Optical limiting in semiconductor nanocrystals in glass

Optics Communications 103 (1993) 405-409 North-Holland

OPTICS COMMUNICATIONS

Optical limiting in semiconductor nanocrystals in glass B.L. Justus, A.J. Campillo, D.G. H e n d e r s h o t and D.K. Gaskill Naval Research Laboratory, Washington, DC, 20375-5338, USA

Received4 June 1993;revised manuscript received20 July 1993

Reportedis opticallimiting in the near-irusingcompositesconsistingof quantum-confinednanocrystalsof InP in porousVycor glass. The nonlinearoptical propertiesof the InP-dopedglasses, investigatedusing the Z-scan technique,werecharacterizedby a negativenonlinearindex, absorption-inducednegativerefractiondue to carriers,and nonlinearloss due to two-photonabsorption and excitedstate absorptionof carriers.

1. Introduction

Nanometer-sized semiconductor crystallites embedded in glass exhibit carrier quantum confinement effects which profoundly influence both their linear and nonlinear optical (NLO) properties. In particular, large enhancements of the third-order susceptibility, Z ~3), have been predicted [ 1-3] and observed in semiconductor-doped glasses both on resonance [4-6 ] and in the single-photon transparency region [7,8]. Consequently, such quantumconfined glass composites may be useful in applications, such as optical limiting, where large bandwidth, high linear transmission, and strong NLO properties are required. In this work, InP quantum wires grown in porous Vycor glass were investigated at 850 nm in an f / 5 optical limiting test-bed and found to effectively clamp the transmitted fluence at values below 2 ~tJ/cm a. Optical limiting with bulk semiconductors has been extensively studied [9 ] and effective limiting with picosecond excitation was achieved over a broad range of wavelengths using a defocusing geometry. The limiting mechanisms were found to be two-photon absorption (TPA) and TPA-generated free carrier refraction. Limiting of nanosecond duration pulses was more difficult to accomplish without incurring damage. Optical limiting in quantum-confined semiconductor-doped glasses has not, to our knowledge, been reported. Previous picosecond studies [ 8 ] of the NLO properties of nanocrystals of ElsevierSciencePublishers B.V.

GaAs fabricated in porous Vycor glass indicated that the TPA coefficient, r, and the nonlinear refractive index, y, were enhanced. This suggested that the use of quantum-confined semiconductor materials in a defocusing optical limiter relying on nonlinear absorption and negative refraction, reinforced by carrier induced defocusing, might demonstrate improved performance over limiting with bulk materials. Since TPA is an intensity dependent process, larger values of fl should improve the limiting for ns pulse durations.

2. Experimental

The composite glasses used in this study were fabricated by impregnating porous Coming Vycor glass with InP using organometallic chemical vapor deposition methods [ 10]. It has been shown previously that quantum-confined nanocrystallites of many semiconductors, including II-VI [ 11,12 ] and III-V [ 8,13,14 ] semiconductors, may be fabricated using Vycor glass. Vycor is a porous silicate glass containing an interconnected network ofnanometersized channels. The channels restrict the growth of the crystals in at least two dimensions to nanometers, but allow growth in the third dimension along the channels, creating an interconnected network of convoluted quantum wire-like nanostructures. The InP nanocrystals were grown in the Vycor glasses by the thermal reaction of trimethylindium and phos405

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phine. Two types of Vycor glass were used in this work. Standard Vycor glass (type 7930) has an average pore size of ~ 40 A, while a second type of glass, differing in the nature of the heat treatment during fabrication, has a larger average pore size, on the order of 150 A. The average crystal size, determined [10] by X-ray diffraction, in the 40 ~ and 150 pore-size glasses were 150 A and 45 A, respectively. The concentration of the InP in the glass has not been accurately determined, however, based on the quantity of trimethylindium introduced, a reasonable estimate is 3 _ 1.5% by volume. The absorption spectra of the InP-doped glasses are shown in fig. 1. For reference, the wavelength corresponding to the bandgap in bulk InP is ~ 916 nm. The broad, structureless absorption spectra indicate the presence of a wide distribution of wire diameters. Limiting measurements were performed at 850 and 880 nm using f / 5 optics in a defocusing geometry and ~ 8 ns duration pulses from a Ti:sapphire laser pumped by a Q-switched Nd:YAG laser (schematic shown in fig. 2). The incident beam was spatially filtered and expanded such that the 10 mm entrance aperture truncated the beam at the 1/e 2 diameter. A 50 mm focal length, single-element piano-convex lens was used to focus the beam. The 1/e 2 radius of the beam at the focus in air, determined by translating a knife-edge and fitting the resulting transmission curve to a gaussian beam profile, was 7.8 pm. This value is ~ 2.5 × greater than that expected for diffraction limited f/5 optics and reflects the presence

