Standing electron plasma wave mechanism of void array formation inside glass by femtosecond laser irradiation

We investigate the mechanism of formation of periodic void arrays inside fused silica and BK7 glass irradiated by a tightly focused femtosecond (fs) laser beam. Our results show that the period of each void array is not uniform along the laser propagation direction, and the average period of the void array decreases with increasing pulse number and pulse energy. We propose a mechanism in which a standing electron plasma wave created by the interference of a fs-laser-driven electron wave and its reflected wave is responsible for the formation of the periodic void arrays.

Cavity Optomechanics at Millikelvin Temperatures

The field of cavity optomechanics, which concerns the coupling of a mechanical object's motion to the electromagnetic field of a high finesse cavity, allows for exquisitely sensitive measurements of mechanical motion, from large-scale gravitational wave detection to microscale accelerometers. Moreover, it provides a potential means to control and engineer the state of a macroscopic mechanical object at the quantum level, provided one can realize sufficiently strong interaction strengths relative to the ambient thermal noise. Recent experiments utilizing the optomechanical interaction to cool mechanical resonators to their motional quantum ground state allow for a variety of quantum engineering applications, including preparation of non-classical mechanical states and coherent optical to microwave conversion. Optomechanical crystals (OMCs), in which bandgaps for both optical and mechanical waves can be introduced through patterning of a material, provide one particularly attractive means for realizing strong interactions between high-frequency mechanical resonators and near-infrared light. Beyond the usual paradigm of cavity optomechanics involving isolated single mechanical elements, OMCs can also be fashioned into planar circuits for photons and phonons, and arrays of optomechanical elements can be interconnected via optical and acoustic waveguides. Such coupled OMC arrays have been proposed as a way to realize quantum optomechanical memories, nanomechanical circuits for continuous variable quantum information processing and phononic quantum networks, and as a platform for engineering and studying quantum many-body physics of optomechanical meta-materials.

However, while ground state occupancies (that is, average phonon occupancies less than one) have been achieved in OMC cavities utilizing laser cooling techniques, parasitic absorption and the concomitant degradation of the mechanical quality factor fundamentally limit this approach. On the other hand, the high mechanical frequency of these systems allows for the possibility of using a dilution refrigerator to simultaneously achieve low thermal occupancy and long mechanical coherence time by passively cooling the device to the millikelvin regime. This thesis describes efforts to realize the measurement of OMC cavities inside a dilution refrigerator, including the development of fridge-compatible optical coupling schemes and the characterization of the heating dynamics of the mechanical resonator at sub-kelvin temperatures.

We will begin by summarizing the theoretical framework used to describe cavity optomechanical systems, as well as a handful of the quantum applications envisioned for such devices. Then, we will present background on the design of the nanobeam OMC cavities used for this work, along with details of the design and characterization of tapered fiber couplers for optical coupling inside the fridge. Finally, we will present measurements of the devices at fridge base temperatures of T_f = 10 mK, using both heterodyne spectroscopy and time-resolved sideband photon counting, as well as detailed analysis of the prospects for future quantum applications based on the observed optically-induced heating.

Fiber-optic integration and efficient detection schemes for optomechanical resonators

With the advent of the laser in the year 1960, the field of optics experienced a renaissance from what was considered to be a dull, solved subject to an active area of development, with applications and discoveries which are yet to be exhausted 55 years later. Light is now nearly ubiquitous not only in cutting-edge research in physics, chemistry, and biology, but also in modern technology and infrastructure. One quality of light, that of the imparted radiation pressure force upon reflection from an object, has attracted intense interest from researchers seeking to precisely monitor and control the motional degrees of freedom of an object using light. These optomechanical interactions have inspired myriad proposals, ranging from quantum memories and transducers in quantum information networks to precision metrology of classical forces. Alongside advances in micro- and nano-fabrication, the burgeoning field of optomechanics has yielded a class of highly engineered systems designed to produce strong interactions between light and motion.

