnS(2) sheets is confirmed by X-ray diffraction measurements. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the incorporated MEH-PPV chains reveal that the morphology of the incorporated chains can be varied through the choice of solvent used for chain intercalation. Incorporation from chloroform results in more extended conformations compared to intercalation from xylene. Even highly twisted conformations can be achieved when the incorporation occurs from a methanol:chloroform mixture. The PL spectra of the MEH-PPV incorporated SnS(2) nanocomposites using the different solvents are in good agreement with the PL spectra of the same solutions, indicating that the conformation of the polymer chains in the solutions is retained upon intercalation into the inorganic host. Therefore, intercalation of conjugated polymer chains into layered hosts enables the study of intra-chain photophysical processes as a function of chain conformation.