Stimulated Emission Cross-section Spectra Research Articles

Characterisation and laser performance of a Yb:LuAG double-clad planar waveguide grown by pulsed laser deposition

We report on the fabrication of a crystalline multi-layer lutetium aluminium garnet planar waveguide, with a ytterbium-doped core, via hetero-epitaxial pulsed laser deposition on an undoped yttrium aluminium garnet substrate. Physical and optical characterization of the device revealed good crystallinity of the grown films and mode propagation investigations confirmed waveguiding properties. The measured fluorescence lifetime and calculated absorption and stimulated emission cross section spectra are found to be comparable with those reported for Yb:LuAG crystals grown by traditional methods. When end-pumped by a diode-laser bar, the crystalline double-clad Yb:LuAG planar waveguide lased using a quasi-monolithic cavity configuration. An output power of > 3 W with a 20% slope efficiency was obtained, limited by a waveguide propagation loss of 1.2 dB cm−1. This first demonstration of a multi-layer LuAG double-clad planar waveguide laser shows great potential for realising compact high-power waveguide lasers and amplifiers.

Spectroscopy of Tb3+ ions in monoclinic KLu(WO4)2 crystal application of an intermediate configuration interaction theory

The spectroscopic properties of Tb3+ ions in monoclinic KLu(WO4)2 double tungstate crystal are studied with polarized light. The absorption spectra in the visible, near- and mid-IR including the transitions to all lower-lying 7FJ (J = 0 … 5) excited states are measured. The maximum absorption cross-section for the 7F6 → 5D4 transition is 3.42 × 10−21 cm2 at 486.7 nm for light polarization E || Nm. The transition probabilities for Tb3+ ions are calculated within the Judd-Ofelt theory modified for the case of an intermediate configuration interaction (ICI). The radiative lifetime of the 5D4 state is 450 μs and the luminescence quantum yield is >90%. The polarized stimulated-emission cross-section spectra for all 5D4 → 7FJ (J = 0 … 6) emission channels are evaluated. The maximum σSE is 11.4 × 10−21 cm2 at 549.4 nm (for E || Nm). Tb3+:KLu(WO4)2 features high transition cross-sections for polarized light being promising for color-tunable visible lasers and imaging.

Czochralski growth and spectral investigations of Er:GSAG laser crystal

Er3+ doped GSAG laser crystal with high optical quality was grown successfully by the Czochralski method for the first time. The structural parameters and the high crystalline quality of this crystal were determined. Judd-ofelt (J-O) parameterization scheme was applied to analyze the absorption spectra at room temperature. Accordingly, the lines strengths, oscillator strengths, transition probabilities, fluorescence branching ratios, and radiative lifetimes were calculated to characterize the laser properties of Er:GSAG crystal. Additionally, the fluorescence spectra and lifetimes of Er:GSAG crystal were measured and studied. The stimulated emission cross-section spectra and gain cross-section spectra were evaluated for 4I13/2 →4I15/2 (1.5–1.6 µm) transitions which indicate high potential of 1 at% Er:GSAG for laser emissions at 1567 and 1605 nm. Furthermore, the possibility of Er:GSAG crystal to realize the visible laser output pumped by the GaN LDs were initially studied and analyzed.

Growth and spectroscopic investigations of the 1.5 at.% Er:GSGG laser crystal

The 1.5 at.% Er3+ doped gadolinium scandium gallium garnet (GSGG) laser crystal with high optical quality was successfully grown by the Czochralski method. The structural parameters were obtained by x-ray Rietveld refinement. A high crystalline quality of Er:GSGG was determined by x-ray rocking curve. The spectroscopic parameters of Er3+ ion were calculated and analyzed using the Judd–Ofelt theory. Furthermore, the stimulated-emission cross-section spectra were investigated for the 4I13/2 → 4I15/2 transitions at 1.5–1.6 µm, which indicates the great potential of Er:GSGG for multi-wavelength emission at 1.5–1.6 µm. The larger emission cross-section and long fluorescence lifetimes around 0.55 and 0.67 µm mean that the 1.5 at.% Er:GSGG crystal is beneficial for green and red laser generation. Meanwhile, the intensity of the emission spectra was compared between the 1.5 at.% and 2.0 at.% Er-doped GSGG. The roles of cross-upconversion, cross-relaxation and the excited-state absorption process were discussed in the 1.5 at.% Er:GSGG crystal for generating visible and 1.5–1.6 µm lasers.

