Ring beam shaping optics fabricated with ultra-precision cutting for YAG laser processing

In this study, a method for generating ring intensity distribution at a refraction-type lens with an aspheric element was proposed, and the beam shaping optical element was finished using only ultra-precision cutting. The shape of the optical element and its irradiance pattern were determined from numerical calculation based on its geometrical and physical optics. An ultra-precision lathe was employed to fabricate beam shaping optical elements, and acrylic resin was used as the material. The transmittance of an optical element (a rotationally symmetrical body) with an aspheric surface fabricated using a single-crystal diamond tool was over 98%, and its surface roughness was 9.6 nm Ra. The method enabled the formation of a circular melting zone on a piece of stainless steel with a thickness of 300 μm through pulse YAG laser (λ 1:06 μm) processing such that the average radius was 610 μm and the width was 100–200 μm. Circular processing using a ring beam shaping optical element can be realized by single-pulse beam irradiation without beam scanning.     

Rapid fabrication of optical volume gratings in Foturan glass by femtosecond laser micromachining

We report on rapid fabrication of optical volume gratings in Foturan glass using a modulated femtosecond laser focused with cylindrical lenses. An optical volume grating with an area of 2 mm ×3 mm and ∼2 mm thickness can be achieved within 10 min by use of this method. Optical micrography confirms the volume nature of the gratings and shows that they consist of 10 μm-thickness planes with a period of 15 μm. The diffraction efficiency is examined to be ∼56%. The limitations and future implementations of the fabricated volume gratings are discussed.     

A vision chip with column-level amplification of optical data signals for indoor optical wireless local area networks

We present a vision chip architecture with column-level photo-amplification of optical data signals for optical wireless local area networks (LANs) to reduce the pixel area. Based on the architecture, we have fabricated a prototype vision chip in a standard 0.8 μm bipolar complementary metal-oxide-semiconductor (BiCMOS) technology. The device offers position detection of other optical transceivers in the LAN and 4-ch concurrent high-speed optical data acquisition. A data rate of 400 Mbps was demonstrated. The pixel size was 125 μm square, which can be shrunk to smaller than around 60 μm square in 0.35 μm or more advanced BiCMOS or CMOS technologies.     

Synthesis and field emission of two kinds of ZnO nanotubes: taper-like and flat-roofed tubes

Two kinds of ZnO nanotubes, including taper-like and flat-roofed tubes, have been successfully fabricated using a simple aqueous solution route by changing the experimental conditions. All the obtained nanotubes have a uniform size of 500 nm in diameter, 10–50 nm in wall thickness, and 2–5 μm in length. The growth mechanism of two kinds of ZnO nanotubes was investigated. Field emission measurements showed that tapering nanotubes have the good field emission performance with a low turn-on field of ∼ 2.1 V μm^-1 and a low threshold field of ∼ 3.8 V μm^-1, which suggests the possible applications of the ZnO tubular structures in field emission microelectronic devices. PACS 73.61.Ga; 73.63. Fg; 85.45.Db     

Optical properties of one-dimensional photonic crystals fabricated by photo-electrochemical etching of silicon

The optical properties of one-dimensional photonic crystals (1D PCs), fabricated by electrochemical etching of periodic wall arrays on n-type (100) Si substrates, are investigated in this study. Several 1D PCs were fabricated with lattice periods varying from 4 to 7 μm and with trench depths in the range 160–210 μm. In-plane reflection spectra of the photonic structures at different depths were registered over a wide spectral range of 1.5–15 μm using Fourier Transform Infra-Red (FTIR) micro-spectroscopy. Some of the features observed in the reflection spectra of the structures investigated are believed to be as a result of interface roughness. A corrugated side-wall surface, an artifact of the fabrication technique, results in the degradation of optical reflection characteristics, principally mainly in the near IR spectral range, and the emergence of optical anisotropy. As a result of the periodicity, modulation of the reflection spectra, that is, the difference between the maxima and minima of the interference fringes, reached a value of 95% in the mid-infrared. The optical properties of the structures investigated indicate that they show promise for microphotonics applications.     

