Volume 107, Issue 3, 01 February 2010
Index of content:
- Lasers, Optics, and Optoelectronics
Terahertz dielectric response and optical conductivity of -type single-crystal ZnO epilayers grown by metalorganic chemical vapor deposition107(2010); http://dx.doi.org/10.1063/1.3294650View Description Hide Description
Using terahertz time-domain spectroscopy, we measured the frequency dependent complex dielectric response and conductivity of -type single-crystal ZnO epilayers with different carrier concentrations over the frequency range from 0.1 to 3.0 THz. The measured complex dielectric response and conductivity are analyzed using Drude model.
Optical properties of multiple quantum wells grown on (111) Si using buffer assisted pulsed-laser deposition107(2010); http://dx.doi.org/10.1063/1.3266171View Description Hide Description
This work investigates the photoluminescence properties of multiple quantum wells(MQWs), which have been fabricated by pulsed-laser deposition on (111) Si substrates using intervening epitaxial buffer layers. In MQWs, the luminescence is dominated by localized exciton (LE) emission throughout the whole temperature range studied. With increasing temperature from 10 to 300 K, the LE emission redshifts by 38 meV. This redshift is believed to be due to the thermalized excitons occupying higher-lying localized states where they emit higher energy radiation and temperature-induced band gap shrinkage. Moreover, the LE emission from the MQWs decays more slowly than exciton emission from ZnO. In addition, the LE emission in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum size effect.
107(2010); http://dx.doi.org/10.1063/1.3277024View Description Hide Description
Upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate, dopedglass, ZBNA (), when pumped at 978 nm via a tapered optical fiber is demonstrated. An ultralow, green lasing threshold of for 550 nm emissions is measured. This is one order of magnitude lower than that previously reported for ZBLAN () microspheres. Optical bistability effects in ZBNA microspheres are reported and the bistable mechanism is discussed and attributed to shifts of the whispering gallery modes due to thermal expansion of the sphere, where heating is achieved by optical pumping around 978 nm. The effect of the bistability on the upconversion lasing is examined and we report multiple bistability loops within the microspheres.
107(2010); http://dx.doi.org/10.1063/1.3296044View Description Hide Description
We report femtosecond transient reflectivity measurements of as-grown and iron-doped . A hybrid vertical Bridgman and gradient freezing directional solidification process was employed for the growth of high quality crystals with the uniform impurity doping concentration necessary for high resistivity and high mobility material. Carrier lifetimes range from for as-grown to for . The high carrier mobility along with high resistivity and subpicosecond carrier lifetimes make an excellent candidate for photoconducting antenna based terahertz emitters.
The challenge of unity wall plug efficiency: The effects of internal heating on the efficiency of light emitting diodes107(2010); http://dx.doi.org/10.1063/1.3285431View Description Hide Description
We develop a self-consistent model to describe the internal heating of high power light emitting diodes(LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits.
107(2010); http://dx.doi.org/10.1063/1.3294652View Description Hide Description
The incorporation of in the ferroelectric fluoride crystal is investigated for its potential application as self-frequency-converter solid state laser. Low temperature high-resolution optical spectroscopy shows that is located at two well distinguished sites referred as A and B. The whole set of energy levels in the excited , , and ground states are identified for both sites and their crystallographic location in lattice is discussed. The radiative lifetime of the metastable laser level is obtained to be 1430 and for site A and B, respectively, and the luminescence quantum efficiency of the metastable state for the dominant A site is 87%. The possibility of annihilating the contribution of the B site in the optical spectrum is demonstrated by means of the incorporation into the crystal.
Interaction of vacuum ultraviolet excimer laser radiation with fused silica. I. Positive ion emission107(2010); http://dx.doi.org/10.1063/1.3253732View Description Hide Description
We report time- and mass-resolved measurements of and emission from ultraviolet-grade fused silica during exposure to pulsed 157 nm excimer laser radiation at fluences below the threshold for optical breakdown. The emission intensities are increased by treatments that increase the density of surface defects, such as abrasion, and are reduced by treatments that reduce the density of surface defects, such as annealing. Ion emission is a sensitive probe of mechanical damage on silicasurfaces. The mean ion kinetic energies are typically several eV: 8–9 eV for and about 4 eV for . Hartree–Fock studies of candidate defect sites suggest that antibonding states excited by 157 nm photons play a critical role in the release of these ions. We propose that positive ion emission from fused silica under these conditions is best explained by a hybrid mechanism involving (a) the excitation of an antibonding chemical state (Menzel–Gomer–Redhead mechanism) and (b) the acceleration of the positive ion by repulsive electrostatic forces due to the photoionization of nearby electron traps.
