Saturable absorbers, which operate at terahertz (THz) frequencies, are beginning to open doors in the development of photonic devices, namely in passively mode-locked THz micro-sources. Now, a team of Researchers from Italy and the UK have used graphite exfoliation, transfer coating and inkjet processed to create a THz saturable absorber composed of graphene.
Ultra-short laser pulses that operate in the THz frequency range are thought to have the potential to span many industries. This includes, but not limited to, application in information and communication technologies, security and spectroscopy devices and as tools to probe light–matter interaction phenomena.
There are existing technologies which use such laser pulses and whilst some designs are better than others, they lack the performance required for wide commercial use and suffer from many drawbacks. However, there has been some promising developments that generates ultra-short pulses directly from THz quantum cascade lasers, such as passive mode-locking, but the realization of these devices requires the addition of an appropriate saturable absorber.
Saturable absorbers operate through either transmission or reflection and are commonly used to mode-lock lasers. This enables the laser to derive a set of short pulses from the continuous wave operation. The main parameters for obtaining a useful saturable absorber material are the operation wavelength range, recovery time and saturation fluence (the pulse energy density required to achieve saturation).
Traditionally, semiconductors have been used as saturable absorber materials. The Researchers in this instance graphene was choosen as it possesses similar properties to semiconductors, but generally these properties are enhanced against their semiconductor counterparts, and more beneficial properties can be realized with graphene.
The Researchers used a liquid-phase exfoliation method, using graphite as the starting material, to form two inks- a water-based ink and a surfactant-free, low boiling point, ethanol-based ink. The Researchers took these inks and demonstrated THz saturable absorption through vacuum filtration and inkjet printing (Fujifilm DMC-11610 and Fujifilm Dimatix, DMP-2800).
The Researchers formed the water-based ink through ultrasonication (Fisherbrand FB15069), dispersion, ultracentrifugation (Sorvall WX100, TH-641 swinging bucket rotor), vacuum filtration and film transfer techniques.
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The method process was pretty similar for the formation of the ethanol-based ink. However, it did include extra steps of carbon dioxide and methane cracking using a plasma torch. The formation of the ethanol-based ink also uses different equipment, where a Beckman Coulter Proteomelab XL-A centrifuge with a SW 32 Ti swinging bucket rotor and a Cambridge Nanosystems GR1 ultrasonic device were used.
Both inks were inkjet printed on a substrate and were characterized using the pendent drop method (First Ten Angstroms FTA1000B), rheometers (DHR rheometer TA instruments and a Bohlin C-VOR Rheometer), high-resolution transition electron microscopy (HRTEM, Tecnai T20), atomic force microscopy (AFM, Bruker Dimension Icon) and Fourier transform infra-red spectroscopy (FTIR).
Using the various characterization methods, the Researchers demonstrated that the laser had an 80% transparent modulation, which is nearly one order of magnitude larger than current THz frequency lasers. FTIR measurements also showed evidence of intraband-controlled absorption bleaching.
The high transparent modulation is an important discovery. This property can be combined with the flexibility of graphene to allow the intra-cavity embedding of saturable absorbers into existing THz sources. This provides a unique approach for producing mode-locked and compact THz sources
Traditionally, these types of material suffer from non-saturable losses which arise from scattering at the interfaces between the layers. However, it has been shown that using graphene with the long-range THz wavelengths only yields a minor non-saturable loss.
The Researchers also found that the graphene saturable absorber is dominated by intraband transitions and the dynamics of the in the THZ frequency range is governed by ultrafast intraband dynamics.
The research paves the way for the integration of graphene saturable absorbers with existing technologies, such as electrically pumped THz semiconductor micro-sources, to realize ultrafast, mode-locked lasers and passive ultrafast components across the THz frequency range and improve the commerciality of these technologies. It is thought that the addition of these graphene saturable absorbers could be used in wide-reaching applications, including in time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications.
“Terahertz saturable absorbers from liquid phase exfoliation of graphite”- Bianchi V., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15763