The InGaAs surface quantum dots grown on GaAs surface without a capping layer (surface quantum dots, SQDs) are expected to play an important role for sensor applications due to their special surface sensitive properties. In this research, we investigated the photoluminescence (PL) characteristics of such In0.35Ga0.65As/GaAs SQDs with a layer of buried InGaAs QDs (BQDs) as reference. The uncapped InGaAs SQDs are integrated into a hybrid nanostructure with SQDs and buried quantum dots (BQDs) spaced by a 70 nm GaAs layer. Due to this thick GaAs spacer, we assert there is no quantum coupling between the SQDs and BQDs so that each layer of QDs has independent emission. The PL spectra show that the SQD PL intensity is far less than BQDs at low temperature but exceeds BQDs at high temperature, indicating a possible carrier transfer between the SQDs and surface states. With increasing excitation intensity, the PL spectra show clearly broaden on the high energy side and a blueshift for both the SQDs and BQDs. Therefore, there is lateral carrier transfer among each layer of QDs due to their high areal density. The intra-layer carrier transfer among SQDs as well as the inter-layer carrier transfer between SQDs and surface states attribute to carriers dynamics that make the SQDs having optical performance very different from the BQDs.
Recently, to sandwich InAs QDs with GaAs1-xSbx layers have attracted enormous attention. It is expected to achieve a transition from type-I to type-II band alignment at ~ x=12%. The type-II InAs/GaAs1-xSbx QDs are predicted to be one of the optimizing active region materials for achieving high efficient intermediate-band solar cells.
We first investigate PL properties of InAs/GaAs1-xSbx QD structures of different Sb compositions (x=0, 0.15, and 0.25) in the GaAs1-xSbx capping layer. By capping the InAs QDs with GaAs0.85Sb0.15 and GaAs0.75Sb0.25 layer, we are able to gain type II QDs. These type II QDs exhibit a clear multi-peaks characteristic under increasing laser excitation intensity, which stems from different carriers recombination routes due to the combination of InAs QDs with GaAs1-xSbx capping layer. Time-resolved PL measurements further confirm our assignment for the mnulti-peaks in the PL spectra. We then study carrier coupling inside vertically aligned InAs/GaAs and InAs/GaAs1-xSbx QD pairs. The features of InAs/GaAs1-xSbx QD pairs are very different from the traditional InAs/GaAs QD-pairs. Again, clear multi-peaks characteristic are observed under stronng laser excitation intensity, which stems from different carriers recombination routes from the top InAs/GaAs1-xSbx QDs.
Our investigations indicate that the optical behavior and carrier dynamics in type-II InAs/GaAs1-xSbx QDs and QD-pairs are much more complicated than the InAs/GaAs QDs counterparts. This study provide useful information for understanding the band structure and carrier dynamics of the InAs/GaAs1-xSbx QDs for high efficiency solar cell applications.
The type-I to type-II band alignment transition in InAlAsAs/AlGaAs/GaAs self-assembled quantum dots (QDs) is investigated when the Al-composition in QDs and barrier are changed. In particular, the In0.46Al0.54As/Ga0.46Al0.54As/GaAs QDs show unique optical properties. The PL peak energy has a blue-shift of >40 meV when the laser intensity increases by four orders of magnitude, indicating a type-II band alignment of the QDs. The formation of the type-II band alignment is explained by that the quantum-confinement effect pulls up the minimum electron energy level in the QDs and the Γ→X transition in the Ga0.46Al0.54As barrier. The time-resolved PL (TRPL) spectrum of QDs at peak wavelength exhibits a double-component decay behavior, suggesting the possibility of type-I and type-II band alignment coexistence in this QD sample. The continuum state of the QDs is also investigated. Emission associated with the continuum states of the QDs is directly observed in PL spectra. The PL excitation (PLE) and TRPL spectra reveal an efficient carrier relaxation from the AlGaAs barrier into the InAlAs QD ground state via the continuum states. The carrier recombination in the continuum states can compete with that in the QDs due to the long recombination lifetime in the type-II QDs. This feature of continuum state emission can not be observed for normal InGaAs/GaAs QDs with the type-I band structure.
