Orchi Hassan

Effect of temperature on Quantum Cascade Laser emission as a function of cavity length

Quantum Cascade Laser (QCL) is extremely unique in its characteristics due to its narrow bandwidth resulting from unipolar operation, mid-infrared emission, and wavelength tunability by quantum mechanical engineering. While lasing, the change in temperature affects the cavity length of the QCL, which in turn may affect the emission wavelength. In this work, we have studied the tunability of QCL emission when the operating temperature varies. We have found that the temperature-dependent tunability of the emission wavelength is sensitive to the cavity length. The tunability is greater for smaller cavity lengths. However, the emission wavelength may change sharply at different temperatures for larger cavities due to mode hopping. In any case, the overall change in the emission wavelength will not affect the performance if the target molecules do not have absorption peaks separated by less than 2 cm-1, which makes QCL an outstanding mid-infrared spectroscopy source.

Quantum cascade laser wavelength tuning due to temperature-dependent index of refraction

In this work, we study the change in the emission wavelength of quantum cascade lasers when the index of refraction of the lasing medium changes due to a change in the temperature. We find that the gain of the laser and the wavelength of the longitudinal optical modes in the laser cavity change significantly as the temperature changes. As a result, the output wavelength of QCLs changes as the temperature changes. When the wavelength of the lasing mode changes so much that one of the neighboring modes becomes the closest mode to the peak of the gain spectrum, the lasing hops to the neighboring mode. We also find that the mode-tuning and mode-hopping depend on the longitudinal length of the laser cavity. The tuning range increases for a smaller cavity, while the mode-hopping becomes more frequent for a longer cavity as the temperature increases.

Study of design-dependent electroluminescence linewidth of quantum cascade lasers

Electroluminescence linewidth is an important characteristic of quantum cascade lasers (QCLs) because the gain and the emission spectra of QCLs significantly depend on electroluminescence linewidth. Electroluminescence linewidth depends on the different intra-subband and inter-subband electron scattering mechanisms. Since the electron scattering rates due to different mechanisms vary when the applied electric field, temperature, and the quantum mechanical design vary, so does the electroluminescence linewidth of QCLs. In this work, we have studied the change in electroluminescence linewidth of QCLs when the quantum mechanical design and the material composition vary. We find that the linewidth of QCLs with diagonal lasing transition is greater than the linewidth of QCLs with vertical lasing transition; the linewidth of QCLs with strain-compensated material system is greater than the linewidth of QCLs with lattice-matched material system; and the linewidth of QCLs built on InGaAs/InAlAs material system is greater than the linewidth of QCLs built on GaAs/AlGaAs material system.

Injection ratio and storage time of a non-uniformly doped Schottky barrier diode

The minority carrier injection and excess minority carrier stored charge of a non-uniformly doped n-Si Schottky barrier diode (SBD) considering carrier recombination and blocking properties of the low-high (n-n+) interface are analysed in this work. Based on the result of numerical analysis of slow variation of minority carrier current within the quasi-neutral Si, minority carrier current density is represented by a polynomial, and using that equation a solution of minority carrier profile is obtained. For the first time, a closed form expression for minority carrier profile p(x) for non-uniformly doped n-Si SBD is obtained, which is applicable for all levels of injection. Storage time and injection ratio can be obtained from minority carrier profile. Present analysis shows that charge storage time and current injection ratio depend on peak doping density, No and the logarithmic slope of doping profile, α. The storage time and injection ratio increase with α and decrease with increase of No. The results for Storage time and injection ratio obtained numerically, together with those obtained using the present model are compared and both results are found in good agreement.

Bias-dependent intersubband electroluminescence linewidth of quantum cascade lasers

The electroluminescence linewidth is a significant performance indicator of quantum cascade lasers (QCLs) as it determines peak gain, degree of monochromaticity, and wavelength tunability. In this work, we have modeled the intersubband electroluminescence linewidth of QCLs considering the intersubband and intrasubband electron-LO phonon scattering and electron-interface roughness scattering of the two lasing levels. We have studied the change in the electroluminescence linewidth as the applied bias changes. We find that, in QCLs those employ vertical radiative transition in real space, the linewidth decreases as the applied bias increases since the wavefunctions of the two lasing levels overlap more in active region quantum wells with increased bias. By contrast, we find that, in QCLs those employ diagonal radiative transition in real space, the linewidth increases since the wavefunctions of the two lasing levels overlap less in active region quantum wells with increased bias. We also find that the interface roughness is the dominant scattering mechanism to contribute to linewidth. The results that we have found agree with the experimental observations.

Short pulse dynamics in quantum cascade lasers

Since the first ever demonstration of the quantum cascade lasers (QCLs) in 1994, there has been extensive research on its improvement for various reasons. One of them is to produce ultrashort optical pulses. The nonlinearity in the index of refraction of a lasing medium can be manipulated to produce ultrashort laser pulses which have applications in the communication field, IC technology and even in surgical procedures. In this paper, we have modeled this dependency and have found that the pulse duration significantly increases in time when the pulse pulse intensity increases; however, the pulse duration significantly decreases in time when the pulse intensity increases.

Bias dependence of gain spectrum and output emission characteristics of two phonon resonance design quantum cascade lasers

Quantum cascade lasers (QCLs) are the most promising light sources for optical spectroscopy in the molecular finger-print mid-infrared region. To this purpose, QCLs are required to be wavelength selective and the change of emission wavelength needs to be known beforehand as the operating conditions, such as the applied bias changes. In this paper, we study the change in the gain spectrum and the output emission characteristics of QCLs as the applied bias changes. We find that the gain spectrum becomes narrower and the wavelength of the peak gain is blue shifted as the applied bias increases. As a result, the emission wavelengths at particular biases change sharply as the lasing hops between neighboring modes. For the QCL that we study, we find that the emission wavelength tunes from 8.862 μm to 8.538 μm as the applied bias changes from ~ 44 kV/cm to 64 kV/cm.