Quantum dynamics of optical phonons generated by optical excitation of a quantum dot

The study of the fundamental properties of phonons is crucial to understand their role in applications in quantum information science, where the active use of phonons is currently highly debated. A genuine quantum phenomenon associated with the fluctuation properties of phonons is squeezing, which is achieved when the fluctuations of a certain variable drop below their respective vacuum values. We consider a semiconductor quantum dot (QD) in which the exciton is coupled to phonons. We review the fluctuation properties of the phonons, which are generated by optical manipulation of the QD, in the limiting case of ultra-short pulses. Then, we discuss the phonon properties for an excitation with finite pulses. Within a generating function formalism, we calculate the corresponding fluctuation properties of the phonons and show that phonon squeezing can be achieved by the optical manipulation of the QD exciton for certain conditions even for a single-pulse excitation where neither for short nor for long pulses squeezing occurs. To explain the occurrence of squeezing, we employ a Wigner function picture providing a detailed understanding of the induced quantum dynamics.     

Single Gold Nanorod Detection Using Confocal Light Absorption and Scattering Spectroscopy

Gold nanorods have the potential to be employed as extremely bright molecular marker labels for fluorescence, absorption, or scattering imaging of living tissue. However, samples containing a large number of gold nanorods usually exhibit relatively wide spectral lines. This linewidth limits the use of the nanorods as effective molecular labels, since it would be rather difficult to image several types of nanorod markers simultaneously. In addition, the observed linewidth does not agree well with theoretical calculations, which predict significantly narrower absorption and scattering lines. The discrepancy could be explained by apparent broadening because of the contribution of nanorods with various sizes and aspect ratios. We measured native scattering spectra of single gold nanorods with the confocal light absorption and scattering spectroscopy system, and found that single gold nanorods have a narrow spectrum as predicted by the theory, which suggests that nanorod-based molecular markers with controlled narrow aspect ratios, and to a lesser degree size distributions, should provide spectral lines sufficiently narrow for effective biomedical imaging.     

Inhomogeneous exciton broadening and mean free path in In1-xGaxAsyP1-y-InP heterostructures

Absorption and electroabsorption spectra are investigated at low temperature to derive broadening parameters of excitonic absorption edges in the alloy crystals. A strong increase of the inhomogeneous linewidth of excitons is observed for quaternary material which exceeds significantly the width expected from compositional fluctuations. The linewidth decreases again for phosphorous rich samples. Franz-Keldysh oscillations above the absorption edge contain information on the coherence length of band states. They show a corresponding decrease of the mean free path from 150 nm for ternary material to 70 nm for quaternary material     

Excitonic photoluminescence in CuAlS2 powders

Copper aluminum disulfide (CuAlS₂) powders were synthesized in the evacuated ampoule. The luminescence properties of obtained powders were evaluated by photoluminescence (PL) at various temperatures. The visible emission peaks at 2.16 and 1.90 eV, and weak ultraviolet emission peak at 3.47 eV were obtained in the room temperature PL. The temperature dependence of the PL revealed that the ultraviolet emission at room temperature was considered to be free-exciton. Several emission peaks related to free-exciton, bound-exciton and phonons were observed in the high-resolution PL at 12 K, reflecting that the obtained powders had high crystallinity. Emission peaks at 3.459 and 3.300 eV for CuAlS₂ were observed for the first time. The former emission might be originated from the bound exciton and the latter from the donor–acceptor pair recombination.     

Numerical approach to field fluctuations and spectral lineshape in InGaAsP laser diodes

This paper reports on a numerical approach to model the field fluctuations, spectral lineshape and linewidth in semiconductor lasers. The approach is based on numerical solution of the laser rate equations augmented by Langevin noise sources that account for fluctuations in the lasing field. The paper newly examines contributions of intensity and frequency noises to the spectral characteristics of the lineshape and its linewidth over a wide range of injection current. The model is applied to InGaAsP lasers emitting in a wavelength of 1.5 μm as the most representative light sources in optical communication systems. Accuracy of approximated models of calculating linewidth from low‐frequency components of the frequency noise is checked. Effect of non‐linear gain suppression on the lineshape is also explored. The spectral lineshape promotes and the linewidth decreases as the laser is injected far from the near‐threshold region. The lineshape changes mainly with changes in the frequency noise spectrum while the linewidth is sensitive to variation in the low‐frequency levels of both intensity and frequency noises. Copyright © 2004 John Wiley & Sons, Ltd.     

