Single CdTe Nanowire Optical Correlator for Femtojoule Pulses

Jul 14, 2016 - Chenguang Xin†, Shaoliang Yu†, Qingyang Bao†, Xiaoqin Wu†, Bigeng Chen†, Yipei Wang†, Yingxin Xu†, Zongyin Yang‡, and L...
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Single CdTe Nanowire Optical Correlator for Femtojoule Pulses Chenguang Xin,† Shaoliang Yu,† Qingyang Bao,† Xiaoqin Wu,† Bigeng Chen,† Yipei Wang,† Yingxin Xu,† Zongyin Yang,‡ and Limin Tong*,† †

State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, United Kingdom S Supporting Information *

ABSTRACT: On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a singlenanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy. KEYWORDS: Semiconductor nanowires, second-harmonic generation, optical correlator, ultrashort pulse measurement, femtojoule pulses

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In this paper, we demonstrate a single CdTe nanowire optical correlator for ultrashort pulses characterization with input pulse energy down to ∼2 fJ/pulse, which is more than 20folds lower than that of the CdS nanowire correlator.16 The significant reduction in pulse energy originates from the much higher second-order nonlinearity and refractive index of the CdTe nanowire than the CdS nanowire. The CdTe nanowires were synthesized by a thermal evaporation process.29−31 Electron microscope characterization confirms that they have a uniform diameter (Figure 1a), an excellent smooth surface (Figure 1b), and a hexagonal symmetry structure (inset of Figure 1b). The measured optical loss for waveguiding 1064 nm light is about 6.4 dB/mm (see Supporting Information), which is typical for optical loss of subwavelength-diameter semiconductor nanowires.32,33 Highresolution transmission electron microscope image (Figure 1c) and electron diffraction pattern (Figure 1d) confirm that the nanowires are single crystalline with wurtzite crystal structure. Compared with nanowaveguide material (e.g., CdS) used in previous work, the CdTe has a much larger second-order nonlinearity (109 pm/V for CdTe versus 19.1 pm/V for CdS at 1064 nm wavelength).16,34−36 Also, the higher refractive index (2.8 for CdTe versus 2.3 for CdS at 1064 nm wavelength), makes it possible to offer higher optical confinement for smaller

ltrashort optical pulses have broad applications spanning from optical communication, fluorescence microscopy to biological medicine.1−4 So far, a variety of techniques have been developed for ultrashort pulse measurement, such as standard autocorrelators, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric-field reconstruction (SPIDER).5−7 While most of these techniques are based on optical nonlinear effects in bulk crystals, they typically require elaborate setups with precision optical stages that takes a fair amount of space.1,8 Recent interest in developing integrable and energy-efficient photonic circuitry has created a compelling need for miniaturization of optical devices.9−11 For characterizing ultrashort pulses within ultracompact footprints, transversal second harmonic generation (TSHG) and third harmonic generation (THG) in optical waveguide have been attracting continuous attentions,12−17 owing to their attractive features such as high scalability and compatibility with integrated photonic circuits and chips. Semiconductor nanowire offers an excellent platform for nanoscale nonlinear optics, relying on its high refractive index for tight optical confinement18,19 and large optical nonlinearity.20−25 Recently, based on TSHG, optical correlation using a single CdS nanowire has been successfully demonstrated with minimum input energy of about 50 fJ/pulse,16 which is comparable to conventional correlators using bulk crystals,8 while optical correlation with lower pulse energy is highly desired for broad applications including optical interconnection and communication.26−28 © XXXX American Chemical Society

Received: March 1, 2016 Revised: July 1, 2016

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DOI: 10.1021/acs.nanolett.6b00893 Nano Lett. XXXX, XXX, XXX−XXX

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can be detected in the direction perpendicular to the nanowire axis, as required by the wave-vector matching condition.14−16 The signal was first collected by a 50× objective (NA = 0.55) and then directed to an EM-CCD (iXon DU-897, Andor Inc.) and a spectrometer (iHR550, HORIBA Inc.) after passing through a 1064 nm blocking notch filter (Edmund, Inc.). To verify the validity of the system, we launched a 10 mW 1064 nm continuous wave (CW) light into a 800 nm diameter CdTe nanowire from both ends and captured the green TSH emission along the whole length of the nanowire, as shown in Figure 2c. The nonuniform emission pattern may be caused by surface irregularity of the nanowire and high order waveguiding modes in the nanowire.38 For reference, Figure 2d gives comparison between TSHG of a CdTe nanowire and a CdS nanowire with similar diameter, which were connected in series on MgF2 substrate. The much stronger TSHG of CdTe nanowire than CdS nanowire (4.3 times) shows much higher second-order nonlinearity. To operate the nanowire as an optical correlator, we used 1064 nm ultrashort pulses generated from a Ti:sapphire femtosecond laser source (Coherent, Inc., pulse width, 200 fs; repetition rate, 76.5 MHz) as the input signal. The time delay between the two counterpropagating pulses was adjusted by the free-space optical path to approximately be zero when they meet each other around the central part of the nanowire. Considering that the time interval between the two neighboring pulses is about 13 ns (about 4 m in free-space interval), there is only one pair of pulses that overlaps in the nanowire at a time. The optical micrographs of the TSH emission (taken by the EMCCD) with different average input power are shown in Figure 3a−c. From the Gaussian fits of the TSHG signal in spatial intensity (Figure 3d−f), we measured that the full width at half-maximum (fwhm) of all the three cases (average input power of 15, 1.5, and 0.16 μW, respectively) were about 40 μm. With a calculated group index of 3.47 for an 800 nm diameter CdTe nanowire at 1064 nm, it corresponds to a pulse duration of 601 fs. Considering the measured input pulse duration to be about 200 fs in free space, a 401 fs pulse broadening was caused by optical fiber and fiber tapers.38,39 It is worth to mention that because of the high nonlinearity and tight optical confinement of the CdTe nanowire the input power can be reduced to as low as 0.16 μW (Figure 3c, f), corresponding to a pulse energy of about 2.1 fJ/pulse, which is much lower than that of CdS nanowire correlator reported previously (corresponding to pulse energy of 50 fJ/pulse, or input power of 3.8 μW).16 Considering the pulse width is 601 fs in the CdTe nanowire and 200 fs in the CdS nanowire, the peak pump power is 3.41 and 250 mW respectively16 (see Supporting Information). Within the broad spectral range, except for the two peaks corresponding to the fundamental pumping light and the TSHG signal there are no other observable peaks. By calculating the calibrated intensity ratio between the TSHG signal and the pumping light, we obtained a TSHG efficiency of about 10−10 for peak input power of 30 mW or peak intensity of about 8.5 × 106 W/cm2 (see Supporting Information). To directly measure the group index of the pumping light guided along the CdTe nanowire, we changed the optical path difference between the two counterpropagating beams by tuning the free-space optical path and measured the spatial shift of the TSHG emission spot with respect to the free-space optical path difference. As shown in Figure 4, 1040 nm counterpropagated pulses (pulse width of about 100 fs in free space) were coupled into an 800 nm diameter CdTe nanowire,

