Rajesh Bera

Shortwave infrared (SWIR) light‐emitting diodes (LEDs) are essential for next‐generation applications in machine vision, remote sensing, and communications; however, their performance in conventional bottom‐emission architectures is hampered by optical trapping in substrate modes and limited thermal dissipation on glass. In this work, we report high‐performance top‐emission quantum‐dot light‐emitting diodes (TQLEDs) based on colloidal lead sulphide (PbS) quantum dots that overcome these limitations. Critical to this architecture is the development of a high‐performance transparent conductive electrode optimized for the SWIR region. Integrating these electrodes into a low‐Q microcavity led to the modification of the far‐field radiation pattern, manifested by enhanced forward‐directed light emission and narrowed emission linewidth. Furthermore, the use of high‐thermal‐conductivity silicon as the carrier substrate enabled the devices to sustain significantly higher current densities compared to glass‐based counterparts. The synergy between resonant optical engineering and superior thermal management enabled a record‐high radiance exceeding 100 W sr ⁻¹ m ⁻² . Finally, we demonstrate the practical utility of these high‐flux emitters through the demonstration of active SWIR imaging of a see‐through undoped silicon wafer illuminated solely by active quantum‐dot LEDs (QLEDs). This work establishes top‐emission architectures on silicon as a viable pathway for integrated, high‐power infrared QLEDs.

In search of heavy metal-free mid-IR active colloidal materials, self-doped silver selenide colloidal quantum dots (CQDs) can be an alternative offering tunable mid-IR wavelength with a narrow bandwidth. One of the challenges in the study of the intraband transition is developing a method to widen the intraband transition energy range as well as reducing the toxicity of the materials. Here, we present Ag_xSe (x > 2) CQDs exhibiting an intraband transition up to 0.39 eV, produced by the cation exchange (CE) method from PbSe CQDs. The major electronic transition efficiently changes from the SWIR band gap of PbSe CQDs to the mid-IR intraband transition of the Ag_xSe CQDs by the CE. The intraband exciton is verified by examining the absorption and emission of the CE Ag_xSe CQDs as well as their applications on electrochemical mid-IR luminescence and mid-IR intraband photodetectors.

Realizing bright colloidal infrared emitters in the midwavelength infrared (or mid-IR), which can be used for low-power IR light-emitting diodes (LEDs), sensors, and deep-tissue imaging, has been a challenge for the last few decades. Here, we present colloidal tellurium nanowires with strong emission intensity at room temperature and even lasing at 3.6 μm (ω) under cryotemperature. Furthermore, the second-harmonic field at 1.8 μm (2ω) and the third-harmonic field at 1.2 μm (3ω) are successfully generated thanks to the intrinsic property of the tellurium nanowire. These unique optical features have never been reported for colloidal tellurium nanocrystals. With the colloidal midwavelength infrared (MWIR) Te nanowire laser, we demonstrate its potential in biomedical applications. MWIR lasing has been clearly observed from nanowires embedded in a human neuroblastoma cell, which could further realize deep-tissue imaging and thermotherapy in the near future.

Intraband transitions of colloidal semiconductor nanocrystals, or the electronic transitions occurring in either the conduction band or valence band, have recently received considerable attention because utilizing the intraband transitions provides new approaches for applications such as photodetectors, imaging, solar cells, lasers, and so on. In the past few years, it has been revealed that observing the intraband transition is not limited for temporal measurement such as ultrafast spectroscopy but available for steady-state measurement even under ambient conditions with the help of self-doped semiconductor nanocrystals. Considering the large absorption coefficient of the steady-state intraband transition comparable to that of the bandgap transition, the use of the intraband transition will be promising for both fundamental and application studies. Here, we summarize the recent progress in studies on intraband photoluminescence of self-doped semiconductor nanocrystals and discuss key questions to be addressed in future research.