Quantum Dot Emitter for Broadband IR

Broadband light emission in the infrared has proven to be of paramount importance for a large range of appli­cations that include food quality and product/process moni­toring, recycling, environmental sensing and monitoring, multispectral imaging in automotive as well as safety and security. With the advent of IoT and the increasing demand in adding more func­tionalities to portable devices the intro­duction of on-chip spectro­meters for health monitoring, allergen detection food quality inspection, to name a few, is expected to happen soon. But in order to have such func­tionalities easily integrated and implemented in mass production consumer electronics, several prere­quisites need to be met. More specifically, the light source needs to be compact, highly efficient and ideally CMOS integrated to guarantee low-cost and high volume manufacturing.

The multi-stack of CQDs of different size are built on top of a flexible plastic substrate that is later deposited onto a commercial visible LED to produce broadband IR light. (Source: ICFO)

So far, broadband light emitters in the shortwave infrared range in which these aforementioned applications work, are based on previous-century tech­nology, which is actually based on incan­descent light sources, i.e. black body radiators. Even though their cost of production is low, their func­tionality is based on the principle of heating, which does not allow minia­turization of those sources, ending up in bulky form factors. Furthermore heat dissi­pation becomes a major issue when it comes to integration in compact portable systems. What makes matters even worse is the fact that these sources are uncon­trollably broadband, emitting across a spectrum that is far broader than usually needed, which means that they are highly ineffi­cient since most of the generated light is essen­tially useless.

To address this challenge, ICFO researchers Santanu Pradhan and Mariona Dalmases led by Gerasimos Konstantatos, developed a new class of broadband solid state light emitters based on colloidal quantum dot (CQD) thin film technology. Now, CQDs offer the advan­tages of low-cost solution processa­bility, easy CMOS inte­gration and a readily tunable bandgap. By leveraging these properties, the researchers designed and engi­neered a multi-stack of CQDs of different size, which showed to be capable of emitting light with a spectrum that depends on the size of the emitting QDs.

The sequence and thickness of the layers was optimized to maximize the photo­conversion efficiency of this down-converting nano­phosphor type of thin film. The stacks were built on top of a flexible plastic substrate which was then glued on top of a LED that emits in the visible range. This LED emits visible light that is then absorbed and converted by the CQDs to infrared light with a desired spectrum and, more impor­tantly, with an outstanding photon conversion effi­ciency of 25 %. They showed that the shape of the emission spectrum can be tuned by choosing the appro­priate popu­lations of CQD sizes. For this particular case, the researchers developed a broadband light source covering an emission range between 1100 – 1700 nm with a FWHM of 400 nm.

Then, by exploiting the conductive nature of the CQD thin films, the researchers were able to take a step further in their experiment and also construct elec­trically driven active broadband LEDs with a FWHM in excess of 350 nm and quantum efficiency of 5 %. Such achieve­ment represents the first monolithic electrically driven broadband Short Wave Infrared (SWIR) LED that does not need to rely on external light sources for excitation. This is a remarkable discovery since current available techno­logies based on III-V semiconductors not only are CMOS incom­patible, but also require the use of multiple InGaAs chips in the form of an array to deliver a broadband spectrum, which adds complexity, cost and device volume increase.

Finally, to demons­trate how suitable this technology could be for market appli­cations based on spectro­scopy techniques, the team of researchers searched for several real case examples that could be good candidates for such techno­logy. They took their CQD light source setup and by putting it together with commercially available spectro­meters, they were able to distin­guish between different types of plastics, liquids and milks that have distinct spectral signatures in the SWIR. The successful results open a new realm for the field of SWIR spectro­scopy since they prove that this technology could defi­nitely be used for appli­cations that range from plastic sorting in recycling process, to health and safety or even food inspection, to name a few. (Source: ICFO)

Reference: S. Pradhan et al.: Solid‐State Thin‐Film Broadband Short‐Wave Infrared Light Emitters, Adv. Mat., online 30 September 2020; DOI: 10.1002/adma.202003830

Link: Functional Optoelectronic Nanomaterials, ICFO ‐ Institut de Ciències Fotòniques, Barcelona Institute of Science and Technology, Castelldefels, Spain

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