Single-Mode Laser in the Telecom Range by Deterministic Amplification of the Topological Interface Mode

Abstract

Photonic integrated circuits are paving the way for novel on-chip functionalities with diverse applications in communication, computing, and beyond. The integration of on-chip light sources, especially single-mode lasers, is crucial for advancing those photonic chips to their full potential. Recently, novel concepts involving topological designs introduced a variety of options for tuning device properties, such as the desired single-mode emission. Here, we introduce a novel cavity design that allows amplification of the topological interface mode by deterministic placement of gain material within a topological lattice. The proposed design is experimentally implemented by a selective epitaxy process to achieve closely spaced Si and InGaAs nanorods embedded within the same layer. This results in the first demonstration of a single-mode laser in the telecom band using the concept of amplified topological modes without introducing artificial losses.

Figure 1

Topological photonic nanorod lattice. (a) Two dimerized nanorod lattices with equal unit cells are overlapped on one edge to create a symmetric topological cavity. As indicated by the coloring, the symmetricity implies a switching of the two sites within the unit cell. (b–d) 3D-FDTD simulations of this structure. (b) Spectrum containing a pronounced photonic band gap spanning 150  nm and the cavity mode centered in it. (c) Mode profile corresponding to the interface mode overlaid with the outline of the device. (d) Band structure simulation verifying the existence of a fully open photonic band gap.

Figure 2

Fabrication of the hybrid III–V/Si cavity. (a) Process flow for template-assisted selective epitaxy. The structure is patterned by Si dry etching (I), then encapsulated in SiO₂ (II). The template is created by locally removing the oxide on one side of the Si structure (III) and then selectively etching out the Si (IV). The resulting hollow template is filled with III–Vs by MOCVD (V). Steps II–IV can be repeated to remove the Si seed on the other side (VI). (b) Scanning electron microscopy image of the fabricated structure. The III–V nanorods are visible inside the SiO₂ template, alternating with Si nanorods, both of which are covered by oxide. Additional features at both ends of each III–V nanorod result from the regrowth procedure: The sacrificial nanorods were designed longer such that they can be accessed on both sides for the etch-back; their outline remains visible as the sidewall of the now empty SiO₂ template.

Figure 3

Lasing from the single cavity peak. (a) Photoluminescence spectra for excitation of the cavity center with increasing pump power, where a strong peak emerges from the spectrum. Contrarily, several weak peaks appear under excitation of an off-center position, corresponding to the photonic band edges. (b) Power-dependent integrated emission intensity for both the emerging peak and the remaining PL signal. (c) Evolution of the peak wavelength and its full width at half-maximum over increasing pump power.

Figure 4

Time-resolved photoluminescence response. A transition in carrier lifetime is measured in the time-resolved PL at different pump powers, decreasing from 37 ps for spontaneous emission to 20 ps above the lasing threshold.

Figure 5

Comparison of different device designs. (a) Tunability of the emission wavelength by varying the lattice constant, peak positions are indicated by stars. (b) False-colored scanning electrode images of two structures with inverted positioning of the Si and III–V nanorods. (c) Photoluminescence spectra for those two devices excited in the cavity center, showing the strong lasing mode for A and several weak peaks for B, whereby their positions correspond well to the simulated photonic band gap center and first band, respectively.