To optimize the performance of Er3+/Yb3+ co-doped double cladding fiber laser (EY-DCFL), the output characteristics of
DBR EY-DCFLs with different parameters are investigated theoretically and experimentally. The output powers as
functions of the launched pump power, the gain fiber length, as well as the reflectivity of the output mirror are presented
by numerical simulation based on rate equations and power propagation equations. Experimental study on the output
power, the spectral properties and the time-domain stability of DBR EY-DCFLs with different reflectivity is carried out.
In the optimum condition, up to 2W output power at 1550.8nm is obtained with a slope-efficiency of 53.8% and a 3dB
bandwidth of about 0.02nm.
The Y2Si2O7:Tb3+ phosphor was synthesized by high temperature solid state method. The crystal structure and
luminescent properties of phosphors were studied by XRD pattern, excitation and emission spectra in this paper. XRD
pattern showed that the sample was single phase Y2Si2O7 crystal and the crystal lattice constants a=0.806nm, b=0.934
nm, and c=0.692 nm. The excitation spectrum is composed of a broad band centered 290nm and three narrow bands
corresponding to 4 - 4 transition of Tb3+ centered 378 nm, 400nm and 420nm, respectively. The emission peaks of
phosphor were located at 487nm, 546nm, 584nm and 623nm, which were corresponding to 5D4-7F6, 5D4-7F5, 5D4-7F4 and
5D4-7F3, respectively. The influences of Tb3+ concentration on the luminescent intensity of Y2Si2O7:Tb3+ phosphor was
studied. The results indicated that this phosphor could act as a candidate green phosphor for UV-excited white LED.
Eu3+ doped yttrium orthosilicate (Y2SiO5) phosphor was prepared by the sol-combustion method using citric acid as
complexing agent in this experiment. The X-ray diffraction (XRD) pattern, excitation and emission spectra were used to
investigate the crystal structure and luminescent properties of the phosphor. XRD pattern showed that pure Y2SiO5:Eu3+
phosphor was obtained. The excitation spectrum was composed of a broad band from 200-350 nm and a series of narrow
bands from 350-500 nm, in which the excitation peaks at 400 nm and 470 nm were stronger. The emission spectrum
showed the most intense emission peak was located at 613 nm, which corresponded to the 5D0→7F2 transition of Eu3+.
The results showed that this phosphor could be excited by UV or blue light and emit red light. The luminescent intensity
depends on the concentration of Eu3+ and it reached the maximum when the molar concentration of Eu3+ was 4 mol%. In
this study, we found that the emission intensity reached maximum when the ratio of citric acid and Y3+ was 1.5:1. The
results indicated that Y2SiO5:Eu3+ is a potential red-emitting candidate phosphor for white light-emitting diodes.
Proc. SPIE. 7658, 5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optoelectronic Materials and Devices for Detector, Imager, Display, and Energy Conversion Technology
This work studied the formation of spatial solitons in a SBN: 75 photorefractive crystal by 532 nm continuous-wave
laser beam from a single frequency solid laser. It was found that the behaviors of the spatial solitons varied with
background light. While white light and 632 nm red light are used as background, the solitons perform like first-order
light soliton. The compressed-amplitude, in measured with laser beam's size before and after passing the crystal, is
proportional to the applied voltage of a dc electric field, i.e., they are quasi-steady-state solitons formed in a time
window. However, as the background is changed to an incoherent 532 nm light output from the same laser, the solitons
vary periodically and the compressed-amplitude is not proportional to the voltage of applied dc electric field any more.
