Nonlinear optics (NLO) is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light. The nonlinearity is typically observed only at very high light intensities (values of the electric field comparable to interatomic electric fields, typically 108 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.
Nonlinear optics is a key enabling technology of our modern society, such as in imaging and high-speed data communication. Military uses of nonlinear optical materials range from laser beamsteering and control of beam quality to eye-protection and guided-wave photonic devices and components.
But the traditional devices suffer from relatively small nonlinear optical coefficients of conventional optical materials. Researchers have discovered that monolayer molybdenum disulfide, a unique two-dimensional (2-D) layered material similar to graphene, has an extremely large nonlinear optical response, which can efficiently convert low-energy photons into coherent high-energy photons.
Metamaterials for nonlinear optics
Scientists from the University of Massachusetts (UMass) Lowell, King’s College London, Paris Diderot University, and the University of Hartford have found that several materials with poor nonlinear characteristics can be combined together to form a new metamaterial that exhibits state-of-the-art nonlinear properties.
The team illustrated two incoming (red) photons being converted into one reflected (green) photon as result of light interaction with the nanowire structure in the metamaterial. The nanowires are about 100 nanometers apart from center to center, which is about one-fifty-thousandth the diameter of human hair.
The enhancement comes from the way the metamaterial reshapes the flow of photons,” said professor Viktor Podolskiy, the project’s principal investigator at UMass Lowell. The new class of metamaterial can be structurally tuned to change the color of the light, resulting in a photon that exhibits a different level of energy. Enabling the interaction of photons is key to faster information processing and optical computing, said Podolskiy. “Unfortunately, this nonlinear process is extremely inefficient, and suitable materials for promoting the photon interaction are very rare,” he said.
The team was able to show that reshaping of electromagnetic fields in metamaterials with plasmonic components could be used to transform second-harmonic generation (SHG) from a surface-dominated to volume-dominated regime and to engineer a strong tunable bulk nonlinear response in plasmonic composites. The researchers demonstrated tunable SHG from plasmonic nanorod metamaterials; developed a theoretical description of the observed phenomena; and showed that the nonlinear response could be engineered by changing structural parameters of the composite material.
The work demonstrates the emergence of a structurally tunable nonlinear optical response in plasmonic composites and presents a new nonlinear optical platform that could be suitable for integrated nonlinear photonics. This technology could someday enable on-chip optical communication in computer processors, leading to smaller, faster, cheaper, more efficient chips with wider bandwidth and better data storage.
MIT introduce nonlinearities in silicon photonics which will enable new class of complex devices
The telecom devices like modulators depend on second-order nonlinearities for complex signal modulations. Now MIT researchers present a practical way to introduce second-order nonlinearities into silicon photonics that would make optical signal processing more efficient and reliable .
They also report prototypes of two different silicon devices that exploit those nonlinearities: a modulator, which encodes data onto an optical beam, and a frequency doubler, a component vital to the development of lasers that can be precisely tuned to a range of different frequencies. Frequency doublers can be used to build extraordinarily precise on-chip optical clocks, optical amplifiers, and sources of terahertz radiation, which has promising security applications.We now have the ability to have a second-order nonlinearity in silicon, and this is the first real demonstration of that,” says Michael Watts, an associate professor of electrical engineering and computer science at MIT and senior author on the new paper.
“Now you can build a phase modulator that is not dependent on the free-carrier effect in silicon. The benefit there is that the free-carrier effect in silicon always has a phase and amplitude coupling. So whenever you change the carrier concentration, you’re changing both the phase and the amplitude of the wave that’s passing through it. With second-order nonlinearity, you break that coupling, so you can have a pure phase modulator. That’s important for a lot of applications. Certainly in the communications realm that’s important. Today, the design kits include standard cells for waveguides, couplers, 10G and 25G Mach Zehnder modulators, electro-absorption modulator, ring modulator, spot size converters, input output grating coupler, photodetectors and more.
Researchers develop nonlinear material using meta-materials consisting of arrays of antennas
Team of scientists – including postdoctoral researcher Dr Sebastian Schulz of Tyndall National Institute and the Centre for Advanced Photonics and Process Analysis (CAPPA) at Cork Institute of Technology have created a breakthrough in materials for photonics applications that could result in faster internet speeds and could also simultaneously reduce power consumption.
The scientists combined meta-materials consisting of arrays of antennas and a thin film of a non-linear material to form the new meta-material has a much stronger optical response than is available in natural materials. In this team’s case, the antenna array is made from gold and the thin film from indium tin oxide (a transparent conductor typically used in solar cells and touchscreens), but the concept can also be used with a wide range of other materials.
