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Actuation of thin polymeric films via electron irradiation is a promising avenue to realize devices based on strain engineered two-dimensional materials. Complex strain profiles demand a deep understanding of the mechanics of the polymeric layer under electron irradiation; in this article, we report a detailed investigation on electron-induced stress on a poly-methyl-methacrylate (PMMA) thin film material. After an assessment of stress values using a method based on dielectric cantilevers, we directly investigate the lateral shrinkage of PMMA patterns on epitaxial graphene, which reveals a universal behavior, independent of the electron acceleration energy. By knowing the stress-strain curve, we finally estimate an effective Young's modulus of PMMA on top of graphene, which is a relevant parameter for PMMA-based electron-beam lithography and strain engineering applications. Published under license by AIP Publishing.

An "injectorless" quantum-cascade (QC) laser is presented. The requirement of using injector regions to transport electrons from the lower laser level and other low-lying energy levels of one active region to the upper laser level of the next electron-downstream active region was eliminated by using an appropriately designed double-quantum-well "chirped" superlattice active region. The major advantage of the "injectorless" QC laser is the close packing of the active regions and the concomitant large optical confinement factor. Using a cascade of 75 consecutive active regions, designed for emission at lambda = 11.5 mum, a pulsed peak output power of 270 mW is achieved at 7 K and approximately 10 mW at the maximum operating temperature of 195 K. (C) 2001 American Institute of Physics.

The local temperature of quantum-cascade lasers operating in continuous wave mode is reported. This information is extracted from the thermal shift of the band-to-band photoluminescence peaks in the AlInAs and InP cladding layers of quantum-cascade laser facets using a high-resolution microprobe setup. Interpolation by means of a two-dimensional heat diffusion model allows to obtain the temperature profile and the thermal conductivity in the waveguide core. Comparison between substrate and epilayer-side mounted lasers shows the superior thermal dissipation capability of the latter, and explains their better performance with respect to threshold current and maximum operating temperature. (C) 2001 American Institute of Physics.

The longest wavelength III–V semiconductor laser to date has been realized employing a quantum cascade configuration for the active material. It operates at λ∼19 μm on interminiband transitions in graded superlattices. The waveguide is based on surface plasmon modes confined at a metal–semiconductor interface. The devices emit peak output powers of 14 mW per facet at 10 K and laser action is achieved up to the maximum temperature of 145 K.

Single-mode tunable quantum-cascade distributed feedback lasers emitting at 4.6–4.7 μm wavelength are reported. The lasers employ strained heterostructure material with global strain compensation to provide the large band offset needed for high-performance short wavelength operation. Pulsed, continuously tunable single-mode emission is achieved from 90 to 300 K with a tuning range of 65 nm. Peak output power levels of 100 mW at room temperature are obtained. In continuous-wave operation, current tunable single-mode emission is demonstrated around liquid-nitrogen temperature with a tuning range of 20 nm (over a current range of 450 mA). The maximum output power in continuous wave at 80 K is 150 mW.

We report the realization of a semiconductor injection laser based on intraband transitions with emission wavelengths extending beyond the atmospheric windows. The structure uses the quantum cascade scheme with “chirped” superlattices as active material. Laser action in pulsed operation is achieved at λ≃17 μm up to 150 K, with peak output powers of ∼12 mW at cryogenic temperatures.

The realization of the first noncascaded intersubband injection lasers based on a single optical transition is reported. The unipolar lasers are based on an active region consisting of three InGaAs quantum wells closely coupled by thin AlInAs barriers. The lasers emit at λ≈7.7 μm wavelength and operate in pulsed mode up to 110 K. Peak power levels of 20 mW at 10 K and 4 mW at 110 K are obtained. The low-temperature threshold current density is 25.6 kA cm−2 in good agreement with calculations. Several advantages arise from this novel type of intersubband laser. First, only few layers are necessary to build the active region, simplifying sample preparation. Second, low operating voltages can be achieved, which is essential for many applications. Finally, the noncascaded intersubband laser allows studying fundamental properties of quantum cascade lasers without possible artifacts introduced by the sequential stacking of many active regions.

Quantum cascade (QC) lasers emitting at λ≈8 μm with a power performance equal to short-wavelength (λ≈5 μm) QC lasers are reported. The device improvement is mainly achieved by a design of the injector/relaxation region, which at laser threshold allows resonant carrier injection between the ground state of the preceding and the upper laser level of the subsequent active region. In pulsed operation a peak output power of 1.3 W per facet has been measured at 100 K. At room temperature a record peak power of 325 mW and a record slope efficiency of 180 mW/A have been measured. In continuous-wave operation the maximum power at 30 K was 510 mW per facet and still 200 mW per facet at 80 K. The high values of the output power and slope efficiency demonstrate the validity of the cascading scheme, in which electrons above threshold generate one photon per each active region they successively traverse.

We demonstrate the preparation of narrow‐band porous‐silicon reflectors integrated on porous‐silicon layers by electrochemical etching. By carefully tuning the resulting photon cavity mode around the maximum of the porous silicon photoluminescence, we have obtained both a narrowing and enhancement of the emission line, and a highly concentrated radiation pattern. These results show that the porous silicon spontaneous emission is modified because of the coupling with the photon cavity mode.

Conductive metallic contacts can significantly affect the operation of field effect transistors fabricated starting from semiconductor nanowires deposited on a dielectric substrate. Screening effects can also lead to systematic errors in the estimates of transport parameters obtained on the basis of simple uniform capacitive models. We study the role of contacts in both back- and lateral-gate transistor geometries and provide rules of thumbs to predict screening effects in real devices. Additionally, we show how the contacts influence charge density profiles within the wire, focusing in particular on their evolution when transistors nonlinear properties are addressed.

We present a surface emitting distributed feedback terahertz laser resonator based on a dual-slit configuration. With the spacing of the slits as a parameter, the surface losses of the lasers can be chosen over a large range, while keeping slits narrow for optimum current injection. Quantum cascade devices based on a double metal waveguide showed slope efficiencies up to 40 mW/A and peak powers of more than 20 mW. The efficient radiation out-coupling does not depend on boundary conditions, and therefore would allow the application to large-area devices, making this an ideal resonator for high-power surface emission.

We explore the use of quantum cascade structures as amplifiers for terahertz radiation. For this purpose, two section devices were fabricated, in which the output from a nu similar to 2.3 THz laser section is amplified in a subsequent stage. For relatively short devices we obtain an amplification of 1.5x, in good agreement with the calculated level. Based on these results we predict that amplifications of up to 13 dB could be obtained for such structures using increased amplifier lengths of a few millimeters.(C) 2007 American Institute of Physics.

The conduction‐band discontinuity in Zn1−xCdxSe/ZnSe multiple‐quantum‐well structures grown by molecular‐beam epitaxy and pseudomorphically strained to ZnSe were evaluated via electrical and intersubband absorption measurements for 0.23≤x≤0.33. A variation between 165±14 and 253±14 meV is found in the cadmium concentrations range analyzed. Our results are consistent with a valence‐band contribution to the band‐gap difference Qv=0.33±0.03 in the alloy series.