Letter pubs.acs.org/NanoLett
Single Nanowire Optical Correlator Huakang Yu,† Wei Fang,† Xiaoqin Wu,† Xing Lin,† Limin Tong,*,† Weitao Liu,*,‡ Aimin Wang,§ and Y. Ron Shen‡,⊥ †
State Key Laboratory of Modern Optical Instrumentation, Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Physics, Fudan University, Shanghai 200433, China § Institute of Quantum Electronics, State Key Laboratory of Advanced Optical Communication System and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China ⊥ Department of Physics, University of California, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Integration of miniaturized elements has been a major driving force behind modern photonics. Nanowires have emerged as potential building blocks for compact photonic circuits and devices in nanophotonics. We demonstrate here a single nanowire optical correlator (SNOC) for ultrafast pulse characterization based on imaging of the second harmonic (SH) generated from a cadmium sulfide (CdS) nanowire by counterpropagating guided pulses. The SH spatial image can be readily converted to the temporal profile of the pulses, and only an overall pulse energy of 8 μJ is needed to acquire a clear image of 200 fs pulses. Such a correlator should be easily incorporated into a photonic circuit for future use of onchip ultrafast optical technology. KEYWORDS: Semiconductor Nanowire, Second-Harmonic Generation, Optical Correlator, Ultrashort Pulse Measurement, Group-Velocity Measurement
U
previously for optical correlation measurements in an optical waveguide,15,16 but strong loss in the material used has prevented its further development.17−19 The CdS nanowire of our correlator has low loss and high second-order nonlinearity. Its wide bandgap (∼2.42 eV) and zero cutoff of the fundamental waveguide mode allow the operation over a very broad bandwidth. Another advantage of our correlator is that the optical alignment is not critical. The time delay between the counterpropagating pulses does not have to be very precise as long as they overlap in the nanowire. The detection of SHG perpendicular to the wire also avoids the huge background usually due to the remnant fundamental input, which is now tightly confined in the nanowire. Experimentally, CdS nanowires were synthesized by a thermal evaporation process.20 Electron microscope characterization of a typical 500 nm diameter CdS nanowire confirmed that it has the wurtzite structure (Figure 1b), an excellent smooth surface, and a uniform diameter (Figure 1c). The fundamental guided mode at 1310 nm was highly confined to the nanowire (Figure 1d) because of the large refractive index of CdS (∼2.3 at 1.310 μm).21 The effective mode area was about 0.1 μm2, and a 1-pJ 200 fs pulse propagating in the nanowire can have a peak intensity of ∼5 GW/cm2. The high refractive index of the nanowire also
ltrafast optical technology has broad applications ranging from ultrafast spectroscopy, ultrabandwidth telecommunications, and material processing to biomedicine.1−3 To characterize ultrashort pulses, optical correlators are generally used.4 Most conventional correlators are based on correlated nonlinear optical wave mixing processes in bulk crystals.4 They require an elaborate setup with precision optical stages that takes a fair amount of space.5 Recent interest in developing integrable and energy-efficient photonic circuitry has called for miniaturization of optical components or devices.6−8 For construction of a miniature optical correlator, photonic nanowires appear to be ideal. Semiconductor nanowires have single-crystal structure, uniform diameter, high refractive index, and large optical nonlinearity.9 They offer an excellent platform for nanoscale nonlinear optics.10−12 Tight confinement of light propagating along a nanowire greatly enhances light−matter interaction and associated nonlinear optical effects.13,14 We demonstrate in this paper a simple, ultracompact optical correlator based on second harmonic generation (SHG) in a CdS nanowire. The device is schematically described in Figure 1a. Laser beams are coupled into the nanowire in a counterpropagating configuration, and SHG is detected in the directions perpendicular to the nanowire, as required by wave-vector matching. When the inputs are ultrashort pulses, the spatial profile of the SH intensity distribution in the pulse overlapping region can then be converted into the time domain to yield the temporal profile of the pulses (see Supporting Information). Very low pulse energy is needed for the measurement. The same scheme was used © XXXX American Chemical Society
Received: March 20, 2014 Revised: May 5, 2014
A
dx.doi.org/10.1021/nl5010477 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Figure 1. SHG in a single nanowire. (a) Schematic illustration of SHG (2ω) in a nanowire with two counterpropagation beams (ω). The phase matching of SHG is described in the inset. (b) High-resolution transmission electron microscope image of a CdS nanowire and the corresponding electron diffraction pattern (inset) revealing the [001] growth-direction of along the axis of the nanowire. (c) Scanning electron microscope image of a 500 nm diameter CdS nanowire (taken by a Zeiss Ultra-55 field emission scanning electron microscope with acceleration voltage 3 kV). Scale bar, 1 μm. (d) Calculated cross-sectional energy distribution of a guided 1310 nm light in 500 nm diameter nanowire (calculated using a finite element method based software Comsol Multiphysics 4.2a with RF module). Scale bar, 500 nm.
