- Email: [email protected]
Femtosecond nonlinear optical properities of PbS nanoparticles
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 549–551 www.elsevier.com/locate/jlumin
Femtosecond nonlinear optical properities of PbS nanoparticles Dan Lia,, Chunjun Lianga, Ye Liub, Shixiong Qianb a
Department of Physics, Northern Jiaotong University, Beijing 100044, PR China Department of Physics and Surface Physics Lab (National Key Lab), Fudan University, Shanghai 200433, PR China
b
Available online 10 March 2006
Abstract We report on the ultrafast carrier dynamics and the nonlinear optical properties of PbS nanoparticles at various energy fluences. At low pump fluences both photo-induced absorption of biexciton and photobleaching of the exciton were observed at the same time. On the contrary, the induced absorption dominates the temporal behavior in the femtosecond domain, indicating that biexciton effect was strongly dependent on the density of excited carriers and started to play a significant role at the multiple e–h pair excitation per nanoparticle. The magnitude of w(3) for PbS nanoparticles doped SiO2 thin film was calculated to be 5.1 1010 esu, which came from the contribution of biexciton effect at high pump intensity. r 2006 Elsevier B.V. All rights reserved. Keywords: PbS; Nanoparticles; Femtosecond; Optical nonlinearities
Enhanced nonlinear optical properties (NLO) and ultrafast carrier dynamics in quantum dots (QD) have been attracting much interest in both fundamental research and device applications. In particular NLO of semiconductor QDs are expected to be greatly enhanced in the strong-confinement regime [1]. Strong confinement can be easily achieved in the narrow band-gap semiconductors, in which the excitons have large Bohr radii [2]. For example, in PbS, both the electron and hole Bohr radii are about 10 nm, and the Bohr radius of the exciton is 18 nm, so PbS QDs offer opportunities for achieving strong quantum confinement of both carriers as well as the exciton. Banyai et al. [1] predicted that for a given particle size, the nonlinearity of PbS would be 30 times larger than that of GaAs, and three orders of magnitude larger than that of CdS. In this letter, we report on temporal profiles of nonlinear absorption of PbS nanoparticles at excitation energy fluences ranging from 48 mJ/cm2 to 4 mJ/cm2. At low pump fluences, the contributions of biexciton and exciton to the temporal behavior of nonlinear absorption were observed at the same time. On the contrary, the photo-induced absorption dominates the temporal behavior in the Corresponding author. Tel.: +86 10 51683773.
E-mail address: [email protected] (D. Li). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.214
femtosecond domain, indicating the dominant role of the biexciton effect at the multiple e–h pair excitation per nanoparticle. The origins of third-order NLO were studied by fs optical Kerr effect (OKE) measurement at various excitation energy fluences. The magnitude of the third-order NLO susceptibility (w(3)) is calculated to be 5.1 1010 esu at a pump intensity of 4 mJ/cm2, which arises from the dominant role of biexciton effect. PbS nanoparticles doped SiO2 thin films were prepared by the sol–gel method and the dip-coating technique. The method has been used to prepare CdS nanoparticles doped zirconia films [3]. An average particle diameter of 5 nm was determined from images taken with a transmission electron microscope (TEM). The sample under investigation was PbS nanoparticles sol–gel film of about 150 nm thick on glass substrate. Its linear absorption spectrum is shown in Fig. 1. The band gap of bulk PbS is 0.41 eV, and it shifts to higher energy for PbS nanoparticles due to the quantum confinement effect. The absorption spectrum of sample exhibits a distinct band peaked at 640 nm, which is attributed to 1Se–1Sh transitions [4]. Transient dynamics and OKE measurements were performed at room temperature, utilizing a 120 fs by using a Ti:sapphire laser system, which produces 800 nm photons at a repetition rate of 82 MHz (oscillator) or 1 kHz (amplifier), respectively. Details of the experimental set-up
ARTICLE IN PRESS D. Li et al. / Journal of Luminescence 122–123 (2007) 549–551
550
1.6
1
1.4
0 1
1.0 ∆T (a.u.)
