Coherent heterodyne-assisted pulsed spectroscopy: sub-Doppler two-photon spectra of krypton, characterizing a tunable nonlinear-optical ultraviolet light source

A chirp-minimized, nanosecond-pulsed system has been developed to generate tunable coherent ultraviolet light at ∼212.5 nm by fourth-harmonic conversion of output from an amplified, injection-seeded optical parametric oscillator (OPO). Our CHAPS (coherent heterodyne-assisted pulsed spectroscopy) method is used to characterize the frequency stability and optical bandwidth of the system’s output radiation by recording sub-Doppler two-photon excitation spectra of krypton. In our new scanned-reference variant of CHAPS, the central frequency for each amplified OPO pulse is logged by the optical-heterodyne chirp-analysis software, with the frequency of the seed laser slowly tuned and monitored by a high-resolution wavemeter, unlike our previous implementation of CHAPS where the seed-laser frequency was fixed. For the amplified, up-converted pulses at ∼212.5 nm, our CHAPS measurements indicate an optical bandwidth of ∼100 MHz, consistent with the Fourier-transform limit of their duration (∼4.5 ns).     

Self-grown fiber fabrication by two-photon photopolymerization

We demonstrate a new fiber growth mechanism in a photocurable resin by ultrafast laser illumination. A high-repetition rate (∼1 MHz) ultrafast laser beam at the wavelength of ∼523 nm was focused into an ultraviolet photocurable resin to trigger two-photon photopolymerization process. Time-resolved shadowgraphs and scattered light imaging revealed that the curing commenced in the neighborhood of the geometric focal point of the laser beam and that the fiber growth progressed mostly towards the laser source. The cured fiber was thinner and longer than the profile of the focused laser beam, facilitated by nonlinear propagation and absorption of the ultra-fast laser beam. The achieved aspect ratio of the fiber was higher than 180 with ∼10 μm mean diameter, and the average growth rate was up to ∼2 mm/s.     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

A femtosecond stimulated raman spectrograph for the near ultraviolet

The design of a femtosecond stimulated Raman spectrograph (FSRS) optimized for measurements in the near UV is presented. It features a 387.5 nm Raman pump pulse causing a (pre-)resonance enhancement for molecular absorbing in the UV. As many photoreactive molecules absorb there, the set-up is particularly suited to study photochemical reactions. The 387.5 nm pulses are generated by frequency adding two femtosecond laser pulses with linear chirps, equal in magnitude but opposite in sign. This results in intense and spectrally narrow (∼10 cm^-1) Raman pump pulses which allow recording of Raman spectra with a good spectral resolution. The power of the spectrograph to trace ultrafast photoreactions is illustrated by measurements on the photochemistry of o-nitrobenzaldehyde. PACS 33.20.Fb; 39.30.+w     

Femtosecond properties of photorefractive polymers

Photorefractive polymers allow to reversibly record holograms over a broad spectral range. This capability offers the possibility to store the information contained in ultrafast optical pulses (i.e., time domain) in the frequency domain. We demonstrate a storage bandwidth of >80 nm around 800 nm (i.e., >36 THz), giving a temporal resolution for Gaussian pulses of 13 fs at room temperature. Time reversal of a pulse train of 130 fs pulses confirms these capabilities.     

Multiple-wavelength lasing by multiform self-frequency conversion in Nd<Superscript>3+</Superscript>-doped La<Subscript>2</Subscript>CaB<Subscript>10</Subscript>O<Subscript>19</Subscript> crystals

The self-sum-frequency process was firstly demonstrated in Nd³⁺-doped La₂CaB₁₀O₁₉ (LCB) crystal. In addition, simultaneous participation of the fundamental laser in multi-self-frequency conversion including the self-frequency doubling was reported. Emissions at five different wavelengths (525, 529, 533.6, 1050 and 1069 nm) were simultaneously obtained. The output power of the three green visible lasers (525, 529 and 533.6 nm) generated by multi-self-frequency-conversion of the fundamental laser is up to 26.64 mW, and the light-light conversion efficiency is up to 4.85%. The lasers at 525 and 529 nm are very close to the green primary color G (526.3 nm) defined by the 1964CIERGB system, which has potential applications in laser-based high brightness display. The large frequency differences of 4–6 THz between 525 and 529 nm, 529 and 533.6 nm, as well as 1050 and 1069 nm, are also potential useful in generating THz waves by difference frequency generation (DFG) technique in a nonlinear optical (NLO) crystal.     