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1 December 1993 L2

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Fig. 2. Schematicof optical limiting apparatus. BS: beamspliner; PM: pyroelectricenergymeter probe; A1: entrance aperture; L1: focusing lens; S: semiconductor-dopedVycorglass sample; A2: collectingaperture; L2: imaginglens; EM: energymeter. of spherical aberrations typically found in pianoconvex optics. A variable collecting aperture was placed before the imaging lens and was alternately adjusted to pass 100% or 90% of the low intensity light transmitted by the sample. Before each limiting measurement a scan was made in the z direction to determine the optimum sample location that gave minimum transmission. The reference and transmitted pulse energies were measured with calibrated pyroelectric energy meters with accuracy to 4-_0.1 gJ (Laser Precision RJP-735 probes).

3. Results and discussion The optical limiting characteristics of the 40 A pore-size InP-doped Vycor glass sample measured at 850 nm are shown in fig. 3. The linear system transmission, determined by the product of the sample transmission and the collecting aperture transmission, is represented by the straight line. The data given by open circles were obtained with the collecting aperture set for 100% transmission of the low intensity light. The data shown by diamonds were obtained by reducing the diameter of the collecting aperture to a point where it had 90% transmission of the low intensity light. The system transmission line was drawn assuming a 90% transmitting collecting aperture. Each data point represents a single shot measurement, and the sample was translated to a previously unexposed spot for each measurement. The transmitted energy was clamped at ~ 1.3 g J,

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OPTICS COMMUNICATIONS

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Fig. 4. Optical limiting data, similar to that of fig. 3, obtained with InP-doped, 150 A pore size, Vycor glass sample measured at 880 nm.

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g corresponding to a fluence through the collecting aperture o f ~ 2 ~tJ/cm 2, for incident energies up to 20 ~tJ, the threshold for p l a s m a b r e a k d o w n and sample damage. Refractive defocusing appears responsible for a significant p o r t i o n o f the limiting effectiveness. In particular, a 10% reduction in the aperture transmission results in a significant i m p r o v e m e n t in the limiting, an i n d i c a t i o n o f significant refractive defocusing a n d / o r possible diffraction resulting from T P A absorbing a greater fraction o f the Airy pattern center rather than the wings. S i m i l a r limiting at 880 n m for the I n P - d o p e d 150/~ pore-size glass is shown in fig. 4. Z-scan experiments [ 17,18 ] were also p e r f o r m e d on the 40 A pore-size I n P - d o p e d Vycor glass at 850 n m in o r d e r to elucidate the various p h e n o m e n a contributing to the N L O behavior. Longer focal length optics ( f / 4 0 ) were chosen to insure that the Rayleigh length at the focus was greater than the 0.4 m m sample thickness in these studies. The focal b e a m radius in air, again d e t e r m i n e d with the knife-edge technique, was 35 ~tm. The e x p e r i m e n t a l a p p a r a t u s was quite similar to that used in the limiting experiments except that a voltage p r o p o r t i o n a l to the ratio o f the t r a n s m i t t e d a n d reference energies was prov i d e d by the energy m e t e r a n d was plotted during the scan by an x - t plotter. The samples were trans-

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Fig. 5. Z-scan data obtained with f/40 optics for InP-doped Vycor glass sample measured at 850 nm. The bold-dashed curve is the open-aperture scan, the dashed curve is the closed-aperture scan, and the solid line is the ratio of the closed- and open-aperture scans. lated using a stepper-motor-controlled translation stage while the laser was fired at a repetition rate o f 10 Hz. The Z-scan plots were digitized by generating their b i t - m a p p e d image using an optical scanner. ASCII text files o f the traces were created a n d the digitized traces were subsequently analyzed. Figure 5 shows Z-scan traces o b t a i n e d at an incident intensity o f 340 M W / c m 2. The b o l d - d a s h e d curve was m e a s u r e d with the collecting aperture open, the d a s h e d curve was o b t a i n e d with the collecting aperture set at S = 0.4 transmission [ 17 ] a n d the solid 407

