Saeid Zamiri

Remote photoacoustic imaging on solid material using a two-wave mixing interferometer

We report on remote and contactless photoacoustic imaging (PAI) for the inspection of solid materials using a two-wave mixing interferometer. In this Letter, a semitransparent sample was excited with picosecond laser pulses. The local absorption of the electromagnetic radiation led to generation of broadband ultrasonic waves inside the sample. Ultrasonic waves arriving at the sample surface were detected utilizing a two-wave mixing interferometer. After data acquisition, the initial pressure distribution was reconstructed using a Fourier space synthetic aperture technique algorithm. We show the potential of PAI for the inspection of semitransparent solid materials.

Fullerene Sensitized Silicon for Near- to Mid-Infrared Light Detection

Here, we report on a novel light sensing scheme based on a silicon/fullerene-derivative (methano-fullerene [6,6] phenylC61 butyric acid methyl ester – PCBM) heterojunction that allows the realization of optoelectronic devices for the detection of nearto mid-IR light. Despite the absent absorption of silicon and the fullerene-derivative in the IR a heterojunction of these materials absorbes and generates a photocurrentin the spectral range from 1.1 to 0.55 eV. Besides its scientific relevance, the simple fabrication process of the heterojunction (e.g., the fullerene-derivative is deposited by spin coating on Si) as well as its compatibility with the established complementary metaloxide semiconductor (CMOS) technology makes the presented hybrid approach a promising candidate for widespread applications.

Quasi-balanced two-wave mixing interferometer for remote ultrasound detection

We present an improved detection scheme for a two-wave mixing interferometer with a Bi12SiO20 crystal. The proposed detection scheme allows quasi-balanced detection of ultrasonic signals whereby electrical disturbances are suppressed. Quasi-balancing is achieved by changing the polarity of the high voltage at the photorefractive crystal, leading to an inversion of the optical interference signal, in combination with inversion of the detector signal using a signal inverter before the data acquisition device. The polarity of the high voltage is changed by utilizing an H-bridge consisting of five high-voltage relays. Microcontrollers are used to synchronize the reversion of the high voltage at the photorefractive crystal and the inversion of the measured signals. We demonstrate remote measurement of ultrasonic waves and shown that electrical disturbances are suppressed using the quasi-balanced mode.

Quality factor optimization of photonic crystal cavities through multiple multipole expansion technique and power loss integral

The Local Density of photonic States (LDOS) and Multiple Multipole Expansion technique (MME) are powerful tools in the study of spontaneous emission and calculation of photon confinement as well as efficient calculation of stationary field in planar photonic crystals. We bridge between optimization of Purcell factor and Q-factor in photonic crystal micro-cavities on one hand, and cavity power loss on the other hand. The quality factor calculated through a pulse response technique based on Finite Difference Time Domain (FDTD) simulations are compared with quality factor calculated by other approaches of LDOS and power loss. It turned out that the latter methods are more accurate and computationally less expensive. The cavity power loss is defined as the surface integration of energy density flow projected toward outside of the effective cavity volume. It is shown that size changes and shifting the neighboring rods or holes have a large impact on the mode volume and confinement. The quality factor optimization is performed for a H1- photonic crystal cavity, and mode volume investigations carried out for high Q factor arrangements. These investigations are resulted in effective structural design rules and geometrical freedom contour plots for the neighboring rods in the vicinity of the micro-cavity. These generalized design rules are suitable for further studies in other photonic micro-cavities.

Laser ultrasonic velocity measurement for phase transformation investigation in titanium alloy

We report on a contactless laser ultrasonic (LUS) method to monitor the phase transformation in a titanium alloy (Ti-6%Al-4%V) sheet by using a simple and high resolution LUS interferometer, based on two wave mixing in a fast BSO photorefractive crystal. The temperature dependent phase transformation from the α to the β phase in the Ti samples was observed around 1000°C. During heating the Ti sheets, the velocity of the laser generated longitudinal and shear ultrasonic wave, 5820 and 2950 m/s respectively, decreases fast up to 980°C which shows the start of the phase change region. At 980°C to 1020°C the bulk wave velocity is approximately constant (4820 and 2600 m/s) indicating the phase transformation process from the α to the β phase. Due to microstructural changes in the β phase at higher temperatures, the velocity of the bulk waves decreases slowly in comparison to that of the α phase.

