Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15-60 min range.A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.     

Luminescence properties of spark-processed Si in air, O2, and N2 with low pressure

This paper reports the luminescence properties of spark-processed Si (sp-Si) prepared with different atmospheres such as air, O₂, and N₂ in low vacuum range (50-760 Torr). Three main luminescence bands are observed from spark-processed Si (sp-Si). In addition to the well-known two luminescence bands in the blue/violet peaking at 410 nm and green peaking at 500 nm, a novel UV luminescence band is detected for the sp-Si prepared in N₂. The temperature dependence of photoluminescence (PL) characteristics of the newly detected UV luminescence band is examined. Further studies of photoluminescence excitation (PLE) have been performed and origins of luminescence are discussed based on the experimental results.     

Photoluminescence of spark-processed metals

The photoluminescence (PL) properties of spark-processed (sp) metals were systematically investigated. It was found that spark-processing of Cr, Cu, Fe, Mg, and Mn essentially yielded no PL at room temperature in the visible spectral range. In contrast, sp-Ti, Ta, Al, Ni, Mo, and Zn showed strong PL, which is in many respects comparable to sp-Si. It was further found that the PL intensity of the sp metals depends exponentially on the nitrogen concentration, which is contained in the spark-processing ambient (along with oxygen). This behavior was also found for sp-Si. In contrast, the PL intensities for native oxides (containing essentially no nitrogen) is several orders of magnitude smaller than those for sp-metals and the respective PL peaks are situated at higher energies. The results are discussed in the light of the self-trapped exciton model.     

Variation of spark-processing parameters on the photoluminescence properties of spark-processed silicon

Electrical and physical parameters, which influence the photoluminescence (PL) properties of spark-processed silicon (sp-Si), were systematically varied in order to obtain optimal PL emission. Among these parameters are the average spark current, the pulse width of the spark events, the frequency of the pulses, the processing time, the electrode diameter, the distance between the electrodes, the spark-processing environment, and the gas ambient pressure. It was found that for optimal PL emission the processing current needs to be between 20 and 40 mA, and the pulse frequency of the sparks between 10 and 15 kHz. Further, the N₂/O₂ ratio of the processing environment needs to be about 7:3 and the ambient gas pressure and the processing time as large as feasible. The conditions that are favorable for green PL are a small pulse width, a small counter electrode diameter, a small gap between electrodes, a relatively large nitrogen concentration in the processing chamber, and a comparatively large spark frequency. In the opposite cases, a UV/blue PL is predominantly observed. The results are discussed in terms of various thermal effects on the resulting molecules or defects, which are believed to be important for the PL emission.     

Near-infrared emission bands of Er-doped YAP and LSO crystals

The ground state absorption (GSA), photoluminescence (PL) and photoluminescence excitation (PLE) spectra for Er(1.0 at%):YAP and Er(0.5 at%):LSO were measured at room temperature. Based on the GSA spectra, the radiative transition rates and luminescence branch ratios of erbium ions were determined by the Judd-Ofelt (J-O) method. In the range of 1400-1700 nm Er(1.0 at%):YAP has intense absorption at 1509 nm (0.96×10⁻²⁰ cm²), which is almost two times larger than the peak absorption of Er(0.5 at%):LSO. From the PL and PLE spectra, four intense emission bands around 850 nm (⁴S_3/2→⁴I_13/2), 980 nm (⁴I_11/2→⁴I_15/2), 1230 nm (⁴S_3/2→⁴I_11/2) and 1520 nm (⁴I_13/2→⁴I_15/2) were observed. The stimulated emission cross-sections of the four bands were calculated by the Fuchtbauer-Ladenberg (F-L) equation. The results suggest that Er(1.0 at%):YAP has potential to realize laser oscillation at 858 nm because of the relatively large simulated emission cross-section (1.76×10⁻²⁰ cm²). The temperature dependences of the PL spectra for the two crystals were also investigated in the range of 290-12 K. The ∼1520 nm emission presents continuous increase with temperature, while the emissions around 850, 1230 and 980 nm firstly increase with temperature, then reach their own largest values at the transition temperatures (about 100 K), and finally decrease with temperature. These results were well interpreted by the temperature dependence of multi-phonon process.     

