Plasmonic Optical Tweezers
Contactless manipulation of nano-matter
Laser physics gives us the ability to pick up and move a single cell or molecule without physically touching it by the use of optical tweezers, which employ enhanced forces at the focal point of a tightly focused laser beam to trap, accelerate, or slow down micro and nanoparticles. Making use of recent advances in nanotechnology, novel optical nanotweezers were developed by researchers of the Theoretical and Physical Chemistry Institute at the National Hellenic Research Foundation (NHRF) in Athens, Greece, in collaboration with researchers of the Department of Physics and Astronomy at the University of Southampton, UK. These novel nano-tweezers employ the collective oscillatory motion of surface electrons in sharp metallized silicon nanostructures, which act like tiny antennas and amplify the electromagnetic fields near them. This way, they enable high-precision trapping and manipulation of sensitive nanoparticles and biomolecules, such as DNA or proteins, at the focal point of a laser beam near the metallized nanostructures at low photon flux to avoid photodamage. In order to adopt high-performance optical tweezers in real-world applications, it is essential to develop large-scale fabrication processes without compromising the trapping performance. In this study, simple, single-step, tabletop laser fabrication methods were used for the development of the optical nano-tweezers, which facilitate scalability. The achieved trapping performance is ten times higher than that of conventional optical tweezers and comparable to the highest ones reported to date. From NATIONAL HELLENIC RESEARCH FOUNDATION THEORETICAL & PHYSICAL CHEMISTRY INSTITUTE a more fundamental aspect, the performance improvement is conclusively attributed to the excitation of collective surface electron oscillations in the underlying metallized nanostructures, through careful distance- and wavelength-dependent measurements. Metallized nanostructured silicon is a promising platform for large-scale parallel optical tweezer applications that will broaden the range of optical manipulation in nanoengineering, biology, and the study of collective biophotonic effects. Highperformance optical tweezers, in combination with other scientific techniques, such as laser microsurgery or spectroscopy, may find important applications in nanomedicine and medical diagnostics. Article Reference: “Plasmon enhanced optical tweezers with gold-coated black silicon” Domna G. Kotsifaki, Maria Kandyla, and Pavlos G. Lagoudakis.
Trapping tiny particles: A versatile tool for nanomanipulation
Nanoparticles are tiny. At just 1/1000th of a millimeter, they’re impossible to see with the naked eye. But, despite being small, they’re extremely important in many ways. If scientists want to take a close look at DNA, proteins, or viruses, then being able to isolate and monitor nanoparticles is essential.
Trapping these particles involves tightly focusing a laser beam to a point that produces a strong electromagnetic field. This beam can hold particles just like a pair of tweezers but, unfortunately, there are natural restrictions to this technique. Most notable are the size restrictions – if the particle is too small, the technique won’t work. To date, optical tweezers have been unable to hold particles like individual proteins, which are only a few nanometers in diameter.
Now, due to recent advances in nanotechnology, researchers in the Light-Matter Interactions for Quantum Technologies Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have developed a technique for precise nanoparticle trapping. In this study, they overcame the natural restrictions by developing optical tweezers based on metamaterials—a synthetic material with specific properties that do not occur naturally. This was the first time that this kind of metamaterial had been used for single nanoparticle trapping.
“Being able to manipulate or control these small particles is crucial for advances in biomedical science,” explained Dr. Domna Kotsifaki, staff scientist in the OIST Unit and first author of the research paper published in Nano Letters. Dr. Kotsifaki went on to explain that trapping these nanoparticles could enable researchers to see the progression of cancer, to develop effective drugs, and to advance biomedical imaging. “The potential applications for society are far-reaching.”
This novel technique has two sought after abilities – it can stably trap the nanoparticles using low intensity laser power and it can be used for a long period whilst avoiding light damage to the sample. The reason for this was the metamaterial that the researchers chose to use. This metamaterial is highly sensitive to changes in the surrounding environment and, therefore, allows for the use of low intensity laser power.
“Metamaterials have unusual properties due to their unique design and structure. But this makes them very useful. Over the last few years, a whole new era of devices with novel concepts and potential applications has been created from them,” explained Dr. Kotsifaki. “From the metamaterial, we fabricated an array of asymmetric split rings using a beam of ions – tiny, charged particles – on a 50 nm gold film.”
To test whether the technique worked, the research group illuminated the device with near infrared light and trapped 20 nm polystyrene particles at certain regions on it.
Dr. Kotsifaki and colleagues were looking for the trap stiffness, which is a measurement of trapping performance. “The achieved trapping performance was several times better than that of conventional optical tweezers and the highest reported to date as far as we know,” she explained. “As the first group to use this device for precision nanoparticle trapping, it has been rewarding to contribute to such progress in this research area.”
The research team now plans to tweak their device to see if these tweezers can be used in real-world applications. Specifically, in the future, this device could be utilized to create lab-on-chip technologies, which are hand-held, diagnostic tools that can provide results efficiently and economically. Alongside its applications in biomedical science, this research has provided new and fundamental insights into nanotechnology and light behavior at the nanoscale.
As well as Dr. Domna Kotsifaki, the research group consisted of Professor Síle Nic Chormaic, who leads the OIST Unit, and staff scientist, Dr. Viet Giang Truong.
By Lucy Dickie