Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm^-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H₂O, CO₂, C₂H₂ and C₂H₆O at a pressure of 10 bar. Received: 5 August 2002 / Revised version: 23 September 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-608/265-2316, E-mail: ssanders@engr.wisc.edu     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk     

Efficient tunable laser operation of diode- pumped Yb,Tm:KY(WO4)2 around 1.9 μm

A new laser medium – Yb,Tm:KY(WO₄)₂ – for diode pumped solid state laser applications operating around 1.9 to 2.0 μm has been investigated and the main laser characteristics are presented. Diode pumping at 981 nm and around 805 nm was realised. For 981-nm pumping, the excitation occurs into Yb³⁺ ions followed by an energy transfer to Tm³⁺ions. A slope efficiency of 19% was realised. For pumping around 805 nm, the excitation occurs directly into the Tm³⁺ ions. Here a maximum slope efficiency of 52%, an optical efficiency of 40%, and output powers of more than 1 W were realised. Using a birefringent quartz plate as an intracavity tuning element, the tunability of the Yb,Tm:KY(WO₄)₂ laser in the spectral range of 1.85–2.0 μm has been demonstrated. The possibility of laser operation in a microchip cavity configuration for this material has also been shown. Received: 12 March 2002 / Revised version: 20 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-531/592-4116, E-mail: stefan.kueck@ptb.de     

Large-scale synthesis of crystalline β-SiC nanowires

Large quantities of high-purity crystalline β-SiC nanowires have been synthesized at relatively low temperature via a new simple method, the chemical-vapor-reaction approach, in a home-made graphite reaction cell. A mixture of milled Si and SiC powders and C₃H₆ were employed as the starting materials. The results show that the nanowires with diameters of about 10–35 nm are single crystalline β-SiCwithout any wrapping of amorphous material, and the nanowire axes lie along the 〈111〉 direction. Some unique properties are found in the Raman scattering from the β-SiC nanowires, which are different from previous observations of β-SiC materials. A possible growth mechanism for the β-SiC nanowires is proposed. Received: 27 August 2002 / Accepted: 28 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-29/8491-000, E-mail: zjli-sohu@sohu.com     

Efficient generation of blue light by frequency doubling of a Nd:YAG laser operating on 4 F 3/24 I 9/2 transitions

Efficient room-temperature operation of ⁴ F _3/2⁴ I _9/2 transitions in diode-end-pumped Nd:YAG lasers at 946 nm and 938.5 nm is reported. 7.0-W continuous-wave output power at 946 nm and 3.9 W at 938.5 nm have been obtained. An analytical model has been developed for the quasi-three-level laser including the influence of energy-transfer upconversion. Frequency doubling of these transitions in periodically poled KTP generated blue light at 473 nm and 469 nm. Both single-pass extra-cavity as well as intracavity schemes have been investigated. Received: 31 July 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +46-8/750-5430, E-mail: stefan.bjurshagen@acreo.se     

Catalytic growth and characterization of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

β-Ga₂O₃ nanowires have been synthesized using Ga metal and H₂O vapor at 800 °C in the presence of Ni catalyst on the substrate. Remarkable reduction of the diameter and increase of the length of the Ga₂O₃ nanowires are achieved by separation of Ga metal and H₂O vapor before they reach the substrate. Transmission electron microscopy analyses indicate that the β-Ga₂O₃ nanowires possess a single-crystalline structure. Photoluminescence measurements show two broad emission bands centered at 290 nm and 390 nm at room temperature. Received: 27 June 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +886-6/234-4496, E-mail: wujj@mail.ncku.edu.tw     

Electronic properties of Gd@C<Subscript>82</Subscript> metallofullerene peapods: (Gd@C<Subscript>82</Subscript>)<Subscript>n</Subscript>@SWNTs

