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Neumann, Alexander

Publications and source records attributed to Neumann, Alexander.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Thermal Stability of the Dot-in-Well Gain Medium for Photonic Crystal Surface Emitting Lasers

Self-assembled quantum dots (QDs) embedded in InGaAs quantum wells (QWs) are used as active regions for photonic-crystal surface-emitting lasers (PCSELs). An epitaxial regrowth method is developed to fabricate the dot-in-well (DWELL) PCSELs. Here, the epitaxial regrowth starts with the growth of a partial laser structure containing bottom cladding, waveguide, active region, and the photonic crystal (PC) layer. The PC layer is patterned to realize the cavity. Subsequently a top cladding layer is regrown to complete the laser structure. During the regrowth of the top cladding layer, the partial laser structure is subjected to high growth temperatures in excess of 600 °C resulting in an unintentional annealing of the active region. This annealing of the active region can alter the QDs by changing their size resulting in a blue shift in photoluminescence (PL) and narrowing PL emission. This effect results in the misaligning of the gain peak and the cavity resonance, resulting in sub-optimal lasing performance. DWELL active regions are known to have better thermal stability compared to both QDs and QWs and could be an ideal candidate for regrown PCSELs. We successfully demonstrate an optically-pumped epitaxially-regrown DWELL PCSEL with an emission wavelength of 1230 nm operating at room temperature. Furthermore, the DWELL active region shows excellent emission wavelength stability and intensity despite the high temperature regrowth process.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Comparing Pr 3+ and Nd 3+ for deactivating the Er 3+ : 4 I 13/2 level in lanthanum titanate glass

Erbium lanthanum titanate glasses were prepared by levitation melting for the spectroscopic study of ways to promote the mid-infrared fluorescence. Two series of heavily erbium doped glasses (15 wt%) were prepared with the addition of either Pr 3+ or Nd 3+ in amounts relative to Er 3+ of 0.05, 0.1, and 0.2. Both ions quench the lower Er 3+ laser level with the Pr 3+ doing so more rapidly. Although high co-dopant concentrations result in higher energy transfer, as clearly evidenced in upconversion and downconversion fluorescence measurements, the mid-infrared lifetime also suffers a reduction and, therefore, a balance must be struck in the co-dopant concentration. Lifetime and spectral measurements indicate that, at a fixed relative co-dopant amount, Pr 3+ is more effective than Nd 3+ at removing the bottleneck of the Er 3+ 4 I 13/2 level. Moreover, consideration of the lifetimes alongside the absorption data of the individual ions indicates that despite the large absorption cross-section of Nd 3+ at 808 nm, the concentration needed to yield more absorbed power than utilizing direct 976 nm excitation of Er 3+ results in unfavorable lifetimes of the mid-infrared transition. In the end, Pr 3+ prevails as the superior co-dopant in terms of the effects on fluorescence lifetimes as well as potential laser system design considerations. In a unique self-doping approach, a reducing melt atmosphere of Ar instead of O 2 creates a small fraction of Ti 3+ . In 5Er 2 O 3 -12La 2 O 3 -83TiO 2 glass, the presence of Ti 3+ quenches the 4 I 13/2 emission about 2.6 times more than the 4 I 11/2 when lifetimes are compared to an O 2 melt environment. As an additional means of increasing the mid-infrared emission, the effect of temperature on the mid- and near- infrared lifetimes of a lightly doped lanthanum titanate composition is investigated between 77-300 K. The mid-infrared lifetime increases by ∼30% while the near-infrared lifetime increases by ∼10%, which suggests in addition to co-doping, active cooling of the gain media will further enhance performance.

Materials Science↗
DOE OSTI2024

Site‐selective fluorescence and spectroscopic properties of Yb‐doped lanthanum titanate glasses

Abstract Ytterbium‐doped lanthanum titanate glasses were prepared by levitation melting for the detailed characterization of the spectroscopic properties in the rare‐earth titanate glass host. Low‐temperature fluorescence spectroscopy reveals distinct site‐selectivity in both static and lifetime fluorescence measurements suggesting an absence of clustering as well as significant variation of local ytterbium environments. Typical site‐selectivity behavior of a shrinking Stark manifold with lower excitation energy is observed. At 77 K, both the mean emission frequency and the fluorescence lifetime initially increase as the excitation energy decreases from about 11100 to 10750 and then slightly decrease at lower excitation energy. Temperature‐dependent lifetime measurements between 77 and 420 K show a decreasing lifetime with increasing temperature and are well described by a two‐level thermal activation model. The temperature‐dependent fluorescence spectroscopy coupled with a room temperature white light absorption measurement allow the determination of the Stark energy levels of in lanthanum titanate glass as well as the calculation of the laser cross‐sections.

Materials Science↗
DOE OSTI2024

Nonlinear optical properties of lanthanum titanate glasses prepared by levitation melting

Lanthanum titanate glasses (17 La 2 O 3 -83 TiO 2 ) are fabricated via aerodynamic levitation melting. Here, nonlinear refraction and absorption were measured with the Z scan technique using ps optical pulses at 532 nm wavelength with peak intensities in the range of 0.25–2.5 GW/cm 2 . The two-photon absorption coefficient (β) and the effective nonlinear refraction coefficient (⁠$n^{eff}_{2}$) are found to be, respectively, 2.19 cm/GW and 152 × 10 -16 cm 2 /W (85 × 10 -13 esu). The Raman gain is measured to be 106 × 10 -11 cm/W. The nonlinearity strength is found to be nearly 60 times larger in lanthanum titanate glasses relative to silica.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Mid-infrared luminescence properties of erbium and dysprosium doped lanthanum titanate glasses

Glasses of composition x RE 2 O 3 -(17− x ) La 2 O 3 -83 TiO 2 were prepared by levitation melting from x = 0.1 to 9 for RE=Er and at x =0.1 and 1 for RE=Dy. The glasses have high transition temperature, exhibit low OH, and 1 mm thick discs are transparent out to 6 μ m. Mid-infrared emission lineshapes and lifetimes are comparable to what is seen in tellurite glasses containing Er 3+ and Dy 3+ . For x fixed at 1, the Er 3+ : 4 I 11/2 → 4 I 13/2 transition at 2716 nm has a fluorescence lifetime of 254 μ s and the Dy 3+ : 6 H 13/2 → 6 H 15/2 transition at 2957 nm has a fluorescence lifetime of 9.09 μ s. The results indicate that doped lanthanum titanate glasses offer the thermal stability of a ’hard’ glass with the host properties typically associated with ’soft’ glasses. Problems with the measurement of spectral features in the mid-infrared that could erroneously be assigned as resulting from rare-earth ion emissions are presented and discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Visible (400- to 700-nm) chirped-grating-coupled waveguide spectrometer

An integrable on-chip spectrometer, based on a transversely-chirped-grating waveguide-coupler for the 400- to 700-nm visible spectral range is demonstrated. For a fixed angle of incidence, the coupling wavelength is dependent on the local grating period and the waveguide structure. The transversely-chirped-input grating is fabricated on a SiO 2 -Si 3 N 4 -SiO 2 waveguide atop a Si substrate by interferometric lithography in two sections on a single silicon substrate. A uniform period grating, separated from the input coupler by a propagation region, is provided for out-coupling to a 2048 element CMOS detector array. The incident light with wavelength spanning 400- to 700-nm is coupled into waveguide at 33.5° through the chirped grating coupler. A resolution of ∼ 1.2 nm is demonstrated without any signal processing reconstruction.

Wang, Xuemei↗