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Exhaled breath analysis holds great promise for the development of noninvasive, frequently repeatable diagnostic and monitoring tools. For clinical breath analysis to advance beyond its current state, however, much closer multidisciplinary collaboration needs to be not only recognized but also effected. Therefore, this paper reviews the current state of clinical breath analysis from the perspective of the challenges the field faces medically (biomarker uncertainties, sampling methods, dynamics of exogenous compounds within the body, and standardization), technologically (the need for an affordable, user-friendly, real-time, point-of-care instrument for accurate identification of breath volatiles and their concentrations), biochemically (the need to link exhaled compounds with specific diseases by understanding the volatile products particular to relevant pathogenic processes), and in terms of data interpretation (quality, quantity, and complexity of data), collaboration (the need for a more integrated approach to breath analysis, including public health input), and development from research to accepted clinical use (funding challenges peculiar to the medical/technological interface, achieving standards of effectiveness and cost-effectiveness). Having thus increased awareness and aligned expectations among relevant disciplines, this paper provides a course of action for closer collaboration, better understanding, and more productive dialogue between these disciplines, including an iterative sensor development process that is integrated with clinical trials, formation of goals that transcend individual disciplines, creation of multidisciplinary research teams and a cross-disciplinary student exchange program, and collaborative funding options. [ABSTRACT FROM PUBLISHER]

Quantum cascade lasers (QCLs) are complicated unipolar semiconductor devices based on intersubband transitions and resonant tunneling. In this study, femtosecond mid-infrared (Mid-IR) pulses are employed to investigate the nature of carrier transport through the active and injector regions of a room temperature, pulse biased ultrastrong coupling design QCL. Despite the low average power (<1 mW) of femtosecond Mid-IR pulses, the efficient coupling of these pulses into the QCL waveguide made the study of nonlinear effects in QCLs possible. Biased just below threshold, we observed ultrafast gain recovery within the first 200 f s mainly contributed by electrons resonant tunneling through a much thinner injector barrier than that of conventional designed QCLs, which overcomes the interface-roughness-induced detuning of resonant tunneling. Oscillation or overshooting within the first picosecond is caused by electron relaxation from continuum region excited by strong pump beam, as well as coherent electron tunneling transport from injector to active region. The former feature is supported by the observation of second harmonic generation (SHG) with emission of λ ≈ 2.2 μm pulses and measured positive photoconductivity. The transport of electrons through the injector region contributes to a slower gain recovery. A much longer recovery (hundreds of picoseconds) can be explained as electrons are depleted from upper stages down to lower stages in real space. [ABSTRACT FROM PUBLISHER]

A four-wavelength quantum-cascade (QC) laser source that operates using a single current channel is presented. The source includes two different heterogeneous cascade QC lasers, one with emission wavelengths of 7.0 μm and 11.2 μm, and the other with 8.7 μm and 12.0 μm. For 3.0-mm and 3.5-mm cavity lengths, QC lasers with emission wavelengths of 8.7, 11.2, and 12.0 μm have threshold current densities within less than a factor of 2, which allows them to be conveniently driven in series by a single current source. [ABSTRACT FROM AUTHOR]

The voltage tunability of three types of quantum cascade laser designs is investigated. The tuning coefficients and tuning ranges of electroluminescence and laser emission from all designs are measured and compared with the calculated results. A reduced tunability was observed in all lasers above threshold. This is attributed to the decrease of resistance across the laser active region (AR) as the photon density increases. A resumed tunability high above threshold occurs in all lasers with anticrossed injector ground and upper laser states. Lasers based on the anticrossed diagonal transition are tunable above threshold, with a tuning range of about 30 cm-1 (~3% of the laser emission wavenumber), i.e., a tuning rate of 750 cm-1V-1 . period-1 of the AR and the injector. [ABSTRACT FROM AUTHOR]

The study of the electron distribution and intersubband transitions in quantum cascade (QC) lasers provides fundamental insights into the laser operation. Measurements on intersubband transitions are often performed on QC lasers under nonoperating conditions, i.e., at zero electric field. In this study, a single-pass transmission technique that allows for probing of QC lasers under operating conditions was used. The measurements and analysis were done on QC lasers with vertical and diagonal transition active region designs operating at λ ~ 10 μm. The single-pass modal gain coefficients, extracted for the vertical transition design, were 24.04 cm/kA at 100 K, 21.36 cm/kA at 125 K, 14.25 cm/kA at 150 K, and 5.26 cm/kA at 175 K. [ABSTRACT FROM AUTHOR]

Temperature-dependent optical gain and waveguide loss have been measured for continuous-wave operated quantum- cascade lasers with wavelengths between 8.2 and 10.2 μm up to room temperature using the Hakki-Paoli method. The gain co-efficient decreases with increasing temperature, and is close to the designed value for vertical transition lasers, but smaller than the designed value for diagonal transition lasers. The waveguide loss, however, is two to three times higher than calculated from free carrier absorption, and can be nearly constant, increase or decrease with temperature depending on sample design, which indicates that it is dominated by another mechanism other than plain free carrier absorption. One likely factor resulting in high wave-guide loss is intersubband resonant absorption into higher lying states. [ABSTRACT FROM AUTHOR]

A computational and experimental analysis of rollover in high-performance A 84um quantum-cascade (QC) lasers is presented. In addition to conventional, thermal rollover, which is also a common cause of power rollover in diode lasers, "Stark-effect" rollover is observed. While both effects can occur in the same QC laser design, thermal and Stark-effect rollover are shown to be the dominating factor for high-temperature continuous wave operated, and pulsed low-temperature operated and low-doped lasers, respectively. Additionally, the role of the continuum above the wells and barriers is discussed for both effects. [ABSTRACT FROM AUTHOR]