Low Power Cw Research Articles

GRIN lens and lens array fabrication with diffusion-driven photopolymer

We introduce a new method to make gradient index (GRIN) lenses and GRIN lens arrays by exposing diffusion-driven photopolymers using a low-power CW laser. By changing the size and power of the laser beam and the exposure time the index profile of the GRIN lens can be controlled. Combined with the self-developing nature of the photopolymer, this enables rapid "on-demand" printing of arbitrary micro-optics. We also introduce a new form of scanning phase microscopy that is conveniently integrated into the lens exposure hardware, providing accurate process monitoring.

“Triggering” effect of second harmonic generation in centrosymmetric α-BaB 2O 4 crystals

We have discovered an occurrence of optical second harmonic generation at 1064 nm fundamental wavelength for the α-BaB 2O 4 (ABO) crystal illuminated by 7 ns nanosecond nitrogen laser of 337 nm. The effect is observed only during illumination and achieves the maximum SHG output after 15–20 min of treatment. After switching off of the UV laser treatment the SHG decreases during about 5 min. The maximum second order susceptibility achieved was equal to about 2.1 pm/V at 1064 nm after about 20 min of UV laser treatment. The observed SHG is explained by local photothermal heating of the crystals and photo stimulated photothermal anisotropy that favors an occurrence non-centrosymmetry of the local charge density. An additional investigations of the low-power (cw) He–Ne laser light indicate the absence of additional light scattering, which unambiguously indicates an absence of any additional microcrystalline states including non-cenrosymmetrical β-BaB 2O 4 phases. Probably photostimulated enhancement of thermal expansion anisotropy the main reason for the observed non-centrosymmetry.

Nonlinear Confocal Microscopy for High-Resolution Measurement

We demonstrate cell imaging with a new confocal nonlinear optical microscope using a low-power cw laser. The confocal nonlinear optical microscope, employing degenerate four-wave mixing geometry, can detect the fine structure of submicron objects with nanoscale contents such as biological cells. The optical signal, which is given by the third-order susceptibility tensor, is confined to the focal region of the focusing incident beam, because the absorption of the object is dependent on the third power of the excitation laser intensity. We have observed a thylakoid membrane in a chloroplast by scanning the tensor element inside.

Design of a low power optical limiter based on a new nanocomposite material incorporating silica-encapsulated phthalocyanine in Nafion

We report on the design of a stable optical limiter in the low laser power regime based on the thermal variation of refractive index in a novel nanocomposite material. The optical material, chloroaluminium-phthalocyanine (ClAlPc), is embedded in SiO2-Nafion nanocomposite membrane (ClSNf) and its thermally induced nonlinear refractive index is characterized by the Z-scan technique with a low power cw He–Ne laser as the source. The value of nonlinear refractive index coefficient, n2, is found to be about 1.11 × 10−11 m2 W−1. The experiment is repeated with the dye doped in pure Nafion membrane (ClNf) and the results are compared with those of ClAlPc doped SiO2–Nafion nanocomposite membrane. The value of n2 is found to be 1.36 × 10−11 m2 W−1 and is larger than that of the ClAlPc embedded SiO2–Nafion nanocomposite membrane. The photostability of the dye-embedded membrane is studied by exposing the sample to cw He–Ne laser and monitoring its fluorescence emission intensity continuously. The samples are found to show large thermal lens effect and demonstrated to be good optical limiters in the low power regime. Whereas the optical properties of dye-doped Nafion appear to be slightly better than those of the dye embedded in silica and incorporated in Nafion, the latter is found to offer excellent photostability.

Large third-order optical nonlinearity and optical limiting in symmetric and unsymmetrical phthalocyanines studied using Z-scan

The third-order nonlinear optical and optical limiting properties of 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine, 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine zinc(II) ( symmetric) and 2(3)-(butane-1,4-dioic acid)-9(10),16(17),23(24)-tri tert-butyl phthalocyanine zinc(II) ( unsymmetrical) have been investigated using a continuous wave laser at 633 nm. We have employed the Z-scan technique to evaluate the sign and magnitude of nonlinear refractive index and the nonlinear absorption coefficient. Optical limiting based on nonlinear refraction was performed and limiting thresholds were estimated for all the three samples. The magnitude of the third-order nonlinearities measured were one of the highest reported in the cw regime till date. Unsymmetrical phthalocyanine had better limiting characteristics and its nonlinear coefficients were comparatively high. Results of the nonlinear optical studies performed with these organic dyes doped in Poly(methylmethacrylate) are also presented. Our studies indicate that these phthalocyanines are potential candidates for optical limiting applications in low power cw regime.

Optical amplification in Ho3+-doped transparent oxyfluoride glass ceramics at 750nm

Positive transient optical gain has been demonstrated in Ho3+-doped transparent oxyfluoride glass-ceramics. A pump and probe experiment has been designed to show this result. High power laser pulses at 532nm were used as the pump source to strongly populate the Ho3+S25:F45 level due to nonresonant ground state absorption. Low power cw laser radiation at 750nm was used as the probe beam. The signal beam stimulates the emission associated with the Ho3+S25:F45→I75 electronic transition at 750nm. In addition to this, the high power pump pulses provide population inversion between the S25:F45 and I75, initial and final states of the transition, respectively, giving rise to the optical amplification of the signal beam. A gain coefficient of 3.7cm−1 (∼16dB∕cm) was obtained for a pump energy density of about 135mJ∕cm2 and a signal beam power density of 6μW∕cm2.