The strength of non-linear interaction is measured by the change in the refractive index that can be achieved. Usually this ranges around 0.001, but with this new material the team measured a refractive index change of 2.5. Additionally, this change occurs on an extremely short timescale – within 1 picosecond (or one-millionth of one-millionth of a second).
Short response times are important, as the response time limits how many times a system can be switched per second and, therefore, the amount of data that can be processed. The short response time of this system makes it suitable for operation at terahertz (THz) speeds, 10 times faster than current core internet links. Schulz explained that, because the non-linear response of most materials is typically extremely weak, non-linear optics have rarely been used up to now.
“Yet, non-linear optical systems exist, where the behaviour does depend on the amount of light entering the system. In such systems, depending on the power, light can change colour, direction or even its speed, and the team would like to use these effects to manipulate light – for example, to imprint data on light to reduce the power consumption of the internet.”
Extraordinarily strong nonlinear optical graphene-like material could renovate nonlinear photonics
An interdisciplinary team of scientists from Aalto University, University of Eastern Finland, University of Arizona, Cambridge University, University of Ottawa, Italian Institute of Technology, and National University of Singapore, discovered that monolayer molybdenum disulfide, a unique two-dimensional (2-D) layered material similar to graphene, has an extremely large nonlinear optical response, which can efficiently convert low-energy photons into coherent high-energy photons.
“This unusual property can be used for highly miniaturized on-chip photonic devices, such as high-resolution imaging and efficient optical data switching applications,” tells Prof. Zhipei Sun from Aalto University, Finland.
The researchers also observed that the nonlinear multiphoton processes of this material are very sensitive to the number of layers and crystal orientation. The researchers demonstrated that these nonlinear optical processes could also be exploited for rapid and reliable characterization of similar atomically thin materials. This is of great interest in the research and industry.
“Our demonstrated multiphoton approach is a few orders of magnitude faster than the conventional optical microscopy methods. This clearly shows its potential for industrial high-volume and large-size material and device characterization for next generation electronics and photonics,” says Prof. Harri Lipsanen from Aalto University.
Interestingly, the international team also found that the high-order nonlinear optical processes are stronger than the low-order ones. This is contrary to intuition, and is quite surprising, since the intensity of non-linear processes usually decreases with the order in the textbook. Prof. Nasser Peyghambarian, the Finland Distinguished Professor from College of Optical Sciences at the University of Arizona, USA, highlights:
“Such a unique nonlinear optical response is not only interesting for fundamental physics, but also very noteworthy for practical applications, such as, microscopy, therapy, and data switching.”
Tantala is exciting new material for nonlinear integrated photonics
Waveguide-based integrated photonics can be based on a number of different optical materials, including silicon, indium phosphide, polymers, glass (including chalcogenides), and silicon nitride. Some materials have advantages for certain purposes: for example, silicon nitride has a high nonlinear index, making it suitable for nonlinear optical processes (such as continuum generation from laser input).
Some of these materials have the advantage of being compatible with CMOS fabrication processes, making them suitable for inexpensive large-scale production. Now, researchers at the National Institute of Standards and Technology (NIST; Boulder, CO) are exploring the uses of a new material, tantalum pentoxide (Ta2O5, or tantala), which they say is a promising CMOS-compatible nonlinear-optical material with desirable mechanical and linear-optical properties as well.
The researchers characterized both the broadband refractive index and extinction coefficient of tantala. The material is transparent from 320 nm in the ultraviolet to 8 μm in the infrared, with a real refractive index of a little over 2 over most of this range, making it well suited for tightly confined waveguiding. Using finite-element electromagnetic field simulations of nanophotonic waveguides, the researchers calculated the group-velocity dispersion (GVD) for various waveguide cross-section geometries of tantala on silicon dioxide with air cladding for the other waveguide sides; some geometries led to anomalous GVD, which is desirable for nonlinear effects. They then fabricated supercontinuum-generation waveguides and ring resonators with top air cladding (although tantala’s high index makes oxide cladding possible, too); the waveguide propagation losses were as low as 0.1 dB/cm.
Pulsed laser light with a 1560 nm wavelength, repetition rate of 100 MHz, and pulse duration of 80 fs was coupled into a tantala waveguide via a microscope objective lens; light was collected from the waveguide’s other end with a multimode fiber. The resulting supercontinuum for a first prototype waveguide with 1.6 μm width spanned the 800–2500 nm spectral range. More-complex waveguide designs can increase the supercontinuum bandwidth, say the researchers. Prototype tantala ring resonators had quality factors (Q) of 1 × 106. Reference: J. A. Black et al., Opt. Lett. (2021); https://doi.org/10.1364/ol.414095.
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