Recent Publications (since 2009)

December 29, 2010

Characterizing ultrashort optical pulses using second-order nonlinear nanoprobes

H. Li, Z. Zhang, Q. Xu, K. Shi, Y. Jia, B. Zhang, Y. Xu, and Z. Liu, “Characterizing ultrashort optical pulses using second-order nonlinear nanoprobes,” Appl. Phys. Lett. 97, 261108 (2010).

We report a second-order nonlinear nanoprobe for characterizing ultrafast optical near fields. The proposed nanoprobe comprises second harmonic nanocrystals attached to a carbon nanotube, which is in turn attached to a silica fiber taper. We demonstrate in situ pulse characterization directly in the air core of a photonic crystal fiber. Further, it is shown that nanoprobes containing a single nanocrystal in the tip of the nanotube can be fabricated by auxiliary focused ion beam nanomilling. These results indicate that the proposed nanoprobe can open an avenue for probing the evolution of ultrafast optical fields in complex three-dimensional micro- or nanostructures.

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December 2, 2010

Supercontinuum trap stiffness measurement using a confocal approach

Zhe Zhang, Haifeng Li, Peng Li, Kebin Shi, Perry Edwards, Fiorenzo Omenetto, Mark Cronin-Golomb, Guizhong Zhang, and Zhiwen Liu, "Supercontinuum trap stiffness measurement using a confocal approach," Opt. Express 18, 26499-26504 (2010).

We report a novel method for characterizing the stiffness of white light supercontinuum tweezers, in which the nonlinear photonic crystal fiber used for supercontinuum generation is also utilized as an effective confocal pinhole to track the motion of a trapped bead and as a scan head to realize rapid scanning of the optical trap. By measuring the phase of the bead’s motion in following the trap, a lateral stiffness value of about 7.9 μN/m was obtained with supercontinumm power of about 75mW. Our technique can potentially allow for trap stiffness calibration along an arbitrary direction in three dimensions.

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October 27, 2010

Chromatic second harmonic imaging

Chuan Yang, Kebin Shi, Haifeng Li, Qian Xu, Venkatraman Gopalan, and Zhiwen Liu, Optics Express 18, 23837-23843 (2010).

We report a non-axial-scanning second harmonic imaging technique, in which the chromatic aberration of a Fresnel lens is exploited to focus different wavelengths of a fundamental beam into different axial positions to effectively realize axial scanning. Since the second harmonic signals at different axial positions are generated by different fundamental wavelengths and hence accordingly have different center wavelengths, they can be resolved and detected in parallel by using a spectrometer without axial mechanical scanning. We have demonstrated a system capable of achieving about 8 μm effective axial scanning range. Proof-of-concept imaging results are also presented.

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October 26, 2010

Demonstration of a PDMS based hybrid grating and Fresnel lens (G-Fresnel) device

Chuan Yang, Kebin Shi, Perry Edwards, and Zhiwen Liu, Optics Express 18, 23529-23534 (2010).

A hybrid device that we term G-Fresnel (i.e., grating and Fresnel) is demonstrated. It fuses the functions of a grating and a Fresnel lens into a single device. We have fabricated the G-Fresnel device by using polydimethylsiloxane (PDMS) based soft lithography. Three-dimensional surface profilometry has been performed to examine the device quality. We have also conducted optical characterizations to confirm its dual focusing and dispersing properties. The G-Fresnel can be useful for the development of miniature optical spectrometers as well as emerging optofluidic applications.

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April 5, 2010

Inline holographic coherent anti-Stokes Raman microscopy

Qian Xu, Kebin Shi, Haifeng Li, Kerkil Choi, Ryoichi Horisaki, David Brady, Demetri Psaltis, and Zhiwen Liu, Optics Express 18, 8213-8219 (2010).

We demonstrate a simple approach for inline holographic coherent anti-Stokes Raman scattering (CARS) microscopy, in which a layer of uniform nonlinear medium is placed in front of a specimen to be imaged. The reference wave created by four-wave mixing in the nonlinear medium can interfere with the CARS signal generated in the specimen to result in an inline hologram. We experimentally and theoretically investigate the inline CARS holography and show that it has chemical selectivity and can allow for three-dimensional imaging.

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March 4, 2010

Coherent anti-Stokes Raman holography for chemically selective single-shot non-scanning 3D imaging

Kebin Shi, Haifeng Li, Qian Xu, Demetri Psaltis and Zhiwen Liu, Physical Review Letters 104, 093902 (2010).

We propose and develop a new way for nonscanning coherent anti-Stokes Raman scattering (CARS) imaging by using CARS holography, which can capture, at a single shot, both the amplitude and the phase of a coherent Raman image. It hence allows for chemically selective three-dimensional imaging at a laser pulse duration limited speed, which is not possible to achieve by using any of the existing laser scanning based Raman imaging techniques. We have demonstrated single-shot (pulse duration: ~500 ns) holographic CARS imaging of microspheres. Our results show that coherent anti-Stokes Raman holography can open a new path for nonscanning label-free imaging.

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August 4, 2010

Development of functional nanoprobes for optical near-field characterization

Y. Jia, H. Li, B. Zhang, X. Wei, Z. Zhang, Z. Liu and Y. Xu, Journal of Physics: Condensed Matter, vol. 22, no. 33, p. 334218, 2010.

