Efficient low-power photon upconversion in core/shell heterostructured semiconductor nanowires

. Photon energy upconversion, i.e. the conversion of several low-energy photons to a photon of higher energy, offers significant potential for nano-optoelectronics and nanophotonics applications. The primary challenge is to achieve high upconversion efficiency and a broad device performance range, enabling effective upconversion even at low excitation power. This study demonstrates that core/shell semiconductor nanowire heterostructures can exhibit upconversion efficiencies exceeding what was previously reported for semiconductor nanostructures even at a low excitation power of 100 mW/cm 2 , by a two-photon absorption process through conduction band states of the narrow-bandgap nanowire shell region. By engineering the electric-field distribution of the excitation light inside the NWs, upconversion efficiency can be further improved by eight times. This work showcases the effectiveness of the proposed approach in achieving efficient photon upconversion using core/shell NW heterostructures, resulting in some of the highest upconversion efficiencies reported in semiconductor nanostructures. Additionally, it offers design guidelines for enhancing energy upconversion efficiency.

Photon energy upconversion, which converts multiple low-energy photons into a single high-energy photon, is valuable in diverse research fields like biological imaging, remote drug release, infrared detection, and integrated photonic applications such as microscale wavelengthdivision multiplexing and optoelectronic devices.It is particularly relevant for renewable energy generation, as upconversion materials can enhance solar energy harvesting for water splitting and third-generation photovoltaic devices.[1] Semiconductor nanostructures like nanowires (NWs) from III-V compounds offer strong, broad-band light absorption, wide energy tunability, integration with Si-technology, and formation of high-quality latticemismatched heterostructures, making them desirable for nano-optoelectronics and photonics applications.However, the upconversion efficiency in semiconductor nanostructures remains relatively low, typically below 0.1%, particularly at low excitation powers.[2] In this work, we push the limits of low-power upconversion in semiconductor nanostructures, by designing core/shell GaAs(P)/GaNAs(P) heterostructured nanowires where the shell consists of a dilute nitride alloy which is ideally suited for bandgap tuning in the NIR spectral region.From these nanowires we demonstrate upconversion efficiencies up to 15% under low-power (100 mW/cm 2 ) conditions.[3] The investigated GaAs(P)/GaNAs(P) nanowires were grown by molecular beam epitaxy (MBE) on (111) Si substrates.The fabricated samples were found to form dense nanowire arrays containing 2−4.5 μm long wires with diameters approximately 300-400 nm.The schematic in Figure 1a illustrates the electronic structure of the nanowires (black lines), where the nanowire core (shell) region has a larger (smaller) band gap energy.By exciting the nanowire with high-energy light above the wide band gap of the nanowire core, the resulting photoluminescence (PL) spectrum shows two distinct peaks (Figure 1b, red curve) corresponding to carrier recombination in the core and shell regions, respectively.Remarkably, when the energy of the excitation light is tuned below the core band gap energy, the PL emission from the core region can still be observed (the yellow curve on the high-energy side).This anti-Stokes shifted PL corresponds to the upconverted light.Interestingly, we see that the ratio of core PL under upconversion and Stokes-shifted conditions exceeds 1%, indicating a very efficient upconversion process.The power dependence of the upconverted PL (Figure 1c) is linear, which confirms that the upconversion occurs a real intermediate state.By investigating the excitation-energy dependent upconversion efficiency (Figure 1d), we see that the efficiency dramatically decreases when the excitation energy is tuned below also the shell band gap energy, which is a clear indication that the intermediate states in the upconversion process are the band states of the nanowire shell region (as shown schematically in Figure 1a).
To better understand the dynamics of the upconversion process, we performed time-resolved PL spectroscopy (Figure 2a) of the core PL under upconversion (yellow) and Stokes-shifted (red) conditions.Comparing the rise-times of the PL transients, which represent the rate of exciton formation in the core region either from direct optical generation (in the Stokesshifted case) or from upconversion via the shell states (in the upconversion case), it is clear that no difference in exciton formation rate is observed within the timeresolution of the measurement.This shows that the shellcore energy transfer-rate of the upconversion process is very fast, which is consistent with the proposed model in Figure 1a.
Based on the PL transient measurements, we are able to perform a rate equation analysis of the upconversion process, to determine the main limiting factors of the upconversion process.[3] We find a strong dependence of the upconversion efficiency on (i) the carrier lifetime in the shell region and also on (ii) the relative carrier generation rates in the core and shell regions of the nanowire.[3] To establish the effect of carrier lifetime (i) experimentally, we first compare the effects on the upconversion efficiency by varying the percentage of incorporated phosphorous, which is known to greatly affect the exciton lifetime.We find that by incorporating [P]=24%, the upconversion efficiency is increased by a factor of ~3 compared to phosphorous-free material.[3] The influence of shell carrier lifetime also manifests as a reduction of the upconversion efficiency with increasing temperature (Figure 2b), due to the increased rate of non-radiative recombination at elevated temperatures.
A relative increase in the rate of carrier generation in the shell is expected to increase the upconversion efficiency (ii).To investigate this, we note that the electric field distribution by the excitation laser can be affected by the nanowire substrate.By simulating the electric field distribution in a nanowire placed on SiO2 or gold substrates, we find that the electric field can be better concentrated in the nanowire shell region by using a gold substrate.Experimentally, we see an exceptionally high upconversion efficiency as high as 15% for nanowires placed on a gold substrate, compared with ~2% for nanowires on SiO2.[3] In conclusion, this research explores the potential of GaAs(P)/GaNAs(P) heterostructured nanowires for photon energy upconversion, a process with significant applications in various fields, including renewable energy generation and optoelectronics.By designing core/shell nanowires with a dilute nitride alloy shell, we have achieved upconversion efficiencies of up to 15% under low-power conditions.Our analysis highlights the importance of carrier lifetime in the shell region and the relative carrier generation in the core and shell regions as key factors influencing upconversion efficiency.Furthermore, we demonstrate that by incorporating phosphorous and selecting the proper substrate, the upconversion efficiency can be significantly improved.These findings provide valuable insights into enhancing upconversion performance in semiconductor nanostructures for a wide range of applications.

Fig. 1 .
Fig. 1.(a) a schematic showing the upconversion process in the core/shell nanowire.(b) PL spectra acquired with excitation light energy exceeding (red) and below (yellow) the nanowire core band gap energy.For below band gap excitation, the excitation energy is marked with an arrow.The upconverted PL is scaled by a factor 100 for visibility.(c) Excitation power dependence of the upconverted PL (UPL), with a linear curve fit (the solid line) indicating a power factor of 1.1.(d) Excitation energy dependence of the upconversion efficiency.The gray dashed line shows the shell band gap energy.All data are from GaAsP/GaNAsP nanowires with [P]=24% and [N]=1.1%.