Yuxiang Liu

Label-free imaging of semiconducting and metallic carbon nanotubes in cells and mice using transient absorption microscopy

As interest in the potential biomedical applications of carbon nanotubes increases, there is a need for methods that can image nanotubes in live cells, tissues and animals. Although techniques such as Raman, photoacoustic and near-infrared photoluminescence imaging have been used to visualize nanotubes in biological environments, these techniques are limited because nanotubes provide only weak photoluminescence and low Raman scattering and it remains difficult to image both semiconducting and metallic nanotubes at the same time. Here, we show that transient absorption microscopy offers a label-free method to image both semiconducting and metallic single-walled carbon nanotubes in vitro and in vivo, in real time, with submicrometre resolution. By using appropriate near-infrared excitation wavelengths, we detect strong transient absorption signals with opposite phases from semiconducting and metallic nanotubes. Our method separates background signals generated by red blood cells and this allows us to follow the movement of both types of nanotubes inside cells and in the blood circulation and organs of mice without any significant damaging effects.

Electromagnetically induced transparency and wideband wavelength conversion in silicon nitride microdisk optomechanical resonators.

We demonstrate optomechanically mediated electromagnetically induced transparency and wavelength conversion in silicon nitride (Si3N4) microdisk resonators. Fabricated devices support whispering gallery optical modes with a quality factor (Q) of 10(6), and radial breathing mechanical modes with a Q=10(4) and a resonance frequency of 625 MHz, so that the system is in the resolved sideband regime. Placing a strong optical control field on the red (blue) detuned sideband of the optical mode produces coherent interference with a resonant probe beam, inducing a transparency (absorption) window for the probe. This is observed for multiple optical modes of the device, all of which couple to the same mechanical mode, and which can be widely separated in wavelength due to the large band gap of Si3N4. These properties are exploited to demonstrate frequency up-conversion and down-conversion of optical signals between the 1300 and 980 nm bands with a frequency span of 69.4 THz.

Investigation of inclined dual-fiber optical tweezers for 3D manipulation and force sensing

Optical tweezers provide a versatile tool in biological and physical researches. Optical tweezers based on optical fibers are more flexible and ready to be integrated when compared with those based on microscope objectives. In this paper, the three-dimensional (3D) trapping ability of an inclined dual-fiber optical tweezers is demonstrated. The trapping efficiency with respect to displacement is experimentally calibrated along two dimensions. The system is studied numerically using a modified ray-optics model. The spring constants obtained in the experiment are predicted by simulations. It is found both experimentally and numerically that there is a critical value for the fiber inclination angle to retain the 3D trapping ability. The inclined dual-fiber optical tweezers are demonstrated to be more robust to z-axis misalignment than the counter-propagating fiber optical tweezers, which is a special case of th former when the fiber inclination angle is 90 masculine. This inclined dual-fiber optical tweezers can serve as both a manipulator and a force sensor in integrated systems, such as microfluidic systems and lab-on-a-chip systems.

Far-field superfocusing with an optical fiber based surface plasmonic lens made of nanoscale concentric annular slits

We present experimental demonstration of light superfocusing by using an optical fiber based surface plasmonic (SP) lens with nanoscale concentric annular slits. A far-field, sub-diffraction-limit sized focus was achieved with an optical fiber based device. The performance of SP lenses with three and four annular slits was experimentally characterized. Guidelines and suggestions on designing the SP lens are provided. As a microscale device with nanoscale features, the fiber-based SP lens can provide a solution to bridging nanophotonics and conventional optics.

Wide Cantilever Stiffness Range Cavity Optomechanical Sensors for Atomic Force Microscopy

We report on progress in developing compact sensors for atomic force microscopy (AFM), in which the mechanical transducer is integrated with near-field optical readout on a single chip. The motion of a nanoscale, doubly clamped cantilever was transduced by an adjacent high quality factor silicon microdisk cavity. In particular, we show that displacement sensitivity on the order of 1 fm/(Hz)(1/2) can be achieved while the cantilever stiffness is varied over four orders of magnitude (≈0.01 N/m to ≈290 N/m). The ability to transduce both very soft and very stiff cantilevers extends the domain of applicability of this technique, potentially ranging from interrogation of microbiological samples (soft cantilevers) to imaging with high resolution (stiff cantilevers). Along with mechanical frequencies (> 250 kHz) that are much higher than those used in conventional AFM probes of similar stiffness, these results suggest that our cavity optomechanical sensors may have application in a wide variety of high-bandwidth AFM measurements.