In this case, solitons form instantaneously at applied field of 900 V/cm and hold the shape for 7 seconds which is less
than the one with white or red light as background, and then break slowly. We tried to present reasonable explanation for
The diffraction efficiency of the volume grating written and read out in Ce:KNSBN crystal by using orthogonally
polarized light beams (solid state laser with 532nm) is experimentally studied, which exhibits a loop versus the variation
of the fringe modulation. And compare with the extraordinary polarization, the diffraction efficiency is improved 20%
with mutually orthogonal polarized wave while the angle between the incident plane and the polarization direction of the
pump beam equals to 30°. The properties of two-wave coupling edge-enhancement under different intensity ratio of the
reference beam to the object beam are experimentally investigated by using the setup of the Fourier-transform hologram
real-time writing and reading with Ce:KNSBN crystal as the recording medium. It is found that the effect of the image
edge-enhancement strongly influenced by the intensity ratio of the reference beam to the object beam. There is no
edge-enhancement with the intensity ratio of the reference beam to the object beam of 50:1, the high frequency
component in the object beam is enhanced greatly and the low frequency component is obviously weakened with the
intensity ratio of the reference beam to the object beam of 3:1. Along with the decreasing of the intensity ratio of the
reference beam to the object beam, the effect of the image edge-enhancement is still obvious even when the intensity
Wavelength tunable high power lasers are desired for many applications, such as spectroscopy, sensing and nonlinear
optics, etc. A tunable Yb3+-doped photonic crystal fiber laser based on a blazed grating in Littrow configuration is
demonstrated. The active fiber used in the experiment is a double-clad Yb3+-doped PCF which contains an Yb3+-doped
core of 23 μm in diameter and a holy inner clad of 420 μm in diameter. The laser resonator is composed of a dichroic
reflective mirror and a blazed grating which is set in Littrow configuration. To reduce reflection the fiber end near to the
grating is polished into an angle of 8 degree. Wavelength tunable output with a range form 1035.425 nm to 1111.770nm
is realized. The output power of the laser at different wavelengths is flatten within 0.8 dB. At the available maximum
pump power of 12.11 W, the maximum output power of 3.45 W is obtained at 1064.085 nm, which corresponds to a
slope efficiency of 35.9%. Degree of polarization of the output at different pump level are all more than 0.87.
We investigate the quantum interference effects in a cyclic three-level system with a microwave field driving transition between two low levels. By solving the relative density matrix equations of motion, we obtain the absorption profile of the probe field and identify the conditions under which gain may develop. We demonstrate numerically that the absorption line shape depends on the ratio of the intensities of coupling and driving microwave field. When the intensity of coupling field is much weaker than that of driving field, there is a multi-EITs in the probe absorption spectrum. However, if the intensity of both fields is strong, amplification without inversion occurs in different regime of probe frequency. In addition, we predict that larger amplification is obtained when the coupling field is detuned from exact resonance.
In this experiment, the dynamic two-wave coupling properties of extraordinarily polarized He-Ne 632.8nm laser beam in a Ce:KNSBN crystal were studied with a real-time data-acquisition system. The transmitted pump beam intensity Ip' and the transmitted signal beam intensity Is' were recorded as a function of time. While the incident beams of two-wave coupling are both with extraordinary polarization, a great fanning will generate and it would affect the two-wave coupling dynamic properties. The dynamic evolution of two-wave coupling had great differences while the total incident beam intensity and the incident beam intensity ratio were different. The coupled-wave equations were modified according to the beam-fanning threshold phenomena and the experimental data. The theoretical calculation results obtained from the modified coupled-wave equations were agree with the experimental data very well.
In this paper the SRS characteristic of a phosphorus-doped fiber made for RFLs is experimentally investigated. At an averaged pump power of 0.5W we observe three spectral lines in the output spectrum, which correspond to the first order Stokes of silica at 1.12μm, the third order Stokes of silica or the first order Stokes of P2O5 at 1.24μm, and the fourth order Stokes of silica at 1.31μm, respectively. When the pump power is increased to the maximum value of 2W, there in the spectrum appears a continuum near 1.54μm with a FWHM of more than 50nm. The expected fifth Stokes of silica at 1.39μm and the second Stokes of P2O5 at 1.48μm are not observed. We demonstrate that the fiber loss in the region of 1-1.5μm plays an important role in the development of SRS in phosphorus-doped fibers.
