IEEE Journal of Selected Topics in Quantum Electronics

Nonequilibrium Green’s Function Model for Simulation of Quantum Cascade Laser Devices Under Operating Conditions

A simulation scheme based on nonequilibrium Green’s functions for biased periodic semiconductor heterostructure devices is presented in detail. The implementation can determine current and optical gain both for small and large optical fields. Specific results for superlattices, quantum cascade lasers, and quantum cascade detectors are shown which demonstrate the capabilities of the approach.

Theoretical Analysis of GeSn Alloys as a Gain Medium for a Si-Compatible Laser

In this paper, a theoretical analysis of unstrained GeSn alloys as a laser gain medium was performed. Using the empirical pseudopotential method, the band structure of GeSn alloys was simulated and verified against experimental data. This model shows that GeSn becomes direct bandgap with 6.55% Sn concentration. The optical gain of GeSn alloys with 0–10% Sn concentration was calculated with different n-type doping concentrations and injection levels. It is shown theoretically that adding Sn greatly increases the differential gain owing to the reduction of energy between the direct and indirect conduction bands. For a double-heterostructure laser, the model shows that at a cavity loss of 50 cm
$^{-1}$, the minimum threshold current density drops 60 times from Ge to Ge
$_{0.9}$ Sn
$_{0.1}$, and the corresponding optimum n-doping concentration of the active layer drops by almost two orders of magnitude. These results indicate that GeSn alloys are good candidates for a Si-compatible laser.

Synchronized Carrier Dynamics in Quantum Dot-in-a-Well (QDWELL) Laser Under an Optical Injection

We used the Lüdge–Schöll (LS) model of the optically injected quantum dot (QD) in a well (QDWELL) laser and carried out the numerical simulations for the LS rate equation system. We have shown that due to the optical injection, the electron and hole dynamics in QDs is synhcronized, the modulation frequency is enhanced, and the QDWELL laser manifests a high performance at the repetition frequency of
$hbox{20 Hz}$, drastically deteriorating at
$hbox{30 GHz}$ .

Terahertz-Wave Generation Using Graphene: Toward New Types of Terahertz Lasers

This paper reviews recent advances in terahertz-wave generation in graphene toward the creation of new types of terahertz lasers. First, fundamental basis of the optoelectronic properties of graphene is introduced. Second, nonequilibrium carrier relaxation or recombination dynamics in optically or electrically pumped graphene is described to introduce a possibility of negative dynamic conductivity in a wide terahertz range. Third, recent theoretical advances toward the creation of current-injection graphene terahertz lasers are described. Fourth, unique terahertz dynamics of the 2-D plasmons in graphene are described. Finally, the advantages of graphene materials and devices for terahertz-wave generation are summarized.

Tunable Picosecond Terahertz-Wave Parametric Oscillators Based on Noncollinear Pump-Enhanced Signal-Resonant Cavity

We have succeeded in developing tunable picosecond terahertz (THz)-wave parametric oscillators (ps-TPOs) by employing a noncollinear pump-enhanced signal-resonant cavity. As a parametric gain medium, we use two different shapes of unpoled, 5 mol% MgO-doped lithium niobate (MgO:LiNbO
$_{3}$) crystal: 1) a rectangle for Si-prism output-coupler technique and 2) a trapezoid for surface-emitted configuration. Unlike conventional nanosecond TPOs (ns-TPOs), these ps-TPOs are synchronously pumped by a mode-locked 1.5-ps Ti:sapphire laser operating at 780 nm. To overcome the high pump threshold due to the strong absorption by MgO:LiNbO
$_{3}$ in the THz region, we employ a pump-enhanced cavity which is carefully designed for the noncollinear dual resonance of both pump and signal waves. By slightly translating the position of one of the ps-TPO cavity mirrors, we experimentally find that the THz-wave peak frequency is continuously tunable from 0.9 to 3.3 THz, approximately, with the average output power of dozens of nanowatts.

Stabilization and Mode Locking of Terahertz Quantum Cascade Lasers

We present the stabilization of terahertz (THz) quantum cascade lasers (QCLs) to a harmonic of the repetition rate of femtosecond fiber lasers. The technique that we have developed is 1) inherently broadband, i.e., it allows the stabilization of virtually any THz QCL regardless its emission frequency, and 2) can be implemented using different detection techniques such as electrooptic or photoconductive detection. Using electrooptic detection, we demonstrate phase locking of a 2.5-THz QCL, producing an RF beatnote with 70 dB of dynamic range in a bandwidth of 100 Hz. In addition, we show how the broadband character of the phase-locking technique can be used to coherently sample the emission of a multimode THz QCL that is actively mode-locked by current modulation at the roundtrip frequency.

A Higher Order Lateral Mode Suppression Scheme for Terahertz Quantum Cascade Laser Waveguides

The modal performance of a slot-clad metal–metal waveguide for terahertz quantum cascade lasers (QCLs) is elucidated in this study. Terahertz QCLs, based on metal–metal waveguides having a large ridge width, are susceptible to lase with higher order lateral modes, which may produce poor quality beams and can lead to detrimental consequences for certain applications. By comparing the modal performance of the slot-clad waveguide with previously reported designs, it has been shown that the slot-clad metal–metal waveguide can significantly enhance the degree of suppression of higher order lateral modes.

Introduction to the Issue on Quantum and Nanoscale Photonics

The purpose of this issue is to highlight the recent progress and trends in the development of novel nano- and quantum photonic technologies. The articles published in this issue cover a broad range of areas, including: 1) novel nanophotonic light sources??both classical (lasers) and nonclassical (single- and entangled photon sources); 2) nonlinear optics in nanophotonic structures; 3) quantum optics and quantum information processing based on nanophotonics platform; 4) efficient devices for optical interconnects and optical information processing based on novel nanophotonic structures. This issue contains 25 papers, including eight invited, authored by well-established research groups and promising scientists from all over the world.

Study of Band-Edge Modes in GaN-Based Photonic Crystal Surface-Emitting Lasers by the Multiple-Scattering Method

In this paper, we have investigated the mode patterns in real space and reciprocal space, and threshold gain at different band edges in GaN-based photonic crystal surface-emitting lasers (PCSELs) by using the multiple-scattering method. The characteristics at each band edge
$(Gamma 1$ , K2, and M3) of PCSELs are simulated and discussed. In addition, GaN-based PCSELs operated at different band edges have been fabricated and measured. The experimental results of threshold show good agreement with the simulation results.

Electrically Driven Photonic Crystal Nanocavity Devices

Interest in photonic crystal nanocavities is fueled by advances in device performance, particularly in the development of low-threshold laser sources. Effective electrical control of high-performance photonic crystal lasers has thus far remained elusive due to the complexities associated with current injection into cavities. A fabrication procedure for electrically pumping photonic crystal membrane devices using a lateral p-i-n junction has been developed and is described in this study. We have demonstrated electrically pumped lasing in our junctions with a threshold of 181 nA at 50 K—the lowest threshold ever demonstrated in an electrically pumped laser. At room temperature, we find that our devices behave as single-mode light-emitting diodes (LEDs), which when directly modulated, have an ultrafast electrical response up to 10 GHz corresponding to less than 1 fJ/bit energy operation—the lowest for any optical transmitter. In addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs, and have built fiber taper coupled electro-optic modulators.

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