Electrically pumped surface-emitting amplified spontaneous emission from colloidal quantum dots

CQDs, with tunable emission wavelengths, high photoluminescence quantum yield, and good thermal stability, are regarded as ideal gain media for low-cost, solution-processable laser diodes. In recent years, CQD films have achieved low-threshold optically pumped ASE and lasing. However, realizing electrically pumped ASE requires both high current injection and positive net optical gain, which faces several key challenges: heat accumulation and nonradiative losses induced by large current injection, as well as significant optical losses in QLED structures (such as electrode absorption and insufficient optical confinement). As a result, even when population inversion is achieved under high current, it remains difficult to obtain net gain.

 

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Shuming Chen from State Key Laboratory of Quantum Functional Materials, Department of Electrical and Electronic Engineering, Southern University of Science and Technology, and co-workers focus on addressing these bottlenecks through an electro-thermal-optical co-design strategy to enable efficient carrier injection, effective thermal management, and strong optical confinement, ultimately paving the way for electrically pumped surface-emitting ASE in CQDs.

 

To achieve population inversion–a prerequisite for lasing–CQDs must be driven with sufficiently high current. However, operating under such large current densities generates substantial Joule heating, and in conventional devices (Fig. 1e, left) the peak temperature can soar to 470 K (Fig. 1f, red line), leading to thermal failure. To address this, the team implemented a threefold thermal management strategy across spatial, structural, and temporal dimensions. Spatially, the emission area was minimized to just 0.01 mm², greatly reducing heat generation. Structurally, devices were fabricated directly on high-thermal-conductivity silicon (Si) substrates to efficiently dissipate heat. Temporally, ultrashort current pulses (63 ns) were applied to prevent heat accumulation. With these measures, the device could stably sustain an extraordinary current density of up to 2000 A cm⁻² at low temperature (153 K, Fig. 1g). Strong electroluminescence from the higher-energy 1P states of CQDs was clearly observed (Fig. 1h), confirming the achievement of population inversion, with calculations indicating an average of 5–6 excitons per quantum dot.

 

In such devices, metal electrodes induce severe optical losses, including absorption and surface plasmon polariton (SPP) dissipation, while the use of semi-transparent electrodes required for surface emission further weakens optical confinement. To minimize these losses and enhance net gain, it is essential to confine the optical field as much as possible within the CQD gain region. To this end, the device adopts a top-emitting (TE) Fabry–Pérot (FP) cavity structure (Fig. 1e, right), employing composite electrodes of silver (Ag) and indium zinc oxide (IZO) –with an Ag/IZO bottom reflective electrode and an IZO/Ag semi-transparent top electrode–and performs detailed optical analysis of the architecture. Comparison of power dissipation spectra shows that a thicker IZO layer nearly eliminates the SPP loss caused by the Ag electrode (Fig. 2b). Moreover, IZO functions as a phase-tuning layer, localizing the optical field predominantly within the CQD gain region (Fig. 2e–f). This design achieves an optical confinement factor as high as 0.54, more than twice that of conventional bottom-emitting (BE) devices, ultimately enabling positive net optical gain, a prerequisite for ASE. The strong optical localization in this structure maximizes the interaction between the optical field and the CQD gain medium, generating lateral ASE signals that, through effective scattering in the CQD layer, emerge as detectable surface-emitting ASE.

 

Through the electro-thermal-optical co-design, electrically pumped surface-emitting ASE was ultimately achieved. At 77 K under femtosecond laser pumping, a surface-emitting ASE signal was clearly observed in the photoluminescence (PL) spectrum with a low threshold of only 10 μJ cm⁻² (Fig. 3a–c). At 153 K under nanosecond current pulses, when the current density reached 94 A cm⁻² (Fig. 3d–f), the electroluminescence (EL) spectrum exhibited a sharp new peak at 634 nm; the intensity of this peak increased superlinearly with current density (slope k rising from 0.8 to 1.2); and the spectral linewidth narrowed significantly (from 24 nm to 15 nm). Taken together, these three signatures-emergence of a new peak, superlinear growth, and linewidth narrowing–confirm the realization of EL surface-emitting ASE.

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