Growth, structure and spectroscopic properties of 1 at.% Er3+: GdTaO4 laser crystal

1at% Er3+ doped Er: GdTaO4 laser crystal with high optical quality was successfully grown by the Czochralski method. The structural parameters were obtained by the X-ray Rietveld refinement. A high crystalline quality of Er: GdTaO4 was determined by the X-ray rocking curve. The transition intensity parameters of Er3+ ions were fitted according to the absorption spectra along three principal polarization directions. The relatively larger emission cross section (1.022×10−20cm2), long fluorescence lifetime ( 5.66ms) and relatively weaker reabsorption phenomenon around 1.6µm indicate that 1at% Er: GdTaO4 crystal can be used as a promising NIR laser material. Meanwhile, the stimulated emission cross-section spectra along three principal polarization directions were investigated for 4I13/2 →4I15/2 transitions at 1.5–1.6µm which indicates a great potential of Er: GdTaO4 for multi-wavelength emission at 1.5–1.6µm. In addition, the possibility of generating green and red lasers in Er: GdTaO4 was also analyzed and evaluated.

Temperature-dependent spectroscopy and microchip laser operation of Nd:KGd(WO4)2

High-resolution absorption and stimulated-emission cross-section spectra are presented for monoclinic Nd:KGd(WO4)2 (Nd:KGW) laser crystals in the temperature range 77–450 K. At room-temperature, the maximum stimulated emission cross-section is σSE = 21.4 × 10−20 cm2 at 1067.3 nm, for light polarization E || Nm. The lifetime of the 4F3/2 state of Nd3+ in KGW is practically temperature independent at 115 ± 5 μs. Measurement of the energy transfer upconversion parameter for a 3 at.% Nd:KGW crystal proved that this was significantly smaller than for alternative hosts, ∼2.5 × 10−17 cm3/s. When cut along the Ng optical indicatrix axis, the Nd:KGW crystal was configured as a microchip laser, generating ∼4 W of continuous-wave output at 1067 nm with a slope efficiency of 61% under diode-pumping. Using a highly-doped (10 at.%) Nd:KGW crystal, the slope efficiency reached 71% and 74% when pumped with a laser diode and a Ti:Sapphire laser, respectively. The concept of an ultrathin (250 μm) Nd:KGW microchip laser sandwiched between two synthetic diamond heat-spreaders is demonstrated.

Judd–Ofelt analysis and stimulated-emission cross-sections for highly doped (38 at%) Er:YSGG laser crystal

Stimulated-emission cross-section spectra are determined for the ~1.5μm and 3μm transitions of Er3+ ions in YSGG crystal. For the 4I11/2→4I13/2 channel, the maximum stimulated-emission cross-section σSE is 0.43×10−20cm2 at 2797.1nm. For the 4I13/2→4I15/2 channel, σSE=1.20×10−20cm2 at 1532.8nm. Due to the reabsorption loss, laser operation is expected at ~1644nm. Radiative lifetimes of all excited states of the Er3+ ion from 4I13/2 to 2H9/2 and probabilities of radiative transitions from these states are determined using the Judd–Ofelt theory. Radiative lifetimes of the 4I13/2 and 4I11/2 excited states for Er3+ ions in YSGG are 7.73ms and 9.75ms, respectively. Non-radiative decay is analyzed for lower excited-states of Er3+ ions in YSGG.