Absorbance
1.2
48 µJ/cm2
0.8 0.6
0 1.2 mJ/cm2 -1 0 3 mJ/cm2
0.4 -2 0
0.2 400
500
600
700
800
900
1000
Wavelength (nm)
4 mJ/cm2
-1 0
Fig. 1. Absorption spectrum of PbS nanoparticles.
were described in previous paper [5]. The time-dependent nonlinear absorption measurements, taken for energy fluences ranging from 48 mJ/cm2 to 4 mJ/cm2, are shown in Fig. 2. For energy fluences below 3 mJ/cm2, both the saturation and the photo-induced absorption were observed. In particular, at a fluence of 48 mJ/cm2, a very slow bleaching decay was observed. Comparing with the results at energy fluences below 3 mJ/cm2, there is a pronounced modification of the temporal profile at the highest pump fluence of 4 mJ/cm2 and the saturated absorption no longer appear and the photo-induced absorption dominates the shape of the curve. For PbS nanoparticles, the bleaching is due to the saturation of the exciton transition and the photo-induced absorption may be caused by the biexciton effect, which has been observed on both the high- and low-energy sides of the exciton resonance [6,7]. The ground-state biexciton is theoretically observable as a photo-induced absorption on the low-energy side of the exciton resonance [8]. Similar transient absorption was previously observed in nanoparticles in the case of nonlinearities arising from carriertrapping-induced charge separation [9]. In the case of the DC stark effect, the electric field is generated by the charge separation due to the trapped holes at the surface of the semiconductor nanoparticles. According to the previous report, the photo-induced absorption should be observed even at 300 ps after the photo-excitation. This is not consistent with the fast subpicosecond dynamics of absorption changes measured in our sample. In contrast, as the biexciton effect shows subpicosecond decay dynamics, the induced absorption in our samples could be attributed to the biexciton effect. More experimental results and further analysis about saturation absorption and induced-absorption at various wavelengths will be discussed in another paper. A pump fluence of 48 mJ/cm2 according to the report [10], can be estimated corresponds to the average number of 0.04 e–h pairs excited per nanoparticle. In this case, although biexciton states can be formed when multiple e–h
2
4 6 Delay time (ps)
8
10
12
Fig. 2. Time-dependent nonlinear aborption at various energy fluences for 800 nm wavelength.
pair excitation in a small portion of nanoparticles, due to some inhomogeneous broadening of exciton band, the lowenergy tail of the exciton transition will overlap with the ground-state-biexciton transition. So the photobleaching was observed at low pump fluences at 800 nm wavelengths. This result means that the photo-induced absorption of the biexciton effect is not large enough to surpass the bleaching of the exciton transition under low excitation. As energy fluence increases, the saturated absorption gradually disappeared, and only the photo-induced absorption was observed at a energy fluence of 4 mJ/cm2. In comparison with the low pump intensity data, in the highest energy fluence the photo-induced absorption dominates the ultrafast behavior. This result indicates that biexciton effect starts to play a significant role when the multiple e–h pair per nanoparticle on average was excited. Fig. 3 shows the results of the OKE responses at 800 nm with various energy fluences, from which we can see that, the character of OKE response evolution is similar to that of the pump–probe measurements. At low-energy fluence, the OKE response evolution of PbS nanoparticles includes a positive and a negative component, and a very slow decay was observed. At higher pump fluences, there is a pronounced modification of the temporal profile, the disappearance of the positive component and the appearance of a negative component. The results indicate that the ultrafast optical nonlinearity arises from the contribution of the saturation due to exciton transition and the photoinduced absorption due to biexciton effect. The contributions of the single exciton and that of biexciton to the thirdorder susceptibility are opposite, which agrees with Banyai’s theoretical predict [1]. For the pump fluences of 3 mJ/cm2, the magnitude of OKE signal is compared with that of reference sample CS2. The value of the third-order NLO susceptibility wð3Þ of the sample can be calculated by using the s
ARTICLE IN PRESS D. Li et al. / Journal of Luminescence 122–123 (2007) 549–551
In conclusion, we have studied the transient dynamics and third-order NLO of PbS nanoparticles. The results indicate that when the pump and probe wavelength were chosen at the low-energy side of the exciton resonance and low-energy fluences, photo-induced absorption of the biexciton effect and photobleaching of the exciton were observed at the same time. And the contribution of the exciton to the third-order susceptibility cancels that of the biexciton. On the contrary, at high pump fluence, the photo-induced absorption dominates the ultrafast nonlinear optical response, revealing that biexciton effect starts to play a significant role due to the multiple e–h pair excitation per nanoparticle. At energy fluences of 4 mJ/cm2, the magnitude of w(3) for PbS nanoparticles doped SiO2 thin film is calculated to be 5.1 1010 esu, which arises from the contribution of biexciton effect at high pump intensity.
