Asymmetric contacts or split gate geometries can be used to obtain rectification, electroluminescence (EL) and photocurrent from carbon nanotube field effect transistors. Here, we report devices with both split gates and asymmetric contacts and show that device parameters can be optimised with an appropriate split gate bias, giving the ability to select the rectification direction, modify the reverse bias saturation current and the ideality factor. When operated as a photodiode, the short circuit current and open circuit voltage can be modified by the split gate bias, and the estimated power conversion efficiency was 1×10-6. When using split gates and symmetric contacts, strong EL peaking at 0.86 eV was observed with a full width at half maximum varying between 64 and 120 meV, depending on the bias configuration. The power and quantum efficiency of the EL was estimated to be around 1×10-6 and 1×10-5 respectively.
Photoluminescence (PL) and excitation spectra of Bi melt-doped oxide and chalcogenide glasses are very similar, indicating the same Bi center is present. When implanted with Bi, chalcogenide, phosphate and silica glasses, and BaF2 crystals, all display characteristically different PL spectra to when Bi is incorporated by melt-doping. This indicates that ion implantation is able to generate Bi centers which are not present in samples whose dopants are introduced during melting. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi2+ and Bi clusters.
Yanina Fedorenko, Mark Hughes, Julien Colaux, C. Jeynes, Russell Gwilliam, Kevin Homewood, Jin Yao, Dan Hewak, Tae-Hoon Lee, Stephen Elliott, B. Gholipour, Richard Curry
Doping of amorphous chalcogenide films of rather dissimilar bonding type and resistivity, namely, Ga-La-S, GeTe, and Ge-Sb-Te by means of ion implantation of bismuth is considered. To characterize defects induced by ionbeam implantation space-charge-limited conduction and capacitance-voltage characteristics of amorphous chalcogenide/silicon heterojunctions are investigated. It is shown that ion implantation introduces substantial defect densities in the films and their interfaces with silicon. This comes along with a gradual decrease in the resistivity and the thermopower coefficient. It is shown that conductivity in GeTe and Ge-Sb-Te films is consistent with the two-type carrier conduction model. It is anticipated that ion implantation renders electrons to become less localized than holes leading to electron conductivity in certain cases as, for example, in GeTe.
Direct laser writing has been already demonstrated for the fabrication of under surface "buried" 3D mid-IR waveguides
in chalcogenide glasses by employing a large photo-induced refractive index change in the features formed in the path of
the focused beam from a short pulse laser. In this paper, we report on direct laser writing of relief diffraction gratings
with periods of 6, 14 and 24 μm into the surface of Ge15Ga3Sb12S70 chalcogenide glass by using a 800 nm Ti:saphire
femtosecond pulse laser. The first order diffraction efficiency of the fabricated gratings was over 60 % at 650 nm. We
have also fabricated a "composite" grating composed of three relief diffraction gratings inscribed in the same position,
but with a mutual tilt. Composite grating provided complex multidirectional diffraction of the light in the accordance
with geometrical arrangement and grating period of all the gratings inscribed. The fabrication was implemented on a
computer controlled stage employing surface-to-beam alignment, laser power and raster pattern control. Pulse energies
of 1.5, 3.0 and 4.5 μJ were used, resulting in channel widths of around 4, 5 and 6 μm, respectively, and depths up to 1.7
μm. We propose practical applications including surface relief diffraction micro-gratings at the ends of multimode
chalcogenide optical waveguides or on the surfaces of bare core optical fibers used for chemical sensing.
The absorption and fluorescence characteristics of Er doped and Nd, Er codoped fluoride glasses were investigated under
illumination of the simulated sunlight, laser or a monochromatic light filtered from a Xe lamp. Er was used as a
sensitizing agent enhancing the energy conversion and the emission efficiency of Nd ions in fluoride glass intended for
the sunlight excitation. Er doped fluoride glasses showed four emission peaks under simulated sunlight illumination at
the wavelengths of 550, 848, 980, and 1530 nm attributed to the electronic transitions of Er3+ ions. The quantum
efficiency of the emission from all of the bands had a peak at x = 0.5 mol. % Er and with the maximum of 73 %. The
intensity of each emission band showed different ratios for various ErF3 contents. It is expected that concentration
quenching of 4S3/2 state is easy to occur with high concentration of ErF3 compared to the other states. The energy transfer
from Er to Nd was studied using a monochromatic light illumination which is absorbed by Er3+ ions only. Strong
contribution of Er absorption to the 1.05 μm emission of Nd, Er co-doped fluoride glass was observed. Er was confirmed
as a suitable sensitizer for the enhanced energy conversion and emission efficiency of Nd ions in ZBLAN glasses which
are proposed for highly efficient solar pumped fiber lasers.
