Tunable Ti Research Articles

Time-Resolved Studies of Excitonic Dynamics in a Wide II-VI Quantum Well by a Femtosecond Pump-Probe Reflectivity

The time-resolved reflection was studied on wide quantum wells (QW) based on semimagnetic semiconductors. The QW made of Cd1-xMnxTe were embedded between (Cd,Mg)Te barriers. The use of 160 A wide QW allowed observation of up to 7 excitonic lines associated to excited states of carriers confined in the QW. Several samples with concentration of Mn ions in QWs ranging from 0 to 2.5% were used. The giant Zeeman splitting was primarily used to assign the observed lines to heavy and light hole excitons. The time-resolved reflection experiments were performed in a pump-probe set-up with a tunable Ti:Al2O3 femtosecond pulsed laser as a light source. The delay between pump and probe pulses was precisely controlled by a delay line. The same laser was a source for both the pump and the probe pulses. The probe is an spectrally broad (>40nm) laser pulse taken directly from the laser. The pump is a spectrally narrow line selected from the dispersed laser spectrum. Using the pump pulses narrowed spectrally to about 1nm it was possible to excite resonantly selected states in a quantum well. The independent circular polarizers on the pump and probe beams allowed to study the evolution of populations of states with different spin orientations. The experimental set-up was similar to those used in Refs. [1] and [2] to study ultrafast processes in narrow QW, i.e. the interplay between excitons, trions and carriers in QWs. In the present experiments we extended such studies on the excited states in wide QW. In particular we observed a significant quenching of the absorption of the excited states by the high population of excitons in the fundamental excitonic subband. It was also possible to study the dynamics of the relaxation from the excited levels to the ground states.

Fluorescence-Suppressed Raman Technique for Quantitative Analysis of Protein Solution Using a Micro-Raman Probe, the Shifted Excitation Method, and Partial Least Squares Regression Analysis

A practical Raman analyzing technique with suppression of the strong fluorescent background in order to obtain quantitative information is proposed in the present study. The technique is based on the shifted excitation method and partial least squares regression (PLSR) analysis. The Raman system consists of a single Raman spectrometer, a background-free electrically tunable Ti:Sapphire laser (BF-ETL), and a micro-Raman probe (MRP). The system allows one to obtain reliable shifted excitation Raman spectra with a simple operation. The PLSR analysis successfully provides quantitative information from the obtained spectra with the suppression of random noise including photon shot noise. The present study demonstrates that the technique is effective for extracting quantitative information concealed behind a fluorescent background that is more than 200 times stronger than the Raman signal.

Intermediacy of Poly(l-proline) II and β-Strand Conformations in Poly(l-lysine) β-Sheet Formation Probed by Temperature-Jump/UV Resonance Raman Spectroscopy

Ultraviolet resonance Raman spectroscopy (UVRR) in combination with a nanosecond temperature jump (T-jump) was used to investigate early steps in the temperature-induced alpha-helix to beta-sheet conformational transition of poly(L-lysine) [poly(K)]. Excitation at 197 nm from a tunable frequency-quadrupled Ti:sapphire laser provided high-quality UVRR spectra, containing multiple conformation-sensitive amide bands. Although un-ionized poly(K) (pH 11.6) is mainly alpha-helical below 30 degrees C, there is a detectable fraction (approximately 15%) of unfolded polypeptide, which is mainly in the poly(L-proline) II (PPII) conformation. However, deviations from the expected amide I and II signals indicate an additional conformation, suggested to be beta-strand. Above 30 degrees C un-ionized poly(K) forms a beta-sheet at a rate (minutes) which increases with increasing temperature. A 22-44 degrees C T-jump is accompanied by prompt amide I and II difference signals suggested to arise from a rapid shift in the PPII/beta-strand equilibrium. These signals are superimposed on a subsequently evolving difference spectrum which is characteristic of PPII, although the extent of conversion is low, approximately 2% at the 3 micros time limit of the experiment. The rise time of the PPII signals is approximately 250 ns, consistent with melting of short alpha-helical segments. A model is proposed in which the melted PPII segments interconvert with beta-strand conformation, whose association through interstrand H-bonding nucleates the formation of beta-sheet. The intrinsic propensity for beta-strand formation could be a determinant of beta-sheet induction time, with implications for the onset of amyloid diseases.