Spectral Linewidth Reduction in Widely Wavelength Tunable DFB Laser Array

We have developed an L-band tunable distributed feedback laser array (TLA) with a new design to reduce the spectral linewidth. A wide wavelength tuning range of $sim$40 nm is obtained with a high fiber output power of 20 mW and a high side-mode suppression ratio of ≫50 dB in the TLA module. A narrow linewidth of less than 580 kHz is achieved over the entire tuning range. Furthermore, we investigated the causes of linewidth variation. We found that a TLA with a longer cavity is more tolerant to external feedback, which reduces the variation in linewidth.      

An ultra-line-narrowed high-power F/sub 2/ laser for dioptric design microlithography exposure tools

An ultra-line-narrowed high-power and high-repetition rate F/sub 2/ laser system has been developed for 157-nm microlithography exposure tools with dioptric projection design. The injection locked system (ILS) consists of a low-power seed laser with ultra-narrow spectral linewidth and a high-gain amplifier. More than 25-W output power, a spectral linewidth below 0.2 pm full-width at half-maximum (FWHM) and an energy stability (3-sigma) below 10% have been obtained at a 5-kHz repetition rate and for a delay time range between the two laser stages of about 15 ns. Directly compared with a master oscillator power amplifier system, the ILS had a better performance related to output energy, energy stability, and laser pulse duration.     

High-power 1.55-μm mass-transport-grating DFB lasers forexternally modulated systems

High-output-power operation of 1.55-μm-wavelength distributed-feedback (DFB) lasers with a novel mass-transport grating (MTG) structure which is composed of InAsP buried with InP are reported. To improve high output power characteristics, we have investigated the influence of the width of the active layer on the light output power and the spectral linewidth at high injection current. It is confirmed that the increase of the active layer width is effective to realize high output power and to reduce the linewidth power product. The fabricated lasers show high single-longitudinal-mode output power of 180 mW, which is the highest value reported for 1.55-μm DFB lasers. They also exhibit narrow spectral linewidths less than 0.3 MHz and low noise characteristics of -159 dB/Hz. Moreover, we have obtained the mean time to failure of longer than 10⁵ h with a lifetime test over 200 h at 50°C     

Active Plasmonics: Surface Plasmon Interaction With Optical Emitters

The interaction between surface plasmons and optical emitters is fundamentally important for engineering applications, especially surface plasmon amplification and controlled spontaneous emission. We investigate these phenomena in an active planar metal-film system comprising InGaN/GaN quantum wells and a silver film. First, we present a detailed study of the propagation and amplification of surface plasmon polaritons (SPPs) at visible frequencies. In doing so, we propose a multiple quantum well structure and present quantum well gain coefficient calculations accounting for SPP polarization, line broadening due to exciton damping, and particularly, the effects of finite temperature. Second, we show that the emission of an optical emitter into various channels (surface plasmons, lossy surface waves, and free radiation) can be precisely controlled by strategically positioning the emitters. Together, these could provide a range of photonic devices (for example, surface plasmon amplifiers, nanolasers, nanoemitters, plasmonic cavities) and a foundation for the study of cavity quantum electrodynamics associated with surface plasmons.      