Nonlinear responses and optical limiting behavior of fast green FCF dye under a low power CW He–Ne laser irradiation

The nonlinear responses and optical limiting performance of a dye type acid, called fast green FCF, are investigated under irradiation of 35mW continuous wave He–Ne Laser. The second order refractive index and nonlinear absorption coefficient are measured by use of z-Scan technique. The optical limiting behavior is investigated by transmission measurement through the sample. Linear absorption coefficient (α), nonlinear absorption coefficient (β) and second order refractive index (n2) of fast green FCF are measured at different concentrations.

High-gain diode-pumped amplifier for generation of microjoule-level picosecond pulses

A diode-pumped single-pass amplifier system relying on two grazing-incidence Nd:YVO(4) slabs was developed to increase the energy of low-repetition-rate pulses from a decimated low-power cw mode-locked oscillator. Single-pass unsaturated gain up to 1.3x10(5) was achieved, and amplified pulses of 10-muJ energy and 8.0-ps duration were obtained. Efficient second harmonic generation (SHG) at 532 nm was achieved, as well as traveling-wave parametric conversion in the range 770-1020 nm (signal) and 1110-1720 nm (idler).

Anti-Stokes photoluminescence in ZnO microcrystal

Low temperature (10K) strong anti-Stokes photoluminescence (ASPL) of ZnO microcrystal excited by low power cw 532nm laser is reported here. Energy upconversion of 1.1eV is obtained in our experiment with no conventional nonlinear effect. Through the study of the normal photoluminescence and temperature dependence of ASPL we conclude that the green band luminescence in ZnO is related to deep donor to valance band transition. Using the two-step two-photon absorption model, we provide a plausible mechanism leading to the ASPL phenomenon in our experiment.

Continuous wave ultraviolet light-induced fiber Bragg gratings in few- and single-mode microstructured polymer optical fibers

We report observations and measurements of the inscription of fiber Bragg gratings (FBGs) in two different types of microstructured polymer optical fiber: few-mode and an endlessly single mode. Contrary to the FBG inscription in silica microstructured fiber, where high-energy laser pulses are a prerequisite, we have successfully used a low-power cw laser source operating at 325 nm to produce 1 cm long gratings with a reflection peak at 1570 nm. Peak reflectivities of more than 10% have been observed.

Difference frequency mixing of strongly focused Gaussian beams in periodically poled LiNbO3

Difference frequency mixing of strongly focused Gaussian beams in periodically poled LiNbO3 was investigated. The beams were focused inside the crystal without damage. Mixing of cw and pulsed laser systems was employed to minimize the thermal lensing effects inside this crystal. A low-power cw 1.064‐μm Nd:YAG laser beam was used as the pump, whereas the signal beam came from a picosecond optical parametric oscillator. The two independently focused beams were made to overlap inside the crystal. The focusing parameters of the two beams were about the same. The focusing function developed by Boyd and Kleinman for tightly focused beams was extended to periodically poled crystals in order to fit the temperature-tuning curve. A 0.66%∕W efficiency was obtained for the difference frequency mixing process.

Low-threshold optical power limiting of cw laser illumination based on nonlinear refraction in zinc tetraphenyl porphyrin

The increasing use of low power cw lasers in various applications calls for the design of optical limiters with low thresholds. To this end, the optical nonlinearity exhibited by zinc tetraphenyl porphyrin at low laser powers is utilized to design an optical limiter for low threshold operation. The basic parameter responsible for limiting action, the nonlinear refractive index of the medium, is measured using the Z-scan technique and found to have a value of - 1.4 × 10 - 7 cm 2 / W at the helium–neon laser wavelength of 632.8 nm. The origin of nonlinearity is explained on the basis of the thermal lens model. It is shown that effective optical limiting at desired threshold values can be achieved by the optimal choice of aperture size and experimental geometry.

Pulsed saturation recovery 250 MHz electron paramagnetic resonance spectrometer

Pulsed saturation recovery capability has been added to the pulsed 250 MHz EPR spectrometer previously described. The design of the spectrometer is sufficiently flexible so that the only change needed was an interchangeable plug-in card that directed the VHF pulses and the low-power CW VHF to monitor the spin recovery following saturation. The plug-in module converts the ESE spectrometer to a SR spectrometer. The switch in the plug-in module for pulsed ESE or FID operation with the crossed-loop resonator (CLR) is replaced by a directional coupler. The effect of the directional coupler is to cause the low-level CW observe power to always be incident on the CLR and to direct the high-power RF pulses to the excitation side of the CLR. The pump and observe RF paths do not need to be coherent. The phase differences between the pump and observe RF paths do not have a detectable effect on the observed recovery signal because, unlike an FID or echo measurement, the SR measurement is not sensitive to phase coherence other than in the double-balanced mixer signal detector. The circuit on the plug-in card also provides signal paths for tuning both resonators of the CLR. The SR experiments are described and examples are presented of T1 measurements made possible by this new 250 MHz saturation recovery capability. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 26B: 23–27, 2005.