Optomechanical crystals are one such system in which the patterning of periodic holes in thin dielectric films traps both light and sound waves to a micro-scale volume. These devices feature strong radiation pressure coupling between high-quality optical cavity modes and internal nanomechanical resonances. Whether for applications in the quantum or classical domain, the utility of optomechanical crystals hinges on the degree to which light radiating from the device, having interacted with mechanical motion, can be collected and detected in an experimental apparatus consisting of conventional optical components such as lenses and optical fibers. While several efficient methods of optical coupling exist to meet this task, most are unsuitable for the cryogenic or vacuum integration required for many applications. The first portion of this dissertation will detail the development of robust and efficient methods of optically coupling optomechanical resonators to optical fibers, with an emphasis on fabrication processes and optical characterization.

I will then proceed to describe a few experiments enabled by the fiber couplers. The first studies the performance of an optomechanical resonator as a precise sensor for continuous position measurement. The sensitivity of the measurement, limited by the detection efficiency of intracavity photons, is compared to the standard quantum limit imposed by the quantum properties of the laser probe light. The added noise of the measurement is seen to fall within a factor of 3 of the standard quantum limit, representing an order of magnitude improvement over previous experiments utilizing optomechanical crystals, and matching the performance of similar measurements in the microwave domain.

The next experiment uses single photon counting to detect individual phonon emission and absorption events within the nanomechanical oscillator. The scattering of laser light from mechanical motion produces correlated photon-phonon pairs, and detection of the emitted photon corresponds to an effective phonon counting scheme. In the process of scattering, the coherence properties of the mechanical oscillation are mapped onto the reflected light. Intensity interferometry of the reflected light then allows measurement of the temporal coherence of the acoustic field. These correlations are measured for a range of experimental conditions, including the optomechanical amplification of the mechanics to a self-oscillation regime, and comparisons are drawn to a laser system for phonons. Finally, prospects for using phonon counting and intensity interferometry to produce non-classical mechanical states are detailed following recent proposals in literature.

Spectral narrowing of negatively chirped femtosecond pulse by cross-phase modulation in a single-mode optical fiber

We demonstrate theoretically that the negatively chirped femtosecond laser pulse can be spectrally narrowed by cross-phase modulation. The new view is well Supported by numerical simulation. The negative chirp method in fibers might be useful in all optical wavelength switching applications. (c) 2005 Elsevier B.V. All rights reserved.

Stabilization of optical Fabry-Perot sensor by active feedback control of diode laser

This paper presents the design and characterization of a fiber Fabry-Perot interferometer (FFPI) acoustic wave detector with its Q point being stabilized actively. The relationship between the reflectivity of the F-P cavity facets and cavity length was theoretically analyzed, and high visibility of 100% was realized by optimized design of the F-P cavity. To prevent the drifting of the Q point, a new stabilization method by actively feedback controlling of the diode laser is proposed and demonstrated, indicating the method is simple and easy operating. Measurement shows that good tracing of Q point was effectively realized. (c) 2008 Elsevier B.V. All rights reserved.

Dynamic readout of subdiffraction-limited pit arrays with a silver superlens

Recent theoretical and experimental results suggested that the silver superlens could be constructed through controlling silver thin film thickness and preparation conditions, and applied in subdiffraction-limited optical imaging and optical lithography. In this work, we report another significant application of silver superlens-ultrahigh density optical data storage. With the silver superlens the subdiffraction-limited pit arrays on an optical disk are dynamically read out and the carrier-to-noise ratio can reach 25 dB for the thin film thickness of 46 nm. The readout laser power and readout velocity have little effect on the carrier-to-noise ratio. Additionally, in our experiment the silver thin film thickness needs to be controlled in the range from 20 to 80 nm.

On the dynamic readout of sub-diffraction-limit pit arrays with a silver thin film

Fast moving arrays of periodic sub-diffraction-limit pits were dynamically read out via a silver thin film. The mechanism of the dynamic readout is analysed and discussed in detail, both experimentally and theoretically. The analysis and experiment show that, in the course of readout, surface plasmons can be excited at the silver/air interface by the focused laser beam and amplified by the silver thin film. The surface plasmons are transmitted into the substrate/silver interface with a large enhancement. The surface waves at the substrate/silver interface are scattered by the sinusoidal pits of sub-diffraction-limit size. The scattered waves are collected by a converging lens and guided into the detector for the readout.