Figure 2. SHG in a CdS nanowire by CW light. (a) Schematics of the experimental setup. A CdS nanowire of around 850 nm in diameter is suspended across a microchannel of 200 μm on an MgF2 wafer. CW light is coupled into the nanowire through fiber tapers at both ends. (b) Bright-field optical microscope image of a suspended CdS nanowire. (c)−(e) Optical microscope images of the second harmonic (SH) generated in the nanowire by 1064 nm CW light input. The analyzer before the CCD camera for SHG detection is removed in (c), parallel to the nanowire in (d), and perpendicular to the nanowire in (e). The sumfrequency image generated in another nanowire (∼850 nm) by 1064 and 1310 nm CW light coupled in from the two ends is shown in (f). The scale bar of 50 μm in (b) applies to all images.
makes it usable even in a liquid environment for on-chip chemical or biological spectroscopic analysis without much leakage.22,23 In the experiment, we suspended a CdS nanowire across a slit of an MgF2 substrate (sketched in Figure 2a, with a corresponding micrograph in Figure 2b) in order to avoid leakage of both the fundamental and SH light from the nanowire through the substrate as well as possible nonlinear contribution from the substrate. The input light was coupled into the nanowire from both ends via tapered optical fibers (see Supporting Information).24 The SH output perpendicular to the nanowire was collected by a 20× objective (NA = 0.4) and directed to a CCD camera and a spectrometer. We first tested SHG from the nanowire by launching a 1 mW 1064 nm CW laser beam into the nanowire at both ends. Green light emitted from the side was readily observed along the nanowire (Figure 2c), which was confirmed to be SHG by its spectrum (see Figure S3 in Supporting Information). If the input was launched into the nanowire only from one end, SH emission from the side essentially disappeared (see Supporting Information). The guided wave coupled into the nanowire generally appears in both TE-like and TM-like modes. By coating a thin Au film on the MgF2 substrate, the TM-like mode was damped out (through its coupling to the surface plasmon mode of the Au film),25 leaving only the TE-like mode propagating in the suspended portion of the nanowire. The SH side emission was found to be polarized along the nanowire as seen in Figures 2d and 2e, in agreement with the theory (see Supporting Information). Sum-frequency generation (SFG) from counterpropagating waves of different wavelengths (1064 and 1310 nm in Figure 2f) was also observed, indicating possible applications of the nanowire as a cross-correlator. To operate the nanowire as an optical correlator for ultrashort pulse characterization, we used a train of fs pulses (220 fs, 76 MHz, 1550 nm) generated from an optical parametric oscillator pumped by a fs Ti:sapphire laser. The series of pulses
were split into two, which were coupled into the CdS nanowire from the two ends (Figure 3a). The time delay between the two trains of pulses was set to overlap the counterpropagating pulses roughly in the middle of the nanowire. Although the interval between neighboring pulses in the train is ∼13 ns, only one pair of counterpropagating pulses can appear in the nanowire at a given time. A typical micrograph (taken with 0.1 s exposure time) of the SH generated from a CdS nanowire by the train of pulses with 1 pJ/pulse (corresponding to an overall pulse energy of 7.6 μJ) is presented in Figure 3b. Figure 3c shows the measured spectrum of the SH emission. The roughly symmetric pattern readily provides an estimate on the pulse profile. A Gaussian fit of the SH spatial intensity variation (red line in Figure 3d) yields a full width at half-maximum (fwhm) of 17.6 μm. With a calculated group index of 2.58 for the 850 nm diameter nanowire (Figure 3e),26 this spatial correlation width of 17.6 μm corresponds to a pulse width of 197 fs (see Supporting Information). It is known that pulse broadening occurs in propagating through the optical fiber and pulse narrowing occurs in transmitting from the optical fiber to the nanowire through the taper. In our case, the pulse width changed by −30 fs in the process (see Supporting Information). Therefore, the measured pulse width of 197 fs actually corresponds to an input pulse width of 227 fs, which is close to the 220 fs specified by the laser manufacturer. In real applications, to avoid errors from calculation, the correlator should be first calibrated against a standard correlator. B
dx.doi.org/10.1021/nl5010477 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Figure 4. SNOC for group velocity measurement. (a)−(d) Optical microscope images of SHG spots with 1310 nm incident pulses. The relative delay in the free-space optical path is 100 μm each from (a) to (d). (e)−(h) Corresponding intensity profiles of the SHG trace from (a) to (d), separately. Scale bar, 20 μm.
optical gating (FROG) for retrieving the amplitude and phase of ultrashort pulses.27 To summarize, we have demonstrated a simple and compact optical correlator based on a single nanowire, which can be readily implanted into photonic circuits, chips, or devices with high scalability and compatibility. Possible operation over a wide range of frequencies with very high sensitivity makes it a versatile compact device for characterizing ultrashort pulses in applications spanning from on-chip optical communication to laser spectroscopy.
Figure 3. SNOC for ultrafast pulse measurement. (a) Schematics of the experimental setup. (b) Microscopic image of the SH generated by counterpropagating fs pulses in the nanowire described in Figure 2. Scale bar, 25 μm. (c) Measured spectrum of the SH emission. (d) Intensity profile of the SH image along the nanowire axis (black dots) in (b). The solid red line is the Gaussian fit to the experimental data. (e) Calculated group index of the guided wave around 1.3 μm propagating in the nanowire.
■
The image of Figure 3b is almost background-free, suggesting that the nanowire correlator can be used to characterize ultrafast pulses with very low energy. For example, with 76 MHz repetition rate, the overall energy is ≤8 μJ for 200 fs pulses, or equivalently ≤2 μJ for 100 fs pulses. Considering the fact that the measured single-pulse damage threshold of the CdS nanowire was about 200 nJ, by improving the efficiency and sensitivity of the detection system, as well as optimizing the material and geometries of the nanowire, the SNOC may be operated as a single-shot correlator. The CdS nanowire is long enough to characterize pulses as long as several picoseconds. Unlike the case of conventional correlators, precise control and scanning of the relative path length of the two pulses are not needed for the nanowire correlator. A slight change in the relative path length shows up simply in a small shift of the SH emission spot along the nanowire. This is seen in Figures 4a−d, where it is shown that the autocorrelation spot of the counterpropagated 1310 nm (with its corresponding intensity profiles of the SHG trace shown in Figures 4e−h), 220 fs pulses shifts by 14 μm along a 450 nm diameter nanowire each time when the relative path length of the two pulses is changed by 100 μm. The corresponding group index of this nanowire is (100/2)/14 = 3.5, which roughly agrees with the calculated value of about 3.3 (ref 26). This is a simple, straightforward approach to measure the group index of a nanowire, which could be complicated otherwise. In addition, if we can replace one-side excitation with one known reference pulse, the SNOC may even work as a miniature frequency-resolved
ASSOCIATED CONTENT
S Supporting Information *
Sections: (1) Materials and methods; (2) additional information on transverse second-harmonic generation; (3) calibration coefficient; (4) waveguide dispersion in the fiber and the fiber tapers. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (L. T.). *E-mail:
[email protected] (W. L.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Z. G. Zhang and L. Y. Chen for helpful discussions. This work was supported by the National Key Basic Research Program of China under grant agreements 2013CB328703 and 2012CB921400 and Fundamental Research Funds for the Central Universities. Y.R.S. was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U. S. Department of Energy under Contract No. DE-AC03-76SF00098.
