Mid-infrared (MIR) imaging has emerged as a valuable tool to investigate biological samples, such as tissue histological sections and cell cultures, by providing non-destructive chemical specificity ...without recourse to labels. While feasibility studies have shown the capabilities of MIR imaging approaches to address key biological and clinical questions, these techniques are still far from being deployable by non-expert users. In this review, we discuss the current state of the art of MIR technologies and give an overview on technical innovations and developments with the potential to make MIR imaging systems more readily available to a larger community. The most promising developments over the last few years are discussed here. They include improvements in MIR light sources with the availability of quantum cascade lasers and supercontinuum IR sources as well as the recently developed upconversion scheme to improve the detection of MIR radiation. These technical advances can substantially speed up data acquisition of multispectral or hyperspectral datasets thus providing the end user with vast amounts of data when imaging whole tissue areas of many mm2. Therefore, effective data analysis is of tremendous importance, and progress in method development is discussed with respect to the specific biomedical context.
Abstract
Narrow conduction channels are fabricated from an In
0.75
Ga
0.25
As-InP heterostructure using electron-beam lithography and dry etching. The etched surface is realized to be smooth by ...employing a reactive ion etching. The etching-induced surface conduction is eliminated by removing the damaged surface layer using a diluted HCl solution. The negligible surface depletion for the In-rich quantum well enables to create conducting channels in arbitrary geometries such as in a circular shape. We evidence the presence of a ballistic contribution in the electron transport by demonstrating a rectification of
rf
excitations that is achieved by the magnetic-field-tuned transmission asymmetry in the circularly-shaped channels. The absence of the surface depletion is shown to cause, on the other hand, a surface scattering for the electrons confined in the channels. An increase of the resistance, including its anomalous enhancement at low temperatures, is induced by the gas molecules attached to the sidewalls of the channels. We also report a large persistent photoconduction, which occurs as a parallel conduction in the undoped InP barrier layer.
A theory based on the Bardeen formalism is developed for computing the tunnel current between a metal tip and a semiconductor surface. Tip-induced band bending in the semiconductor is included, with ...the electrostatic potential computed in a fully three-dimensional model whereas the tunnel current is computed in the limit of large tip radii. Localized states forming at the semiconductor surface as well as wavefunction tailing through the semiconductor depletion region are fully accounted for. Numerical results are provided and compared with data obtained from p-type GaAs surfaces, and generalization of the method to semiconductor heterojunctions is discussed.
We compare the interface roughness scattering of electrons at the In0.53Ga0.47As/In0.52Al0.48As heterointerface simultaneously grown on (100) and (411)A oriented InP substrates using gas-source ...molecular-beam epitaxy (MBE). A modulation-doped double quantum well structure is designed to emphasize the effects of interface scattering. The transport properties for both (100) and (411)A orientations are compared for different MBE growth temperatures. The highest mobilities on (411) oriented structures are 40% higher than those on (100) oriented structures, indicating less electron scattering due to interface roughness scattering.
•A double quantum well structure is designed to emphasize interface scattering.•Interface quality is systematically compared between (100) and (411)A structures.•For all growth temperatures (400–550°C), (411)A interfaces are of superior quality.•The highest mobility is 40% higher in the (411)A samples.•QCL, QW-Lasers and HEMTs are expected to benefit from the superior interface quality.
The effect of post-growth annealing of epitaxial gallium phosphide grown on silicon substrates using gas-source molecular-beam epitaxy is described. The epitaxial layers were grown at substrate ...temperatures ranging from 250 to 550°C. After optimizing the growth temperature, the prepared films were thermally annealed. Two thermal annealing methods are compared: annealing at a constant temperature and step-graded annealing, in which the temperature is raised by a constant rate per unit time. The effect of the thermal annealing on the crystal structure was studied by characterizing the epilayer using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). It was found that after thermal annealing, the epilayer exhibited a reconstructed RHEED pattern, and its quality was much improved.
•Epitaxial growth of GaP/Si heterostructure.•New method of thermal annealing is proposed.•Effect of the thermal annealing on the grown films is studied.•XRD and SEM characterizations confirm that this method is applicable.
The influence of substrate temperature during the growth of quantum-cascade lasers using gas-source molecular-beam epitaxy on performance and crystal quality of quantum-cascade laser is investigated. ...Lower substrate temperature is consistently resulting in narrower X-ray diffraction satellites, and lower laser threshold current. This correlation is attributed to increased interface roughness at higher growth temperature.
► The influence of growth temperature of quantum-cascade lasers on laser performance was studied. ► The crystal quality is decreasing with the rise of growth temperature. ► Threshold current density is increasing with the rise of growth temperature. ► For samples with higher growth temperature the operating range and output power are larger.
We describe the realization of buried-heterostructure strain-compensated quantum-cascade lasers that incorporate a very high degree of internal strain and are grown on InP substrates using gas-source ...molecular-beam epitaxy (GSMBE). The active region of the lasers contains AlAs layers up to 1.6nm thick with 3.7% tensile strain; restricting any post-growth processing to temperatures below 600°C to avoid relaxation. We demonstrate that buried-heterostructure devices can be realized by using GSMBE to over-grow the etched laser ridge with insulating InP:Fe at temperatures low enough to preserve the crystal quality of the strain-compensated active region. Two distinct growth techniques are described, both leading to successful device realization: selective regrowth at 550°C and non-selective regrowth at 470°C. The resulting buried-heterostructure lasers are compared to a reference laser from the same wafer, but with SiO2 insulation; all three have very similar threshold current densities, operational thermal stability, and waveguide losses.
► Semi-insulating InP:Fe is grown by gas-source molecular-beam epitaxy. ► Buried-heterostructure strain-compensated quantum-cascade lasers are fabricated. ► Laser threshold and waveguide loss are consistent with those of reference structure.
Diffuse boundary reflection of electrons in the presence of a magnetic field is demonstrated to induce a rectification effect in circular-shaped ballistic nano-channels. The effect is robust as the ...rectification is associated with a transmission asymmetry in the equilibrium transmission predicted by the billiard model, which is possible as the time reversal symmetry is broken by the magnetic field. The rectified current exhibits antisymmetric dependence on the magnetic field despite the two-terminal geometry due to the inherent nonlinearity of the rectification. Unambiguous identification of the origin of the current based on this antisymmetric dependence reveals the rectification effect being sensitive to the extent that extraction of electrical energy out of environmental non-equilibrium noises is achieved.
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