Figure 1. (a,b) Scanning electron microscope image of CdTe nanowires confirm they have smooth surface and uniform diameter (scale bar in (a,b) is 1 μm, scale bar in inset is 200 nm). (c) Highresolution transmission electron microscope image (scale bar, 5 nm) and (d) electron diffraction pattern of a CdTe nanowire reveals it has single-crystal structure.

effective mode area and higher flexibility for using in liquid environment or on-chip chemicals.16 The experiment setup is schematically illustrated in Figure 2a. The CdTe nanowire was suspended across a slit of two

Figure 2. (a) Schematic diagram of the experiment. (b) Bright-field optical microscope image of a CdTe nanowire suspended across a slit of two MgF2 substrates, and the light was coupled into it through fiber tapers at both ends. (c) Optical microscope image of the TSH light generated along the whole nanowire with 10 mW 1064 nm CW light input (scale bar, 50 μm). (d) Optical microscope image of TSHG from series-connected CdTe nanowire and CdS nanowire on a MgF2 substrate taken by EM-CCD (scale bar, 50 μm).

MgF2 substrates to avoid the interaction of the propagating pulses along the nanowire with environmental substrate (Figure 2b).16 The input beam was first coupled into a standard optical fiber (SMF-28, Corning Inc.) and split into two and then coupled into both ends of a CdTe nanowire using fiber-taperassisted evanescent coupling.32,37 The coherent length of TSHG caused by phase mismatch was obtained as 1770 nm, which is larger than the diameter of nanowire (of 800 nm) used here (see Supporting Information). Coupling efficiencies between optical components were adjusted carefully to balance the output power from two fiber tapers. When the counterpropagating pulses overlap in the nanowire, the TSHG signal B

DOI: 10.1021/acs.nanolett.6b00893 Nano Lett. XXXX, XXX, XXX−XXX

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from on-chip optical communication, biophotonics, to ultracompact laser spectroscopy. Considering that the low-loss transmission window of the CdTe crystal extends from nearinfrared (∼1 μm) to about 30 μm,40 this kind of correlator in principle can be operated beyond mid-infrared spectral range, providing that larger diameter nanowires and mid-infrared input system are used.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00893. (1) Fabrication and characterization of fiber tapers; (2) determination of input power; (3) coherent length of TSHG; (4) second-harmonic conversion efficiency; (5) optical loss of the CdTe nanowire; (6) comparison between TSHG from series-coupled CdTe and CdS nanowires; (7) polarization behavior; (8) dispersion property; (9) limitations of the device.(PDF)

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Figure 3. Optical microscope images taken by an EM-CCD with (a− c) 15, 1.5, and 0.16 μW input (the peak-to-background ratio of the SH emission is 84, 14, and 1.5, respectively) and (d−f) corresponding intensity profiles show that calculated pulse width does not changed with input power (fwhm is 39.9, 37.6, and 41.8 μm, respectively). (g) Measured spectrum of the SH signal after passing through a 1064 nm notch filter (there is a weak peak at 1064 nm since exciting light has not been filtered totally). The inset shows measured spectrum of the signal without the filter (scale bar, 20 μm).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Huakang Yu and Xing Lin for their helpful discussion. This research was supported by the National Key Basic Research Program of China (2013CB328703) and National Natural Science Foundation of China (61475140).

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Figure 4. Group index measurement. (a−c) Optical microscope images of SHG spots taken by EM-CCD with 1040 nm wavelength pulses launched. The relative delay in the free-space optical path is 100 μm between each picture. (d−f) Corresponding intensity profiles of the TSHG traces (scale bar, 50 μm).

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DOI: 10.1021/acs.nanolett.6b00893 Nano Lett. XXXX, XXX, XXX−XXX