Jin Hong Lim

Investigation of an optical limiting mechanism in multiwalled carbon nanotubes

We report our investigation of the mechanism that is responsible for the optical limiting behavior in multiwalled carbon nanotubes. We conducted energy-dependent transmission measurements, picosecond time-resolved pump–probe experiment, and nonlinear scattering experiments at 532-nm wavelength on multiwalled carbon nanotube suspension. For comparison, C60–toluene solutions and carbon black suspensions were also studied in the same experiments. The similarities that we observed between the multiwalled carbon nanotubes and carbon black suspension suggest that nonlinear scattering, which is known to be responsible for the limiting action in carbon black suspension, should play an important role in the limiting effect in multiwalled carbon nanotubes.

Z-scan measurement of the ground and excited state absorption cross sections of Cr/sup 4+/ in yttrium aluminum garnet

Z-scan measurements of the ground and excited state absorption cross sections of Cr/sup 4+/-doped yttrium aluminum garnet are reported. These quantities are found to depend on crystal preparation but are independent of temperature from -60/spl deg/C to 98/spl deg/C. Their ratio is shown to weakly determine the energy output of a passively Q-switched Nd:glass laser using this material as the saturable absorber.

POLYMETHINE AND SQUARYLIUM MOLECULES WITH LARGE EXCITED-STATE ABSORPTION

Abstract We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, σ 01 , which effectively populates an excited state that possesses an extremely large ESA cross section, σ 12 . The ratios of σ 12 / σ 01 are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans – cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs.

Nonlinear spectrometry of chromophores for optical limiting

We describe two methods for the spectral measurement of nonlinear absorption and refraction in reverse-saturable absorber materials. In the first, we use a picosecond optical parametric oscillator to perform Z-scan at many different wavelengths to measure excited state refraction and absorption cross sections throughout the visible. The second methods uses a chirped-pulse amplification scheme to produce 100 fs pulses at 840 nm. Focusing these into sapphire generates a white light continuum that is used as a probe in an excite-probe experiment. The excitation beam is derived from the second harmonic of the remaining 840 nm light. By measurement of the transmission spectrum of the probe as a function of excite- probe delay time, we can determine the spectral dependence of the excited-state absorption cross section. Moreover, by use of Kramers-Kronig relations, the excited state refraction can also be extracted from this data. We describe our measurements using both methods in a Zn:tetrabenzporphyrine derivative (TBP). The fact that both methods give excellent agreement not only verifies the utility of continuum measurements, but also reveals some interesting properties of the excited states of TBP.

Optical Limiting With Neutral Nickel Dithiolene Complexes

We report an investigation of optical-limiting behavior in two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, [Ni(medt) 2 ] I (medt = 5,6-dihydro-6-methyl-1,4-dithiin-2,3- dithiolate) and [Ni(phdt) 2 ] 2 (phdt = 5,6-dihydro-5-phenyl-l,4-dithiin-2,3-dithiolate) in benzene solution. The fluence-dependent transmission of the complexes was observed with nanosecond and picosecond laser pulses at 532-nm wavelength. The limiting thresholds of the complexes were ˜0.3 J/cm 2 , when measured with the picosecond pulses. Both picosecond time-resolved pump-probe and Z-scan measurement revealed that the limiting effects should originate from excited-state absorption and refraction. The transparency window (400˜900 nm), observed in the linear absorption spectra of the complexes, indicated that their limiting response should cover a wider range than those of fullerenes and phthalocyanines.

Optical-limiting properties of neutral nickel dithiolenes

Wide applications of lasers have stimulated a great interest in development of optical limiting devices. These devices can be used to protect optical sensor form laser-induced damage because their transmission is high when they are exposed to low-power laser light, and their transmission becomes low when irradiated by intense laser beams. Here we report such a device based on nonlinear optical effects in two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, 1 and 2. The limiting device consisted of a focusing setup and a 1-mm-thick cell, which contained a benzene solution of one of the complexes. THe limiting properties were investigated by both nanosecond and picosecond laser pulses. At 532 nm, the limiting thresholds of complexes 1 and 2 measured by the picosecond laser pulses. At 532 nm, the limiting threshold of complexes 1 and 2 measured by the picosecond laser pluses with a focusing geometry were determined to be approximately 0.3 J/cm2. The linear absorption spectra of the to complexes also indicated that their limiting response should cover the visible and near-IR region. All these result show their limiting performance is superior to the limiting effecting C60. Picosecond time-resolved pump-probe and Z-scan experiments revealed that the observed limiting effects should originate from excited-state absorption and refraction.

Reverse saturable absorption in polymethine dyes

Thenonlinear absorption of a series of polymethine dyes (PDs) have been investigated, and some of them are identified as potential candidates for optical limiting applications. We have demonstrated that very low optical limiting thresholds can be obtained by using polymethine dyes with a large ratio of the absorption cross section ofthe excited to the ground-state: a ratio as large as 200 has been measured in one of the PDs. This value is significantly higher than any previously reported value, while, at the same time, maintaining a large ground-state cross section. It has been shown that nonlinear absorption of PDs in polyurethane acrylate (PUA) polymer is as large as in ethanol solution. In addition, photostability of the PDs and initial measurements ofoptical limiting in these samples are reported.

Investigation of Optical Limiting Mechanism in Multiwalled Carbon Nanotubes

We report our investigation of the mechanism that is responsible for the optical limiting behavior in multiwalled carbon nanotubes. We conducted energy-dependent transmission measurements, picosecond time-resolved pump-probe experiment, and nonlinear scattering experiments at 532-nm wavelength on multiwalled carbon nanotube suspension. For comparison, C(60)-toluene solutions and carbon black suspensions were also studied in the same experiments. The similarities that we observed between the multiwalled carbon nanotubes and carbon black suspension suggest that nonlinear scattering, which is known to be responsible for the limiting action in carbon black suspension, should play an important role in the limiting effect in multiwalled carbon nanotubes.

Z-scan measurement of ground and excited state absorption cross sections in Cr/sup 4+/:YAG at 1064 nm

Crystals of Cr/sup 4+/ doped yttrium aluminum garnet (YAG) have been used as a solid state, saturable absorber Q-switch for 1064 nm. Research has been conducted on its spectroscopic properties, including ground state and excited state absorption cross sections (/spl sigma//sub gs/ and /spl sigma//sub es/). However, the reported values of /spl sigma//sub gs/ and /spl sigma//sub es/ vary more than an order of magnitude. We conducted Z-scan measurement of /spl sigma//sub gs/ and /spl sigma//sub es/ in Cr/sup 4+/:YAG at 1064 nm to provide useful data for use in modeling the performance of passively Q-switched lasers.

Spectral measurements of nonlinear absorption in polymethine and squarylium dyes

We report measurements of these two dyes at wavelengths from 440 to 650 nm. The experimental light source is a singly resonant picosecond optical parametric oscillator (OPO) synchronously pumped by the third harmonic of a flash-lamp-pumped Nd:YAG laser. It is continuously tunable from 400 to 700 nm using two critically phase-matched BBO crystals mounted for walk-off compensation.