In this paper we report emission from Bi doped gallium lanthanum sulphide (Bi:GLS) glass with a full width at half
maximum (FWHM) of 600 nm which is flattened and covers the entire telecommunications window. The excitation
wavelength of this emission was 1020 nm, the quantum efficiency (QE) was 17%, the lifetime was 160 μs and product of
the emission cross section and lifetime (σemτ) was 2×10-25 cm2s. The maximum room temperature QE was 32% at 900
nm excitation. At cryogenic temperatures the FWHM reached 850 nm with 974 nm excitation and we observed two new
bismuth emission bands at 2000 and 2600 nm. The QE reached 40% for both 974 and 808 nm excitation at cryogenic
temperatures. Emission spectra, normalized to the excitation power, taken with excitation wavelengths of 480-1300 nm,
revealed 4 absorption bands at 680, 850, 1020 and 1180 nm. The 1180 nm absorption band was previously unobserved.
Deconvolution of the emission spectra into Gaussians indicated 5 distinct emission bands over the entire excitation
range. The maximum room and cryogenic temperature lifetimes were 175 and 280 μs, respectively. Their respective
emission and excitation wavelengths were ~1500 and 974 nm; and ~1600 and 808 nm. By examining previously
published models of Bi emission in glasses to see if they could account for the 2000 and 2600 nm emission bands, and
reviewing other previously published evidence, we propose that the origin of the emission in Bi:GLS is Bi2 (2-) dimers.
The quantum efficiencies of the emission from the 4F3/2(R) level of Nd doped in tellurite glass were measured
with an integrating sphere using natural sunlight(ηns), simulated sunlight (ηns), and 808 nm laser light (η808),
respectively. The radiative quantum efficiency (ηr) was estimated from the fluorescence lifetime (τf) and the
radiative lifetime calculated by Judd-Ofelt analysis (τr). ηr was almost 100 % for χ ≤ 0.5 mol.%. η808 was
86 % for χ=0.05 mol.% and decreased monotonically with increasing in χ. ηns had a peak at χ=0.5 mol.% and
the maximum was 33 %. It is thought absorption of the excitation light by the host glass limits the quantum
efficiency of the tellurite glass under sunlight excitation. Therefore, it is important to reduce absorption of the
tellurite glass host in order to realize efficient solar-pumped tellurite fiber lasers.
The efficiency of energy transfer from Cr to Nd in silicate glasses has been examined in order to develop a gain medium
for high-efficiency solar pumped fiber lasers (SPFLs). The internal quantum efficiency (QE) of the emission from the
4T2 state of Cr in Cr-doped glasses and from the 4F3/2 state of Nd in Nd-doped and Nd,Cr codoped glasses was measured
using an integrating sphere. For Cr-doped and Nd,Cr codoped glasses, 650 nm excitation was used. For Nd-doped
glasses, 808 nm excitation was used. The QE of Cr-doped glass (ηCr) was 7.5 % for 0.05 mol.% Cr2O3, the QE decreased monotonically with increasing Cr2O3 content. The QE of the Nd-doped glass (ηNd) has a maximum of 43% at 0.2 mol.% Nd2O3. We suggest that absorption of host glass could lower the QE at the low content side of the maximum. The QE of
Nd emission in Nd,Cr codoped glass (ηNd,Cr) excited at 650 nm, which excites the 4A2→4T2 transition of Cr was 5.7 %
for 0.05 mol.% Cr2O3 and 0.2 mol.% Nd2O3 content. The energy transfer quantum efficiency, ηtr, from Cr to Nd which is
defined as the ratio of the ηCr and the ηNd,Cr was calculated from the obtained QEs. The largest ηtr was 13.4 % at 0.01
mol.% Cr2O3, and decreased with increasing Cr2O3 when content of Nd2O3 was fixed by 0.2 mol.%. This tendency is
quite similar to the QEs of the Cr emission in Cr-doped glasses. Thus an increase in the QE of Cr maybe essential to
increase the ηtr.
