Two- and Three-Photon Absorption and Frequency Upconverted

NANO LETTERS

Two- and Three-Photon Absorption and Frequency Upconverted Emission of Silicon Quantum Dots

2008 Vol. 8, No. 9 2688-2692

Guang S. He,* Qingdong Zheng, Ken-Tye Yong, Folarin Erogbogbo, Mark T. Swihart, and Paras N. Prasad Institute for Lasers, Photonics and Biophotonics, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 Received April 6, 2008; Revised Manuscript Received July 22, 2008

ABSTRACT In this communication, we present the experimental results of two- and three-photon excitation studies on silicon quantum dots (QDs) in chloroform (as well as in water) by using femtosecond laser pulses with wavelengths of 778 and 1335 nm and a pulse duration ∼160 fs. The photoluminescence spectral distributions are nearly the same upon one-, two-, and three-photon excitation. With one- and two-photon excitation, the temporal relaxation measurements of photoluminescence emission manifest the same multiexponential decay behavior in the time range from 0.05 ns to 15 µs, characterized by three successive decay constants: 0.75 ns, 300 ns, and 5 µs, respectively. Finally, the two-photon absorption spectrum in the spectral range of 650-900 nm and the three-photon absorption spectrum in the spectral range of 1150-1400 nm have been measured.

Multiphoton excitation studies of various semiconductor quantum-dots (QDs) systems are attracting increasing attention because of their great potential of applications. In multiphoton excitation cases, longer IR wavelengths can be used, which are more suitable for optical communications and biological/medical applications. In addition, the multiphoton excitation can produce frequency-upconverted emission and provide three-dimensional resolution for imaging, data storage, and microfabrication.1 However, the most wellstudied semiconductor QDs systems (such as CdSe, CdTe, and InAs) have the problem of their toxicity that to a certain extent limits their biological and medical applications. Silicon is probably the most studied of all inorganic materials and provides the platform for both the massive integrated circuit industry and most of the photovoltaic industry. Moreover, it is abundant and nontoxic, and processes for refining it have essentially been perfected, so that it can be produced in higher purity than virtually any other material. Perhaps the greatest limitations of silicon arise from its indirect bandgap character. While this poses no problem in electronic devices, it severely limits the use of silicon in photonic applications, where direct band gap compound semiconductors dominate. The extremely low light emission efficiency of silicon can be overcome by preparing it in the form of small nanocrystals or quantum dots. Recently, photoluminescence quantum yields exceeding * Corresponding author. Email: [email protected]. 10.1021/nl800982z CCC: $40.75 Published on Web 08/13/2008

 2008 American Chemical Society

60%, at room temperature, have been demonstrated for organically capped silicon nanocrystals with emission in the near-infrared.2,3 However, multiphoton absorption and multiphoton-excited emission from organically capped silicon nanocrystals have not previously been studied. Even for single photon excited photoluminescence (PL), there are significant discrepancies in the literature regarding the PL lifetimes for silicon nanocrystals. Convincing studies reporting both long (microsecond) lifetimes typical of an indirect transition4 and short (nanosecond) lifetimes typical of a direct transition5 have been presented. This contradiction is most often attributed to differences between the nanocrystal samples prepared by different methods. However, in the work presented here, we observe multiexponential decay that includes both nanosecond and microsecond components, from a silicon quantum dot sample. In the past two decades, some studies related to the thirdorder nonlinear optical properties (two-photon absorption and nonlinear refractive index change) have been reported in various silicon based samples, including Si bulk,6-12 porous silicon,13,14 Si -photonic crystal,15-17 Si waveguide18,19 and Si film.20 However, such studies have not yet been reported for free-standing silicon nanocrystals. As mentioned above, the emission efficiency can be significantly increased in such structures. Moreover, the absorption and emission spectra can be tuned by controlling the nanocrystal size, through quantum confinement effects. Over the past decade, several experimental techniques for preparing such quantum confined