Interaction of vacuum ultraviolet excimer laser radiation with fused silica: II. Neutral atom and molecule emission107(2010); http://dx.doi.org/10.1063/1.3290879View Description Hide Description
We report mass-resolved time-of-flight measurements of neutral Si, O, and SiO from ultraviolet-grade fused silica during pulsed 157-nm irradiation at fluences well below the threshold for optical breakdown. Although the emission intensities are strongly affected by thermal treatments that affect the density of strained bonds in the lattice, they are not consistently affected by mechanical treatments that alter the density of point defects, such as polishing and abrasion. We propose that the absorption of single 157 nm photons cleave strained bonds to produce defects that subsequently diffuse to the surface. There they react with dangling bonds to release neutral atoms and molecules. Hartree–Fock calculations on clusters containing these defects support the contention that defect interactions can yield emission. More direct emission by the photoelectronic excitation of antibonding chemical states is also supported.
Interaction of vacuum ultraviolet excimer laser radiation with fused silica. III. Negative ion formation107(2010); http://dx.doi.org/10.1063/1.3290945View Description Hide Description
We report mass- and time-resolved measurements of negative ions produced by exposing fused silica to 157 nm radiation at fluences below the threshold for optical breakdown. The principal observed negative ions are , , and , in order of decreasing intensity. The peak in the negative ion time-of-flight signals occurs after the peak in the positive ion signal and before the peak in the corresponding neutral atom or molecule signal. The negative ion intensities are strong functions of the degree of overlap between the positive ion and neutral atom densities. We propose that , , and are created after the laser pulse, by electron attachment to these neutral particles and that the electrons participating in attachment events are trapped in the electrostatic potential of the positive ions.
Intense fluorescence of codoped phosphate glass and the three-photon phenomenon of near infrared upconversion luminescence107(2010); http://dx.doi.org/10.1063/1.3294611View Description Hide Description
Transparent glasses codoped with and ions were fabricated by high-temperature melting technique. Strong near infrared and visible upconversion luminescence were observed under 975 nm laser diode excitation, and the luminescence processes were explained in details. For band, the calculated peak emission cross sections is , and it is better than that in germinate, tellurite, silicate, and other phosphate glasses. The near infrared upconversion emission around 828 nm contains two-photon and three-photon processes, and it is attributed to the transition, which enhances the population of the upper level for laser emission and reduces the energy loss due to upconversion. Efficient near infrared fluorescence indicates codoped phosphate glass is a promising laser and amplifier material.
Continuously tunable, phase-locked, continuous-wave terahertz generator based on photomixing of two continuous-wave lasers locked to two independent optical combs107(2010); http://dx.doi.org/10.1063/1.3305324View Description Hide Description
A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology.
107(2010); http://dx.doi.org/10.1063/1.3294615View Description Hide Description
In laser-induced backside wet etching (LIBWE), the liquid absorbent indirectly heats the transparent material, causing explosive phase change and cavitation. Accordingly, the hydrodynamics of the absorbing liquid, including the size of the liquid chamber, is strongly related to the ablation process. Because the hydrodynamics is dependent on the elastic deformation of the sample, the sample thickness also affects the performance of LIBWE. In this work, experimental analyses were performed to elucidate the hydrodynamics in LIBWE and the effect on the etch rate by varying the liquid chamber size and sample thickness. A KrF excimer laser was used to ablate fused silica samples in toluene and the etch rate was quantified using a scanning profilometer. Laser flash shadowgraphy and photodeflection probing techniques were employed for in situ measurement of the laser-induced hydrodynamics and displacement of the sample, respectively, with a time resolution of approximately nanoseconds. To directly observe the effect of increased liquid pressure on LIBWE, the liquid pressure on the etching spot was locally increased by an external shock wave and the etch results are examined. This study confirms that the photomechanical effects from the laser-induced bubble plays a key role in the LIBWE process, revealing a linear relationship between the etch rate the applied recoil momentum. However, the dependence of the etch rate on the chamber size and sample thickness could not be explained by the change in recoil momentum, i.e., by the bubblepressure. Instead, transient deformation of the sample by the pressureimpact was estimated to be mainly responsible for the dependence.