Waveguiding and photoluminescence in Er-doped Ta2O5 planar waveguides

The optimization of erbium-doped Ta₂O₅ thin film waveguides deposited by magnetron sputtering onto thermally oxidized silicon wafer is described. Optical constants of the film were determined by ellipsometry. For the slab waveguides, background losses below 0.4 dB/cm at 633 nm have been obtained before post-annealing. The samples, when pumped at 980 nm yielded a broad photoluminescence spectrum (FWHM∼50 nm) centred at 1534 nm, corresponding to ⁴I_13/2-⁴I_15/2 transition of Er³⁺ ion. The samples were annealed up to 600 °C and both photoluminescence power and fluorescence lifetime increase with post-annealing temperature and a fluorescence lifetime of 2.4 ms was achieved, yielding promising results for compact waveguide amplifiers.     

Excitation mechanisms and localization sites of erbium-doped porous silicon

Porous silicon (PS) is doped with erbium by electrochemical anodisation. The penetration of erbium into the PS layer is confirmed by Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray (EDX) measurements. Efficient green and infrared emissions were observed at room temperature. The investigations are focused on the evolutions versus temperature and pump intensity of the green photoluminescence (PL) corresponding to the ⁴S_3/2 → ⁴I_15/2 transition. It was found that an erbium related level defect can be involved on the excitation and emission processes of erbium. Pump intensity dependent PL studies revealed that for the electrochemical incorporation, most of the Er³⁺ ions are localized inside the Si nanocrystallites and not in stoichiometric SiO₂. The optical cross-section is close to that of erbium in Si nanocrystallites.     

Visible and NIR luminescence of nanocrystalline β-Ga2O3:Er prepared by solution combustion synthesis

In this paper we report on facile solution combustion synthesis of erbium doped β-Ga₂O₃ with urea as fuel. The product was characterized using powder X-ray diffraction and transmission electron microscopy (TEM). X-ray diffraction and TEM showed that the material is nanostructured. Luminescence properties of β-Ga₂O₃:Er are studied with excitation in near infrared (Nd:YAG laser at 1064 nm) and visible (argon laser at 514.5 nm). A strong NIR emission of Er³⁺ in the window of minimal optical loss in silica based optical fibers, due to the ⁴I_13/2→⁴I_15/2 transition at 1.55 μm has been observed. Codoping with Yb³⁺ significantly increases the intensity of that important emission.     

Host dependent frequency upconversion of Er-doped oxyfluoride germanate glasses

Er³⁺-doped oxyfluoride germanate glasses have been synthesized by the conventional melting and quenching method. The Judd-Ofelt intensity parameters were calculated based on the Judd-Ofelt theory and absorption spectra measurements. With the substitution of PbF₂ for PbO, the Ω₂ parameter decreases, while the Ω₆ parameter increases. These change trends indicate that fluoride anions come to coordinate erbium cations and the covalency of the Er-O bond decreases. Structural and thermal stability properties were obtained by Raman spectra and differential thermal analysis, indicating that PbF₂ plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were simultaneously observed at room temperature. With increasing PbF₂ content, the intensity of red (657 nm) emissions increases significantly, while that of the green (525 and 546 nm) emission increases slightly. The results indicate that PbF₂ has more influence on the red (657 nm) emission than the green (525 and 546 nm) emissions in oxyfluoride germanate glasses. The possible upconversion luminescence mechanisms have also been estimated and discussed.     

Broadband photoluminescence from pulsed laser deposited Er2O3 films

Photoluminescence (PL) with the bandwidth of 45 nm (1523-1568 nm at the level of 3 dB) was observed in amorphous Er₂O₃ films grown on to the quartz substrate by pulsed laser ablation of erbium oxide stoichiometric target. Optical transmission spectrum has been fitted to Swanepoel formula to determine the dispersion of refractive index and to extract resonance absorption peaks at 980 and 1535 nm. The maximum gain coefficient of 800 dB/cm at 1535 nm was estimated using McCumber theory and experimental spectrum of the resonance absorption. In 5.7 mm-long waveguide amplifier a theory predicts the spectral gain of 20 dB with 1.4 dB peak-to-peak flatness in the bandwidth of 31 nm (1532-1563 nm) when 73% of Er³⁺ ions are excited from the ground state to the ⁴I_13/2 laser level. Strong broadband PL at room temperature and inherently flat spectral gain promise Er₂O₃ films for ultra-short high-gain optical waveguide amplifiers and integrated light circuits.     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