silicon nanocrystals have been developed, and the size dependence of the emission wavelength and efficiency have been confirmed.2-5,21-24 Silicon nanocrystals therefore have much of the same potential in photonic and biophotonic applications as the CdSe, CdTe, InAs, and other compound semiconductor quantum dots that have been intensively studied over the past two decades. However, silicon nanoparticles do not present the environmental and toxicity concerns that plague those heavy metal based materials, nor does silicon suffer from any supply limitations, as may be encountered for indium and some of the other less common metals. For biological imaging applications, a key limitation of silicon nanocrystals is the need to excite them at short wavelengths, which can induce photodamage and autoluminescence. Even for Si nanocrystals with emission in the red to near-IR, the single photon excitation spectrum typically peaks in the UV, near 340 nm. Two- or three-photon excitation at infrared wavelengths where tissues are relatively transparent would dramatically enhance the applicability of Si nanocrystals in bioimaging applications. Nevertheless, the nonlinear optical properties of Si quantum dots, especially their multiphoton excitation characteristics, have not been fully investigated, largely because of the challenges inherent in fabricating them. There are very few publications mentioning the issue of two-photon absorption (2PA) of Si nanocrystals,25,26 and three-photon absorption (3PA) by Si QDs has not previously been reported. In our recent experimental study, the Si QDs were prepared, as described previously,22,23 by laser-driven pyrolysis of silane, followed by HF-HNO3 etching. The average size of the Si QDs sample used for our study was around 3 nm. Styrene was grafted to the Si QD surfaces, thereby rendering them dispersible in chloroform. The silicon QD dispersion did not show any significant decrease in PL intensity after storage in the dark at 4 °C for more than a few months. These stored QDs can be directly used for nonlinear optical studies without any sign of quenching effects. In addition, Si QDs were encapsulated in phopholipid-polyethylene glycol (PEG) micelles to form aqueous dispersions, as described elsewhere. The linear transmission spectrum of a 4 mm path length Si nanocrystal dispersion in chloroform (CHCl3) of concentration d0 ) 1.43 mg/mL is shown in Figure 1a in comparison with a pure solvent sample of the same optical path length. Figure 1b shows the linear absorption spectra of the 1 mm Si nanocrystal dispersion in chloroform of the same concentration and of a pure solvent sample of the same path length. For these silicon nanocrystals, the strong linear absorption band is located in the UV (e350 nm) range and extends its decaying tail into the 550-600 nm visible spectral range. From Figure 1, one can also see that there are two spectral windows for the Si QDs solution sample with negligible one-photon absorption. One is from ∼650 to ∼1100 nm, which is suitable for two-photon excitation (2PE); the other one is from ∼1160 to 1360 nm, which is suitable for three-photon excitation (3PE). Nano Lett., Vol. 8, No. 9, 2008

Figure 1. (a) Linear transmission spectra of a 4 mm Si QDs in CHCl3 of 1.43 mg/mL concentration (solid line) and a 4 mm pure solvent sample (dotted line); (b) linear absorption spectra of a 1 mm Si QDs solution sample of 1.43 mg/mL (solid line) and a 1 mm pure solvent sample (dotted line).

We used a 778 nm, ∼160- fs pulsed laser for 2PE study and a 1335 nm, ∼160 fs pulsed laser for 3PE study; the former was from a Ti-sapphire oscillator/amplifier system (CPA-2010 from Clark-MXR, Inc.) operating at a repetition rate of 1 kHz, whereas the latter was from an optical parametric generator (OPG) pumped by the Ti-sapphire laser system. For one-photon excitation (1PE) study, the secondharmonic beam of 389 nm from the 778 nm laser beam was used. The loosely focused excitation laser beam passed through a 1 mm sample solution, and the induced fluorescence emission signal was measured at the side direction. The normalized one-photon (at 389 nm), two-photon (at 778 nm), and three-photon (at 1335 nm) excited photoluminescence spectra from the Si QDs in chloroform with 1.43 mg/mL concentration are shown in Figure 2 by three solid line curves, measured by a grating spectrometer (Holo Spec from Kaiser Optical System, Inc.). The emission spectra under one-, two-, and three-photon excitation conditions are nearly identical, with an emission peak wavelength of ∼650 nm and a spectral bandwidth of ∼120 nm, implying that they come from the same upper emitting level(s) of the sample medium under three different excitation conditions. In addition, the normalized emission spectra measured from the dispersion of micelle-encapsulated silicon nanocrystals in 2689

Figure 2. One-, two-, and three-photon induced photoluminescence spectra for Si QDs in CHCl3 (solid lines) and for Si QDs in water (dotted lines).

