Omer Gokalp Memis

A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars

We report a low-cost and high-throughput process for the realization of two-dimensional arrays of deep sub-wavelength features using silica and polystyrene spheres. The pattern size in this method is a weak function of sphere size, and hence excellent size uniformity is achievable. Also, the period and diameter of the holes and pillars formed with this technique can be controlled precisely and independently. Moreover, the patterns can be formed in conventional negative and positive photoresists, and hence this approach is compatible with a wide range of existing processing methods. Although we achieved hole sizes of ~250 nm with a broadband UV source centered at 400 nm, our simulation results show that patterns as small as 180 nm should be achievable at a wavelength of 365 nm.

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.

A photon detector with very high gain at low bias and at room temperature

We report on a photon detector aimed at low light detection, which is based on the combination of small sensing volumes and large absorbing regions. Fabricated devices show stable gain values in the range of 1000–10 000 at bias voltages of ∼1V at 1.55μm at room temperature. Submicron devices show dark current less than 90nA and unity gain dark current density values less than 900nA∕cm2. The noise equivalent power (NEP) is measured to be 4fW∕Hz0.5 at room temperature without any gating, which is similar to NEP of current InGaAs∕InP avalanche photodetectors in gated operation.

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.

Sub-Poissonian shot noise of a high internal gain injection photon detector

The noise performance of an infrared injection photon detector with very high internal gain was investigated at a wavelength of 1.55 mum. The devices showed sub-Poissonian shot noise with Fano factors around 0.55 at 0.7 V at room temperature. Optical to electrical conversion factors of 3000 electrons per absorbed photon were recorded at 0.7 V. The change in noise-equivalent power with respect to bias voltage was evaluated. The optical to electrical conversion factor and Fano factor were measured under increasing illumination and compared to theoretical expectations.

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.

Isolated electron injection detectors with high gain and record low dark current at telecom wavelength

We report on recent performance breakthroughs in a novel short-wave infrared linear-mode electron-injection-based detector. Detectors consist of InP material system with a type-II band alignment and provide high internal avalanche-free amplification mechanism. Measurements on devices with 10-μm injector diameter and 30-μm absorber diameter show internal dark current density of about 0.1 nA/cm2 at 160 K. Compared with our previous reported results, dark current is reduced by two orders of magnitude with no sign of surface leakage limitation down to the lowest measured temperature. Compared with the best-reported linear-mode avalanche photodetector, which is based on HgCdTe, the electron-injection detector shows over three orders of magnitude lower internal dark current density at all measured temperatures. Using a detailed simulation with experimentally measured parameters, dark count rate of 1 Hz at 90% photon detection efficiency at 210 K is anticipated. This is a significantly higher operating temperature compared with superconducting detectors with a similar performance.

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.