OKE signal (µV)
5
0 48 µJ/cm2
0
-50 4 mJ/cm2 -100 0
2 4 Delay time (ps)
551
6
Fig. 3. Transient optical Kerr signal at wavelength of 800 nm with different energy fluences (a) 48 mJ/cm2 and (b) 4 mJ/cm2.
following formula [11]: 1=2 2 ns Lr aL ð3Þ I s s wð3Þ ¼ w , s r Ir nr Ls exp aLs =2 ð1 expðaLs ÞÞ
The work is supported by the National Natural Science Foundation of China (Nos.60406005 and 10374002).
References
(1) where I is the intensity of the OKE signal at zero delay time, a is the linear absorption coefficient of the nm-C glass, n is the refractive index, and L is the interaction length of pump beam and probe beam over the CS2 cell and the sample. The subscript s and r denote sample and CS2, respectively. The value wð3Þ is known to be 0.67 1013 esu for CS2 in fs r domain. Based on the magnitudes of the OKE signal of the sample and the reference under identical experimental condition, the magnitude of wð3Þ for PbS nanoparticles s doped SiO2 thin film is calculated to be 5.1 1010 esu, which is four orders larger in magnitude than that of CS2, and one order magnitude larger than that of the standard optical nonlinear material CdS, which is known to be 5 1011 esu [12]. Based on preparation method of our sample, size distribution of the nanoparticles is usually more than 10%. Wide size distribution results in a strong reduction of the optical nonlinearities. If we can achieve a narrow size distribution and high dopent density, optical nonlinearities of the sample may be further increased.
[1] L. Banyai, Y.Z. Hu, M. Lindberg, S.W. Koch, Phys. Rev. B 38 (1988) 8142. [2] S. Rink, D.A.B. Miller, D.S. Chemla, Phys. Rev. B 35 (1987) 8113. [3] W. Huang, Jian lin Shi, Scripta mater. 44 (2001) 117. [4] J.L. Machol, F.W. Wise, R.C. Patel, D.B. Tanner, Phys. Rev. B 48 (1993) 2819. [5] G.H. Ma, L.J. Guo, J. Mi, Y. Liu, S.X. Qian, D.C. Pan, Y. Huang, Solid State Commun. 118 (2001) 633. [6] Y.Z. Hu, S.W. Koch, M. Lindberg, N. Peyghambarian, E.L. Pollock, F.F. Abraham, Phys. Rev. Lett. 64 (1990) 1805. [7] M.G. Bawendi, W.L. Wilson, L. Rothberg, P.J. Carroll, T.M. Jedju, M.L. Steigerwald, L.E. Brus, Phys. Rev. Lett. 65 (1990) 1623. [8] Y.Z. Hu, M. Lindberg, S.W. Koch, Phys. Rev. B 42 (1990) 1713. [9] D.J. Norris, A. Sacra, C.B. Murray, M.G. Bawendi, Phys. Rev. Lett. 72 (1994) 2612. [10] V.I. Klimov, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Phys. Rev. B 60 (1999) 13740. [11] M.K. Casstevens, M. Samoc, J. Pfleger, P.N. Prasad, J. Chem. Phys. 92 (1990) 2019. [12] R. Reisfeld, in: C.K. Jorgensen, R. Reisfeid (Eds.), Optical and Electronic Phenomena In Sol–Gel Glasses and Modern Application, Structure and Bonding, vol. 85, 1996, Springer Press Inc., Germany, pp. 99–147.