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curve is the ratio of the two curves. Both qualitative and quantitative information about the nonlinearities is readily obtained from fig. 5 and additional Zscan data obtained as a function o f incident intensity. First, the open aperture scans showed there was a significant nonlinear absorption. This nonlinear loss increased in magnitude with increasing incident intensity, typical o f TPA. Second, from the shape of the closed aperture scan (the peak of the transmission preceded the focus and the m i n i m u m transmission occurred near the focus) a negative refractive nonlinearity (self-defocusing) was evident. Third, as the intensity was increased the magnitude of the nonlinearity increased and the sign remained negative, indicating the presence o f carrier-induced negative refraction. Analysis o f the open-aperture Zscan data reveals that the increase in the nonlinear loss as a function of intensity is inconsistent with a single loss mechanism. Additional loss, most likely due to excited-state absorption, was observed at the higher intensities. This interpretation is consistent with similar measurements of bulk semiconductor TPA coefficients in the literature [ 19,20 ] which report that free carrier generation and subsequent free carrier absorption are the cause of apparent overestimates offl when ns pulsed excitation is used. Since excited-state absorption by carriers is apparently not negligible using ns excitation, calculation [ 18 ] of the magnitude offl using the intensity dependent Z-scan data is not possible. An estimate for the value of the nonlinear index can be obtained from the Z-scans acquired at low intensity upon calculating [ 17 ] the value of An/I (where n is the index o f refraction and / i s the intensity o f the incident laser b e a m ) from the ratio o f the closed- and open-aperture scans. The correct value for the nonlinear index is given in the limit as the intensity goes to zero, however, a slightly high estimate is obtained at low intensity where the intensity dependent losses are relatively small and are cancelled upon taking the ratio of the scans. The upper limit for the magnitude (sign is negative) o f the nonlinear index measured in this manner is approximately 3X 10-~2 cmE/W. The corresponding nonlinear refractive index, scaled to bulk and assuming a 3% fill factor, is ~ 10 -1° cmE/W. Additional study o f the nonlinear optical properties o f the InP-doped glasses using picosecond duration pulses, in order to avoid carrier absorption 408

1 December 1993

problems, is planned. The wavelength dependence of the nonlinear properties will be studied to determine whether the dispersion of these properties, as observed and modelled in bulk semiconductors [ 21 ], is affected by quantum confinement. Previous work has indicated that for I I - V I semiconductor doped glasses [ 22 ] the dispersion of the nonlinearity is different from that of the bulk materials, while the nonlinearity measured in I - V I I doped glasses [ 23] is in agreement with the general theory [21 ]. In summary, optical limiting in porous glass composites containing q u a n t u m wire-like nanocrystals of InP is reported. The limiting mechanism is complex, including contributions from negative refraction due to the nonlinear index o f the material, negative refraction due to carriers excited by linear as well as two-photon absorption, and nonlinear losses due to two-photon absorption as well as excited state absorption.

Acknowledgements This work was supported by the Office of Naval Research. The authors acknowledge Ms. S. Hultman, Coming, Inc., for her careful preparation of the porous Vycor glass used in this work.

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[ 12 ] R.D. Stramel, T. Nakamura and J.K. Thomas, J. Chem. Soc., Faraday Trans. I 84 (1988) 1287. [ 13 ] J.C. Luong and N.F. Borrelli, Mat. Res. Soc, Syrup. Proc. 144 (1989) 695. [ 14 ] Y. Wang and N. Herron, Res. Chem. Intermed. 15 ( 1991 ) 17. [ 15 ] C. Jin, J. Yu, W. Qin, J. Zhao, F. Zhou, K. Dou, J. Liu and S. Huang, J. Lumin. 53 (1992) 483. [ 16 ] M. Kull, J.L. Coutaz, G. Manneberg and V. Grivickas, Appl. Phys. Lett. 54 (1989) 1830. [ 17 ] M. Sheik-Bahae, A.A. Said, T-H. Wei, D.J. Hagan and E.W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760,

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