Electronic properties of Gd@C₈₂ metallofullerene peapods, (Gd@C₈₂)_n@SWNTs, were investigated by electron energy-loss spectroscopy (EELS), scanning tunneling microscopy and spectroscopy (STM/STS), and field-effect transistor (FET) transport measurements. The results indicate that the electronic structure of Gd@C₈₂ metallofullerene peapods is completely different from that of intact single-walled nanotubes (SWNTs). For example, Gd@C₈₂-peapod-FETs show ambipolar behavior which is not observed in the empty SWNT-FETs under our experimental conditions. Furthermore, in semiconducting nanotubes the band gap can be varied from ∼0.5 to ∼0.1 eV using inserted Gd@C₈₂ endohedral metallofullerenes with a spatial periodicity of 1.1 to 8.0 nm, depending on the density of the fullerenes. The present findings suggest that metallofullerene peapods may point the way toward novel electronic devices. Received: 6 September 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-52/789-1169, E-mail: noris@cc.nagoya-u.ac.jp     

Structural and optical properties of nanophase zinc oxide

Nanophase zinc oxide samples were synthesized by a two-step solid-state reaction method. The phase structure and microstructure were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The vibrational Raman spectra were compared with those from the bulk and their grain size dependence was also examined. Their photoelectric behavior was studied by X-ray photoelectron spectroscopy (XPS). The peaks at 1044.5 and 1021.4 eV were recorded as corresponding to the respective binding energies of Zn 2p_1/2 and Zn 2p_3/2, and the photoelectron spectrum of O 1s in the as-prepared powder was located at 531.2 eV. A strong visible emission centered at 580 nm was clearly observed in the nanosized zinc oxide at room temperature. Photoluminescence (PL) spectra were investigated as a function of grain size after different heat treatments. The origin of the luminescence is attributed to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Received: 30 June 2001 / Accepted: 20 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Micromagnetism of two-dimensional permalloy particles with different aspect ratios

Micromagnetic properties of the Fe₁₉Ni₈₁ (5 nm)/NiO (50 nm)/Fe₁₉Ni₈₁ (30 nm) structured system have been investigated in a photoemission electron microscope in the magnetic X-ray circular dichroism operating mode. The microstructured Fe₁₉Ni₈₁ (5 nm) film contained two-dimensional islands with the aspect ratio varying from 1:1 to 10:1, and the linear size of their long axis comprised 24, 12 and 6 μm. It is shown that the magnetic domains have the direction of magnetization preferentially parallel and antiparallel to the magnetic field direction in which this system was prepared. Their number is determined by the particles’ sizes, their shape as well as by the direction of the external magnetizing field and can be characterized by a non-monotonic size dependence. The magnetization of domains with different lateral sizes was found to be 0.4 T with an accuracy better than 20%. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: nepijko@mail.uni-mainz.de     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Preparation of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowires by calcining the precursor powders synthesized in a novel inverse microemulsion

Manganese oxide (hausmannite) nanowires were prepared by annealing precursor powders at a temperature of 800 °C for 3 h, which were produced in a novel inverse microemulsion (IμE) system. The microstructures of the as-prepared Mn₃O₄ nanowires were investigated by means of X-ray diffraction, transmission electron microscopy, and Raman spectra. It has been found that the Mn₃O₄ nanowires were relatively straight and their surfaces were smooth with a typical diameter of 75–150 nm. The formation mechanism of the Mn₃O₄ nanowires is discussed. Received: 30 May 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: wangqun@nju.edu.cn     

Fabrication of submicron gratings in fused silica by F<Subscript>2</Subscript>-laser ablation

Submicron surface-relief gratings were fabricated on fused silica by F₂-laser ablation with nanosecond duration pulses from a high-resolution 157-nm optical processing system. A 157 nm wavelength projection mask was prepared by ArF-laser ablation to form a 20-μm period grating of equal lines and spaces. A 25-fold demagnification of the mask by a Schwarzschild objective generated gratings of an 830-nm period and a 250 nm modulation depth, as characterized by SEM, AFM and HeNe-laser beam diffraction. Received: 24 April 2002 / Accepted: 25 April 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503 599, E-mail: jihle@llg.gwdg.de     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Laser-induced nano-patterning by means of interference subpatterns generated by microspheres