Journal of Luminescence 122–123 (2007) 549–551 www.elsevier.com/locate/jlumin
Femtosecond nonlinear optical properities of PbS nanoparticles Dan Lia,, Chunjun Lianga, Ye Liub, Shixiong Qianb a
Department of Physics, Northern Jiaotong University, Beijing 100044, PR China Department of Physics and Surface Physics Lab (National Key Lab), Fudan University, Shanghai 200433, PR China
b
Available online 10 March 2006
Abstract We report on the ultrafast carrier dynamics and the nonlinear optical properties of PbS nanoparticles at various energy fluences. At low pump fluences both photo-induced absorption of biexciton and photobleaching of the exciton were observed at the same time. On the contrary, the induced absorption dominates the temporal behavior in the femtosecond domain, indicating that biexciton effect was strongly dependent on the density of excited carriers and started to play a significant role at the multiple e–h pair excitation per nanoparticle. The magnitude of w(3) for PbS nanoparticles doped SiO2 thin film was calculated to be 5.1 1010 esu, which came from the contribution of biexciton effect at high pump intensity. r 2006 Elsevier B.V. All rights reserved. Keywords: PbS; Nanoparticles; Femtosecond; Optical nonlinearities
Enhanced nonlinear optical properties (NLO) and ultrafast carrier dynamics in quantum dots (QD) have been attracting much interest in both fundamental research and device applications. In particular NLO of semiconductor QDs are expected to be greatly enhanced in the strong-confinement regime [1]. Strong confinement can be easily achieved in the narrow band-gap semiconductors, in which the excitons have large Bohr radii [2]. For example, in PbS, both the electron and hole Bohr radii are about 10 nm, and the Bohr radius of the exciton is 18 nm, so PbS QDs offer opportunities for achieving strong quantum confinement of both carriers as well as the exciton. Banyai et al. [1] predicted that for a given particle size, the nonlinearity of PbS would be 30 times larger than that of GaAs, and three orders of magnitude larger than that of CdS. In this letter, we report on temporal profiles of nonlinear absorption of PbS nanoparticles at excitation energy fluences ranging from 48 mJ/cm2 to 4 mJ/cm2. At low pump fluences, the contributions of biexciton and exciton to the temporal behavior of nonlinear absorption were observed at the same time. On the contrary, the photo-induced absorption dominates the temporal behavior in the Corresponding author. Tel.: +86 10 51683773.
E-mail address: [email protected] (D. Li). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.214
femtosecond domain, indicating the dominant role of the biexciton effect at the multiple e–h pair excitation per nanoparticle. The origins of third-order NLO were studied by fs optical Kerr effect (OKE) measurement at various excitation energy fluences. The magnitude of the third-order NLO susceptibility (w(3)) is calculated to be 5.1 1010 esu at a pump intensity of 4 mJ/cm2, which arises from the dominant role of biexciton effect. PbS nanoparticles doped SiO2 thin films were prepared by the sol–gel method and the dip-coating technique. The method has been used to prepare CdS nanoparticles doped zirconia films [3]. An average particle diameter of 5 nm was determined from images taken with a transmission electron microscope (TEM). The sample under investigation was PbS nanoparticles sol–gel film of about 150 nm thick on glass substrate. Its linear absorption spectrum is shown in Fig. 1. The band gap of bulk PbS is 0.41 eV, and it shifts to higher energy for PbS nanoparticles due to the quantum confinement effect. The absorption spectrum of sample exhibits a distinct band peaked at 640 nm, which is attributed to 1Se–1Sh transitions [4]. Transient dynamics and OKE measurements were performed at room temperature, utilizing a 120 fs by using a Ti:sapphire laser system, which produces 800 nm photons at a repetition rate of 82 MHz (oscillator) or 1 kHz (amplifier), respectively. Details of the experimental set-up
ARTICLE IN PRESS D. Li et al. / Journal of Luminescence 122–123 (2007) 549–551
550
1.6
1
1.4
0 1
1.0 ∆T (a.u.)
