We fabricated semiconductor cadmium selenide (CdSe) quantum dots via the pulsed laser ablation in the superfluid helium. The fabricated quantum dots showed blue-shifted fluorescence due to the strong quantum confinement effect. The fluorescence blinking phenomena were also observed indicating the single photon emission process. Our proposed scheme is a simple, robust, and reliable method to fabricate quantum dots and to introduce the highly fluorescence nanoparticles into superfluid helium appropriate for resonant optical manipulation and nano-tracers for liquid helium visualization.
Superfluid helium having extremely low temperature, negligibly small viscosity, and huge thermal conductivity provides us a unique opportunity to generate a novel cryogenic space for the fabrication of nanostructures and the manipulation of their motion. Here we fabricated metallic nano- and micro-particles by laser ablation in superfluid helium and selectively trapped superconducting particles with a quadrupole magnetic field utilizing perfect diamagnetism caused by Meissner effect. We also discuss the size dependence of the superconducting transition temperatures of the trapped metallic particles by changing the temperature of liquid helium.
We fabricated semiconductor ZnO microspheres via the pulsed laser ablation in the superfluid helium. The scanning
electron microscope observation revealed the high sphericity and smooth surface. We also observed whispering gallery
mode resonances, the electromagnetic eigenmode resonances within the microspheres, in the cathodoluminescence
spectrum, verifying the high symmetry of the fabricated microspheres. Further, we cross-sectioned the microspheres with
using focused ion beam. The scanning electron microscope observation of the cross section uncovers the existence of
small holes within the microspheres. The inner structure examination helps us to understand the microscopic mechanism
of our fabrication method.
We demonstrate the arbitrary control of the carrier-envelope phase of intense few-cycle THz pulses by using a simple passive component with high transmission efficiency based on a parallel metal plate waveguide. In this component, the carrier-envelope phase is altered by using the difference between the group and phase velocities. We demonstrate pulseshape- dependent nonlinear spectroscopy using these passive optics for Ge:Sb, where strong transitions between the shallow acceptor levels are located at 2.0 THz.
One of the most important techniques in modern optical science is the generation of phase-locked pulses. We review two different approaches to achieve the broadband generation and detection: photoconductive antenna and air-plasma method, and show the application to spectroscopy. We investigated dependences of the detection sensitivity on the growth and annealing conditions of antenna substrate, antenna structure, and the gate pulse duration. We successfully generated ultra-broadband phase-locked pulses in the terahertz and infrared regions (up to ∼200 THz) using a combination of organic nonlinear crystals and 5-fs ultrashort laser pulses, which is directly detected by an optimized photoconductive antenna. With a combination of air plasma and intense 10-fs pulses, we also achieved the generation and detection of ultra-broadband phase-locked pulses continuously from the terahertz region to the near-infrared region. The methods are applied to the spectroscopy of superconducting gaps. Our results demonstrate that the broadband phase-locked pulses can easily be generated and detected without explicit carrier envelope phase stabilization, and can be used for broadband spectroscopy.
We succeeded in fabricating ZnO microspheres with high sphericity by laser ablation in superfluid helium. Such
microspheres enable efficient lasing in the whole visible region due to defects with a CW laser at room temperature. The
lasing threshold is found to be around 100 W/cm2. This value is much smaller than those of the recent reports on the
lasing in ZnO microwire. Cathodoluminescence of single ZnO microspheres was also measured.
We have experimentally demonstrated purely optical manipulation of wide-gap semiconductor CuCl quantum dots in
superfluid helium. The superfluidity provides an ideal cryogenic frictionless environment for the manipulation. In order
to introduce the quantum dots into liquid helium, small particles of CuCl with a broad size-distribution ranging from 10
nm to 10 &mgr;m in radius have been fabricated from a bulk sample by laser ablation in a helium cryostat. We irradiated
these particles with laser light covering the excitonic resonance levels of the quantum dots smaller than 50 nm to push
them by using resonant radiation force. As a result, we have found that many quantum dots of which sizes range from 10
to 50 nm were transported and sorted over a macroscopic distance, ~1 cm. Importantly, the excitonic resonance condition
was crucial for this optical manipulation. The result means that the resonant radiation force for the quantum dots is much
stronger than the gravitational force. Feasibility of size-selective manipulation is also discussed.