Diode-pumped Yb:KYW femtosecond laser frequency comb with stabilized carrier-envelope offset frequency

We describe the detection and stabilization of the carrier envelope offset (CEO) frequency of a diode-pumped Yb:KYW (ytterbium-doped potassium yttrium tungstate) femtosecond oscillator that is spectrally centered at 1033 nm. The system consists of a diode-pumped, passively mode-locked femtosecond laser that produces 290 fs pulses at a repetition rate of 160 MHz. These pulses are first amplified, spectrally broadened and temporally compressed to 80 fs, and then launched into microstructured fiber to produce an octave-spanning spectrum. An f-2f nonlinear interferometer is employed with the broadened spectrum to detect and stabilize the CEO frequency through feedback to the pump laser current. These results demonstrate that such a Yb-doped tungstate laser can provide an efficient, compact, high-repetition-rate optical frequency comb with coverage from 650–1450 nm.     

Dielectric-loaded surface plasmon–polariton nanowaveguides fabricated by two-photon polymerization

The design, fabrication, and characterization of dielectric-loaded surface plasmon–polariton nanowave-guides on a gold film are presented. The nanostructures are produced by two-photon polymerization with femtosecond laser pulses, and the minimum ridge height is ∼170 nm. Leakage radiation microscopy shows that these surface plasmon–polariton waveguides are single mode with strong mode confinement at the wavelength of 830 nm. The experimental results are in good agreement with the simulation by the effective-index method.     

Efficient ultrafast ultraviolet generation based on frequency doubling in short-period periodically-poled KTiOPO4 crystal

We have efficiently generated ultrafast coherent ultraviolet (UV) pulses based on second-harmonic generation in a periodically-poled KTiOPO₄ crystal with the period as short as 2.55 μm. The highest output power is measured to be 20.1 mW at 377.1 nm with the conversion efficiency of 15.6%. Such an output wavelength is probably among the shortest ones generated by forward second-harmonic generation in a periodically-poled KTiOPO₄ crystal.     

Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

Gigahertz Harmonic Mode-Locked Fiber Laser Based on Tunable SMS Ultrafast Optical Switch

A novel passively gigahertz harmonic mode-locked all-fiber laser based on hybrid fiber structure (single-mode fiber–graded-index multimode fiber–single-mode fiber (SMS)) is proposed SMS, which coils on the paddles of polarization controller (PC), is demonstrated to modulate the temporal intensity for mode-locking. The nonlinear absorption properties of the optical switch are controllable by adjusting the paddles of PC; such an ultrafast optical switch enables the wavelength switchable harmonic mode-locking operation. Ultrafast pulses with 1.9 ps at 1558.41 nm and 0.95 ps at 1563.08 nm are generated. The maximum repetition rate of the laser up to 1.127 GHz harmonic of fundamental repetition mode-locking at 1563.08 nm, corresponding to 880 order, and the output power is 4.2 mW. Considering its superiority in terms of low cost, easy integration, and high reliability, the findings validate that SMS can be used in harmonic mode-locking.     

Distributed feedback dye laser holographically induced in improved organic–inorganic photocurable nanocomposites

Distributed feedback (DFB) lasing in permanent volume transmission gratings formed in a laser dye-doped organic–inorganic nanocomposite has been investigated. DFB laser cavities were fabricated using one-step two-beam holographic exposure of Pyrromethene 567 (PM567) doped photopolymerizable acrylate monomers containing inorganic (LaPO₄) nanoparticles. Compared to the formulation previously utilized, the material composition presented provides longer lifetime of the laser. Spectral and polarization properties, input–output and stability characteristics of the laser output have been investigated by varying the material composition and the patterning parameters. DFB lasing emission of the second and the third diffraction orders has been demonstrated. The spectral linewidth of ∼0.08 nm has been observed at a pump energy threshold of about 0.2 μJ/pulse for the second-order DFB lasing when pumped with 532 nm 500 ps laser pulses. Spectral tuning of the lasing output over ∼56 and ∼7 nm was obtained by varying the grating period and the content of inorganic nanoparticles in the polymer matrix, respectively.     

Nonlinear optical and optical limiting studies of alkoxy phthalocyanines in solutions studied at 532 nm with nanosecond pulse excitation

We report our results on the nonlinear optical and optical limiting properties of two alkoxy phthalocyanines namely 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine and 2,3,9,10,16,17,23,24-octakis-(heptyloxy) phthalocyanine zinc(II) studied at a wavelength of 532 nm using 6 ns pulses. Using the standard Z-scan technique we observed that both the phthalocyanines exhibited negative nonlinearity as revealed by the signature of closed aperture data. The magnitude of the nonlinear refractive index n₂ evaluated from the closed aperture data was ∼ 1.61×10^-11 cm²/W for the free-base phthalocyanine and ∼ 1.56×10^-11 cm²/W for the metallic phthalocyanine. Open aperture Z-scan data indicates strong nonlinear absorption in both the phthalocyanines with measured nonlinear coefficients of ∼ 1650 cm/GW and ∼ 1850 cm/GW respectively. We also report optical limiting properties of these phthalocyanines with limiting thresholds (I_1/2) of ∼ 0.5 J/cm². Our studies suggest that these phthalocyanines are one of the best molecules for nonlinear optical applications studied recently. PACS 42.65.-k; 42.70.Jk, 42.65.Jx     