Thin-film poly-Si solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), an open-circuit voltage of 0.539 V and a short-current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance. No light-induced degradation is observed. The optical and transport properties of poly-Si cells are summarized. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 7 July 1999     

Study on the p-type QWIP-LED device

A p-type quantum well infrared photodetector (QWIP) integrated with a light-emitting diode (LED) (named QWIP-LED) was fabricated and studied. The infrared photo-response spectrum was obtained from the device resistance variation and the near-infrared photo-emission intensity variation. A good agreement between these two spectra was observed, which demonstrates that the long-wavelength infrared radiation around 7.5 μm has been transferred to the near-infrared light at 0.8 μm by the photo-electronic process in the QWIP-LED structure. Moreover, the experimentally observed infrared response wavelength is in good agreement with the theoretical calculation value of 7.7 μm. The results on the upconversion of the infrared radiation will be very useful for the new infrared focal plane array technology.     

Long-wavelength MBE grown GaInNAs quantum well laser emitting at 1270 nm

In this paper, we report on comprehensive theoretical optical properties analysis and experimental device electrical-optical characterization of long wavelength GaInNAs edge-emitting laser diode. The theoretical analysis demonstrates that a high quality GaInNAs active region and device design are devised, where high material gain near 1.3 μm and optimal optical mode confinement are calculated. Experimentally, room temperature lasing emission around 1.27 μm with threshold current densities of 670–810 A/cm² is obtained from the fabricated broad area GaInNAs edge-emitting laser grown by molecular beam epitaxy technique.     

Ion channeling in natural and synthetic beryl crystals

The transmission of ions by channeling through natural beryl and synthetic emerald has been studied extensively. The transmission ratios depend upon the angle of incidence with a full half width of less than 0.32°. While the maximum ratio obtained up to now is only 4×10⁻⁴ for 350 keV protons through a crystal of 21 μm thickness, the energy of the transmitted ions is high, the loss being in the order of a few keV/μm. About 60–80% of the particles emerging from the rear surface are ionized. By varying the ion species transmission could be observed up to atomic number 9. It is assumed that the transmission is facilitated by the existence of an electron free channel core. Higher transmission ratios can be expected for sufficiently perfect crystals.     

Optimization of electro-optical characteristics of GaAs-based oxide confinement VCSEL

An experimental study has been presented of the oxide-confined vertical-cavity surface-emitting lasers (VCSEL) operating in the 850 nm region of the electromagnetic spectrum. In this regard, various relevant VCSEL samples with numerous oxide aperture sizes have been fabricated and characterized. Thorough investigations of the electrical as well as optical characteristics of the fabricated samples have been performed which includes the overall device performance as a function of the oxidize aperture sizes. It is reported that the VCSEL with oxide aperture size <10 μm require low threshold currents (<1 mA). Further, the differential quantum efficiencies up to 28% were measured for a number of these devices. It is found that devices employing oxide aperture of 10 to 15 μm shows promising electro-optical characteristics for 850 nm oxide VCSEL optimization.     

Influence of multimode-pump propagation on the gain characteristics of erbium–ytterbium doped waveguide amplifier

Channel waveguides with channel opening widths (COWs) from 4.5 to 7.5 μm with increment of 0.5 μm have been fabricated by two-step ion exchange on the same erbium-ytterbium uniformly doped phosphate glass substrate of 4.3-cm length. Experimental results indicate that the gain for 1534-nm small signal light pumped at saturation is maximized and shows a 3.6-dB enhancement for these erbium-ytterbium doped waveguide amplifiers (EYDWAs). The intensity profile overlap between signal and multimode-pump light in waveguide leads to the improvement in the gain of EYDWA. This can be used to explain the experimental results and shows the general dependence of the gain characteristics on the modal behavior of EYDWA. The presence of higher order modes of pump light and the optimization of this intensity profile overlap are significant to improve the gain properties of EYDWA.     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

High-quality MBE growth of AlχGa1-χ As-based THz quantum cascade lasers

High-quality GaAs-based quantum cascade laser (QCL) structures for the terahertz (THz) emission have been grown by solid source molecular-beam epitaxy. Ex-situ high-resolution x-ray diffraction shows that layer thickness and its control is the most critical growth aspect and that the lasing potential of the structure can be determined by the thickness accuracy of the layers. For our samples, the thickness tolerance for working lasing structures emitting approximately 100 μm was determined to be minimally above 1% for a 15 μm active region which was composed of 54.6 nm cascade cells. Increasing interface roughness adversely affects the lasing threshold and power. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

Controlling the properties of silver nanoparticles deposited on surfaces using supercritical carbon dioxide for surface-enhanced Raman spectroscopy

Silver nanoparticles (AgNPs) have been deposited on silicon and glass surfaces via a supercritical carbon dioxide (sc-CO₂) synthesis route for application in surface-enhanced Raman spectroscopy (SERS). Arrhenius plots revealed that nucleation and growth processes in this system depend on both temperature and surface chemistry. Results also demonstrated that temperature and surface chemistry could be varied to control nanoparticle properties, such as the mean nanoparticle size, density, and surface coverage, providing two useful variables for manipulating the properties of AgNPs deposited on surfaces in this system. These data also provide scientific insight into the underlying mechanisms governing heterogeneous AgNP deposition on a substrate in a sc-CO₂ system in addition to engineering insight into the variables that can be used to manipulate AgNP characteristics. The mean particle size could be tuned over the range 20–200 nm, the interparticle distance could be tuned over the range 70 nm–1 μm, and the surface coverage could be tuned over the range 0.035–0.58. Products were analyzed by scanning electron microscopy with image analysis, transmission electron microscopy, X-ray diffraction, and SERS. The silver nanoparticle-coated substrates were successfully applied in SERS, detecting the model analyte Rhodamine 6G at a concentration of 1 μM, a three orders of magnitude improvement over SERS surfaces previously fabricated in sc-CO₂ systems. Such surfaces can find use in trace concentration analyte detection in biomedical, chemical, and environmental applications.     

Processing of micro-particles by UV laser irradiation in a field cage

An electric cage-laser micro-turning lathe was realised and applied to contact-free handling and mechanical processing of micro particles. Since particles with diameters of several micrometers cannot be fixed in mechanical chucks, an octode field cage was used to trap and rotate a single particle in a fluid without any mechanical surface contact. A pulsed nitrogen laser of high beam quality focused to about 1 μm in diameter could be adjusted independently of the cage position. The trapping forces (negative dielectrophoresis) acting on a bead of 5 to 15 μm are up to several hundred pN. This and the surrounding fluid damp down the effect of the laser pulses during bead processing. Examples demonstrating the possibilities of this technique are shown. Microsystems with high optical quality were fabricated photolithographically or by laser direct-write chemical vapor deposition (LCVD). Technical and biotechnological applications are discussed. Received: 20 October 1999 / Accepted: 27 October 1999 / Published online: 10 November 1999     

Design theory and experiment of acousto-optical tunable filter by use of flexural waves applied to thin optical fiber

Spectral response of acoustically induced microbending through thin optical fiber is discussed from mode-coupling of core and cladding modes. The thin fiber is analyzed in three-layered structure (core-cladding-air) to gain insights into acousto-optic modulation. We explained the dependence of core and/or cladding diameters on acoustic source parameters from numerical calculations. According to the calculations, we successfully fabricated all-optical tunable filter using this thin fiber that yields an efficient mode-coupling at flexural wave frequencies less than 1MHz. To increase the acousto-optic effect, we used a specially designed thin optical fiber (80 μm of cladding diameter) in the section where flexural wave is produced, and spliced both ends of the thin fiber to the tapered 125 μm fibers. The frequency and voltage tuning of fabricated filter is also confirmed by changing the driven frequency and applied voltage of the PZT, respectively. This result suggests a possibility of fiber-optic device application as all-optical tunable filter at 1.55 μm.     