Interference patterns generated by a regular lattice of SiO₂ microspheres on a transparent support are used for the surface patterning of polyimide (PI) foils. Using 248 nm excimer-laser radiation, thousands to millions of holes with a full width at half maximum (FWHM) of 160±40 nm can be generated with a single laser pulse. Received: 1 March 2002 / Accepted: 4 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, E-mail: dieter.baeuerle@jku.at     

High-repetition-rate eye-safe optical parametric oscillator intracavity pumped by a diode-pumped Q-switched Nd:YVO<Subscript>4</Subscript> laser

A high-repetition-rate eye-safe optical parametric oscillator (OPO), using a non-critically phase-matched KTP crystal intracavity pumped by an acousto-optically (AO) Q-switchedNd:YVO₄ laser, is experimentally demonstrated. It is found that the average OPO signal power at 1573 nm can be efficiently increased by increasing the pulse repetition rate. Moreover, the intracavity OPO process effectively shortens the pulse width so that it is in the range 5∼8 ns for pulse repetition rates of 10 to 80 kHz. As a result of the relatively short pulse, the peak power at 1573 nm is higher than 2 kW at a pulse repetition rate of 80 kHz. Received: 10 July 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +886-35/729-134, E-mail: yfchen@cc.nctu.edu.tw     

A fast-response near-infrared tunable diode laser absorption spectrometer for in situ measurements of CH4 in the upper troposphere and lower stratosphere

We describe a near-infrared in situ tunable diode laser spectrometer developed for atmospheric measurements of CH₄ in the upper troposphere and lower stratosphere (UT/LS). The instrument is designed to provide fast-response (0.5–1 Hz) measurements and operate autonomously on the NASA WB-57F high-altitude aircraft. A single-mode InGaAsP distributed feedback laser diode operating at 1.6537 μm scans continuously over the R(3) rotation–vibration transition in the 2ν₃ band. We use a direct absorption technique incorporating a custom-designed long path length (252 m) low-volume (3.6 L) astigmatic Herriott cell. The present detection sensitivity is 5×10¹⁰ molecules cm^-3, corresponding to ∼20 ppbv in the UT/LS, with the main limit to instrument precision being background optical interference fringes. In-flight performance is demonstrated by presentation of recent data. Received: 25 January 2002 / Revised version: 5 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5373, E-mail: richard@al.noaa.gov     

Large-scale synthesis of SnO2 nanobelts

Tin dioxide (SnO₂) nanobelts have been successfully synthesized in bulk quantity by a simple and low-cost process based on the thermal evaporation of tin powders at 800 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the nanobelts are uniform, with lengths from several-hundred micrometers to a few millimeters, widths of 60 to 250 nm and thicknesses of 10 to 30 nm. X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and selected-area electron diffraction analysis (SAED) indicate that the nanobelts are tetragonal rutile structure of SnO₂. The SnO₂ nanobelts grow via a vapor–solid (VS) process. Received: 3 June 2002 / Accepted: 10 June 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-551/559-1434, E-mail: gwmeng@mail.issp.ac.cn     

Preparation and characterization of a poled nanocrystal and polymer composite PbTiO3/PEKc film for electro-optic applications

Composite thin films of PbTiO₃ nanocrystals and high-transparency PEKc polymer for applications in electro-optical devices were prepared using the spin coating technique. The size of the PbTiO₃ nanocrystals was estimated to be 30–40 nm using a transmission electron microscope. The transmission technique, a simple method for measuring the electro-optic coefficients of poled composite polymer films was developed. The electro-optic coefficient γ₃₃ of poled PbTiO₃/PEKc composite polymer films was measured to be 18.34 pm  V^-1 at 633 nm under room temperature. The index at 633 nm and the dielectric constant at 100 kHz under room temperature were determined to be 1.65248 and 7.32, respectively. The figure of merit F₂=n⁷γ²/ε was estimated to be 1546, showing very good electro-optical properties. Received: 5 February 2002 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +852/2788-7791, E-mail: eeytc@cityu.edu.hk     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca