2010 Optical Limiting and Stabilization

8/9/2019 2010 Optical Limiting and Stabilization

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932 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 46, NO. 6, JUNE 2010

Fig. 1. (a) Linear absorbance spectra for a 1-mm path-length pure solvent(CHCl ) sample and two CdTe tripods/CHCl solution samples with differentconcentration values. (b) Linear transmission spectra of a 1-cm pure solventsample and a 1-cm tripods solution sample of concentration. The inset in(a) is the TEM image of the tripods.

Ti-sapphire oscillator/amplifier system (CPA-2010 from Clark-

MXR, Inc.) operating at a repetition rate of 1 kHz. The output

wavelength from this OPG could be tuned over the spectral

range from 1100 nm to 1900 nm. The output beam was linearly

polarized with an 3-mm beam size and 0.5-mrad divergent

angle.

III. MULTI-PHOTONINDUCEDEMISSION

The CdTe/CdS/ZnS tripods/CHCl solution sample emits

photoluminescence upon exciting the nanoparticles with one-,

two-, and three-photon absorption at suitable wavelengths. As

an example, Fig. 2(a) shows the emission spectra excited by

laser pulses at three different wavelengths, i.e., 625 nm (for

1PA), 1250 nm (for 2PA), and 1630 nm (for 3PA), respectively.

These spectra were recorded by using a grating spectrometer

(EPP 2000 from StellarNet), and the 625-nm excitation beam

was the second-harmonic generation of the 1250-nm pulsed

laser beam. To avoid the reabsorption influence on the emissionspectral profile, the sample concentration for the emission

spectral measurement was reduced to 1 mg/mL. Fig. 2(b)

shows the curves of relative emission intensity dependence on

the excitation pulse energy, measured at 1250-nm and 1630-nm,

respectively. In a double-logarithm scale, the measured data are

well fitted by a straight line with the slope of 2 for the former

and another straight line with the slope of 3 for the latter,

confirming the 2PA and 3PA features at these two excitationwavelengths. Based on the results shown in Fig. 2(a), one can

conclude that the emission spectral profiles for one-, two- and

three-photon excitation are basically the same with the emission

peak located around -nm position and bandwidth of

45 nm. On the other hand, it is known that the selection rules

and transition pathways are generally different for the cases

of one-, two-, and three-photon excitation processes. Similar

to what observed in organic multi-photon active chromophore

systems, the results shown in Fig. 2(a) reveal that after one-,

two- or three-photon excitation, the emitting centers of the

tripod particles finally relax to the same energy level(s), and

then emit the same spectra.

IV. 2PA-BASEDOPTICALLIMITING ANDSTABILIZATION

To quantitatively characterize the multi-photon absorption

properties of our samples and demonstrate their optical power

limiting performance, we have specifically chosen 1175-nm

and 1580-nm wavelengths for the 2PA- and 3PA-induced

nonlinear transmissivity measurements. At these two wave-

lengths, the linear transmissivity is higher than 90% for the

1-cm path-length samples with two chosen concentration levels

mg/mL and mg/mL). In both cases, the

input excitation pulsed laser beam was focused by an -cm

lens into the center of a 1-cm thick sample solution cuvette.

The input and output pulse energy values were measured bya large-area energy-meter. To avoid thermal lensing influence

on the measurement, the input laser beam was first passed

through a beam chopper with an opening ratio of 1 and a

rotating speed of 75 rounds per second. After passing through

this chopper, the pulse number per second was reduced from

1000 to 150, while the energy or peak power of each individual

pulse remained unchanged.

Fig. 3(a) shows the measured nonlinear transmission versus

the input 1175-nm laser pulse energy for the 1-cm path-length

samples with two different concentration values. According to

the basic 2PA theory, the nonlinear transmission of a one-photon

transparent but two-photon absorbing medium excited by a fo-cused laser beam with a hat-top pulse shape can be expressed as

follows [1]:

(1)

where is the peak intensity of the input laser beam inside

the medium, is the thickness of the sample, and is the 2PA

coefficient of the medium. In Fig. 3(a) the two groups of exper-

imental data can be well fitted by two theoretical curves given

by (1) with two values, i.e., cm/GW for sample

of concentration and cm/GW for sample of .