Growth and decay dynamics of a stable microbubble produced at the end of a near-field scanning optical microscopy fiber probe

Low power cw laser radiation coupled into a near-field scanning optical microscopy fiber probe has been used to generate a stable microbubble in water. A probe tip which was selectively chemically etched and metallized served as a microheater for the generation of the stable bubble. Bubble diameters in the range of 40–400 μm and lifetimes of over an hour have been obtained. The microbubble exhibited a linear growth phase over a period of a few seconds before reaching a maximum diameter which depended on the laser power. When the laser beam was blocked the microbubble decayed with a rate which was inversely proportional to the bubble diameter. The bubble lifetime depended on the square of the initial bubble diameter. Instabilities which transform a large stable bubble into a microjet stream of micron sized bubbles as the laser power was increased is also described.

Holographic data storage on azobenzene photopolymer film using nanosecond laser pulses

A new photopolymer has been developed as a material candidate for high-density holographic data storage applications. Holographic gratings have been recorded on the polymer film using nanosecond laser pulses at 532 nm wavelength with diffraction efficiency measured up to 20%. Further studies have shown 14 line pairs/mm as the material resolution of our photopolymer films at the current development stage. Combined with the erasability results published earlier, the results presented in this paper suggest the possibility of both types of recording (rewritable with low-power CW laser recording and read-only with high-energy pulse laser recording) on this new photopolymer film.

Trapping and mixing of particles in water using a microbubble attached to an NSOM fiber probe

Low power cw laser radiation at lambda=1.32microm was coupled into a chemically etched,metalized Near-Field Scanning Optical Microscopy (NSOM) fiber probe to generate a stable microbubble in water as well as in other fluids.The microbubble,which was attached to the end face of the fiber probe,was used to trap, manipulate and mix micron sized glass,latex and fluorescent particles as well as biological material.

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks.

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.

Evidence of Superexciplex in Dye Molecules

The excited state behavior of dye molecules under intense pulsed laser excitation appears to differ from that under low-power cw Xenon lamp excitation. In the latter case molecules absorb photons (from the lamp), go to the excited state and produce normal fluorescence. This is a one photon–one molecule process. But in the former case, two excited dye molecules combine together to form `superexciplex' and produce fluorescence and also amplified spontaneous emission (ASE) under intense, sharp, laser pulse excitation. This paper gives sufficient evidence verifying the existence of this new molecular species.

Studies in light field of the mesomorphic state of some biological membrane models

The paper presents studies under light beams of some systems which model the biological membrane (BM), namely fatty acids (FAs) and mixtures of fatty acids with cholesterol (Ch), and surfactant systems. Under a weak external electric field, the systems behave as weak dielectrics, with a hysteretic evolution of the current versus the applied voltage, and a negative resistance, which suggest a liquid crystal state. The microscopic aspect of both the systems in water showed a self-assembly of the molecules as a double layer, similar with the membranes structure. The hydrophobic part presents a lyotropic liquid crystal texture that changes in terms of the concentration. Some of the samples exhibit also a thermotropic mesomorphism, that is much influenced by adding cholesterol. Maintained under integral and polarized light, the textures and their colors change in a few seconds. In a low power cw laser beam, the light spreading has been evidenced and some non-linear optical effects were observed. The results seem to lead to the necessity to study both the lyotropic and thermotropic mesomorphism together and in interaction with different external stimuli, among them the light being an important tool for biological investigations.

Real-time studies of mark formation processes in phase-change and magneto-optical media using a two-laser tester.

We have equipped a commercial polarized-light microscope with two laser diodes operating at 643 nm and 680 nm. The two laser beams are focused simultaneously and coincidentally on the sample through the microscope objective lens. We use one of the lasers in the pulsed mode to write marks on the sample, and the other laser in a low-power cw mode to monitor the changes in reflectivity and/or the state of polarization upon reflection from the sample. The system has a liquid crystal wave-plate for adjusting the optical phase-shifts, an electro-magnet, a hot plate, a computerized XY-stage, two polarizing beam-splitters, and both single-ended and differential detection modules for each of the laser wavelengths.The dynamics of phase-transition (i.e., melting, crystallization, and amorphization upon rapid cooling from melt in the case of PC media, and ferrimagnetic-to-paramagnetic in the case of MO media) are readily observed in the 160 MHz-bandwidth signal picked up by the detectors. Similarly, changes in the magnetic state of the MO media under a focused spot (whether due to the change of temperature or caused by magnetization reversal), are monitored in real-time.We describe the dynamics of phase transformation in PC media as well as the observed behavior of magnetization of the MO media within the region of the hot spot. These observations are made over periods ranging from 30 ns up to several microseconds, with a resolution of only a few nanoseconds. We also outline the methods of extracting from the observed data the physical parameters of the media such as the specific heat, thermal conductivity, temperature-dependence of the optical constants, and the probabilities of nucleation and growth.