Muhammad Anisuzzaman Talukder

Self-induced transparency modelocking of quantum cascade lasers in the presence of saturable nonlinearity and group velocity dispersion.

We consider the impact of saturable nonlinearity and group velocity dispersion on self-induced transparency (SIT) modelocking of quantum cascade lasers (QCLs). We find that self-induced transparency modelocking in QCLs can be obtained in the presence of saturable nonlinearity if the saturable loss or gain is below a critical limit. The limit for the saturable loss is significantly more stringent than the limit for the saturable gain. Stable modelocked pulses are also obtained in the presence of both normal and anomalous group velocity dispersion when its magnitude is below a critical value. The stability limit for the saturable loss becomes less stringent when group velocity dispersion is simultaneously present. However, the stability limit for the saturable gain is not significantly affected. All these limits depend on the ratio of the SIT-induced gain and absorpt n to the linear loss. Realistic values for both the saturable nonlinearity and chromatic dispersion are within the range in which SIT modelocking is predicted to be stable.

An efficient plasmonic photovoltaic structure using silicon strip-loaded geometry

We show that a silicon thin-film photovoltaic structure with silicon strips on the top and grooves on the silver back contact layer can absorb incident solar energy over a broad spectral range. The silicon strips on the top scatter the incident light and significantly help couple to the photonic modes in the smaller wavelength range. The grooves on the silver back contact layer both scatter the incident light and help couple to the photonic modes and resonant surface plasmon polaritons. We find an increase of ∼46% in total integrated solar absorption in the proposed strip-loaded structure compared to that in a planar thin film structure of same dimensions. The proposed structure offers simpler fabrication compared to similar plasmonic-inspired designs.

Temperature-dependent coherent carrier transport in quantum cascade lasers

The temperature dependence of coherent carrier transport in quantum cascade lasers (QCLs) is studied in this paper. It was found that coherent carrier transport in QCLs decreases as the temperature increases because the coherence between the injector and active region energy levels decays at a faster rate with increasing temperature. Calculations show that the coherence time decreases by at least a factor of two as the temperature increases from 100K to room temperature. Electron transport from the injector regions into the active regions and vice versa is a highly coherent process that becomes less efficient with decreasing coherence time and hence becomes less efficient with increasing temperature. As a consequence, when the temperature increases, the population of the upper lasing levels in active regions decreases, the population of the lower lasing levels increases and performance suffers.

A proposal and a theoretical analysis of an enhanced surface plasmon coupled emission structure for single molecule detection

We propose a structure that can be used for enhanced single molecule detection using surface plasmon coupled emission (SPCE). In the proposed structure, instead of a single metal layer on the glass prism of a typical SPCE structure for fluorescence microscopy, a metal-dielectric-metal structure is used. We theoretically show that the proposed structure significantly decreases the excitation volume of the fluorescently labeled sample, and simultaneously increases the peak SPCE intensity and SPCE power. Therefore, the signal-to-noise ratio and sensitivity of an SPCE based fluorescence microscopy system can be significantly increased using the proposed structure, which will be helpful for enhanced single molecule detection, especially, in a less pure biological sample.

Modeling of gain recovery of quantum cascade lasers

We present a model to calculate the gain recovery of quantum cascade lasers (QCLs). We implement the model for two QCLs with different material systems and quantum mechanical designs. It is found that both incoherent scattering and coherent tunneling are important for gain recovery, however, their relative importance depends on the material systems, quantum mechanical designs, and operating conditions. Though details of the gain recovery vary for the two QCLs, a complete gain recovery takes ∼2 ps in both cases. The results are consistent with the results found in pump-probe experiments.

Effects of backward-propagating waves and lumped mirror losses on self-induced transparency modelocking in quantum cascade lasers

Work to date on self-induced transparency modelocking in quantum cascade lasers (QCLs) has neglected backward-propagating waves and lumped mirror losses. In this work, we remove these unrealistic assumptions. The qualitative features of the modelocking are unaffected by this improvement in the model, but the parameter regime in which stable modelocked pulses may be found is reduced. This reduction is due to incomplete gain recovery near the edges of the QCL when pulses pass through after reflecting from the mirrors, coincident with the loss of pulse energy at the mirrors. Spatial hole burning is observed in parameter regimes in which continuous waves can grow, but it does not affect the stability of the modelocking.Work to date on self-induced transparency modelocking in quantum cascade lasers (QCLs) has neglected backward-propagating waves and lumped mirror losses. In this work, we remove these unrealistic assumptions. The qualitative features of the modelocking are unaffected by this improvement in the model, but the parameter regime in which stable modelocked pulses may be found is reduced. This reduction is due to incomplete gain recovery near the edges of the QCL when pulses pass through after reflecting from the mirrors, coincident with the loss of pulse energy at the mirrors. Spatial hole burning is observed in parameter regimes in which continuous waves can grow, but it does not affect the stability of the modelocking.

Performance of bi-end compensation in a wavelength-division multiplexed system considering the effect of self phase modulation

An extensive investigation has been carried out, by computer simulation, to evaluate the impact of self phase modulation SPM on residual dispersion for a wavelength-division multiplexed WDM system at a bit rate of 10 Gbit/s. Degradation of the eye opening of the trans- mitted pulses at the output of the transmission fiber due to interplay of the SPM and the group-velocity dispersion effects is investigated for post- and bi-end compensation configurations, including residual disper- sion. Hence the eye-opening penalty and the threshold power limit at 3-dB eye-opening penalty are determined for both the configurations. It is found that the bi-end compensation configuration offers the best per- formance in respect of the SPM effect and that for a WDM system posi- tive residual dispersion should be used.

Quantum cascade structures for efficient thermo-photovoltaic energy conversion

Intersubband transitions in quantum cascade heterostructures have been proposed for thermo-photovoltaic energy conversion for applications in solar cells so that when they are used cascaded with p-n junctions efficiency can be increased. Our designed structures produce wider absorption spectra and faster carrier transport than those have been reported before.

Management of light absorption in extraordinary optical transmission based ultra-thin-film tandem solar cells

Although ultra-thin-film solar cells can be attractive in reducing the cost, they suffer from low absorption as the thickness of the active layer is usually much smaller than the wavelength of incident light. Different nano-photonic techniques, including plasmonic structures, are being explored to increase the light absorption in ultra-thin-film solar cells. More than one layer of active materials with different energy bandgaps can be used in tandem to increase the light absorption as well. However, due to different amount of light absorption in different active layers, photo-generated currents in different active layers will not be the same. The current mismatch between the tandem layers makes them ineffective in increasing the efficiency. In this work, we investigate the light absorption properties of tandem solar cells with two ultra-thin active layers working as two subcells and a metal layer with periodically perforated holes in-between the two subcells. While the metal layer helps to overcome the cu...

Quantum coherent saturable absorption for mid-infrared ultra-short pulses

We theoretically show that quantum coherent saturable absorption can be used to obtain ultra-short pulses from mid-infrared quantum cascade lasers (QCLs). In this proposal, quantum cascade structures are processed as two electrically isolated sections. The two sections will be biased with two different voltages so that one of the sections produces gain as is done in typical QCLs, while the other produces quantum coherent resonant absorption for the propagating waves. The quantum coherent absorbing section is saturable and favors the generation of ultra-short pulses. We find that stable ultra-short pulses on the order of ∼100 ps are created from a two-section QCL when the pumping in the gain and absorbing sections remains within critical limits. The intensity and the duration of the stable pulses can be significantly varied when the pumping in the gain and absorbing sections and the length of the gain and absorbing sections are varied.