These results are reasonable as the 2PA coefficient is propor-

tional to the concentration of the nonlinearly absorbing nanopar-ticles. Furthermore, Fig. 3(b) shows the measured data of output


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HEet al.: OPTICAL LIMITING AND STABILIZATION OF CdTe/CdS/ZnS QUANTUM TRIPODS SYSTEM 933

Fig. 2. (a) Normalized emission spectra excited by one-photon absorption at625 nm, two-photon absorption at 1250 nm, and three-photon absorption at1630 nm; (b) Measured relative emission intensity versus input pulse energy ofthe 1250-nm beam and the 1630-nm beam, showing the quadratic dependenceand cubic dependence respectively.

pulse energy versus the input pulse energy, the theoretical fit-

ting curves with the corresponding values are given by the

following equation:

(2)

where and are the input and output pulse energy, re-

spectively. From Fig. 3(a), one can readily find that the non-

linear transmissivity for the high concentration sample is re-

duced from 1 to 0.2 when the input pulse energy is increased

from 0.1 J to 12 J. This is a typical and efficient power

limiting effect. Also from Fig. 3(b), we can observe that at high

input levels (612 J), a relatively large input pulse energy (in-

tensity) fluctuation will only lead to a much smaller output fluc-

tuation; this is the so-called optical stabilization effect based on

2PA mechanism. Each experimental point shown in Fig. 3 is a

result of average over 100 pulses, the measurement errors arecaused by the input pulse energy fluctuations.

Fig. 3. (a) Measured nonlinear transmission of two solution samples versusthe input energy or peak intensity of 1175-nm laser pulses. (b) Measured outputpulse energy versus input energyof the same laser beam. The solid- and dashed-line are the best fitting curves given by 2PA theory.

Here, for our CdTe/CdS/ZnS quantum tripod sample of

mg/mL concentration, the measured 2PA coef-

ficient is cm/GW. Our previous results obtained

under basically the same experimental conditions showed

that for CdSe quantum dots of 4-nm size and 70 mg/mL

concentration in hexane, the measured cm/GW

(at 775-nm wavelength) [13]. For CdTe quantum dots of

7 nm size and 8 mg/mL concentration in chloroform, the

measured cm/GW (at 1300-nm wavelength) [23].

Moreover, AF350 dye is among the best two-photon active or-

ganic chromophores, at the concentration level of 20 mg/mL in

tetrahydrofuran (THF),its 2PA coefficientis cm/GW

(at 800 nm) [31].

To further demonstrate the application of this nonlinear

effect, we measured the normalized peak power fluctuation

of the input 1175-nm pulses at an average energy level of

10 J, and the fluctuation of the output pulses after passing

through the 1-cm sample of mg/mL concentration.

These results are obtained by using a large-area (4 4 mm )photodetector (SET110 from Thor-Labs) in conjunction with a


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Fig. 4. Displays of the input pulses fluctuation (a) and the output pulsesfluctuation (b) recorded by a high-speed oscilloscope. The input wavelength is1175 nm, pulse energy 10 J, and the 1-cm solution sample concentration is24.6 mg/mL.

500-MHz band-width oscilloscope (Infinum from HP). The ex-perimental results are shown in Fig. 4, indicating that the input

relative fluctuation (standard deviation) is 0.24, whereas the

output fluctuation is reduced to 0.13. This is an experimental

evidence for the improvement of stability of femtosecond laser

pulses, based on a nonlinear absorbing nanoparticles system.

V. 3PA-BASEDLIMITING ANDSTABILIZATION

In a similar way, under the condition of 3PA excitation with

1580-nm laser wavelength, the measured nonlinear transmis-

sion and output energy as a function of the input energy are

shown in Fig. 5(a) and (b) individually; the same 1-cm long

samples with two different concentration values were employed

for these measurements. According to the basic 3PA theory, the

nonlinear transmissivity can be written as

(3)

where is the 3PA coefficient of the sample medium. It is found

that the experimental data shown in Fig. 5 cannot be simply

fitted by using (3). However, these data can be well fitted by

the modified 3PA theory by considering 3PA saturation effect

[32], according to which the nonlinear transmission upon 3PA

excitation is expressed as

(4)

Fig. 5. (a) Measured nonlinear transmission of two solution samples versusthe input energy or peak intensity of 1580-nm laser pulses. (b) Measured outputpulse energy versus input energy of the same laser beam. The solid- and dashed-line are the best fitting curves given by 3PA theory with considering the satura-tion effect.

Here is 3PA saturation intensity parameter that is a

medium constant, and is the unsaturated 3PA coefficient

when the input intensity is much lower than . It is noted

that the similar saturation behavior of 3PA in CdSe quantum

dots solution system was also previously reported [13]. In

our present case for CdTe tripods/CHCl solution system,

the best fitting values are cm /GW for

mg/mL sample, cm /GW for

sample, and GW/cm for both samples.

