Dibyendu Dey

Fabrication of Large Area Periodic Nanostructures Using Nanosphere Photolithography

Large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications, such as photonic band-gap materials, high dense data storage, and photonic devices. We have developed a maskless photolithography method—Nanosphere Photolithography (NSP)—to produce a large area of uniform nanopatterns in the photoresist utilizing the silica micro-spheres to focus UV light. Here, we will extend the idea to fabricate metallic nanostructures using the NSP method. We produced large areas of periodic uniform nanohole array perforated in different metallic films, such as gold and aluminum. The diameters of these nanoholes are much smaller than the wavelength of UV light used and they are very uniformly distributed. The method introduced here inherently has both the advantages of photolithography and self-assembled methods. Besides, it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes.

A Novel Self-aligned and Maskless Process for Formation of Highly Uniform Arrays of Nanoholes and Nanopillars

Fabrication of a large area of periodic structures with deep sub-wavelength features is required in many applications such as solar cells, photonic crystals, and artificial kidneys. We present a low-cost and high-throughput process for realization of 2D arrays of deep sub-wavelength features using a self-assembled monolayer of hexagonally close packed (HCP) silica and polystyrene microspheres. This method utilizes the microspheres as super-lenses to fabricate nanohole and pillar arrays over large areas on conventional positive and negative photoresist, and with a high aspect ratio. The period and diameter of the holes and pillars formed with this technique can be controlled precisely and independently. We demonstrate that the method can produce HCP arrays of hole of sub-250 nm size using a conventional photolithography system with a broadband UV source centered at 400 nm. We also present our 3D FDTD modeling, which shows a good agreement with the experimental results.

Opto-mechanical force mapping of deep subwavelength plasmonic modes.

We present spatial mapping of optical force generated near the hot spot of a metal-dielectric-metal bowtie nanoantenna at a wavelength of 1550 nm. Maxwells stress tensor method has been used to simulate the optical force and it agrees well with the experimental data. This method could potentially produce field intensity and optical force mapping simultaneously with a high spatial resolution. Detailed mapping of the optical force is crucial for understanding and designing plasmonic-based optical trapping for emerging applications such as chip-scale biosensing and optomechanical switching.

Quantum-cascade laser integrated with a metal–dielectric–metal-based plasmonic antenna

Optical nanoantennas are capable of enhancing the near-field intensity and confining optical energy within a small spot size. We report a novel metal-dielectric-metal coupled-nanorods antenna integrated on the facet of a quantum-cascade laser. Finite-difference time-domain simulations showed that, for dielectric thicknesses in the range from 10 to 30 nm, peak optical intensity at the top of the antenna gap is 4000 times greater than the incident field intensity. This is 4 times higher enhancement compared to a coupled metal antenna. The antenna is fabricated using focused ion-beam milling and measured using modified scanning probe microscopy. Such a device has potential applications in building mid-IR biosensors.

Large areas of periodic nanoholes perforated in multistacked films produced by lift-off

The authors report a low-cost and high-throughput method—nanosphere photolithography, for generating periodic subwavelength holes in metals/dielectrics. By combining the self-assembled and focus properties of micro-/nanospheres, the authors utilized the sphere arrays as lenses to produce large areas of nanopillars with a strong undercut in negative photoresist. Using lift-off with the nanopillars of photoresist, the authors demonstrate a large area of uniform nanoholes of as small as 50 nm in diameter at the bottom of ∼160 nm thick metal. The authors also show that the nanohole arrays can be generated in multistacked layers of different materials and these nanoholes can be processed with different sidewall shapes. The technique promises to be an alternative nanopatterning method that is simple, economical, fast, and flexible.

Composite Nano-Antenna Integrated With Quantum Cascade Laser

Exploiting optical nano-antennas to boost the near-field confinement within a small volume can increase the limit of molecular detection by an order of magnitude. We present a novel antenna design based on Au-SiO2-Au single nanorod integrated on the facet of a quantum cascade laser operating in the midinfrared region of the optical spectrum. Finite-difference time-domain simulations showed that for sandwiched dielectric thicknesses within the range of 20-30 nm, peak optical intensity at the top of the antenna ends is 500 times greater than the incident field intensity. The device was fabricated using focused ion beam milling. Apertureless midinfrared near-field scanning optical microscopy showed that the device can generate a spatially confined spot within a nanometric size about 12 times smaller than the operating wavelength. Such high intensity, hot spot locations can be used in increasing photon interaction with bio-molecules for sensing applications.

Injectorless quantum cascade laser with low voltage defect and improved thermal performance grown by metal-organic chemical-vapor deposition

We demonstrate a strain-compensated injectorless quantum cascade laser (I-QCL), grown by metal-organic chemical-vapor deposition, with a very low voltage defect operating up to room temperature. We experimentally study the effect of voltage defect on thermal performance by comparing the rise in core temperature over a 300 ns pulse width of I-QCL and conventional QCL, working in pulsed mode using time-resolved step scan. I-QCL shows approximately eight times lower rate of rise in core temperature compared to conventional QCL.

Integrated all-optical infrared switchable plasmonic quantum cascade laser.

We report a type of infrared switchable plasmonic quantum cascade laser, in which far field light in the midwave infrared (MWIR, 6.1 μm) is modulated by a near field interaction of light in the telecommunications wavelength (1.55 μm). To achieve this all-optical switch, we used cross-polarized bowtie antennas and a centrally located germanium nanoslab. The bowtie antenna squeezes the short wavelength light into the gap region, where the germanium is placed. The perturbation of refractive index of the germanium due to the free carrier absorption produced by short wavelength light changes the optical response of the antenna and the entire laser intensity at 6.1 μm significantly. This device shows a viable method to modulate the far field of a laser through a near field interaction.

Towards an Integrated Chip-Scale Plasmonic Biosensor

Biosensing allows researchers to detect tiny amounts of harmful chemicals before they become major threats. These researchers are using advanced optical technologies to develop the biosensor of the future-a plasmonic-based chip-scale device that will allow for compact, inexpensive, ubiquitous and sensitive detection.

A voltage tunable quantum dot photodetector for terahertz detection

A voltage tunable quantum dot (QD) photodetector for terahertz detection based on intersublevel transitions is proposed. The intersublevels are formed by the lateral electrical confinement applied on quantum wells and the transitions between them can be strongly tuned by the confinement. Under normal incidence, the peak detection wavelengths can be tuned from ∼50 to ∼90 µm (6.0 to ∼3.3THz) with a gate voltage range of − 5t o−2V. The peak absorption coefficient of detection is in the order of 10 3 cm −1 at 77K, and the peak detectivity of the photodetector can reach ∼10 9 cm 2 Hz 1/2 W −1 . The proposed approach has the advantage of forming a high uniformity of QD effective sizes and provides an alternative way to detect terahertz radiation. (Some figures in this article are in colour only in the electronic version).