Spectroscopic and laser characterization of Yb,Tm:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO4)2 crystals. Stimulated-emission cross-section spectra corresponding to the 3F4→3H6 transition of Tm3+ ions are determined. The radiative lifetime of the 3F4 state of Tm3+ ions is 0.82ms. The maximum Yb3+→Tm3+ energy transfer efficiency is 83.9% for 5at.% Yb – 8at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO4)2 is 0.45±0.05. Using a hemispherical cavity and 5at.% Yb – 6at.% Tm doped crystal, a maximum CW power of 227mW is achieved at 1.983–2.011μm with a maximum slope efficiency η=14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5at.% Yb– 8at.% Tm doped crystal with a maximum output power of 201mW at 1.99–2.007μm. Operation of Yb,Tm:KLu(WO4)2 as a vibronic laser emitting at 2.081–2.093μm is also demonstrated.

Judd–Ofelt analysis of spectroscopic properties of Eu3+:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic study of monoclinic Eu:KLu(WO4)2 crystal concerning its potential applications in red lasers. Optical absorption and stimulated-emission cross-section spectra are determined for this crystal for the principal light polarizations, E || Np, Nm and Ng. The maximum σSE corresponding to the 5D0→7F4 transition is 1.78×10−20cm2 at 703.5nm (for E || Nm). Spectroscopic properties of Eu:KLu(WO4)2 are modeled within the conventional Judd–Ofelt theory, as well as its modification for systems with an anomalously strong configuration interaction (ASCI), yielding absorption oscillator strengths, luminescence branching ratios for 5DJ→7FJ' transitions and radiative lifetimes of the 5DJ states. Photoluminescent properties of Eu:KLu(WO4)2 are studied under UV/visible excitation. Polarized Raman spectra are measured for Eu:KLu(WO4)2.

Spectroscopic and photoluminescence characterization of Eu3+-doped monoclinic KY(WO4)2 crystal

Monoclinic 2at% Eu-doped KY(WO4)2 is grown by top-seeded solution growth method. Polarization-resolved absorption and stimulated-emission cross-section spectra are determined for this crystal. Spectroscopic properties of Eu:KY(WO4)2 are modeled within conventional Judd–Ofelt theory, as well as theory of f–f transition intensities for systems with anomalously strong configuration interaction, yielding absorption oscillator strengths, luminescence branching ratios and radiative lifetime of 5D0 state. The impact of excited-state absorption from this state on possibility of laser operation is discussed. Photoluminescent properties of Eu:KY(WO4)2 are determined. This crystal provides intense red emission with CIE coordinates x=0.670, y=0.329.

In-band pumped room-temperature Er:KY(WO4)2 laser emitting around 1.6 μm

We present efficient continuous-wave operation of an Er:KY(WO4)2 crystal under in-band pumping by a compact diode-pumped Er, Yb:GdAl3(BO3)4 laser. Maximum slope efficiency of 27% and output power of 35 mW at 1609.5 nm were obtained with beam propagation factor M2 < 1.2. Absorption and stimulated emission cross-section spectra, as well as the radiative lifetime of the 4I13/2 energy level, were determined.

Spectroscopic characterization and pulsed laser operation of Eu3+:KGd(WO4)2 crystal

Europium-doped monoclinic potassium gadolinium tungstate KGd(WO4)2 crystals are grown by the top-seeded solution growth (TSSG) technique. Their absorption spectra are studied in detail for principal light polarizations, E ∥ Np, Nm and Ng. It is accompanied by determination of absorption oscillator strengths by means of the theory of f–f transition intensities for systems with anomalously strong configuration interaction. Spectral and temporal characteristics of luminescence associated with 5D0 → 7FJ transitions are analyzed, and luminescence branching ratios and the radiative lifetime of the 5D0 state are determined. Stimulated-emission cross-section spectra are evaluated for Eu:KGd(WO4)2 crystal. Pulsed Eu:KGd(WO4)2 lasers operating at room temperature at the wavelength of 702.8 nm (5D0 → 7F4 transition) are studied at Eu concentration of 10/25 at.%. Under laser pumping at 533.6 nm, a maximum output energy of 570 μJ is obtained. The main limitation for laser operation at 5D0 → 7F2 transition is strong excited-state absorption via the 5D0 → 5F4 channel.