The output characteristics of a large-core double clad fiber (DCF) laser are experimentally investigated in this paper. The fiber laser is fabricated with 5m Yb-doped DCF that consists of a 30μm diameter core and a 400/340μm D-shape inner clad. The measured spectra show that the wavelength of laser is not stable at low pump levels, and the wavelength tends to be stabilized at 1074nm until the pump power is increased to a level far over the threshold. The maximum output power of 7.2W is obtained at an input pump of 11W with a FWHM of about 6nm. The slope efficiency and the total conversion efficiency are 84% and 65.5%, respectively.
Two-wave mixing properties of a Ce:KNSBN photorefractive crystal is experimentally studied while the ihcoherent erasing technique is applied. As the signal beam is with extraordinary polarization and the polarized direcitn of the pump beam has an angle φ with its extraordinarily polarized direction, the gain of the signal beam is measured with respect to the angle φ. Because of the erasing of the fanning by the ordinary component of the pump beam, the gain enhances at small angle φ and obtains its maximum at φ = 30°. The couple-wave theory with considering of the fanning effect is used to fit the experimental data and the fitting curve is coincident wiht the experimental data very well. The calculated results of the gain as a fucntin of the angle φ for different fanning factor are given out. Finally the enhancement of the gain is experimentally measured with respect to the angle between the signal beam and the normal of the crystal face. While this angle takes θ=11°, the enhancement of the gain obtains its minimum value, which means the minimum fanning.
Two-wave coupling of focused argon laser beam with a wavelength of 514.5nm is experimentally studied in a Ce:SBN photorefractive crystal. The images of the input and output signal of the crystal are provided, which indicate that there are two-wave couplign between the interference fringes inside the crystal and the energy can transfer from one bright peak to another along the negative c-axis direction of the crystal. The fringe intesnties at the output face of the crystal are also investigated with respect to the time, the incident beam intensity ratio, and the total intensity of the incident beams. This research demonstrates a bistable state, which is significant to achieve optic-optic controlling device such as one-to-multi optic-optic switch.
In this paper we present a coherent image processor, which uses the loop-like nonlinear diffraction property of the Ce:KNSBN photorefractive crystal to achieve real-time edge- enhanced optical correlation and optical pattern recognition. The full-width-at-half-maximum of the auto- correlation peak is decreased four times by the edge- enhancement operation. The intensity ratio between auto- correlation peak and the cross-correlation peak is improved nearly two times. As the same time the lobe of the auto- correlation peak and the background 'clutter' noise are obviously suppressed. Therefore the discrimination of this system achieves great improvement.
Based on the nonlinear property of the moving grating in BSO crystal in four-wave mixing architecture at large fringe modulation, i.e. the enhancement of the reflectivity increases as the incident beam ratio increases and the grating with large pump beam ratio can obtain higher enhancement of the reflectivity, we achieve edge-enhancement and edge-enhanced optical correlation of a binary optical image by applying the moving grating in the Fresnel transform four-wave mixing system. The relative intensity of the edge of the object is enhanced nearly 2 times. The Full- Width-at-Half-Maximum of the auto-correlation peak and the fluctuation noise if obviously suppressed, which indicate that a significant improvement in the discrimination capability of the correlator is achieved.
In this research, we experimentally studied the spectral quality of the CUl324.7nm and Cul327.4nm lines of the copper alloy standard samples in argon atmosphere at reduced pressure by a laser microprobe emission spectral analysis (LMESA) system. Compared with the results obtained in air atmosphere with the pressure of 760Torr, the spectral quality of the lines obtained obvious improvement. As the distance between the two graphite electrodes is 4mm and the pressure is 250Torr, the intensity of the spectral lines is about 2-3 times of that in the air atmosphere and the Full-Width-at-Half-Maximum (FWHM) is nearly one third of that in the air atmosphere.