Subwavelength optical imaging can be accomplished by scanning a nanoscale aperture or a nanoprobe containing a locally defined nanoscale optical source. Currently, most such methods, including various implementations of near-field scanning optical microscopy (NSOM), form near-field images by measuring the intensity of optical signals generated by optical transmission, scattering or fluorescence. Here we report the development of a nanoprobe that can extend NSOM functionalities by focusing on the dynamical aspects of light emission (such as fluorescence lifetime measurement) and nonlinear optical processes (such as second harmonic generation). Our nanoprobes consist of a silica fiber taper, a single nanowire or a nanotube, and appropriate functional nano-optical structures. The fabrication, characterization and potential applications of such nanoprobes are discussed.

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February 26, 2010

Optical multistability in a silicon-core silica-cladding fiber

Ivan A. Temnykh, Neil F. Baril, Zhiwen Liu, John V. Badding, and Venkatraman Gopalan, "Optical multistability in a silicon-core silica-cladding fiber," Opt. Express 18, 5305-5313 (2010).

We fabricate a novel silicon-core silica-cladding optical fiber using high pressure chemical fluid deposition and investigate optical transmission characteristics at the telecommunications wavelength of 1550 nm. High thermo-optic and thermal expansion coefficients of silicon give rise to a thermal phase shift of 6.3 rad/K in a 4 mm-long, 6.9 µm diameter fiber acting as a Fabry-Perot resonator. Using both power and wavelength modulation, we observe all-optical bistability at a low threshold power of 15 mW, featuring intensity transitions of 1.4 dB occurring over 0.1 pm change in wavelength. Threshold powers for higher-order multistable states are predicted. Tristability is experimentally confirmed.

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January 11, 2010

Development of a Nonlinear Nanoprobe for Interferometric Autocorrelation Based Characterization of Ultrashort Optical Pulses

Haifeng Li, Yaoshun Jia, Qian Xu, Kebin Shi, Jian Wu, Peter C. Eklund, Yong Xu and Zhiwen Liu, Appl. Phys. Lett. 96, 021103 (2010).

Near-field scanning can achieve nanoscale resolution while ultrashort pulse diagnostic tools can characterize femtosecond pulses. Yet currently it is still challenging to nonperturbatively characterize the near field of an ultrashort optical pulse with nanofemtoscale spatiotemporal resolution. To address this challenge, we propose to develop a nonlinear nanoprobe composed of a silica fiber taper, a nanowire, and nonlinear fluorescent spheres. Using such a nanoprobe, we also report proof-of-principle characterization of femtosecond optical pulse through interferometric autocorrelation measurement.

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November 17, 2009

Electro-optic laser beam shaping by patterned ferroelectric domains

M. Krishnamurthi, P. Li, A. Singh, J. G. Thomas, T. M. Lehecka, Z. Liu, and V. Gopalan, “Electro-optic laser beam shaping by patterned ferroelectric domains,” Appl. Phys. Lett. 95, 202902 (2009).   

We demonstrate laser beam shaping by utilizing the linear electro-optic effect in patterned ferroelectric domains in lithium tantalate. The phase function essential for the conversion of a 633 nm wavelength laser light from a Gaussian to a flat-top intensity profile was computed using the Gerchberg–Saxton algorithm. The corresponding ferroelectric domain pattern required to induce the equivalent phase function was fabricated on a lithium tantalate crystal, and the performance compared with design specifications.

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June 22, 2009

Chromatic two-photon excitation fluorescence imaging

Qian Xu, Kebin Shi, Shizhuo Yin and Zhiwen Liu, Journal of Microscopy 235, 79-83 (2009).   

We report on a chromatic axial scanning method for two-photon excitation fluorescence imaging. Effective axial scanning is achieved by incorporating a Fresnel lens in the system, which has large chromatic aberration and can therefore focus the excitation beam to different axial positions depending on its wavelength. We experimentally demonstrated this technique and used it to image the cross-section of fluorescent microspheres.

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February 2, 2009

Large refractive index modulation tilted holographic planar structured grating based on dichromated gelatin

J. Zheng, S. Yin, Z. Liu, J. Yao, J. H. Kim, and C. Yang, “Large refractive index modulation tilted holographic planar structured grating based on dichromated gelatin,” Optics Communications 282, 1762-1766 (2009).   

A new type grating, tilted holographic planar structured dichromated gelatin (DCG) grating with large refractive index modulation, is presented in this paper. The grating is inscribed by superposing two coherent laser beams from the front and the thin side surfaces, respectively. The relationship between the tilted angle and the three types of coupling modes is analyzed when coupling with fiber. Besides, the couple mode theory and experimental investigation are applied on major characteristics analysis of the radiation mode DCG grating of tilted angle 45°. Due to the tilted planar structured grating has large refractive index modulation, it not only offers convenience for connecting (or integrating) with other fiber or waveguide-based devices, but also provide some special characteristics such as low wavelength selectivity (which is required for some applications such as multiple spectrum biomedical imaging). Therefore, this unique grating device has a wide application in optical fields such as for the waveguide-based, parallel, multiple spectrum imaging system.

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The Pennsylvania State University    Department of Electrical Engineering