Abstract In this papaer, the emission characteristics of InAs/InGaAs quantum dot (QD) microdisk lasers, of different cavity diameters, with a top split electrical contact formed using the focused ion ...beam technique are investigated. The dependences of the threshold currents of two-state lasing (i.e. currents corresponding to the start of the ground- and excited-state lasing) for microdisks of 24 and 28 μ m diameters on the electrical contact area are presented. The contact area was found to influence the threshold currents of two-state lasing in microdisks. It is shown that a decrease in the area of the injected electrical contact leads to a decrease in the current corresponding to the start of the excited-state lasing, while the ground-state (GS) lasing threshold remains virtually unchanged. The temperature evolution of the threshold currents for two-state lasing was also studied in microdisks with different electrical contact areas. We demonstrate that the use of contacts of different areas is a method of controlling the threshold currents of two-state lasing and can be used in engineering of QD lasers intended, for example, for multi-level signal transmission with wavelength multiplexing by switching from the GS to excited-state lasing.
Some highlights of the time-resolved vacuum ultraviolet (VUV) luminescence spectroscopy of solids using synchrotron radiation (SR) are outlined, including studies of the unique phenomenon ...crossluminescence (CL) and the contribution of time-resolved VUV spectroscopy to the understanding of 5d-4f transitions of rare earth ions in solids. The main properties of CL studied at different SR sources are described and some unclear aspects of CL are pointed out. The results of recent studies of some CL-active nanosize materials are presented. We describe the time-resolved experiments which led to the discovery of 5d-4f luminescence in the deep VUV region (near 10 eV) of Gd3+ and Lu3+ ions incorporated into some wide band-gap fluoride hosts. The results of high-resolution (Δλ ∼ 0.5 Å) studies of 5d-4f emission and 4f-5d excitation spectra of Gd3+ and Lu3+, which allowed the detailed analysis of electron-lattice coupling in these systems, are presented. Possible new developments in the femtosecond time-resolved spectroscopy of solids with a free electron laser are discussed.
Ceramic samples of Y
3
MgGa
3
SiO
12
, Y
3
MgGa
2
AlSiO
12
, and Y
3
MgGaAl
2
SiO
12
multicomponent garnets doped with 0.2 at % Cr
3+
have been obtained by high-temperature solid-state synthesis. In ...the luminescence spectra of the synthesized garnet samples, overlapping broadband luminescence is observed in the far red spectral region caused by the
4
T
2
→
4
A
2
transition in Cr
3+
ions, and a narrow band is observed in the range of 690–700 nm, corresponding to the zero-phonon line of the
2
Е →
4
A
2
transition in Cr
3+
. The narrow-band and broad-band parts of the spectra are attributed to radiation from two different types of chromium centers, which are in octahedral coordination with different distortion degrees and strength of the crystal field. This results from the presence of two ions at the octahedral position of these garnets, which differ significantly in crystal chemical properties, namely, Mg
2+
and Ga
3+
(Al
3+
). The studied phosphors, which have broadband luminescence in the phytoactive far red region of the spectrum, have the potential for use in greenhouse LED lamps.
Abstract
Epi-side down bonding on a silicon substrate of AlGaAs/GaAs microdisk lasers is presented. A heterostructure with coupled large optical cavities enables location of an InGaAs quantum dot ...active region at a distance of ∼1
µ
m from the heterostructure surface. The thermal resistance was reduced to 0.2 and 0.1 K mW
−1
for disks of 30 and 50
µ
m in diameter, respectively. The maximum continuous-wave power limited by the thermal rollover is more than doubled after bonding.
—
Ceramic LiAl
5
O
8
lithium aluminate spinel materials doped with trivalent chromium ions have been prepared by high-temperature solid-state synthesis. The intensity, spectral shift, and width of ...two zero-phonon luminescence lines (
R
lines) due to transitions from the
2
E
excited state to the
4
A
2
ground state of Cr
3+
ions have been measured as functions of temperature in the range 80–295 K. Analysis of luminescence and luminescence excitation spectra of the ceramics points to severe local lattice distortion around the Cr
3+
ions, which shows up, in particular, as large inhomogeneous broadening of the zero-phonon Cr
3+
luminescence lines. The temperature dependences of the relative intensity, spectral shift, and width of the two zero-phonon lines can be sufficiently well described in terms of existing models for optical centers in crystals. The feasibility of using these temperature dependences for noncontact luminescence thermometry is assessed. Analysis of the data obtained in this study provides no way to uniquely identify the observed zero-phonon lines as due to transitions from two sublevels of the
2
E
state of a single type of chromium center (that is, as
R
1
and
R
2
lines) or transitions of two inequivalent chromium centers coupled by nonresonant energy transfer.