Two-color photoionization of noble gases using laser and VUV synchrotron radiation

Two-photon double-resonant photoionization of Ar and Xe to their autoionizing states lying between the 2P 3/2 and 2P 1/2 ionic thresholds was carried out with the combination of a laser system and VUV synchrotron radiation from the storage ring at NSRRC. Tuning this radiation of narrow width to a selected Rydberg state was followed by excitation to even-parity levels of autoionization states with a tunable Ti:sapphire laser. The experimental results show two-photon ionization spectra of argon with a large ratio of signal to noise. When the intermediate state 3p 5( 2P 1/2)3d′ [3/2] 1 of argon is excited, the spectrum shows a weak continuous background and three autoionizing series 3p 5 nf′[5/2] 2 ( n = 9–20), 3p 5 np′ [3/2] 2 and 3p 5 np′ [1/2] 0. The latter two series were resolved partly because of improved resolution in this work and partly because their relative intensities vary with the relative polarization of two excitation sources.

Time dependence and excitation spectra of the photoluminescence emission at 1.54μm in Si-nanocluster and Er co-doped silica

Abstract The risetime of the 1.54 μm luminescence emission in Er:Si-nc co-doped glasses provides information on the mechanisms leading to population of the luminescent level, i.e. on the energy transfer time. In this paper we present a detailed study of this risetime in silica glasses after excitation in the wavelength region 360–860 nm by a continuously tunable pulsed Ti:Al 2 O 3 laser. The emission risetime turns out to be dependent on the sample fabrication method used, i.e. implantation or co-sputtering and on the annealing temperature used to precipitate the silicon aggregates. The fastest energy transfer time observed in our samples is τ tr ⩽ 2 μs. The risetime was found to be weakly dependent on the excitation wavelength up to around 700 nm where a sudden increase is observed. This increase is more pronounced in samples where the annealing temperature was such to produce amorphous aggregates rather than crystalline ones. The difference in the nature of electronic states excited by near-infrared pumping in the two types of samples containing crystalline or amorphous aggregates is also evidenced by CW photoluminescence excitation (PLE) measurements which reflect the absorption coefficient of the sensitizing centers. In samples containing amorphous aggregates we observe an exponential dependence of the PL emission intensity versus excitation energy, which is indicative of an Urbach tail due to the disordered structure of aggregates. In the sample containing crystalline aggregates we observe an indirect interband transition behavior, with an energy gap of 1.56 eV.

Mutual quenching of Er 3+ photoluminescence under two laser excitation in GaN:Er

Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He–Cd ( λ = 325 nm ) laser and below-band-gap excitation by a tunable Ti–Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er 3+ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lock-in amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er 3+ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er 3+ emission.

High sensitivity broad-band mode-locked cavity-enhanced absorption spectroscopy: measurement of Ar*(3P2) atom and ion densities

The recently developed broad-band absorption technique of ‘mode-locked cavity-enhanced absorption spectroscopy’ (ML-CEAS) is applied to the diagnostics of argon and nitrogen plasmas. Using a commercial tunable mode-locked Ti : Sa femtosecond laser, this combines the multipass advantage of the cavity-enhanced technique with the simultaneous acquisition over a broad spectral range of classic broad-band absorption spectroscopy. Absorption spectra in the 400 nm range are acquired after frequency doubling of the Ti : Sa femtosecond laser in a BBO crystal. The measurement of the metastable Ar*(3P2) atom density in a low pressure argon glow discharge through its weakly absorbing lines at 394.75 and 394.898 nm allows us to illustrate the importance of the apparatus' spectral resolution for determination of absolute number densities. Absorption spectra of the first negative band of the nitrogen ion, , around 391 nm are recorded in a nitrogen glow discharge and in the afterglow of a flowing μ-wave nitrogen plasma. The absolute ion density (and its rotational temperature) measured in the discharge zone and in the maximum of the short-lived afterglow (SLA) are 1.5 × 1015 ions m−3 (1300 K) and 1.0 × 1015 ions m−3 (800 K), respectively. Compared with the previously measured electron density at the maximum of the SLA, it is concluded that ions are not the dominant positive ions in this zone.

All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation.

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.

Temporal Control of Pulses from a High-Repetition-Rate Tunable Ti:Sapphire Laser by Active Q-switching

We investigated the lasing characteristics of a Ti:sapphire laser pumped by a pulsed high-repetition-rate Nd:YAG laser. The pump laser has a pulsewidth of 450 ns, while the Ti:sapphire laser shows a significantly shorter pulse width of 25 ns for suitably intense pumping. The energy conversion efficiency of the laser is more than 10%. To synchronize different lasers and to avoid multiple spiking during one pump pulse, we use a Brewster-cut Pockels cell in the resonator for Q-switching. The temporal profile and conversion efficiency are determined and compared to theoretical estimates.

