A detailed balance model is used to determine the efficiency of intermediate band solar cell including carrier losses from the intermediate band. The effect of the energy gap of the host semiconductor is examined as a function of the intermediate band position in the energy gap and the host semiconductor energy gap. Generally the optimal intermediate band level decreases within the energy gap to mitigate the carrier losses, and carrier losses are less detrimental to small energy gap materials. We therefore focus on the role of carrier losses in wide bandgap semiconductor intermediate band solar cell systems, such as the GaN semiconductor with an Mn impurity band. Experimentally Mn acceptor level in the GaN energy gap is 1.8 eV above the valence band, which is 199 meV off the ideal intermediate band and reduces the efficiency to 21.36%. We demonstrate how carrier losses can be introduced into the system to shift the optimum IB position. Introducing carrier losses shifts the optimal intermediate band position to 1.8 eV above the valence band and increases the efficiency to 23.41%. We compare this to the effect of alloying GaN and introducing biaxial strain to shift the effective position of the Mn impurity band on the efficiency.

The authors present a numerical study on the influence of wetting layer states and doping on the photovoltage loss of InAs/GaAs quantum dot solar cells. Quantum-mechanical simulations are used to analyse how the reduction of wetting layer by Al(Ga)As overgrowth changes the quantum dot electronic states. Device-level simulations allow to correlate such changes with the achievable open circuit voltage. Almost full open circuit voltage recovery is predicted by combining wetting layer reduction, to realise thermal decoupling of barrier and quantum dot confined states, and doping to suppress radiative recombination through the quantum dot confined states.

Optoelectronic terahertz (THz) generation techniques have helped to narrow the THz gap and have opened up a wealth of new applications for THz technology. However, the development of THz systems into mass market is a major technical challenge, which is attributed to high cost of THz hardware components including sources and detectors. Here, the authors report THz generation from a distributed Bragg reflector (DBR) semiconductor laser diode together with fibre coupled photoconducting antennas. Two fibre coupled ion-implanted gallium arsenide photoconducting antennas were employed to generate and detect THz radiation. Two DBR lasers connected with a Y-shaped waveguide structure were monolithically integrated and used to simultaneously emit two wavelengths in the range of 785 nm. These lasers were employed as pumping source for the photomixers. An optical beat frequency of 286 GHz of the dual wavelengths was obtained from optical characterisation. A corresponding THz frequency was confirmed via photomixing in a homodyne set up. By variation of the operation parameters of the laser, the difference frequency was tuned in the range between 286 GHz to 320 GHz. In summary, they report the implementation of a compact and cost effective fiber coupled Terahertz source based on a monolithically integrated dual wavelength DBR semiconductor laser diode.

This study reports work on optimised THz photoconductive antenna switches based on low-temperature grown materials incorporating distributed Bragg reflectors (DBRs). These materials were characterised electrically using Hall effect, mid-infrared reflectivity measurements and fabricated antennas tested under pulsed excitation in a time-domain spectroscopy system. It is shown that the inclusion of DBR results in enhanced THz peak signals by more than twice across the entire operating frequency range. In addition, the devices exhibit significant improvements in the relative THz power, responsivity and optical to electrical efficiency compared with the reference ones.

The effect of optical injection locking (OIL) on modulation dynamics of tunable laser diodes (TLDs) is investigated. The stable locking boundary map of OIL TLD which depends on the lasing wavelength was calculated and the TLD response in different injection states was investigated. Dramatic change in the emission spectra and the relaxation oscillation frequency (ROF) and huge improvement of these characteristics under OIL is demonstrated. Effect of the OIL parameters (frequency detuning and power ratio) on the TLD dynamics was studied for different tuning wavelengths. We show that forward optical injection and positive frequency detuning and blue-wavelength tuning are preferable regimes for enhanced modulation performance of OIL TLDs. Large increase of the ROF (>20 GHz) and the modulation bandwidth (>25 GHz) of the OIL TLD are demonstrated. The novel feature of the modulation response of the OIL TLD compared with usual single-mode lasers emitting at a fixed wavelength is the frequency dispersion of the differential gain in TLDs which increases with wavelength tuning. It is shown that the differential gain increase with wavelength tuning and the optical injection effect on the cavity resonance frequency shift contribute to a substantial enhancement of the modulation response of the OIL TLD.

The modulation dynamic performance of a three-section bulk InGaAsP/InP tunable laser diode (TLD) operating at 1550-nm under direct intensity modulation during discontinuous tuning is investigated with the travelling-wave approach, using commercial software tools VPI and PICS3D. The authors demonstrate a strong effect of the gain spectra shape on the modulation response of the TLDs. Two models have been developed for simulation of the modulation response which incorporate real gain spectra of TLDs obtained either from experiment or ab-initio calculations: (i) the VPI + PICS3D integrated model, and (ii) the fitted parabolic shape gain model. The results obtained for both models are in good agreement. A significant ∼3 times increase of the relaxation oscillation frequency and the corresponding modulation bandwidth was observed under blue wavelength tuning of the TLD over a 21-nm range from the initial 1550-nm lasing wavelength. The authors show that the main physical reason for this increase is a dispersion of the differential gain which increases about 4 to 5 times when the lasing wavelength decreases over the above tuning range. The reported enhancement of the modulation response of TLDs is important for their practical applications.

Tunable diode lasers are essential components in various optical systems. The authors present a concept and simulations of a four-section, widely tunable GaAs-based sampled-grating (SG) distributed-Bragg reflector (DBR) laser emitting at 976 nm. This work includes the design approach of the SG reflectors and the simulation of the behaviour of the modes in the active cavity during the wavelength tuning process. Parameters such as the lasing wavelength, threshold current and the gain of the lasing and of the adjacent side modes during the tuning are presented. The numerical results presented suggest a tunability of at least 17 nm, with a threshold current change of only 2.5 mA and single-mode operation over the entire tuning range, without the need of simultaneous adjustments of the phase section. Finally, the authors present early experimental results of a developed GaAs-based vertical structure, providing a high coupling coefficient of , thus suitable for implementation of an SG-DBR laser design.

In this study, the authors present experimental results on 6 mm long monolithic passively colliding-pulse mode-locked (CPM) lasers. The AlGaAs-based devices with In _x Ga_1−x As _y P_1−y double-quantum-well (DQW) active regions emit between 860 and 880 nm. To optimise the laser performance and to shorten the pulses the optical gain is spectrally broadened to increase the number of longitudinal lasing modes by using a chirped DQW structure where the In-content of the QWs differs by Δx = 0.08. Lasers having the chirped DQW with lasers having a conventional, unchirped DQW active region are compared. The mode locking operation is investigated in dependence on gain current, absorber voltage and absorber length. According to the experimental results, the lasers with the chirped DQW exhibit an overall better performance and generate shorter pulses with higher average optical output powers over a wider range of parameters. For an optimum set of parameters, the generated pulses have a full width at half maximum of the autocorrelation function of 1.9 ps at a repetition frequency of 12.9 GHz.

Experimentally, mutually coupled quantum dot lasers demonstrate a greatly reduced modal linewidth when mutually phase-locked. The authors investigate this phenomenon theoretically. Specifically, they examine a delay differential equation coupled phase model and use it to find the laser linewidth in several configurations. The numerical results are in excellent agreement with previously reported experimental findings. They show that while the model predicts a restoring force on the phase difference of the coupled lasers, this translates to an effective restoring force on the individual laser phases. The phenomenon should be robust to changes in device type and in particular for modern, integrated coupled lasers.