The spectral and kinetic properties of luminescence of fluorozirconate glasses 57ZrF
4
⋅33BaF
2
⋅ 6CeF
3
⋅4AlF
3
doped with 2–4 mol % of manganese ions have been studied. It has been demonstrated ...that increasing the doping ion concentration from 2 to 4 mol % is accompanied by the red shift of the
4
T
1
(
G
) →
6
A
1
band of Mn
2+
luminescence from 570 to 580 nm. The red shift is associated with an increase (with increasing concentration) in the number of Mn
2+
–Mn
2+
pairs, which give a longer wavelength luminescence than individual Mn
2+
ions. Analysis of the Mn
2+
luminescence kinetics showed no appreciable concentration quenching of Mn
2+
luminescence at doping concentrations up to 4 mol %. It has been shown that these glasses exhibiting broadband luminescence in the orange region of the spectrum have potential for practical application in white LEDs.
Chromium trifluoride-doped fluoride glasses in the ZrF
4
–BaF
2
–LaF
3
–AlF
3
–NaF (ZBLAN) system with partial substitution of fluorine for chlorine have been synthesized. The spectral data obtained ...confirm that chromium ions enter the glass structure and exhibit broadband luminescence caused by the
4
T
2
→
4
A
2
transition in the Cr
3+
ion. The observed long-wavelength shift of the broadband luminescence band and Cr
3+
absorption bands in fluoride–chloride glass compared to fluoride glass corresponds to the expected behavior of the Cr
3+
luminescence and absorption spectra when fluoride ions are replaced by chloride ions, which should lead to a weakening of the strength of the crystal field acting on Cr
3+
ions. At room temperature, the luminescence of Cr
3+
ions at 888 and 908 nm is strongly quenched due to the thermally stimulated nonradiative transition from the
4
T
2
excited state to the
4
A
2
ground state.
A model is proposed that makes it possible to analytically analyze the speed performance of a waveguide
p
–
i
–
n
photodiode with a light-absorbing region representing a multilayered array of quantum ...dots separated by undoped spacers. It is shown that there is an optimal number of layers of quantum dots, as well as an optimal thickness of the spacers, which provide the widest bandwidth. The possibility of achieving a frequency range (at the level of –3 dB) above 20 GHz for waveguide photodiodes based on InGaAs/GaAs quantum well-dots is shown.
Ceramic lithium aluminum spinels doped with manganese ions have been synthesized by a high-temperature solid-state reaction. Samples fabricated by annealing at 1000, 1100, 1200, or 1300°C in air for ...4 h have been identified as LiAl
5
O
8
, which is cubic spinel belonging to the space group
P
4
1
32. After annealing at 1000–1200°C, samples with the lattice parameter
a
= 7.923–7.925 Å show intense red luminescence of Mn
4+
ions, which has a narrow luminescence band with a maximum at 662 nm. The excess charge of Mn
4+
ions replacing Al
3+
ions in the octahedral sites of the LiAl
5
O
8
lattice is compensated for by an excess amount of lithium ions (compared to the stoichiometry) substituting for Al
3+
ions adjacent to Mn
4+
. However, annealing at a higher temperature (1300°C) leads to the almost complete disappearance of the Mn
4+
luminescence at 662 nm, although X-ray powder diffraction shows that the crystal structure of these ceramics remains the same one (space group
P
4
1
32), but the lattice parameter
a
becomes 7.908 Å, which exactly corresponds to the lattice parameter of the LiAl
5
O
8
single crystal. It is suggested that ceramics with a larger lattice parameter obtained at lower temperatures are solid solutions in which some of the Al
3+
ions are replaced by larger Li
+
ions. Such solid solutions are not stable and lose Li
+
ions upon annealing at a high temperature to convert to the stoichiometric compound LiAl
5
O
8
. In stoichiometric LiAl
5
O
8
, Mn
4+
ions cannot be stabilized in the octahedral sites due to the lack of a charge compensation mechanism, and there is no red luminescence of Mn
4+
ions in such samples.
A method to synthesize spinel (MgAl
2
O
4
)–based compounds doped with magnesium ions was developed, which makes it possible to create red-emitting phosphors (band peak at 651 nm) or green-emitting ...phosphors (band peak at 525 nm) due to the luminescence of Mn
4+
or Mn
2+
, respectively, and also phosphors that have both (red and green) emission bands. Both types of luminescence exhibit quite high thermal stability; particularly, the green luminescence does. Namely, the temperature at which the luminescence intensity halves in comparison with the maximum intensity is
T
0.5
= 448 and 737 K for the red (Mn
4+
) and green (Mn
2+
) luminescence, respectively. On the other hand, for the phosphors that have both emission bands, the temperature dependences of the intensities of the red and green luminescence more closely resemble each other because of the energy transfer from Mn
4+
ions to Mn
2+
ions. Both emission bands are efficiently excited in the blue spectral region, which enables the use of the conventional three-color RGB (red–green–blue) method to create LED white-light sources based on a combination of a blue light-emitting diode and a phosphor containing manganese ions with an optimally chosen ratio between the intensities of the green and red emission bands.