Si3N4 optomechanical crystals in the resolved-sideband regime

We demonstrate sideband-resolved optomechanical crystals supporting 10^5 quality factor optical modes at 980 nm, coupled to  GHz frequency mechanical modes with quality factors of ≈3000. Optomechanical electromagnetically induced transparency and absorption are observed at room temperature and in atmosphere with intracavity photon numbers in excess of 10^4.

Phase modulation with micromachined resonant mirrors for low-coherence fiber-tip pressure sensors

This letter presents a simple phase modulation scheme for interrogation of low-coherence interferometry based fiber-tip pressure sensors to enable real-time monitoring and miniaturization of the entire sensor system. The key idea is to introduce a sinusoidal modulation signal and retrieve the sensing cavity length change using a simple algorithm, without resorting to any time information. In experiments, phase modulation has been achieved by using a silicon-micromachined tunable Fabry-Pérot interferometer, which is integrated with a light source and a photodiode onto a single chip. Compared with the conventional interrogation methods, this scheme possesses the merits of being less susceptible to disturbance, easy control and easy miniaturization, making it particularly suitable for sensing in constrained spaces and harsh environments.

Wireless fiber optic sensor system for strain and pressure measurements on a rotor blade

Experimental measurements of the strain and pressure of rotor blades are important for understanding the aerodynamics and dynamics of a rotorcraft. This understanding can help in solving on-blade problems as well as in designing and optimizing the blade profiles for improved aerodynamics and noise attenuation in the next generation rotorcraft. The overall goal of our research is to develop a miniature wireless optical sensor system for helicopter on-blade pressure and strain measurements. In this paper, leveraging past and current experiences with fiber optic sensor development, a proof-of- concept of fiber optic pressure/strain sensor system with wireless data acquisition and transfer capability is demonstrated. The recently developed high-speed, real-time fiber optic sensor demodulation techniques based on low coherence interferometry and phase-shifting interferometry is used. This scheme enables a Spatial Division Multiplexing configuration that consists of multiple Fabry-Perot strain and pressure sensors. Calibration of the strain and pressure sensors is carried out by using commercially available sensors as references. Spin chamber testing of the sensor system for simultaneous on-blade pressure and strain field measurements is also performed. It is expected that such a sensor system will result in enhanced robustness and performance for on-blade pressure and strain field measurements.

Multiple traps created with an inclined dual-fiber system

Multiple optical traps allow one to manipulate multiple particles simultaneously, to characterize interactions in colloidal systems, and to assemble particles into complex structures. Most of the current multiple optical traps are realized with microscope objective-based optical tweezers, which are bulky in size. In this article, we created multiple optical traps with an inclined dual-fiber optical tweezers setup. One 3D trap and two 2D traps were formed at different vertical levels with adjustable separations and positions. We demonstrated that this fiber-based trapping system can be used as a simple block to perform multiple functions, such as particle grouping, separation, and stacking. Moreover, we found that multiple beads can be trapped and stacked up in three dimensions. Compared with those formed with objective-based optical tweezers, the multiple traps presented here are small in size and independent of the objective or the substrate, and hence hold the promise to be integrated in microfluidic systems. This fiber-based multiple traps can be used for on-chip parallel manipulation, particle separation, and characterization of interactions of colloidal and biological systems.

Subwavelength optical trapping with a fiber-based surface plasmonic lens

We demonstrate three-dimensional (3D) optical trapping of subwavelength polystyrene beads and bacteria with a surface plasmonic lens fabricated on the endface of an optical fiber. To the best of our knowledge, this is the first demonstration of 3D trapping of subwavelength particles with single fiber optical tweezers. The optical power for achieving a stable 3D trap is smaller compared with conventional optical tweezers, indicating a stronger trap. Compared with surface plasmon tweezers, the trap enabled by our fiber tweezers is located ≈6 wavelengths away from the fiber endface, reducing thermal effects due to the metal absorption and preventing physical contact with the trapped objects.