■
REFERENCES
(1) Diels, J.; Rudolph, W. Ultrafast laser pulse phenomena; Academic Press: New York, 1996. C
dx.doi.org/10.1021/nl5010477 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
(2) Knox, W. H. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 1273− 1278. (3) Weiner, A. M. Ultrafast Optics; Wiley: Hoboken, NJ, 2009. (4) Ippen, E.; Shank, C. Ultrashort Light Pulses. Springer: 1984. (5) Trebino, R.; Bowlan, P.; Gabolde, P.; Gu, X.; Akturk, S.; Kimmel, M. Laser Photonics Rev. 2009, 3, 314−342. (6) Foster, M. A.; Salem, R.; Geraghty, D. F.; Turner-Foster, A. C.; Lipson, M.; Gaeta, A. L. Nature 2008, 456, 81−84. (7) Eggleton, B. J.; Luther-Davies, B.; Richardson, K. Nat. Photonics 2011, 5, 141−148. (8) Moss, D. J.; Morandotti, R.; Gaeta, A. L.; Lipson, M. Nat. Photonics 2013, 7, 597−607. (9) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18− 27. (10) Nakayama, Y.; Pauzauskie, P. J.; Radenovic, A.; Onorato, R. M.; Saykally, R. J.; Liphardt, J.; Yang, P. Nature 2007, 447, 1098−101. (11) Barrelet, C. J.; Ee, H. S.; Kwon, S. H.; Park, H. G. Nano Lett. 2011, 11, 3022−5. (12) Liu, W.; Wang, K.; Liu, Z.; Shen, G.; Lu, P. Nano Lett. 2013, 13, 4224−4229. (13) Foster, M. A.; Turner, A. C.; Lipson, M.; Gaeta, A. L. Opt. Express 2008, 16, 1300−1320. (14) Gu, F.; Yu, H.; Fang, W.; Tong, L. Opt. Express 2012, 20, 8667− 8674. (15) Normandin, R.; Stegeman, G. Appl. Phys. Lett. 1980, 36, 253−255. (16) Otomo, A.; Stegeman, G. I.; Flipse, M. C.; Diemeer, M. B.; Horsthuis, W. H.; Möhlmann, G. R. J. Opt. Soc. Am. B 1998, 15, 759− 772. (17) Chen, C.; De Castro, A.; Shen, Y. Opt. Lett. 1979, 4, 393−394. (18) Beaulieu, Y.; Janz, S.; Dai, H.; Frlan, E.; Fernando, C.; Delâge, A.; Van Der Meer, P.; Dion, M.; Normandin, R. J. Nonlinear Opt. Phys. Mater. 1995, 4, 893−927. (19) Degen, C.; Jennemann, G.; Fischer, I.; Elsäßer, W.; Leu, S.; Rettig, R.; Stolz, W. Opt. Quantum Electron. 2002, 34, 707−716. (20) Kar, S.; Chaudhuri, S. Synth. React. Inorg., Met.-Org., Nano-Met. Chem. 2006, 36, 289−312. (21) Palik, E. D. Handbook of optical constants of solids; Academic Press: New York, 1991. (22) Haacke, S.; Schenkl, S.; Vinzani, S.; Chergui, M. Biopolymers 2002, 67, 306−9. (23) Yan, R.; Park, J. H.; Choi, Y.; Heo, C. J.; Yang, S. M.; Lee, L. P.; Yang, P. Nat. Nanotechnol. 2012, 7, 191−196. (24) Tong, L.; Gattass, R. R.; Ashcom, J. B.; He, S.; Lou, J.; Shen, M.; Maxwell, I.; Mazur, E. Nature 2003, 426, 816−819. (25) Dong, C. H.; Zou, C. L.; Ren, X. F.; Guo, G. C.; Sun, F. W. Appl. Phys. Lett. 2012, 100, 041104. (26) Tong, L.; Lou, J.; Mazur, E. Opt. Express 2004, 12, 1025−1035. (27) O’shea, P.; Kimmel, M.; Gu, X.; Trebino, R. Opt. Lett. 2001, 26, 932−934.
D
dx.doi.org/10.1021/nl5010477 | Nano Lett. XXXX, XXX, XXX−XXX