Absorbance
1.2
48 µJ/cm2
0.8 0.6
0 1.2 mJ/cm2 -1 0 3 mJ/cm2
0.4 -2 0
0.2 400
500
600
700
800
900
1000
Wavelength (nm)
4 mJ/cm2
-1 0
Fig. 1. Absorption spectrum of PbS nanoparticles.
were described in previous paper [5]. The time-dependent nonlinear absorption measurements, taken for energy fluences ranging from 48 mJ/cm2 to 4 mJ/cm2, are shown in Fig. 2. For energy fluences below 3 mJ/cm2, both the saturation and the photo-induced absorption were observed. In particular, at a fluence of 48 mJ/cm2, a very slow bleaching decay was observed. Comparing with the results at energy fluences below 3 mJ/cm2, there is a pronounced modification of the temporal profile at the highest pump fluence of 4 mJ/cm2 and the saturated absorption no longer appear and the photo-induced absorption dominates the shape of the curve. For PbS nanoparticles, the bleaching is due to the saturation of the exciton transition and the photo-induced absorption may be caused by the biexciton effect, which has been observed on both the high- and low-energy sides of the exciton resonance [6,7]. The ground-state biexciton is theoretically observable as a photo-induced absorption on the low-energy side of the exciton resonance [8]. Similar transient absorption was previously observed in nanoparticles in the case of nonlinearities arising from carriertrapping-induced charge separation [9]. In the case of the DC stark effect, the electric field is generated by the charge separation due to the trapped holes at the surface of the semiconductor nanoparticles. According to the previous report, the photo-induced absorption should be observed even at 300 ps after the photo-excitation. This is not consistent with the fast subpicosecond dynamics of absorption changes measured in our sample. In contrast, as the biexciton effect shows subpicosecond decay dynamics, the induced absorption in our samples could be attributed to the biexciton effect. More experimental results and further analysis about saturation absorption and induced-absorption at various wavelengths will be discussed in another paper. A pump fluence of 48 mJ/cm2 according to the report [10], can be estimated corresponds to the average number of 0.04 e–h pairs excited per nanoparticle. In this case, although biexciton states can be formed when multiple e–h
2
4 6 Delay time (ps)
8
10
12
Fig. 2. Time-dependent nonlinear aborption at various energy fluences for 800 nm wavelength.
pair excitation in a small portion of nanoparticles, due to some inhomogeneous broadening of exciton band, the lowenergy tail of the exciton transition will overlap with the ground-state-biexciton transition. So the photobleaching was observed at low pump fluences at 800 nm wavelengths. This result means that the photo-induced absorption of the biexciton effect is not large enough to surpass the bleaching of the exciton transition under low excitation. As energy fluence increases, the saturated absorption gradually disappeared, and only the photo-induced absorption was observed at a energy fluence of 4 mJ/cm2. In comparison with the low pump intensity data, in the highest energy fluence the photo-induced absorption dominates the ultrafast behavior. This result indicates that biexciton effect starts to play a significant role when the multiple e–h pair per nanoparticle on average was excited. Fig. 3 shows the results of the OKE responses at 800 nm with various energy fluences, from which we can see that, the character of OKE response evolution is similar to that of the pump–probe measurements. At low-energy fluence, the OKE response evolution of PbS nanoparticles includes a positive and a negative component, and a very slow decay was observed. At higher pump fluences, there is a pronounced modification of the temporal profile, the disappearance of the positive component and the appearance of a negative component. The results indicate that the ultrafast optical nonlinearity arises from the contribution of the saturation due to exciton transition and the photoinduced absorption due to biexciton effect. The contributions of the single exciton and that of biexciton to the thirdorder susceptibility are opposite, which agrees with Banyai’s theoretical predict [1]. For the pump fluences of 3 mJ/cm2, the magnitude of OKE signal is compared with that of reference sample CS2. The value of the third-order NLO susceptibility wð3Þ of the sample can be calculated by using the s
ARTICLE IN PRESS D. Li et al. / Journal of Luminescence 122–123 (2007) 549–551
In conclusion, we have studied the transient dynamics and third-order NLO of PbS nanoparticles. The results indicate that when the pump and probe wavelength were chosen at the low-energy side of the exciton resonance and low-energy fluences, photo-induced absorption of the biexciton effect and photobleaching of the exciton were observed at the same time. And the contribution of the exciton to the third-order susceptibility cancels that of the biexciton. On the contrary, at high pump fluence, the photo-induced absorption dominates the ultrafast nonlinear optical response, revealing that biexciton effect starts to play a significant role due to the multiple e–h pair excitation per nanoparticle. At energy fluences of 4 mJ/cm2, the magnitude of w(3) for PbS nanoparticles doped SiO2 thin film is calculated to be 5.1 1010 esu, which arises from the contribution of biexciton effect at high pump intensity.