107(2010); http://dx.doi.org/10.1063/1.3305339View Description Hide Description
We studied the photorefractive effect of lithium niobatedoped with Mg,Zn, In, Hf, or codoped with Mg and Fe at an ultraviolet (UV) wavelength down to 325 nm. It is found that the UV photorefraction of doped with Mg,Zn, In, or Hf was enhanced significantly as compared to that of the nominally pure . Our results show that the property of resistance against photorefraction in highly Mg,Zn, In, or Hf doped is true only in the visible and near-infrared wavelength range. By contrast, these crystals exhibit excellent photorefractive characteristics at UV wavelength of 325 nm, even better than those at 351 nm. For example, the photorefractive two-wave coupling gain coefficient and the photorefractive recording sensitivity at 325 nm were measured to be and 37.7 cm/J, respectively, in a crystal doped with Zn. The photorefractive response time of a with a Mg was measured to be 73 ms with a total recording intensity of at 325 nm. In highly Mg,Zn, In, or Hf doped crystals, diffusion dominates over photovoltaic effect and electrons are the dominant charge carriers in UV photorefraction at 325 nm. The results are also of interest to the study on the defect structure of near to the absorption edge.
107(2010); http://dx.doi.org/10.1063/1.3305807View Description Hide Description
We have investigated the influence of a pulsed electric field on the intracenter population inversion between phosphorus donor states in silicon. Terahertz-range electroluminescence resulted from the populated excited donor states. It grows linearly above the impurity breakdown field but saturates at excitation powers exceeding 100 W at pulse length. An electric field applied to the optically pumped silicon laser reduces the efficiency of the phosphorus transition already at voltages below the impurity breakdown. The appearance of a current through the laser sample results in a fast quenching of the laser emission that indicates a reduction in the inversed population between the laser states.
107(2010); http://dx.doi.org/10.1063/1.3298500View Description Hide Description
We prepared very thin amorphous InSe films and investigated the thickness dependence of the nonlinear absorption by pump-probe and open aperture Z-scan techniques. While thinner films (20 and 52 nm) exhibit saturable absorption, thicker films (70 and 104 nm) exhibit nonlinear absorption for 4 ns, 65 ps, and 44 fs pulse durations. This behavior is attributed to increasing localized defect states in the energyband gap as the film thickness increases. We developed a theoretical model incorporating one photon, two photon, and free carrier absorptions and their saturations to derive the transmission in the open aperture Z-scan experiment. The theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method was used to fit the experimental curves. Nonlinear absorption coefficients along with saturation intensity thresholds were extracted from fitting the experimental results for all pulse durations. The lowest saturation threshold was found about for 20 nm film thickness with nanosecond pulse duration and increased about four orders of magnitude for 104 nm film thickness.
107(2010); http://dx.doi.org/10.1063/1.3295903View Description Hide Description
A bright metastable helium beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch.Patterning of the wafer with the was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of light was used to channel and focus the atoms into lines separated by . The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable.
- Plasmas and Electrical Discharges
Self-masking controlled by metallic seed layer during glass dry-etching for optically scattering surfaces107(2010); http://dx.doi.org/10.1063/1.3290969View Description Hide Description
During reactive ion etching in a high density plasma, nonvolatile halogen compounds are generated on the glasssurface and act as statistically distributed micromasks. As a consequence surface roughness occurs, which can be used as additional surface functionality in certain classes of applications, where defined optical scattering is desired. The glassetch process described in this contribution enables user-defined scattering characteristics. The deposition of a thin metallic layer of Cu on top of the samples before dry-etching provides an additional seed for the production of micromasks, resulting in higher reproducibility. By varying the etch parameters a multitude of different surface morphologies can be realized both on borosilicate glasses and on fused silica.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
107(2010); http://dx.doi.org/10.1063/1.3298457View Description Hide Description
Reducing the thermal conductivity of nanowires may enhance their already exciting efficiency of thermoelectric energy conversion. Using molecular dynamics simulations, we demonstrate that the thermal conductivity of siliconnanowires could be significantly decreased by patterning (or etching) induced roughness of the nanowiresurfaces. The type, amplitude, and wavelength of the surface roughness all have profound effects, and the thermal conductivity could be reduced more when the wavelength is smaller or the amplitude is larger. Such an effect of roughness on the thermal conductivity is furthermore found to be coupled with the effects of nanowire cross-sectional size and length. Typically, the roughnesseffect is more prominent in longer and larger nanowires.
107(2010); http://dx.doi.org/10.1063/1.3298499View Description Hide Description
Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies and are found to shift linearly to higher energies as the gallium content increases.
107(2010); http://dx.doi.org/10.1063/1.3298502View Description Hide Description
The structures of liquid (, Er, Ho, Dy, Y, Sm, Gd, Nd, and Ce) alloys were investigated by ab initio molecular dynamics simulation. The results show that the chemical affinity of Fe-RE and RE-B may influence the glass forming ability more than the atomic size of RE atom in these alloys. As expected, the polyhedron dominates around B atoms and is significantly enriched by adding RE elements to the liquid alloy. The good glass formers do not correspond to the larger percentages but to more RE atoms in the shell of polyhedron. These features suggest that the effect of the chemical composition of polyhedron on the glass forming ability may be larger than that of its quantity in these alloys.