Laser-assisted generation of self-assembled microstructures on stainless steel

Utilising a Nd:YVO₄ laser (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) and a Nd:YAG laser (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz), it was found that during the pulsed laser ablation of metal targets, such as stainless steel, periodic nodular microstructures (microcones) with average periods ranging from ∼30 to ∼50 μm were formed. This period depends on the number of accumulated laser pulses and is independent of the laser wavelength. It was found that the formation of microcones could occur after as little as 1500 pulses/spot (a lower number than previously reported) are fired onto a target surface location at laser fluence of ∼12 J/cm², intensity of ∼1.5 GW/cm². The initial feedback mechanism required for the formation of structures is attributed to the hydrodynamic instabilities of the melt. In addition to this, it has been shown that the structures grow along the optical axis of the incoming laser radiation. We demonstrate that highly regular structures can be produced at various angles, something not satisfactorily presented on metallic surfaces previously. The affecting factors such as incident angle of the laser beam and the structures that can be formed when varying the manner in which the laser beam is scanned over the target surface have also been investigated.     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

Absolute phase effect in ultrafast optical responses of metal nanostructures

We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical responses of metal nanostructures significantly depend on the absolute (carrier–envelope) phase of excitation pulses. Strong enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers and atoms, molecules, and condensed matter at relatively low intensities. PACS  78.67.-n; 78.47.+p; 79.60.Jv; 73.20.Mf     

300 microJ noncollinear optical parametric amplifier in the visible at 1 kHz repetition rate

We demonstrate an order-of-magnitude energy scaling of a white-light seeded noncollinear optical parametric amplifier in the visible. The generated pulses, tunable between 520 and 650 nm with sub-25-fs duration, had energies up to 310 microJ with 20% blue-pump-to-signal energy conversion efficiency at 540 nm. This new ultrafast source will make possible numerous extreme nonlinear optics applications. As a first application, we demonstrate the generation of tunable vacuum ultraviolet pulses.     

Nonlinear optical absorption and refraction of epitaxial Ba0.6Sr0.4TiO3 thin films on (001) MgO substrates

Highly epitaxial Ba_0.6Sr_0.4TiO₃ (BST) ferroelectric thin films were fabricated on (001) MgOsubstrates by pulsed laser deposition. The nonlinear optical absorption coefficients (β) and refraction indices (γ) of the BST thin films on (001) MgO substrates were investigated using the single beam Z-scan technique with femtosecond laser pulses at the wavelengths of 790 nm and 395 nm, respectively, at room temperature. The nonlinear absorption coefficients of BST thin films were measured to be ∼0.087 cm/GW and ∼0.77 cm/GW at 790 nm and 395 nm, respectively. The nonlinear refraction indices of BST thin films exhibit a strong dispersion from a positive value of 6.1×10^-5 cm²/GW at 790 nm to a negative value of -4.0×10^-5 cm²/GW at 395 nm near band gap. The dispersion of γ is roughly consistent with Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance. These results show that the BST film is a promising material as a candidate for nonlinear optical applications. PACS 42.70.Mp; 78.20.-e; 81.05.-t     

Performance of actively Q-switched Nd:LiLuF<Subscript>4</Subscript> crystal end-pumped by a 792 nm laser diode

An efficient laser diode end-pumped continuous-wave (CW) and AO Q-switched laser of Nd:LiLuF₄ crystal with dual central wavelengths of 1053.1 and 1054.7 nm is reported for the first time. The maximum CW output power of 6.22 W was obtained at absorbed pump power of ∼14.6 W with the output transmission of 2%. The optical conversion efficiency is ∼43%, corresponding to a slope efficiency of about 48% with respect to the absorbed pump. For the Q-switched operation, the shortest pulse width of 17 ns was obtained at the pulse repetition frequency (PRF) of 0.5 kHz, resulting in a pulse energy of 2.24 mJ and peak power of 131.8 kW.     

Controlling aspect ratios of suspended nanorods fabricated by multi-photon polymerization

Nanorods are building blocks of three-dimensional photonic crystals and other nanostructures fabricated by multi-photon polymerization with femtosecond laser pulses. The aspect ratios of their cross sections are critical to the in-plane and the interlayer rod distances, which greatly affect the performance. Here we demonstrate the control of aspect ratios from ∼3 to 0.85. At a high scanning speed, aspect ratios can be smaller than unity with a lateral size of ∼150 nm. The results indicate that cylindrical nanorods can be polymerized by the commonly used transverse scanning method to improve the qualities of three-dimensional nanostructures.