Near-infrared polymer semiconductor laser

Near-infrared laser emission from a polymer semiconductor has been demonstrated. A cylindrical resonator is formed with a luminescent polymer layer self-assembled on the surface of a quartz fiber 50 μm in diameter. The polymer gain layer consists of poly(9-vinylcarbazole) doped with an electron-transport material and a near-infrared-emitting molecule. The microcavity exhibits whispering-gallery-mode laser emission at 825 nm upon pulsed photoexcitation at 532 nm. A laser threshold analysis based on the absorption and stimulated emission cross-section spectra shows that the threshold minimum is found at around 824 nm. This is consistent with the experimentally observed laser emission at 825 nm.

Spectroscopy of the Bi 4Si 3O 12:Er 3+ glass for optical amplification and laser application

Spectroscopic characterization for optical amplification and laser application of the Er 3+-doped Bi 4Si 3O 12 glass is performed through its absorption and luminescence spectra. The spontaneous radiative transition probabilities from the first seven excited states to the ending states are calculated to obtain the luminescence branching ratios and radiative lifetimes. Intense infrared to visible luminescence of the Er 3+ ion is observed with 4I 9/2 state excitation (800 nm) under a 200 mW laser excitation power. The value obtained for the ratio of the 4I 11/2 → 4F 7/2 excited-state absorption oscillator strength to the 4I 15/2 → 4I 11/2 ground state absorption oscillator strength is in accordance with those reported for Er 3+-doped materials pumped efficiently at 4I 11/2 level excitation wavelengths. Gain cross-section spectra as a function of the inverted population rate and the stimulated emission cross-section spectrum were determined for the 4I 13/2 → 4I 15/2 infrared (IR) laser transition. The large gain bandwidth of the 4I 13/2 → 4I 15/2 IR emission has resulted in being close to those reported for Er 3+-doped oxide glasses suitable for broadband amplifiers.

Spectroscopic and lasing characterisation of a luminescent phenylene/bipyridine polymer

The luminescent polymer, poly[2,2′-bipyridine-5,5′-diyl-(2,5-dihexyl-1,4-phenylene)] (abbreviated PPBpy) dissolved in tetrachloroethane (TCE) and as neat film is characterised by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes are determined. A lasing characterisation is carried out by pumping with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm, duration 35 ps). The excited-state absorption at the pump-laser wavelength is determined by saturable absorption measurements. Laser oscillation of the polymer in TCE in an uncoated rectangular cell was achieved by transverse pumping using the cell windows for light feedback. From the laser threshold pump pulse energy density the effective stimulated emission cross-section at the lasing wavelength is extracted. The travelling-wave lasing behaviour of the polymer as neat film is studied by analysis of the amplification of the transverse pumped spontaneous emission. Surface emitting distributed-feedback lasing was achieved with a neat film on corrugated second-order periodic gratings.

Linear and nonlinear optical spectroscopic characterisation of triphenylamine and 1,2,3-tris(3-methylphenylphenylamino)benzene

The molecule triphenylamine (TPA) in tetrahydrofuran (THF) and the starburst triphenylamine oligomer 1,3,5-tris(3-methylphenylphenylamino)benzene (m-MTDAB) in THF and as neat film are characterized. Absorption and emission spectroscopic parameters are determined. The S 0–S 1 transition is found to be weak (nπ * transition) with small fluorescence quantum yield and efficient non-radiative decay (singlet–triplet intersystem crossing). The S 0–S 1 absorption cross-section spectra in the tails of the broad first absorption bands of the compounds are separated out by radiative lifetime determination and applying the mirror-image relation between absorption and emission. Excimer formation is observed for m-MTDAB neat films, its photo-dynamics is studied, and monomer and excimer stimulated emission cross-section spectra are extracted. Reverse saturable absorption is observed for TPA and m-MTDAB in picosecond laser nonlinear transmission measurements (laser duration 35 ps, laser wavelength 347.15 nm) and the responsible excited-state absorption cross-sections are determined. A new method is developed to calculate the excited-state absorption cross-section spectra of the samples in the fluorescence spectral region by amplification/attenuation of spontaneous emission measurements. The excited-state absorption is found to be larger than the stimulated emission for TPA in THF and for m-MTDAB films excluding their lasing ability. For m-MTDAB in THF the stimulated emission is found to be slightly larger than the excited-state absorption at the wavelength position of peak fluorescence emission.