OPTIMIZATION OF QUALITY FACTOR IN PHOTONIC CRYSTAL CAVITIES THROUGH FINITE DIFFERENCE TIME DOMAIN AND MULTIPOLE EXPANSION TECHNIQUE

Local density of photonic states calculation based on multipole expansion method is a powerful tool for studying spontaneous emission and calculation of photon confinement in photonic crystal cavities. Using multipole expansion method, we calculate local density of states and quality factor of a two-dimensional three angle photonic crystal cavity. We also compare this quality factor result with the one calculated using finite difference time domain of a pulse response. It turns out that the local density of states calculation is more accurate and computationally less expensive. It is shown that shifting and changing the size of neighboring cylinders in the vicinity of photonic crystal cavity has a large impact on the mode volume and confinement. It is also described how the increasing of quality factor can be split up into local optimization of neighboring rods and the effect of increasing the number of photonic crystal layers, which exponentially increases the quality factor. This finding strongly suggests that the number of layers can be excluded from an optimization procedure. We also present structural design rules and geometrical freedom contour plots for the neighboring cylinders. These design rules can be used in further optimization of photonic crystal cavities.

Nano-silicon based photonic crystal stamps with electron beam lithography (EBL) technology

We report on using e-beam lithographically technology for enabling the mass replication of custom-designed and prepared Nano-structures via establishing nanoimprint processes for pattern transfer into UV curable prepolymes. By EBL, the new nano-fabrication technology based on the concept of disposal master technology (DMT) is suitable for mass volume manufacturing of large area arrays of sub-wavelength photonic elements. We will present some kinds of PhC and waveguides for fabrication of nanoimprint Electron beam lithography stamps.

Design and fabrication of si-based photonic crystal stamps

In this work we report on the technology for enabling the mass replication of custom-designed and e-beam lithographically prepared structures via establishing UV-NIL nanoimprint processes for pattern transfer into UV curable pre-polymers. The new nano-fabrication technology based on the concept of disposal master technology (DMT) is suitable for mass volume manufacturing of large area arrays of sub-wavelength photonic elements. To show the potential of the application of the new nanoimprint technologies we choose as an example the fabrication of a photonic crystal (PhC) structure with integrated light coupling devices for low loss interconnection between PhC light wave circuits and optical fiber systems. We present two kinds of PhCs for fabrication of nanoimprint lithography stamps in Si. (a) A photonic crystal of Si-rods in air. This structure was fabricated with electron beam lithography (EBL) in resist and after lift-off, there is a mask of Cr on Si, then the pattern transfer into Si is performed using reacting ion etching (RIE) with SF6 as etch gas. We use 260 nm of positive resist (950K PMMA) for EBL exposure. Resist thickness, exposure dose, development time and parameters for etching have been optimized in this method. (b) In the second method lift-off was performed and metal mask was used as master. The subsequent steps for fabricating the master will be presented detail in our contribution.

Design and fabrication of Si-based photonic crystal stamps with electron beam lithography (EBL)

The ability of fabrication structure in nano-scale with high precise has established technologies like nanoimprinting via hard stamps, where the stamps are usually produced via Electron Beam Lithography (EBL) for applications in the microelectronic industry. On the other hand, nanopatterning with self ordered structures or via holographic patterns provide the basis for large area imprints. In this work we report on a technology for enabling the mass replication of custom-designed and e-beam lithographically prepared structures for pattern transfer into UV curable pre-polymers. The new nano-fabrication technology is based on the concept of Disposal Master Technology (DMT) capable of patterning areas up to 1 x 1 m2 and is suitable for mass volume manufacturing of large area arrays of sub-wavelength photonic elements. As an example to show the potential of the application of the new nanoimprint technologies, we choose the fabrication of a photonic crystal (PhC) structure with integrated light coupling devices for low loss interconnection between PhC light wave circuits and optical fiber systems. In experiment we use 260nm of positive resist 950K PMMA for EBL exposure. Resist thickness, exposure dose, development time and parameter for etching have been optimized and a photonic crystal of air-holes in silicon was fabricated, then use this sample as master stamp to fabricate imprinted photonic crystal on UV curable resist.