The application of ultrashort pulsed lasers for silicon scribing enables precise control of the ablation depth and generally reduces thermal side effects compared to ns-pulses. However, the formation of periodic holes with a depth of several µm can be observed at the bottom of the scribed trenches. The goal of this study is to investigate the influence of the pulse energy and the scan speed on the depth and average pitch of these holes. For this purpose, a simple model was developed to calculate the number of scans to achieve a specific cutting depth for different pulse energies and scan speeds. Then, wafers with a thickness of 525 μm were scribed to a depth of 50 µm using a fs-laser with a pulse duration of 380 fs and a wavelength of 520 nm. The pulse energy was increased from the minimum pulse energy necessary to achieve a scribing depth of 50 μm,1.6 μJ, up to 8 μJ. In addition, the scan speed was varied between 20 mm/s and 2000 mm/s. Finally, the wafers were broken along the cut and the side walls were investigated with scanning electron microscopy. It was found that the average pitch of the holes decreases and the depth of the holes increases with the pulse energy, while the scan speed has no influence. These findings suggest that the roughness at the trench bottom can be minimized by reducing the pulse energy to the minimum value necessary to achieve the desired cutting depth.
We present a new waveguide concept for terahertz quantum-cascade laser. The double-metal waveguide confines the active region between two metallic layers. Thereby, a modal confinement of almost 100 % is achieved. However, these metal layers are also one of the dominating loss mechanisms. Replacing the conventional metal with a superconductor helps to reduce the total losses. A surface plasmon is formed at the interface between the superconductor and the semiconductor. It can be maintained even for photon energies above the superconducting band gap. In this work we use niobium with a band gap of 2.8 meV to confine the active region of a THz-QCL emitting at 9 meV.
We report the realization of microdisk and microring quantum-cascade lasers (QCLs) emitting in the terahertz (THz) region between 3.0 THz and 3.8 THz. The GaAs/Al<sub>0.15</sub>Ga<sub>0.85</sub>As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900 A/cm<sup>2</sup> at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K. Finite-Difference Time-Domaine (FDTD) simulations were performed in a strong field limit to obtain the field distribution within a microdisk THz QCL resonator.