Topological BIC enables terahertz quantum cascade lasers

Electrically pumped semiconductor lasers are indispensable core devices in the fields of optical communication, optical sensing and quantum information. Compact semiconductor lasers have important scientific value and application significance for ultra-large-scale integrated high-performance photonic chips.

The fundamental purpose of achieving a compact laser is to reduce the volume modulus of the optical mode in the optical cavity while maintaining the quality factor of the optical mode. The defective state of flat plate photonic crystal is formed in the band gap of the photonic crystal and has a very small volume modulus, which is widely used in low-threshold semiconductor lasers. However, the defective state of photonic crystals is easily affected by structural deviations, and the output power is generally low. Flat plate photonic crystals with bound states in the continuous spectrum (photonic crystal edge mode with very high quality factors) can effectively increase the output power of the laser and have some robustness, but the device size usually requires tens or hundreds of cycles to enhance the in-plane localization of the optical mode.

In recent years, photonic crystal structures with banded topological characteristics have been a research hotspot in photonics. For photonic structures with special topological properties, defects and disorder in space only cause local parameter changes and do not affect the global properties of the space. As a result, semiconductor lasers with integrated photonic topology achieve unprecedented robustness. However, as far as electrically pumped semiconductor lasers are concerned, improving the performance of lasers on the basis of ensuring the robustness of the optical mode of the device, such as beam shaping, polarization control, high power output, and large-area single-mode output, has always been an important challenge in this field.

Recently, Professor Qi Jie Wang’s team from the School of Electrical and Electronic Engineering at Nanyang Technological University in Singapore and his collaborators have simultaneously realized terahertz quantum cascade lasers with single-mode and vector light field output in the full dynamic range by constructing bound states in the continuum (BICs) in topological band-flipped volumetric continuums.

The results were published in Light: Science & Applications under the title “Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum.” Han Song, a postdoctoral fellow at the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore (now a researcher at Hangzhou International Science and Technology Innovation Center of Zhejiang University and School of Information and Electronic Engineering, Zhejiang University), is the first author of the paper, and Cui Jieyuan and doctoral student Yunda Chua are co-first authors. Professor Qi Jie Wang, School of Electrical and Electronic Engineering, Nanyang Technological University, is the first corresponding author of the paper, and Han Song is the co-corresponding author. This work was supported by Professor Zeng Yongquan of Wuhan University, Dr. Huliang Province and Professor Chuan Sen Tan of Nanyang Technological University, Dr. Lianhe Li, Professor Alexander Giles Davies, Professor Edmund Harold Linfield of the University of Leeds, and Professor Yuri Kivshar of the Australian National University.

As shown in Figure 1a, the topological BIC quantum cascade laser consists of a topological non-trivial photonic crystal structure as an optical resonator, and a topological mediocre photon crystal structure is surrounded by a topological mediocre photonic crystal structure. Due to different topological phases, the optical mode in the non-trivial photonic crystal structure cannot cross the topological interface to form the local mode field of the photonic crystal plate surface. The photonic crystal laser cell structure is etched cylindrical air holes in the center and six apex corners of the regular hexagonal lattice. Since the cascaded quantum well is covered with metal, this topological photonic crystal excites the energy band of TM polarization (out-of-plane electric field). Topological non-trivial photonic crystals exhibit band flipping with two quadrupole modes of orthogonal degeneration occurring at low frequencies, as shown in Figure 1d.

Figure 1. a, b Topological BIC laser processing sample diagram and c device construction schematic. where N is the number of transverse cycles of the non-trivial photonic crystal cavity. d By adjusting the hole size of the hexagonal lattice of the photonic crystal, its body band edge mode is flipped (that is, the quadrupole mode appears at low frequencies), which is a topological non-trivial phase; e Reverse is topological mediocrity. It is worth noting that this quadrupole mode shows an infinite quality factor at the Brillouin center.

It is worth noting that its quadrupole mode shows an infinite quality factor, i.e., a bound state in the continuous spectrum, in both topological and non-trivial bands, at the Brillouin center. Designing a topological trivial photonic crystal structure to cover a non-trivial photonic crystal resonator has the following advantages:

1. The difference between topological mediocrity and non-trivial topological phase makes the optical mode in the resonator realize high-intensity local domain in the plane dimension;

2. Posture BIC enables the optical mode in the resonant cavity to achieve high-intensity localization in the out-of-plane dimension, combined with the first point, so that the Posture BIC has maximum gain competition and ensures the single-mode output characteristics of the laser;

3. The photonic crystal BIC has far-field polarization topological charge, so that the laser radiation light field therefore has non-trivial vector spot characteristics;

4. The body BIC with flipping appears at low frequencies, which can further reduce the size of the device.

Figure 2 shows the experimental measurement results of the topological BIC quantum cascade laser. The experimental processing device N=7, the laser frequency is around 2.93 THz, and the lateral size of the pump area of the laser device is about 3 laser wavelengths, as shown in Figure 2b. The laser maintains a good single-mode output over full dynamic range: at maximum output, the side-mode rejection ratio can reach 20 dB, as shown in Figures 2b and 2c. The far-field spot measured by the experiment has vector characteristics, that is, the central energy of the light field is the weakest, and a linear polarizer is inserted between the laser and the detector, and the spot shows a stable lobe distribution that changes with the polarization angle, as shown in Figure 2d.


Figure 2. a Experimentally measured continuous change curves of light intensity-current density-voltage. b Single-mode characteristics of topological BIC lasers with edge-mode rejection ratios of up to 20 dB at the strongest output. c Single-mode output in full dynamic pumping range. d Far-field spot distribution and vector light field analysis.

This study effectively combines the band flipped BIC and the trivial-non-trivial topology to achieve the vertical and horizontal dimension of the light field localization, thereby obtaining a compact electrically pumped terahertz quantum cascade laser. Due to the largest quality factor of the BIC mode with flipping and the polarization topology of the far field, the laser presents single-mode laser and vector light field output in the full dynamic range. In addition, the characteristics of topological BIC under large-scale conditions such as mode field uniformity, high-power single-mode output, and high-purity columnar vector light field radiation are discussed. This achievement is expected to become the core active device in 6G wireless communication, and plays an important role in terahertz super-resolution imaging, terahertz lidar, and terahertz biochemical sensing and other related fields. (Source: China Optics)

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