Once again, the linear relationship between the value and

the concentration value holds. In Fig. 5(b), the fitting curves

are obtained by using the same relationship of ,

where is given by (4).

Upon comparing Fig. 5 to Fig. 3, we can see that in our 3PA

excitation case, the decrease of nonlinear transmission is get-

ting slower at high input levels ( GW/cm ) due to the

3PA saturation effect, which is undesirable for optical limiting

and stabilization considerations. In Fig. 6, we present the mea-

sured fluctuation of the input 1580-nm pulses for 3PA excita-

tion (a) and the output pulse fluctuation of the same beam afterpassing through the 1-cm sample of concentration (b). The


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HEet al.: OPTICAL LIMITING AND STABILIZATION OF CdTe/CdS/ZnS QUANTUM TRIPODS SYSTEM 935

Fig. 6. Displays of the input pulses fluctuation (a) and the output pulses fluc-tuation (b) recorded by a high-speed oscilloscope. The input wavelength is 1580nm, pulse energy 9 J, and the 1-cm solution sample concentration is 12.3mg/mL.

average input pulse energy was 9 J. In spite of the saturation

effect, we still see certain degree of improvement of the output

stability; the standard deviation for input pulses around their av-

erage value is 0.31, whereas that for output pulses is reduced to

0.25.

Efficient carrier multiplication in quantum dots has been re-

ported for many nanoparticles including CdSe, PbS, PbSe, and

PbTe. However, very few studies are available on CdTe systems.

In this process, absorption of a single energetic photon leads to

generation of two or more electron/hole pairs. The interest in

this phenomenon arises from the fundamental importance of un-

derstanding the role of quantum confinement from the nanocrys-

tals in comparison to the bulk materials. The current studiesof multi-photon excitation properties of our samples and other

semiconductor nanoparticles systems may be helpful for ex-

plore other applications. For example, CdTe tripod nanocrystals

might be a good candidate as efficient carrier multiplication for

solar power applications. We speculate that this material will

allow more complete conversion of photon energy into electric

potential through the generation of multiple electronhole pairs.

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Guang S. Heis a Senior Research Scientist with the Photonics Research Lab-oratory, State University of New York at Buffalo, in 1987 and the Institute

for Lasers, Photonics and Biophotonics since 1999. His major research activ-ities cover the areas of laser physics, nonlinear optics, and multi-photon activematerials. His current research efforts are focused on multi-photon excitationprocess and novel nonlinear optical materials and devices. He has publishedmore than 130 scientific articles and co-authored five monographs includingthe bookPhysics of Nonlinear Optics (World Scientific, 2000).

Ken-Tye Yong received the B.S. and Ph.D. degrees from the State Universityof New York at Buffalo in 2001 and 2006, respectively.

Currently, he is a Postdoctoral Research Associate in the Institute of Lasers,Photonics, and Biophotonics at SUNY Buffalo. His research interest is in thedevelopment of nanomaterials for early cancer detection, diseases diagnosis,and therapy.

Jing Zhureceived the B.S. degree in electronic science and engineering from

SoutheastUniversity, China, in 2003, wherehe is nowpursuing the Ph.D.degreein physical electronics.

Currently, he is a visiting research scholar in the Institute for Lasers, Pho-tonics and Biophotonics, State University of New York at Buffalo.

Hai-Yan Qin received the B.S. degree in information engineering from Zhe-jiang University, Hangzhou, China, in 2005. She is now a joint Ph.D. studentin optical communication technology at Zhejiang University, China, and inbiotechnology at the Royal Institute of Technology, Sweden.

She was a visiting research scholar in the Institute for Lasers, Photonics andBiophotonics, State University of New York at Buffalo.

Paras N. Prasad is Executive Director of the Institute for Lasers, Photonics andBiophotonicsand a DistinguishedProfessor of chemistry, physics,medicineandelectrical engineering at the State University of New York at Buffalo. He alsoholds the Samuel P. Capen Chair. He has published more than 500 scientificpapers and co-edited 5 books. He co-authored a monograph, Introduction to

Nonlinear Optical Effects in Molecules and Polymers(Wiley) and authored tworecent monographs, Introduction to Biophotonics (Wiley) and Nanophotonics(Wiley).

Dr. Prasad is a Fellow of the American Physical Society and a Fellow of theOptical Society of America. He is also a recipient of the prestigious Sloan andGuggenheim fellowships.

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2010 Optical Limiting and Stabilization