Wavelength Tuning of Double-Cavity Micromachined Filter with Electrical and Thermal Actuations

We demonstrate double-cavity micromachined tunable filters with electrical actuations for both blue and red wavelength tuning. Electrical actuations for both shifts can be achieved by combining electrostatic and electro-thermal actuations below and above the breakdown voltage of p-n junctions in the micromachined cantilevers. The measured tuning range with the actuation is 8 nm for both tuning sides and the micromachined filter with a cantilever length of 80 μm exhibits a switching time of less than 115 μs for both turn-on and turn-off states. We also present the theoretical and measured temperature dependence of fabricated micromachined filters at various cantilever lengths, which experimentally gives us a temperature dependence of 0.14 nm/K for a cantilever length of 100 μm.     

Ultrabroad gain in an optical parametric generator with periodically poled KTiOPO4

An optical parametric gain bandwidth of 115 THz at full-width half maximum is generated from a picosecond Ti:sapphire pumped degenerate optical parametric generator. This ultrabroad bandwidth could be obtained by first identifying the wavelength where the nonlinear optical material has zero group-velocity dispersion (GVD). By pumping at half this wavelength the degenerate signal–idler pairs can accommodate ultrabroad bandwidths. The explanation for this is that the group velocities of the signal and the idlers are approximately matched and the GVD is small. However, in order to thoroughly investigate the degeneracy region around 1700 nm we fabricated several periodically poled KTiOPO₄ (PPKTP) crystals with different periods, and also one periodically poled RbTiOPO₄ (PPRTP). Both collinear and noncollinear configurations were employed for broadband parametric generation in this region. It was found that the optimum pump wavelength is in the region between 800 nm to 850 nm for PPKTP, and we could also conclude that a similar performance was found for PPRTP. This work will allow the design of optical parametric devices for generating few-cycle pulses in the spectral region between 1.1 μm and 3.8 μm. PACS 42.65.Re; 42.65.Ky; 42.65.-k     

Photoacoustic spectroscopy of thin films of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and GeSe<Subscript>2</Subscript>

Photoacoustic spectroscopy (PAS) is one of the important branches of spectroscopy, which enables one to detect light-induced heat production following the absorption of pulsed radiation by the sample. As₂S₃, As₂Se₃ and GeSe₂ exhibit a wide variety of photo-induced phenomena that enable them to be used as optical imaging or storage medium and various electronic devices, including electro-optic information storage devices and optical mass memories. Therefore, accurate measurement of thermal properties of semiconducting films is necessary to study the memory density. The thermal conductivity of thin films of As₂S₃ (thickness 100 μm and 80 μm), As₂Se₃ (thickness 100 μm and 80 μm) and GeSe₂ (thickness 120 μm and 100 μm) has been measured using PAS technique. Our result shows that the thermal conductivity of thicker films is larger than the thinner films. This can be explained by the thermal resistance effect between the film and the surface of the substrate.      

Optical modulator using metal-oxide-semiconductor type Si ring resonator

Electric-field drive optical modulators using a Si ring resonator were fabricated on silicon-on-insulator (SOI) wafers. The fabricated resonators consisted of Si waveguides with width and thickness of 1.0 and 0.3 μm, respectively. In order to induce the linear electro-optic (EO) effect in the Si core layer, the strain was applied by covering the layer with Si₃N₄ film (0.26 μm thick) deposited by low pressure chemical vapor deposition (LPCVD) at 750 °C. The vertical electric-field was applied to the Si waveguide through the top and bottom cladding layers, and the optical output from the drop port at the resonance wavelength was measured. At a wavelength of 1501.6 nm, the optical modulation of 33% was obtained at 200V (electric-field at the silicon surface ∼3 × 10⁵ V/cm, nearly the breakdown field). The resonance wavelength was shifted toward the short wavelength side by applying both positive and negative voltages, this shift was explained by carrier concentration modulation. The linear EO effect in the Si core layer was not observed, presumably because the strain in the Si core layer was too small.