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Focused ion beam (FIB) systems have become very useful tools used in the nanotechnology because they provide easy prototyping or post-processing customization of individual devices. A practical application of such post-processing is modification of monolithic quantum cascade laser (QCL) to obtain the structure with coupled cavities (CC-QCL), enabling its single mode performance, critical in many QCL applications. In-situ electrical measurements of the test structures showed that the main problem is avoiding a secondary deposition of the milled material. The redeposition can significantly reduce the performance and reliability of lasers. An avoidance of FIB imaging after final FIB cleaning of walls was revealed as an important factor leading to high quality of structures. The paper describes the solution by developing appropriate patterning procedure. This enabled the formation of exemplary single mode CC-QCLs with excellent performance at 9.45 μm wavelength with 30 dB side-mode suppression ratio, operating at room temperature. Performance of obtained single-mode coupled cavity quantum cascade lasers was stable, repeatable and no reliability problems were observed.

Publisher's version (útgefin grein), The terahertz time-domain spectroscopy (THz-TDS) technique has been used to obtain transmission THz-radiation spectra of polymer nanocomposites containing a controlled amount of exfoliated graphene. Graphene nanocomposites (1 wt%) that were used in this work were based on poly(ethylene terephthalate-ethylene dilinoleate) (PET-DLA) matrix and were prepared via a kilo-scale (suitable for research and development, and prototyping) in-situ polymerization. This was followed by compression molding into 0.3-mm-thick and 0.9-mm-thick foils. Transmission electron microscopy (TEM) and Raman studies were used to confirm that the graphene nanoflakes dispersed in a polymer matrix consisted of a few-layer graphene. The THz-radiation transients were generated and detected using a low-temperature-grown GaAs photoconductive emitter and detector, both excited by 100-fs-wide, 800-nm-wavelength optical pulses, generated at a 76-MHz repetition rate by a Ti:Sapphire laser. Time-domain signals transmitted through the nitrogen, neat polymer reference, and 1-wt% graphene-polymer nanocomposite samples were recorded and subsequently converted into the spectral domain by means of a fast Fourier transformation. The spectral range of our spectrometer was up to 4 THz, and measurements were taken at room temperature in a dry nitrogen environment. We collected a family of spectra and, based on Fresnel equations, performed a numerical analysis, that allowed us to extract the THz-frequency-range refractive index and absorption coefficient and their dependences on the sample composition and graphene content. Using the Clausius-Mossotti relation, we also managed to estimate the graphene effective dielectric constant to be equal to ~7 ± 2. Finally, we extracted from our experimental data complex conductivity spectra of graphene nanocomposites and successfully fitted them to the Drude-Smith model, demonstrating that our graphene nanoflakes were isolated in their polymer matrix and exhibited highly localized electron backscattering with a femtosecond relaxation time. Our results shed new light on how the incorporation of exfoliated graphene nanoflakes modifies polymer electrical properties in the THz-frequency range. Importantly, they demonstrate that the complex conductivity analysis is a very efficient, macroscopic and non-destructive (contrary to TEM) tool for the characterization of the dispersion of a graphene nanofiller within a copolyester matrix., This research was funded in part by the PumpPrimerII Program at the University of Rochester. A.N.V. acknowledges support from the Air Force Office of Scientific Research (FA9550-16-1-0020). M.E.F. acknowledges support from the Polish National Centre for Research and Development (PBS1/A5/2/2012).

In this paper, the optimization of ohmic contacts for semiconductor lasers based on InGaAs/GaAs/GaAlAs layers is reported. Transmission electron microscopy (TEM) and electrical methods were used to study extensively the Pt/Ti/Pt/Au metallization system. The contact fabrication technology was optimized towards achieving the lowest electrical resistance. The technological control and optimization concerned the contact annealing temperature and thickness of metallic layers that form the contact. The average specific contact resistance was below 5×10−6 Ω cm2 (with the record value of 8×10−7 Ω cm2) for the 10 nm Pt/20 nm Ti/30 nm Pt/150 nm Au system. The presented system was used in fabrication of continuous wave (CW) operated laser diodes. The chips mounted on passively cooled copper block achieved optical powers over 1 W, threshold current density values of 140–160 A/cm2 and differential efficiencies above 1 W/A. The value of the characteristic temperature T0 for discussed lasers varied in the range of 180–200 K.

We report our research on processing of AlGaAs/GaAs structures for THz quantum-cascade lasers (QCLs). We focus on the processes of fabrication of Ti/Au claddings for metal-metal waveguides and the wafer bonding with indium solder. We place special emphasis on the optimum technological conditions of these processes, leading to working devices. The wide range of technological conditions was studied, by use of test structures and analyses of their electrical, optical, chemical and mechanical properties, performed by electron microscopic techniques, energy dispersive X-ray spectrometry, secondary ion mass spectroscopy, atomic force microscopy, fourier-transform infra-red spectroscopy and circular transmission line method. On the basis of research a set of technological conditions was selected, and devices lasing at the maximum temperature 130K were fabricated from AlGaAs/GaAs structures grown by molecular beam epitaxy (MBE) technique. Their threshold-current densities were about 1.5kA/cm2. Additionally we report our initial stage research on fabrication of Cu-based claddings, that theoretically are more promising than the Au-based ones for fabrication of low-lossy waveguides for THz QCLs.

We report research results with regard to AlGaAs/GaAs structure processing for THz quantum-cascade lasers (QCLs). We focus on the processes of Ti/Au cladding fabrication for metal–metal waveguides and wafer bonding with indium solder. Particular emphasis is placed on optimization of technological parameters for the said processes that result in working devices. A wide range of technological parameters was studied using test structures and the analysis of their electrical, optical, chemical, and mechanical properties performed by electron microscopic techniques, energy dispersive x-ray spectrometry, secondary ion mass spectroscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, and circular transmission line method. On that basis, a set of technological parameters was selected for the fabrication of devices lasing at a maximum temperature of 130 K from AlGaAs/GaAs structures grown by means of molecular beam epitaxy. Their resulting threshold-current densities were on a level of 1.5 kA/cm2. Furthermore, initial stage research regarding fabrication of Cu-based claddings is reported as these are theoretically more promising than the Au-based ones with regard to low-loss waveguide fabrication for THz QCLs.