Improving Microlasers with Upconversion Nanoparticles
23-08-2021 | By Liam Critchley
There are many different types of microlaser in use. They are found in several commercial applications, from intracellular biosensors to all-photonic microprocessors.
Microlasers have become a staple technology in the bio-photonic and on-chip application sectors. These technologies deliver coherent photons to tiny target volumes with precisely controlled wavelengths.
The success of these technologies in other applications has given rise to a research pursuit where there is a desire to continually create smaller and more efficient microlasers. Specifically, interest in creating continuous-wave microlasers with a low threshold, wavelength-tunable, and narrow linewidth. Especially for high-resolution spectroscopy and interferometric imaging wavelength-multiplexing applications.
Upconversion microlasers show promise for realising this synergy of properties. They have a sizeable anti-Stokes shift (when an emitted photon has more energy than the absorbed photon, the anti-Stokes shift is the energy difference). Despite their potential in these applications, upconversion microlasers are not as well developed as other microlasers because of their high pump power requirements and challenges surrounding the miniaturisation of the optical cavities (resulting in energy loss). But researchers are now looking to lanthanide-doped upconversion nanoparticles (UCNPs) to solve these challenges.
What are Upconversion Nanoparticles (UCNPs)
UCNPs are specialist nanoparticles that can convert photons from a low energy state into a higher energy state. It is common for UCNPs to absorb photons from the infrared range of the electromagnetic spectrum and emit them in the visible or ultraviolet (UV) energy range. UCNPs can be made of different chemical constituents, but they are typically composed of an inorganic host doped with another material, typically lanthanides.
UCNPs are seen as a type of luminophore/fluorophore but behave very differently from other types of fluorescent/luminescent particles. Many different processes promote a single electron from its ground state to its excited state. However, UCNPs use multiple low energy pump photons (intermediate metastases) to accumulate low energy excitation photons that can be changed into higher energy states.
The upconversion process is most successful with lanthanides with a 3+ ionic charge because this ionisation leaves a partially filled 4f electron sub-shell. This enables the lanthanide ions to act as a sensitiser that absorbs the incoming light, upconvert it, and send it to an emitter within the system to emit the higher energy photons.
UCNPs for Microlasers
The features that UCNPs bring to lasers—such as large anti-Stokes shift emissions—make them an attractive gain medium for anti-Stokes-shift microlasers. UCNPs can be used to create laser gain mediums that have the potential to provide a deeper penetration depth and a higher signal-to-noise ratio, as well as causing more minor damage to the device, compared to conventional Stokes-shift microlasers.
Given the potential for anti-Stokes-shift microlasers over the status quo, many attempts have been made to create microlaser systems that utilise UCNPs. This has typically been achieved by coating UCNPs onto the surface of a spherical or cylindrical cavity-based microresonator. However, many of these microlasers have lasing thresholds around 102 Wcm-2, which is two orders of magnitude higher than Stokes-shift laser operations and has been attributed to the high amount of pump power required to absorb multiple photons.
Lower thresholds have been achieved with different UCNP architectures. However, to date, many efforts to create UCNP-enhanced gain mediums have resulted in scattering loss of photons at the interface, small and limited area pump-to-gain interactions, and long-term stability issues. There is still a lot of potential for UCNPs to help create a low threshold and continuous wave anti-Stokes-shift microlasers, and new research set out to try and overcome some of these challenges.
Lanthanide Doped UCNPs for Microlasers
Recent research from researchers in South Korea has yielded a continuous-wave, wavelength-tuneable, upconversion microlaser that lases at an ultralow threshold of 4.7 Wcm-2. This was achieved by laser-induced liquefaction of the UCNPs and rapidly quenching the molten UCNPs, in a process known as ‘liquid quenching’. This process facilitated the complete integration of the UCNPs into a monolithic microsphere which provided:
- A high pump-to-gain interaction.
- Low intracavity losses.
- Efficient light coupling properties for whispering-gallery-mode (WGM) resonators.
The gain material, made of the liquified UCNPs, was found to have highly disordered microenvironments at the atomic scale. The amorphous matrix was found to effectively suppress any phonon-assisted energy back transfer (EBT) from the activator dopants (i.e., erbium and thulium) to the sensitiser dopant (ytterbium). This process enabled the gain media to achieve an efficient population inversion, i.e., the redistribution of energy levels so that the laser can operate optimally. Any narrow laser line was also spectrally tuned by injection pump power and operation temperature adjustments, and the microlaser achieved linewidths as narrow as 0.27 nm.
The microlaser was also operated with different lanthanide doping ions to showcase its potential for expanding into different output wavelengths. The laser output wavelengths from the microlaser can also be tuned to be as large as 3.56nm by thermally expanding the cavity size, which could be controlled readily by changing the injection pump power and operating temperature. This tuning process is much more straightforward than the tuning processes in conventional semiconductor lasers when looking to tune the output over a wide spectral range.
The low threshold, narrow linewidth, tuneability and continuous-wave properties of the microlaser created by the research team are competitive against state-of-the-art Stokes-shift microlasers. Given the interest in these types of lasers, research here opens the door to using anti-Stokes shift microlasers in high-resolution atomic spectroscopy, biomedical quantitative phase imaging, and high-speed optical communications, as it alleviates some of the common challenges with other UCNP and anti-Stokes microlasers. Time will tell if they become commercially competitive as well scientifically competitive.