Degradation behavior of narrow-spectral-linewidth DFB lasers forsuper-wide-band FM conversion

The feasibility of using narrow-spectral-linewidth distributed-feedback lasers as subcarrier multiplexing video distribution sources in optical heterodyne FM converters is shown by investigating degradation modes of the lasers. There is no change in the dynamic properties, such as the FM response and the composite second-order and composite triple-beat figures, though spectral linewidth is increased by degradation. The main degradation appears as an increase in spectral linewidth due to 1/f noise caused by buried heterointerface degradation. The FM response, composite second-order, and composite triple-beat, however, hardly change in the degradation process     

Highly luminescent CdSe nanoparticles embedded in silica thin films

Thin films of luminescent CdSe nanoparticles with and without silica capping were prepared using sol-gel method. The blue shift observed in the optical absorption spectra suggested quantum confinement effect in the prepared films. The films showed photoluminescence emission in the range of 510–590 nm depending upon the particle size of CdSe particles. The emission intensity increased when CdSe particles were embedded in silica matrix. The emission intensity was found to decrease with aging for the films containing CdSe particles without silica capping when they were exposed in relatively humid air (relative humidity 80%). The films containing CdSe nanoparticles embedded in silica matrix showed more stable behavior. The emission intensity practically remained constant with aging in humid atmosphere.     

Miniature erbium:ytterbium fiber Fabry-Perot multiwavelength lasers

We demonstrate stable simultaneous lasing of up to 29 wavelengths in miniature 1- and 2-mm-long Er³⁺:Yb³⁺ fiber Fabry-Perot lasers. The wavelengths are separated by 0.8 (100 GHz) and 0.4 nm (50 GHz), respectively, corresponding to the free spectral range of the laser cavity. The number of lasing wavelengths and the power stability of the individual modes are greatly enhanced by cooling the laser in liquid nitrogen (77 K). The polarization modes and linewidth of each wavelength are measured with high resolution by heterodyning with a local oscillator. The homogeneous linewidth of the Er³⁺:Yb ³⁺ fiber at 77 K is determined to be ~0.5 nm, from spectral-hole-burning measurements, which accounts for the generation of a stable multiwavelength lasing comb with wavelength separations of 0.4 nm     

Spectral narrowing and temporal expanding of femtosecond pulses by use of quadratic nonlinear processes

A nonlinear system capable of expanding pulse with spectral narrowing, an analogy to a spatial beam expander in linear optics, is studied theoretically and experimentally. The system, with features of high efficiency and maintaining near Fourier-transform limit (FTL), is constructed by using quadratic nonlinear processes with chirped pulses. The spectral and temporal characteristics of such a pulse expander are investigated analytically and computationally. It shows that group-velocity mismatch of nonlinear crystal plays a detrimental role, which leads to a deviate operation of pulse expander from its ideal case, e.g., temporal shortening, spectral broadening, and a deviation from the FTL of the expanded pulses. The criteria for designing a near aberration-free pulse expander are given based on these analyses. As a demonstration, we experimentally expand broadband 70-fs pulses from a Ti:sapphire regenerative amplifier to narrowband 60-ps longer pulses. The conversion efficiency from pump to idler of the nonlinear pulse expander is currently limited to a few percent and can be practically improved to 10% to 20%.     

Optomechanical model of surface micromachined tunableoptoelectronic devices

Linewidth is a critical performance parameter for many optoelectronic devices. We have developed a combined optical and mechanical simulation tool and demonstrate its application to micromachined vertical-cavity tunable optoelectronic devices. The deformation of the mirror surface is calculated from the area moment method. The optical field distribution is calculated by the Fox-Li method, and the diffraction losses are estimated from second-order perturbation theory. By comparison to experimental results, we find that the deformation of the central plate is well predicted by our theory. While deformation can be a major source of linewidth broadening in MEMS tunable optoelectronic devices, it is not the primary source in our devices     

A Space Dependent Wigner Equation Including Phonon Interaction

We present a kinetic equation which is obtained after a hierarchy of approximations from the generalized Wigner function equation which accounts for interaction with phonons. The equation treats the coherent part of the transport imposed by the nanostructure potential at a rigorous quantum level. It is general enough to account for the quantum effects in the dissipative part of the transport due to the electron-phonon interaction. Numerical experiments demonstrate the effects of collisional broadening, retardation and the intra-collisional field effect. The obtained equation can be regarded as a generalization of the Levinson equation for space dependence. An analysis shows that the equation is nonlocal in the real space. This quantum effect is due to the correlation between the interaction process and the space component of the Wigner path.     

