Alireza Bonakdar

Plasmonic enhanced quantum well infrared photodetector with high detectivity

We report a normal-incident quantum well infrared photodetector (QWIP) strongly coupled with surface plasmon modes. A periodic hole array perforated in gold film was integrated with In0.53Ga0.47As/InP QWIP to convert normal-incident electromagnetic waves into surface plasmon waves, and to excite the intersubband transition of carriers in the quantum wells. The peak responsivity of the photodetector at ∼8 μm was ∼7 A/W at the bias of 0.7 V at 78 K with the peak detectivity as high as ∼7.4×1010 cm Hz1/2/W. The full width at half maximum of the response spectrum was only ∼0.84 μm due to a narrow plasmonic resonance.

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.

Deep-UV microsphere projection lithography.

In this Letter, we present a single-exposure deep-UV projection lithography at 254-nm wavelength that produces nanopatterns in a scalable area with a feature size of 80 nm. In this method, a macroscopic lens projects a pixelated optical mask on a monolayer of hexagonally arranged microspheres that reside on the Fourier plane and image the masks pattern into a photoresist film. Our macroscopic lens shrinks the size of the mask by providing an imaging magnification of ∼1.86×10(4), while enhancing the exposure power. On the other hand, microsphere lens produces a sub-diffraction limit focal point-a so-called photonic nanojet-based on the near-surface focusing effect, which ensures an excellent patterning accuracy against the presence of surface roughness. Ray-optics simulation is utilized to design the bulk optics part of the lithography system, while a wave-optics simulation is implemented to simulate the optical properties of the exposed regions beneath the microspheres. We characterize the lithography performance in terms of the proximity effect, lens aberration, and interference effect due to refractive index mismatch between photoresist and substrate.

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.

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.

Hybrid optical antenna with high directivity gain

Coupling of a far-field optical mode to electronic states of a quantum absorber or emitter is a crucial process in many applications, including infrared sensors, single molecule spectroscopy, and quantum metrology. In particular, achieving high quantum efficiency for a system with a deep subwavelength quantum absorber/emitter has remained desirable. In this Letter, a hybrid optical antenna based on coupling of a photonic nanojet to a metallo-dielectric antenna is proposed, which allows such efficient coupling. A quantum efficiency of about 50% is predicted for a semiconductor with volume of ~λ³/170. Despite the weak optical absorption coefficient of 2000 cm(-1) in the long infrared wavelength of ~8 μm, very strong far-field coupling has been achieved, as evidenced by an axial directivity gain of 16 dB, which is only 3 dB below of theoretical limit. Unlike the common phased array antenna, this structure does not require coherent sources to achieve a high directivity. The quantum efficiency and directivity gain are more than an order of magnitude higher than existing metallic, dielectric, or metallo-dielectric optical antenna.

Novel high-throughput and maskless photolithography to fabricate plasmonic molecules

Fabrication of nanostructures for applications such as plasmonics and metamaterials is typically low throughput, due to the required submicron feature sizes. Therefore, rapid production of optically engineered structures with low cost and large area is an enabling technology for many applications, such as light harvesting, solid state lighting, disposable biosensing, and metamaterials. Here, the authors propose a simple technique, based on microsphere nanolithography, to fabricate arrays of optical elements, or so-called plasmonic molecules, at about one third of exposure wavelength. This method is capable of producing many symmetric/asymmetric array of submicron arrangement of circles and is compatible with high-throughput nanomanufacturing schemes such as roll-to-roll production. The gap size between disks is precisely controllable by the angle of exposure. Here, the authors demonstrate the capabilities of this method in producing an array of complex plasmonic molecules over a large area. The periodicit...

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.

On the Sensitivity of Electron-Injection Detectors at Low Light Level

We present the signal-to-noise performance of a short-wave infrared detector, which offers an internal avalanche-free gain. The detector is based on a similar mechanism as the heterojunction phototransistor and takes advantage of a type-II band alignment. Current devices demonstrate a noise-equivalent sensitivity of ~670 photons at 260 K and over a linear dynamic range of 20 dB. While this level of sensitivity is about an order of magnitude better than an ideal p-i-n detector attached to the same low-noise amplifier, it was still limited by the amplifier noise (~2600 electrons root mean square) due to the insufficient device gain. Performance comparison with other SWIR detector technologies demonstrates that the so-called electron-injection detectors offer more than three orders of magnitude better noise-equivalent sensitivity compared with state-of-the-art phototransistors operating at similar temperature.