water are also shown in Figure 2 by two dotted-line curves and are nearly identical to the spectra from the chloroform dispersion. Thus, the results in Figure 2 suggest that the solvent effect on the emission spectra of the Si QDs is small. We have measured the quantum yield (QY) of one-photon excited photoluminescence of our Si QDs sample in comparison with a Rhodamine B solution in methanol as a standard sample with a known QY value of ∼0.7. Both samples were excited at wavelength 466 nm, and the measured QY value for Si QDs sample is ∼0.24 ( 0.05. As mentioned above, that the emission states of the Si QDs sample are the same for either one-, two-, or three-photon excitation, the corresponding QY values for these three excitation processes can also be recognized nearly the same.1 To verify that the observed photoluminescence is the result of 2PE at 778 nm and 3PE at 1335 nm, we measured the emission intensity as a function of the input excitation intensity (energy) at each of these two wavelengths separately. The results are shown in Figure 3a,b. The basic theories of 2PE and 3PE predict a quadratic or cubic dependence of the emission intensity on the excitation intensity under our experimental conditions; therefore, on logarithmic scales, there should be a straight line connecting the measured data with a slope of factor 2 for 2PE and a slope of factor 3 for 3PE, respectively. In Figure 3a, the slope of the best-fitting straight line is 1.9, and in Figure 3b, the measured slope is 3.1. These results confirmed the 2PE mechanism for 778 nm excitation and 3PE mechanism for 1335 nm excitation, respectively. 2690

Figure 3. (a) Quadratic dependence of two-photon induced photoluminescence on the excitation intensity; (b) cubic dependence of three-photon induced photoluminescence on the excitation intensity.

The dynamic relaxation processes of the nonlinear absorbing medium after one- and multiphoton excitation with ultrashort femtosecond laser pulses are of interest from both fundamental and technological perspectives. These dynamics can provide insight into the emission mechanism and determine the best strategies for using these particles in applications like bioimaging. Shorter emission lifetimes allow higher photon throughput, but in bioimaging applications, long emission lifetimes can be useful because they allow fast-decaying autofluorescence to be separated from the nanocrystal emission. Both short (nanosecond)5 and long (microsecond to millisecond)4 emission lifetimes have been reported. In this study, we find that, in addition to a slow decay in the microsecond regime, there is also a very fast emission decay observed in the nanosecond regime. To measure the fast decay behavior, a high-speed streak camera system (C5680-22 from Hamamatsu) was employed to record the temporal decay of the photoluminescence signal from the Si QDs chloroform solution excited by the ultrashort laser pulses of ∼160 fs duration. As an example, Figure 4a shows the measured decay curve (solid line) of the emission signal excited by one-photon absorption (1PA) at 389 nm. This measured overall decay behavior in the nanosecond regime Nano Lett., Vol. 8, No. 9, 2008

It is noted that the multiexponential decay behavior of silicon nanocrystals in hexane is also reported very recently by Sykora et al.27 Based on a time-resolved fluorescence upconversion spectroscopy technique, they indicated that there were three different emission processes with different decay characteristics roughly covering the time scales of e1∼2 ns, >200 ns, and >100 µs, respectively. These results are qualitatively consistent with our triple exponential results with three decay constants of 0.75 ns, 300 ns, and 5 µs, respectively. The multidecay behavior of Si QDs samples originates from different emission mechanisms, which are determined by both the intrinsic energy state structure of the core and the extrinsic surface state property of the dots. The deeper understanding of these mechanisms and processes is the subject of further studies. For a given new nonlinear absorbing material, it is important to know the most effective wavelengths for multiphoton excitation. However, systematic 2PA and 3PA spectral data for silicon nanocrystals have not previously been reported. Here, we present the results of our preliminary measurements of 2PA coefficient as well as 3PA coefficient as a function of the excitation wavelength. The nonlinear transmissivity of a two-photon absorbing or a three-photon absorbing medium can be described by28,29 1 (2PA) 1 + β(λ)I0L 1 T(I0, λ) ) (3PA) √1 + 2γ(λ)I20L T(I0, λ) )

Figure 4. (a) Fast temporal decay of one-photon excited photoluminescence in the nanosecond regime; (b,c) slow decay of oneand two-photon induced emission in the microsecond regime, respectively.