Elucidation of the Nature of the Conformational Changes of the EF-interhelical Loop in Bacteriorhodopsin and of the Helix VIII on the Cytoplasmic Surface of Bovine Rhodopsin: A Time-resolved Fluorescence Depolarization Study

The conformation of the AB-loop and EF-loop of bacteriorhodopsin and of the fourth cytoplasmic loop (helix VIII) of bovine rhodopsin were assessed by a combination of time-resolved fluorescence depolarization and site-directed fluorescence labeling. The fluorescence anisotropy decays were measured employing a tunable Ti:sapphire laser/microchannel plate based single-photon counting apparatus with picosecond time resolution. This method allows measurement of the diffusional dynamics of the loops directly on a nanosecond time-scale. We implemented the method to study model peptides and two-helix systems representing sequences of bacteriorhodopsin. Thus, we systematically analyzed the anisotropic behavior of four different fluorescent dyes covalently bound to a single cysteine residue on the protein surface and assigned the anisotropy decay components to the modes of motion of the protein and its segments. We have identified two mechanisms of loop conformational changes in the functionally intact proteins bacteriorhodopsin and bovine rhodopsin. First, we found a surface potential-dependent transition between two conformational states of the EF-loop of bacteriorhodopsin, detected with the fluorescent dye bound to position 160. A transition between the two conformational states at 150 mM KCl and 20 °C requires a surface potential change that corresponds to Δσ≈−1.0 e −/bacteriorhodopsin molecule. We suggest, that the surface potential-based switch of the EF-loop is the missing link between the movement of helix F and the transient surface potential change detected during the photocycle of bacteriorhodopsin. Second, in the visual pigment rhodopsin, with the fluorescent dye bound to position 316, a particularly striking pH-dependent conformational change of the fourth loop on the cytoplasmic surface was analyzed. The loop mobility increased from pH 5 to 8. The midpoint of this transition is at pH 6.2 and correlates with the midpoint of the pH-dependent equilibrium between the active metarhodopsin II and the inactive metarhodopsin I state.

Enhancement of the thermal lens signal induced by sample matrix absorption of the probe laser beam.

The effect of the absorption of the probe laser beam by the sample matrix on the thermal lens signal of a solute was investigated for aqueous solutions of Tb(III), Yb(III), and Nd(III). The measurements were performed with a thermal lens instrument in which the pump and the probe beam were derived from a tunable Ti:sapphire laser. Thermal lens signals were found to be enhanced in the region where the probe beam overlapped with the absorption band of the sample matrix. The observed enhancement was confirmed further with samples of the same solutes (lanthanide ions) but in D20, which does not absorb in the same spectral region as water. The enhancement may be due to the fact that absorption by the sample matrix led to a change in its refractive index and the production of a temperature gradient. In addition to fundamental importance, the observed enhancement can be used to improve the sensitivity of the thermal lens measurements by judiciously selecting a solvent that absorbs the probe laser beam.

High-resolution LIF measurements on hyperfine structure and isotope shifts in various states of Lu I using the second and third harmonic of a cw Ti:sapphire laser

With the second and third harmonic of a tunable single-mode cw Ti:S laser, generated inside external enhancement cavities, high-resolution LIF measurements on several states of Lu I in various parts of the electromagnetic spectrum are performed. From these measurements, hyperfine structure A and B constants for both 175Lu and 176Lu as well as isotope shifts have been determined for all levels observed in the single-step excitation process. From the measured A constants, the magnetic hyperfine structure anomaly has been derived for various states.