Characteristics of spectral-hole burning of InAs self-assembledquantum dots

Spectral-hole burning of InAs self-assembled quantum dots (QDs) embedded in pin-diode was observed. At 5 K, a narrow hole with width of less than 1 mm was observed and the hole depth increased as electric field increased with the writing light power of 8 mW. The hole was observed up to 40 K. The spectral hole was broadened as the writing light power increases from 8 to 20 mW. Spectral-hole width at the 8 mW was well fitted with the convolution integral of Gaussian distribution for reading light and Lorentzian distribution for absorption change taking into account homogeneous broadening of InAs QDs of ⩽80 μeV. Spectral-hole lifetime at the 8 mW was estimated to be in the order of 10^-6 s. Optical absorption spectrum of 15-stacked InAs QD structure was also observed at 77 K and 300 K     

Redshifting and broadening of quantum-well infrared photodetector'sresponse via impurity-free vacancy disordering

The partial intermixing of the well and barrier materials offers unique opportunities to shift locally the bandgap of quantum-well (QW) structures. We have demonstrated redshifting and broadening of the wavelength responses of bound-to-continuum GaAs and InP based quantum-well infrared photodetectors (QWIP's) after growth via impurity-free vacancy disordering (IFVD). A comprehensive set of experiments is conducted on QWIP's fabricated from both as-grown and multiple-quantum-well (MQW) structures. Compared to the as-grown detector, the peak spectral responses of the disordered detectors were shifted to longer wavelengths. The peak absolute response of the disordered GaAs based QWIP is lower by almost a factor of four. However, the responsivity characteristics of the disordered InP based QWIP show no major degradation. In general, with the spectral broadening taken into account, the overall performance of the disordered QWIP's has not dropped significantly. Thus, the postgrowth control of the QW composition profiles by impurity-free vacancy disordering offers unique opportunities to fine tune various aspects of a photodetector's response. Theoretical calculations of the absorption coefficient spectrum are in excellent agreement with the experimental data     

High-Power All-Fiber-Based Narrow-Linewidth Single-Mode Fiber Laser Pulses in the C-Band and Frequency Conversion to THz Generation

Based on highly Er/Yb codoped phosphate fibers, we have implemented all-fiber-based narrow-linewidth single-mode (SM) pulsed fiber lasers in master oscillator and power amplifier configuration. Two approaches were used to achieve the narrow-linewidth pulsed fiber laser seeds: 1) an all-fiber-based active Q-switched fiber laser using an actuator and 2) a directly modulated single-frequency continuous-wave fiber laser. Both the fiber laser seed pulses from the two approaches have the transform-limited spectral linewidth. Based on a newly developed large-core SM highly Er/Yb codoped phosphate fiber, the peak power of SM pulses can be scaled to more than 50 kW with transform-limited linewidth and diffraction-limited beam quality. These high-power narrow-linewidth SM fiber laser pulses have been successfully used to generate coherent terahertz (THz) waves based on parametric processes in a nonlinear optical crystal. The peak power of this fiber-based THz source can reach 26.4 mW.     

Epigenetic reprogramming in cloned embryos

Although nuclear transfer (NT) techniques are used to clone animals, their efficiency is very low. Moreover, NT has resulted in offspring with severe developmental problems, probably due to incomplete nuclear reprogramming. Nuclear reprogramming is characterized by functional modification of the transferred nucleus to allow it to direct normal embryo development with the potential to grow to term. Although the nature of the reprogramming factor(s) in mammals is not clear, various nuclear as well as cytoplasmic components are involved in the processes. In this article we review recent data on factors associated with the nuclear reprogramming of cloned embryos.