can be well-fitted by a double exponential curve (dashed line) that is mainly attributed to two different single exponential decay processes: the fast one possessing a decay constant of ∼0.75 ns with a statistical weight factor of 0.87, and the slow one having a decay constant of ∼300 ns with a statistical weight factor of 0.13. To further clarify the slow decay behavior in the microsecond regime, a 500 MHz digital oscilloscope (Infinum from Hewlett-Packard) was used in conjunction with a high speed photodiode detector to record the slow decay, excited by 1PA at 389 nm or by two-photon absorption (2PA) at 778 nm. Such measured decay curves (solid lines) are shown in Figure 4b,c, respectively with a temporal resolution of 1 ns. It is found that the experimental curves measured in the microsecond regime are essentially the same for both excitation wavelengths and can be wellfitted by a double exponential curve (dashed line). In this case, the fitting curve is attributed to two components: one with a decay constant of ∼300 ns and the other with a much larger decay constant of ∼5 µs. Nano Lett., Vol. 8, No. 9, 2008

(1)

Here, I0 is the incident light intensity, L is the path length of the sample, β(λ) is the 2PA coefficient (in units of cm/GW), and γ(λ) is the 3PA coefficient (in units of cm3/GW2). Both of these two coefficients depend on the excitation wavelength and the density (N′) of nonlinear absorbing centers (e.g., the number of either nonlinear absorbing molecules or nanoparticles per cm3) of the medium. We have the following simple expressions β(λ) ) σ2N′ (2PA) γ(λ) ) σ3N′ (3PA)

(2) 4

where σ2(λ) is the 2PA cross section (in units of cm /GW) and σ3(λ) is the 3PA cross section (in units of cm6/GW2) of the nonlinear absorbing medium. From eq 1, one can see that, for a given excitation wavelength (λ0) and input laser intensity value (I0), the corresponding nonlinear absorption coefficient β(λ0) or γ(λ0) can be determined by measuring the nonlinear transmissivity value, T(I0, λ0) for a given sample medium. Furthermore, a complete 2PA or 3PA spectral structure (or curve) can be obtained by continuously or sequentially changing the excitation wavelength. In practice, two alternative techniques based on nonlinear transmission measurement can be used to obtain the degenerate 2PA and 3PA spectra of a given sample medium. One is to use a single and spatially dispersed intense white-light continuum beam to measure the multiphoton absorption spectra, without the need of wavelength tuning requirement.30 The other one is to use a tunable laser source (or an OPG) to perform multipoint measurements. In the present study, we have used the first method to measure the 2PA spectrum 2691

Acknowledgment. This work was partially supported by the Directorate of Chemistry and Life Sciences of the U.S. Air Force Office of Scientific Research, Washington, D.C. The authors also acknowledge funding from the National Science Foundation, through the international Sweden: US collaboration program Grant DMR-0307282. References

Figure 5. (a) 2PA spectrum of Si QDs in CHCl3 measured by using the white-light continuum generation technique; (b) 3PA spectral data measured by using multipoint nonlinear transmission technique.

in the spectral range from 650 to 900 nm. An intense whitelight continuum beam was generated from a 10 cm long cuvette filled with heavy water that was pumped by a strong 778 nm laser beam, described in detail previously.30,31 The result is shown in Figure 5a, which shows that the 2PA coefficient (β) decreases a value of 0.1 cm/GW at ∼650 nm to ∼0.02 cm/GW at the 750-800 nm range. To measure the 3PA spectrum from 1150 to 1400 nm, we performed nonlinear transmission measurements at 7 different excitation wavelengths. The measured data of 3PA coefficient as a function of the excitation wavelength are shown in Figure 5b, which shows a slight decrease of the γ value from ∼0.9 × 10-5 cm3/GW2 at ∼1200 nm to ∼0.5 × 10-5 cm3/ GW2 at ∼1380 nm. In conclusion, our experimental results demonstrate the multiphoton excited frequency upconversion capability of silicon QDs in chloroform and in water. In addition to providing fundamental multiphoton properties of this important material, these studies reveal the potential for use of silicon QDs in multiphoton excited biological imaging and medical applications. The 2PA and 3PA spectral data presented here will also allow rational selection for other potential application such as optical power limiting and stabilization.32,33 2692

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NL800982Z Nano Lett., Vol. 8, No. 9, 2008