Competition between green and infrared emission in Er:YLiF4 upconversion lasers

The competition between two laser transitions in Er:YLiF₄ (⁴S_3/2 → ⁴I_15/2 at 551 nm and ⁴S_3/2 → ⁴I_13/2 at 850 nm) is studied using a model based on rate equations. The laser emission is pumped by upconversion at 795 nm; for comparison, we also discuss upconversion pumping by another mechanism, at 970 nm. The conditions that favor laser emission in various regimes on these two transitions are found.     

Absorption and emission of light in spark-processed silicon

Spark-processed Si (sp-Si) exhibits blue, green and red photoluminescence at around 385, 525 and 650 nm, depending on the wavelength of excitation. Its optical absorption spectrum reveals bands peaked approximately at 245, 277, 325 and 389 nm. The centers where absorption takes place were modeled as Si and silica clusters in an amorphous SiO_xN_y matrix using various embedding schemes. Geometry optimizations were applied prior to calculations of the absorption spectra of the clusters. The measured absorption spectrum of sp-Si and calculated absorption spectra were compared. Best agreement is achieved for Si particles embedded in amorphous SiO_xN_y matrix. The importance of the various embedding schemes is discussed and conclusions for the centers of emission are established.     

Infrared and visible spectroscopic studies of the ytterbium doped borate Li6Y(BO3)3

The spectroscopic study of trivalent ytterbium doped Li₆Y(BO₃)₃ is conducted in the UV-visible and infrared range. An excitation in the charge transfer band of ytterbium has been selected in order to reduce the reabsorption effect on the IR emission intensity. The maximum of the emission is located at 972 nm for an excitation at 230 nm. The energy level assignment has been successfully conducted using vibrational spectroscopy to distinguish the pure electronic transitions from the phonon-assisted ones. The splitting of the ²F_5/2 and ²F_7/2 components is equal to 523 cm⁻¹ and 676 cm⁻¹, respectively. The decay time dependence as a function of the concentration is also reported. The calculated value τ_rad is about (1.03 ± 0.01) ms for the 1% doped material. For the highest concentration, an IR excitation gives rise to the observation of a blue-green luminescence caused by two mechanisms: an erbium emission at 550 nm after upconversion and a cooperative luminescence of ytterbium ions.     

Optical spectroscopy of heavily Ho-doped BaY2F8 crystals

The ultraviolet, visible, and near IR (0.8-2.4 μm) luminescence spectra of BaY₂F₈ single crystals heavily doped with Ho³⁺ ions (10 and 30 mol%) have been investigated at room temperature and 12 K, together with the luminescence decay curves (up to 300 μs) of the visible emission. Excitation in the visible region gives rise to very strong emission bands originating from the first ⁵I₇ level and located around 2070 nm. However the ⁵I₇ emission is not observed upon excitation at wavelengths shorter than 300 nm. The inter-ionic processes are found to shorten the decay times of the levels emitting in the visible region with respect to the corresponding radiative lifetimes.     

1.54 μm room temperature emission from Er-doped Si nanocrystals deposited by ECR-PECVD

In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er³⁺ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy.Room temperature emission bands centered at ∼1.54 and at 0.75 μm have been obtained for all samples. The most intense emission band at ∼1.54 μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75 μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality.     