In this paper we present the development process of a glass based on bismuth and aluminum co-doped germanate (GAB)
glass. Other workers have demonstrated broadband optical gain in GAB glass. We found the addition of PbO to this
glass (GAPB glass) increased the quantum efficiency (QE). Excitation of GAPB glass at 808 nm produced emission
peaking at 1230 nm with a width of 260 nm, indicating the glass may be useful as a broadband optical amplifier covering
the second telecoms window. The composition of each component of the GAPB glass was optimized for the QE and
lifetime of this emission to produce an optimized composition of 87.4GeO2-6Al2O3-6PbO-0.6Bi2O3 (% molar) which had
a QE of 67%. However, this composition contained many white inclusions which x-ray diffraction measurements
indicated were a phase separation. This would make the glass unsuitable for fibre fabrication. We tried a variety of
melting condition which did not improve the glass, the melting conditions did however affect the QE and lifetime. The
optimum melting conditions were 1250°C for 30 min in a N2 atmosphere. Next we tried adding various quantities of a
variety of glass additives including SiO2, LiO, ZnO and SrCO3. All these additives reduced the QE, and all except SiO2
produced a transparent glass in the appropriate quantity. The quantity of these additives which gave a transparent glass
and their QE's were LiO: 10% and 6%, ZnO: 10% and 43%, SrCO3: 5% and 27% respectively. Therefore the optimized
composition for QE and transparency is 74.4GeO2-10ZnO-6Al2O3-6PbO-0.6Bi2O3.
In this paper we present the fluorescence decay profiles of vanadium and titanium doped gallium lanthanum sulphide
(GLS) glass at various doping concentrations between 0.01 and 1% (molar). We demonstrate that below a critical doping
concentration the fluorescence decay profile can be fitted with the stretched exponential function: exp[-(t/&tgr;)&bgr;], where &tgr; is
the fluorescence lifetime and &bgr; is the stretch factor. At low concentrations the lifetime for vanadium and titanium doped
GLS was 30 &mgr;s and 67 &mgr;s respectively. We validate the use of the stretched exponential model and discuss the possible
microscopic phenomenon it arises from. We also demonstrate that above a critical doping concentration of around 0.1%
(molar) the fluorescence decay profile can be fitted with the double exponential function: a*exp-(t/&tgr;1)+ b*exp-(t/&tgr;2),
where &tgr;1 and &tgr;2 are characteristic fast and slow components of the fluorescence decay profile, for vanadium the fast and
slow components are 5 &mgr;s and 30 &mgr;s respectively and for titanium they are 15 &mgr;s and 67 &mgr;s respectively. We also show
that the fluorescence lifetime of vanadium and titanium at low concentrations in the oxide rich host gallium lanthanum
oxy-sulphide (GLSO) is 43 &mgr;s and 97 &mgr;s respectively, which is longer than that in GLS. From this we deduce that
vanadium and titanium fluorescing ions preferentially substitute into high efficiency oxide sites until at a critical
concentration they become saturated and low efficiency sulphide sites start to be filled.
In this paper we report the spectroscopic data for samples of 0.031% iron, 0.017% nickel, 0.01% chromium and 0.017% cobalt (molar) doped gallium lanthanum sulphide (GLS) glass. Photoluminescence (PL) with a full width half maximum (FWHM) of around 500 nm and peaking between 1120 nm and 1460 nm is observed when excited using wavelengths of 850 nm and 1064 nm. The emission lifetime for nickel-doped GLS at 300 K was measured to be 40 μs. Photoluminescence excitation (PLE) peaks for chromium-doped GLS at 700 nm and 1020 nm have been observed. By comparisons of our spectroscopic data to that of transition metals doped into other hosts we determine the oxidation states of the transition metal ions and propose transitions for the observed spectroscopic peaks.
The infrared (IR) spectrum is of significant importance in many defence applications including free-space communication, thermal imaging and chemical sensing. The materials used in these applications must exhibit a number of suitable properties including mid-IR transparency, rare-earth solubility and low optical loss. When moving towards miniaturised optical devices one tends to adopt the concepts introduced by integrated optics; multiple devices operating harmoniously on a single photonic chip. Our work focuses on the use of a laser to directly write into a novel chalcogenide glass to engineer optical waveguide devices. Our material of choice is gallium lanthanum sulphide (Ga:La:S) glass, an exceptional vitreous chalcogenide material possessing these aforementioned properties as well as a broad range of other properties. These Ga:La:S glasses have a wide transmission window between 0.5 to 10 μm. Furthermore, these low-phonon energy glasses have a high transition temperature (Tg = 560°C), high refractive index, the highest reported non-linearity in a glass, excellent rare-earth solubility with well documented near-mid IR spectroscopic properties. We report on low loss single-mode active channel waveguides in Ga:La:S glass engineered through direct laser writing (λ= 244 nm). We discuss laser operation at 1.075 µm (neodymium) and IR emission at 1.55, 2.02 and 2.74 µm (erbium) from these waveguides.
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