Spectroscopic and travelling-wave lasing characterisation of tetraphenylbenzidine and di-naphtalenyl-diphenylbenzidine

The organic light emitting diode (OLED) hole transport molecules N,N,N’,N’-tetraphenylbenzidine (TPB, triphenylamine dimer TAD or TPD) and N,N’-bis(2-naphtalenyl)-N,N’-bis(phenylbenzidine) (β-NPB, naphtyl-diphenylamine dimer β-NPD), dissolved in tetrahydrofuran (THF) and as neat film, are characterized by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarization, and fluorescence lifetimes are determined. The lasing behaviour is studied by picosecond laser pulse excitation (excitation wavelength 347.15 nm, pulse duration 35 ps). The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurement. Low-Q laser oscillation of TPB in THF is achieved by transverse pumping of the dye in a cell. The excited-state absorption of TPB in THF at the laser wavelength is extracted from the laser threshold. In TPB neat films, wave-guided travelling-wave lasing was obtained. No laser action was achieved for β-NPB because of small S1-S0 stimulated emission cross-section, and the presence of excited-state absorption in the fluorescence wavelength region. The TPB and β-NPB results are compared with the corresponding spectroscopic and lasing behaviour of the related methyl-substituted triphenylamine dimers, 3-methyl-TPD and 4-methyl-TPD, which are well established OLED hole transport materials.

Spectroscopic and lasing characterisation of a dicarbazovinylene-MEH-benzene dye

The dye 1,4-bis(9-ethyl-3-carbazovinylene)-2-methoxy-5-(2′-ethyl-hexyloxy)-benzene (abbreviated 2CzV-MEH-B) dissolved in tetrahydrofuran (THF) and as neat film is characterised by optical absorption and emission spectroscopy. The absorption and stimulated emission cross-section spectra, the fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes are determined. A lasing characterisation is carried out by pumping with single second harmonic pulses of a mode-locked ruby laser (wavelength 347.15 nm, pulse duration 35 ps). The excited-state absorption at the pump laser wavelength is determined by saturable absorption measurements. Laser oscillation of the dye in THF in a rectangular cell is achieved by transverse pumping using the uncoated cell windows for light feedback. From the emission behaviour around threshold the excited-state absorption cross-section spectrum in the laser active spectral region is extracted. The wave-guided travelling-wave lasing behaviour of the dye as neat film is studied by analysis of the amplification of the transverse pumped spontaneous emission. Surface emitting distributed-feedback lasing was achieved with a neat film on corrugated second-order periodic gratings.

Spectral kinetic properties of Yb3+:Na4Y6F22 and Yb3+:LiLuF4 crystals

Yb3+ (2.7 at.%):Na4Y6F22 and Yb3+(1 at.%):LiLuF4 crystals were grown by the Bridgman-Stockbarger method. We measured the temperature dependences of the thermal conductivity of the crystals and the absorption spectra. We determined the radiative lifetime of the Yb3+ ion in these crystals (1.94 msec and 2.13 msec) and calculated the stimulated emission cross section spectra.

Influences of radiation trapping on spectroscopic properties of Er^3+-doped fluorophosphate glasses

Fluorophosphate glasses with different contents of ErF3 were prepared. Due to the radiation trapping of Er, concentration dependence of the fluorescence lifetime is subject to distortion, and the stimulated-emission cross section calculated by the Fuchtbauer-Ladenburg equation is underestimated. The influence of radiation trapping on the measured fluorescence lifetime and width are investigated quantitatively. By comparing the intensity ratio of the 1556-1532 nm peak in the fluorescence spectrum with that in the stimulated-emission cross-section spectrum obtained according to the McCumber theory, the distortion ratio of fluorescence spectrum due to radiation trapping is obtained. An empirical way to quantitatively evaluate the influences of radiation trapping on fluorescence lifetime and width is proposed.