OKE signal (µV)
5
0 48 µJ/cm2
0
-50 4 mJ/cm2 -100 0
2 4 Delay time (ps)
551
6
Fig. 3. Transient optical Kerr signal at wavelength of 800 nm with different energy fluences (a) 48 mJ/cm2 and (b) 4 mJ/cm2.
following formula [11]: 1=2 2 ns Lr aL ð3Þ I s s wð3Þ ¼ w , s r Ir nr Ls exp aLs =2 ð1 expðaLs ÞÞ
The work is supported by the National Natural Science Foundation of China (Nos.60406005 and 10374002).
References
(1) where I is the intensity of the OKE signal at zero delay time, a is the linear absorption coefficient of the nm-C glass, n is the refractive index, and L is the interaction length of pump beam and probe beam over the CS2 cell and the sample. The subscript s and r denote sample and CS2, respectively. The value wð3Þ is known to be 0.67 1013 esu for CS2 in fs r domain. Based on the magnitudes of the OKE signal of the sample and the reference under identical experimental condition, the magnitude of wð3Þ for PbS nanoparticles s doped SiO2 thin film is calculated to be 5.1 1010 esu, which is four orders larger in magnitude than that of CS2, and one order magnitude larger than that of the standard optical nonlinear material CdS, which is known to be 5 1011 esu [12]. Based on preparation method of our sample, size distribution of the nanoparticles is usually more than 10%. Wide size distribution results in a strong reduction of the optical nonlinearities. If we can achieve a narrow size distribution and high dopent density, optical nonlinearities of the sample may be further increased.
[1] L. Banyai, Y.Z. Hu, M. Lindberg, S.W. Koch, Phys. Rev. B 38 (1988) 8142. [2] S. Rink, D.A.B. Miller, D.S. Chemla, Phys. Rev. B 35 (1987) 8113. [3] W. Huang, Jian lin Shi, Scripta mater. 44 (2001) 117. [4] J.L. Machol, F.W. Wise, R.C. Patel, D.B. Tanner, Phys. Rev. B 48 (1993) 2819. [5] G.H. Ma, L.J. Guo, J. Mi, Y. Liu, S.X. Qian, D.C. Pan, Y. Huang, Solid State Commun. 118 (2001) 633. [6] Y.Z. Hu, S.W. Koch, M. Lindberg, N. Peyghambarian, E.L. Pollock, F.F. Abraham, Phys. Rev. Lett. 64 (1990) 1805. [7] M.G. Bawendi, W.L. Wilson, L. Rothberg, P.J. Carroll, T.M. Jedju, M.L. Steigerwald, L.E. Brus, Phys. Rev. Lett. 65 (1990) 1623. [8] Y.Z. Hu, M. Lindberg, S.W. Koch, Phys. Rev. B 42 (1990) 1713. [9] D.J. Norris, A. Sacra, C.B. Murray, M.G. Bawendi, Phys. Rev. Lett. 72 (1994) 2612. [10] V.I. Klimov, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Phys. Rev. B 60 (1999) 13740. [11] M.K. Casstevens, M. Samoc, J. Pfleger, P.N. Prasad, J. Chem. Phys. 92 (1990) 2019. [12] R. Reisfeld, in: C.K. Jorgensen, R. Reisfeid (Eds.), Optical and Electronic Phenomena In Sol–Gel Glasses and Modern Application, Structure and Bonding, vol. 85, 1996, Springer Press Inc., Germany, pp. 99–147.