Spectroscopic investigations of Nd-, Er-, Er/Yb-, and Tm-ions doped SiO2-Al2O3-CaF2-GdF3 glasses

This paper reports on the absorption, visible and near-infrared luminescence properties of Nd³⁺, Er³⁺, Er³⁺/2Yb³⁺, and Tm³⁺ doped oxyfluoride aluminosilicate glasses. From the measured absorption spectra, Judd-Ofelt (J-O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. Decay lifetime curves were measured for the visible emissions of Er³⁺ (558 nm, green), and Tm³⁺ (650 and 795 nm), respectively. The near infrared emission spectrum of Nd³⁺ doped glass has shown full width at half maximum (FWHM) around 45 nm (for the ⁴F_3/2→⁴I_9/2 transition), 45 nm (for the ⁴F_3/2→⁴I_11/2 transition), and 60 nm (for the ⁴F_3/2→⁴I_13/2 transition), respectively, with 800 nm laser diode (LD) excitation. For Er³⁺, and Er³⁺/2Yb³⁺ co-doped glasses, the characteristic near infrared emission bands were spectrally centered at 1532 and 1544 nm, respectively, with 980 nm laser diode excitation, exhibiting full width at half maximum around 50 and 90 nm for the erbium ⁴I_13/2→⁴I_15/2 transition. The measured maximum decay times of ⁴I_13/2→⁴I_15/2 transition (at wavelength 1532 and 1544 nm) are about 5.280 and 5.719 ms for 1Er³⁺ and 1Er³⁺/2Yb³⁺ (mol%) co-doped glasses, respectively. The maximum stimulated emission cross sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and Er³⁺/Yb³⁺ are 10.81×10⁻²¹ and 5.723×10^-21 cm². These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification at low-loss telecommunication windows.     

~ 1.2 μm near-infrared emission and gain anticipation in Ho doped heavy-metal gallate glasses

Ho³⁺-doped low-phonon-energy heavy-metal gallate glasses (LKBPBG) have been prepared and efficient 1.199 μm emission originating from the ⁵I₆ → ⁵I₈ radiative transition has been observed under 900 nm excitation. The spontaneous emission probability and the maximum stimulated emission cross-section were derived to be 294.31 s^− 1 and 3.46 × 10^− 21 cm², respectively. The ratio of quantum yields between ~ 1.2 and ~ 2.0 μm emissions was identified to be 16%, demonstrating that the ⁵I₆ → ⁵I₈ transition is favorable for optical amplification. The maximum gain coefficient of 1.84 dB/cm at 1.199 μm wavelength was anticipated in the ideal status. These results indicate that the Ho³⁺-doped LKBPBG glasses have a promising potential for the development of ~ 1.2 μm signal amplifier devices.     

Kinetics of luminescence of interface defects and resonant Er ions in nanostructured SnO2:SiO2

Silica glass with SnO₂ nanocrystals and Er³⁺ ions are prepared by the sol-gel route and treatment above 1000 °C. Transmission electron microscopy evidences a homogeneous dispersion of nanoclusters 4-6 nm in size in the amorphous silica matrix. Photoluminescence spectra excited at 3.5 eV, outside erbium transitions, show an inhomogeneous spectral distribution of light emission from interface defects, in the range 1.9-2.4 eV, resonant with transitions of erbium ions. The analysis of kinetics and temperature dependence of luminescence allows to quantify the efficiency of the energy transfer channel between nanoclusters and erbium ions.     

Effect of ZnO as modifier on up and downconversion properties of Ho/Yb doped tellurite glasses

Optical absorption and photoluminescent properties of Ho³⁺/Yb³⁺ co-doped tellurite and zinc tellurite glasses are investigated. The effect of zinc oxide as a modifier on the luminescence properties of above mentioned samples has been explored. Two intense upconversion emission bands centered at 546 (⁵F₄ + ⁵S₂ → ⁵I₈) and 660 nm (⁵F₅ → ⁵I₈) are observed on excitation with 976 nm diode laser. Zinc oxide acts as a quencher for 976 nm excited upconversion emission. The up and downconversion emission spectra are recorded with 532 nm excitation source also. In this case zinc oxide improves the up and downconversion emissions. A large enhancement in upconversion intensity has been observed when Ho³⁺ ion is co-doped with Yb³⁺ ion. The dependence of upconversion intensities on excitation power and on temperature has also been studied. The power dependence study shows a quadratic dependence of the fluorescence intensity on the excitation power while a decrement in emission intensity of all the transitions at different rates with increase in temperature is observed in temperature dependence study. The possible mechanisms are also discussed in order to understand the upconversion and energy transfer processes.