Ultrafast lasers are a type of laser which emit many short pulses. For these lasers to work, they need an optical switch that can switch the laser on and off to create the short pulses. Lasers have typically relied on resonator technologies to act as an optical switch, but there has been a change in recent years towards using 2D materials (often as saturable absorbers). In this article, we look at why 2D materials are ideal materials for optical switches.
What are Optical Switches
An optical switch is type of Q switch that uses the optical properties of a material to switch between different states. This changing of states enables a laser beam to pass through, and the optical switch can be turned on and off to create short laser pulses which are essential for ultrafast lasers. For optical switches, there are two kinds; those that use acousto-optic principle and those that use electro-optic principles.
Acousto-optic switches usually involve the creation of a sound wave that partially diffracts a light beam. The waves can be sent through many different mediums, but the most common are glass and crystal, with higher optical intensities being more stable in crystal mediums. These are a lower-tech optical switches and are not often used for ultrafast lasers, unlike materials based on electro-optical principles.
Electro-optic switches manipulate the electronic and optical properties of a material to switch between on and off states. Many of the lasers that use these principles modify the electro-optic effect of a material by changing the polarization state of light, which can the be modulated using a polarizer within the laser system.
Why 2D materials Can Be Used as Optical Switches
2D materials are well known for exhibiting a wide range of beneficial properties, especially in the electronic and optical domain. Many 2D materials, from graphene to transition metal dichalcogenides (TMDCs), exhibit strong light-material interactions, a high nonlinearity, a broadband optical response, a fast relaxation time and controllable optoelectronic properties. This makes 2D materials an attractive choice for photonic applications, including in optical switches used in ultrafast laser systems.
The structure of a 2D material allows the carrier density to be easily tuned by electrical or optical means. This tuning causes a physical change in the structure of the 2D material, which instantly changes their optical response. By using this tunable mechanism, 2D materials can be implemented as optical switches in laser systems. It should be noted that the structure and properties of 2D materials lend these materials for use as optical switches based on electro-optical principles, not electro-acoustic.
Examples of 2D Optical Switches
There are many optical switches out there that use 2D materials, and the number is growing year on year. This is in part because of the properties these materials can exhibit, but also because the field of 2D materials is expanding and researchers are finding new ways to create and use different 2D materials. Here we look at a few examples coming out of academia in the last few years.
2D materials have been tested as optical switches for a while, and back in 2010, Sun et al created an ultra-fast laser using a graphene-polymer composite as the optical switch. The graphene composite was used to mode-lock an erbium-doped fiber laser. The laser operated at a 1559 nm, with 5.24 nm spectral bandwidth and a 460 fs pulse duration. The researchers were able to use the optical non-linearity properties of graphene without needing to engineer its band gap.
In 2015, Chen et al, used black phosphorus as an optical switch in Erbium-doped fiber lasers. The black phosphorus was fabricated as a saturable absorber material at the telecommunication band. The incorporation of black phosphorus enabled the laser to pulse at 946 fs using passive Q-switching and mode-locking mechanisms.
There have been a lot of new developments between 2017 and the start of 2018, including one by Lu et al who have demonstrated that bismuthene could potentially be used as an optical switch. The researchers were able to harness its saturable absorber properties to deliver a 652 fs optical pulse centered at 1559.18 nm. Many saturable absorber materials are used as optical switches, and the recent results show that bismuthene could join the many other 2D materials currently being employed as optical switches.
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“2D Materials for Optical Modulation: Challenges and Opportunities”- Yu S., et al, Advanced Materials, 2017, DOI: 10.1002/adma.201606128
“Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and mode-locking laser operation”- Chen Y., et al, Optics Express, 2015, DOI: :10.1364/OE.23.012823
“Graphene Mode-Locked Ultrafast Laser”- Sun Z., et al, ACS Nano, 2010, DOI: 10.1021/nn901703e
“Two-dimensional materials for ultrafast lasers”- Wang F., Chin. Phys. B., 2017, DOI: 10.1088/1674-1056/26/3/034202
“Few‐layer Bismuthene: Sonochemical Exfoliation, Nonlinear Optics and Applications for Ultrafast Photonics with Enhanced Stability”- Lu L., et al, Laser Photonics and Reviews, 2018, DOI: 10.1002/lpor.201700221