2W Yb-doped double-clad fiber superfluorescent source with 42 nm 3 dB bandwidth

The output spectrum of Yb-doped double-clad fiber superfluorescent source (SFS) is tailored by placing a broadband dichroic mirror in the pump end of conventional single-pass forward configuration, which constitutes double-pass forward configuration. The 3 dB bandwidth is increased from I I to 42 nm. A maximum output SFS power of 2.12 W and a slope efficiency of 43.2% are obtained. The double-clad fiber is 25 in and the pump power is adequate to saturate the fiber as far as the feedback-induced lasing appears. (c) 2004 Elsevier Ltd. All rights reserved.

Kilohertz electro-optic Q-switched Nd : YAG ceramic laser

We investigate the lasing characteristics of a laser-diode-array side-pumped electro-optic Q-switched Nd:Y3Al5O12 ceramic laser operating at 1000 Hz pulse repetition rate. Using a YAG poltcrystalline rod with Nd3+ concentration at 1 at.% as the gain medium, pumping with 808 nm laser-diode-arrays, the Q-switched laser output at 1064 nm wavelength with 23 mJ pulse energy and less than 12 ns FWHM pulse width are obtained at a pumping power of about 400 W, the slope efficiency is around 15%, the output beam divergence angle is about 1.2 mrad.

Double-cladding Yb3+ fiber for free-space optical communication

The experiment result of Nd:YVO4 laser pumped by laser diode that was amplified by double-cladding Yb3+ fiber is reported. Stable mode-locking pulses are obtained at repetition rate of 320 MHz and the output power is 15 mW. When laser power is amplified by Yb3+- doped double-cladding fiber amplifier, its power can get to 600 mW. Based on these, experiment of double-frequency is carried out, and green laser with power of 4 mW is obtained. (c) 2007 Wiley Periodicals, Inc.

Beam quality improvement of laser diode array by using off-axis external cavity

A novel off-axis external cavity is designed for laser diode array to improve the beam quality. In this external cavity, a circle aperture with variable size is used as a spatial filter. The diameter of aperture is optimized to 1.2mm and the off-axis angle of external cavity is optimized at 2.6 deg. In the optimal case, the beam parameter product (BPP) of laser diode array is reduced to 121 mm. mrad from 1050 mm. mrad with external cavity optical efficiency of 81%. (C) 2007 Optical Society of America.

A four-passed ytterbium-doped fiber amplifier

In this paper, a four-passed ytterbium-doped fiber amplifier (YDFA) is discussed. The gain and the pump and the signal light propagation characteristics of the four-passed YDFA are described. It is found that, while using a shorter length of the fiber, a four-passed fiber amplifier can realize the same output power as a single-pass fiber amplifier, and, for the same fiber lengths, a four-passed fiber amplifier offers a significantly higher power than its single-pass counterpart. (C) 2006 Elsevier Ltd. All rights reserved.

Self-organized microvoid array perpendicular to the femtosecond laser beam in CaF2 crystals

Microvoid arrays were self-organized when femtosecond laser beam was tightly focused at a fixed point inside CaF2 crystal sample. Except void array grown below the focal point which had been reported before, we found another void array grown vertical to the laser propagation direction. This result has potential application in the fabrication of integrated micro-optic elements and photonic crystals. The possible mechanism of the phenomenon was proposed and verified experimentally.

beam-shaping microstructure optical fiber

Microstructure optical fiber with uniform intensity distribution of the fundamental mode is proposed. The design guide line and characteristics of this kind fiber are demonstrated. The relationship between refractive index profile and structure parameters is investigated. The mechanism of forming uniform fundamental mode in these fibers is analyzed.

Periodic nanovoid structures via femtosecond laser irradiation

We have observed periodically aligned nanovoid structures inside a conventional borosilicate glass induced by a single femtosecond (fs) laser beam for the first time, to our knowledge. The spherical voids of nanosized diameter were aligned spontaneously with a period along the propagation direction of the laser beam. The period, the number of voids, and the whole length of the aligned void structure were controlled by changing the laser power, the pulse number, and the position of the focal point.