Upconverters

An upconverter is a device or material that converts electromagnetic radiation at a lower energy (longer wavelength) into radiation at a higher energy (shorter wavelength). In practice, the term is used for two broad classes: optical upconverters, which emit higher-energy photons in response to lower-energy excitation, and electronic or electrical upconverters, which translate a signal from one radio frequency (RF) band to a higher one for processing.

Optical upconverters rely on nonlinear or energy-transfer processes in suitable materials. Notable examples are lanthanide-doped crystals and nanocrystals (for instance, erbium- or thulium-doped hosts) that absorb multiple low-energy photons and emit a single higher-energy photon, a mechanism known as upconversion luminescence. Energy-transfer upconversion and excited-state absorption enable emission in the visible or near-infrared from near-infrared excitation. These materials can be embedded in films or particles (upconversion phosphors or upconversion nanoparticles).

Electronic upconverters are mixer-based circuits that use a local oscillator to mix a baseband or intermediate-frequency signal with a high-frequency carrier, producing sum and difference frequencies. They are core components in receivers, radar, and satellite communications, often implemented with nonlinear devices such as diodes or transistors and designed for low noise and high linearity.

Applications and challenges: optical upconverters enable bioimaging with reduced autofluorescence, night vision, security inks, and spectral conversion to improve solar cell efficiency; however, efficiency is typically low and performance depends on temperature and material quality. Electronic upconverters trade off some passband performance for compactness and integration with CMOS; challenges include noise, LO leakage, and achieving broad tuning ranges. Ongoing research seeks higher quantum yields, more stable hosts, biocompatible coatings for nanoparticles, and integrable RF upconverters.