Z-형태의 대칭형 레이저 공진기 구조를 갖는 연속 발진 및 Kerr-렌즈 모드-록킹되는 티타늄 사파이어 레이저의 설계와 동작 특성

티타늄 사파이어 레이저에 대한 활용도를 높기 위하여 4개의 거울로 구성된 Z-형태의 대칭형 레이저 공진기를 구성하여 넓은 파장가변성을 갖는 고효율 연속 발진 티타늄 사파이어 레이저를 제작하였다. 또한 Kerr-렌즈 모드-록킹(KLM)을 일으키기 위한 KLM 강도 및 공진기 모드 크기가 Kerr 매질을 포함하는 Z-형태의 대칭형 공진기에 대하여 공진기 내의 위치, 내부 공진기 레이저 출력 및 안정 파라메터 등에 의존함을 확인하였고, 이를 근거로 하여 펌핑 효율이 높고 KLM 강도가 강한 KLM 티타늄 사파이어 레이저를 제작하여 출력 특성, 펄스폭 및 스펙트럼 반치폭 등을 측정하였다. 그 결과 연속 발진 티타늄 사파이어 레이저의 출력에 대한 기울기 효율은 31.3%이고, 5W의 Ar-이온 펌핑 레이저 출력에 대해 최대 1420㎽의 평균 출력을 얻을 수 있었으며, 파장가변 영역은 730㎚~908㎚까지 모두 700㎽ 이상의 평균 출력을 보였다. 자체-조리개 효과에 의한 KLM 동작 또한 쉽게 얻을 수 있었으며, KLM된 티타늄 사파이어 레이저로부터 출력에 대한 기울기 효율은 16%이고, 5W 펌핑 출력에 대해 최대 500㎽의 평균 출력을 얻었다. 또한 중심 파장 807㎚에서 스펙트럼 반치폭이 33㎚이며, 펄스 반복률이 82㎒인 27fs의 안정된 짧은 펄스를 얻을 수 있었다. We have constructed a high efficiency and broad tunable cw Ti:sapphire laser with a four-mirror symmetric Z-type laser cavity to increase the laser usability. From theoretical analyses and experimental data for a symmetric Z-type laser cavity containing a Kerr medium, the cavity mode size and the Kerr-lens mode-locking (KLM) strength for KLM lasers can be confirmed as function of the position in the cavity, the intracavity laser power, and the stability parameter. As a result, the slope efficiency and the maximum average output power of cw Ti:sapphire laser at 5 W pumping power of Ar-ion laser were 31.3% and 1420 ㎽ respectively. The tunablility was ranged from 730 ㎚ to 908 ㎚ with average output power above 700 ㎽. We obtained the KLM operation easily by self-aperturing effect in the Kerr medium and the slope efficiency and the maximum average output power of KLM Ti:sapphire laser was 16% and 550 ㎽ respectively. The spectral bandwidth was 33 ㎚ at the center wavelength of 807 ㎚ and the pulse width was 27 fs with a repetition rate of 82 ㎒.

Impurity-induced resonance Raman scattering at the (e,A 0) threshold in lightly carbon-doped p-type GaAs at [formula omitted

In lightly carbon-doped p-type GaAs the impurity-induced resonance Raman scattering (RRS) by one longitudinal-optic (LO) phonon has been studied at low temperature by using a tunable Ti–sapphire laser. In spite of involvement of strong luminescence in the spectral region of the Raman scattering, a pronounced scattering resonance at the (e,A 0) threshold has been well identified. Both the extrinsic LO-Raman scattering and the luminescence of (e,A 0) LO phonon replicas have been analyzed in a quantitative way. We find that carbon-acceptor-induced extrinsic scattering is fairly intense compared to the intrinsic Raman scattering. It is interpreted in terms of the large oscillator strength of the (e,A 0) transition. Compared to the intrinsic scattering due to free electron–hole pairs, the large (e,A 0) oscillator strength leads to about two orders of magnitude enhancement in the Raman intensity.

파장가변 티타늄 사파이어 레이저로 펌핑하는 연속발진 Nd:YVO4/KTP 레이저의 출력 특성

선폭 0.2nm인 파장가변 티타늄 사파이어 레이저를 펌핑 레이저로 사용하여 펌핑 레이저의 5편광(E┴<TEX>$\pi$</TEX>)과 P-편광(E∥<TEX>$\pi$</TEX>) 그리고 파장 변화 등에 따라 두께 1 mm인 Nd:YVO<TEX>$_4$</TEX>결정의 흡수율 및 연속 발진 Nd:YVO<TEX>$_4$</TEX>/KTP 레이저의 출력 특성을 측정하였다. 그 결과 S-편광 및 P-편광 펌핑 레이저의 파장에 따른 Nd:YVO<TEX>$_4$</TEX>결정의 최대 흡수율은 각각 809.4 nm일 때 82% 및 808.8 nm일 때 98%로 측정되었으며, 기본파인 Nd:YVO<TEX>$_4$</TEX>레이저(1064 nm) 출력의 기울기 효율은 각각 43% 및 52%로 측정되어 1000 mW 출력의 P-편광 펌핑 레이저에 대하여 최대 516 mW의 Nd:YVO<TEX>$_4$</TEX>레이저 출력을 얻을 수 있었다. 또한 P-편광 펌핑 레이저에 대한 내부공진기 진동수 배가된 제2고조파 Nd:YVO<TEX>$_4$</TEX>/KTP 녹색 레이저(532 nm)출력 기울기 효율은 23%로 측정되었으며 1000 mW 펌핑 출력에 대하여 빔질 파라메터 M<TEX>$^2$</TEX>=1.42인 최대 205 mW의 출력을 얻을 수 있었다. We measured the absorption rate of a Nd:YVO<TEX>$_4$</TEX>crystal with a thickness of 1 mm and the output power characteristics of a cw Nd:YVO<TEX>$_4$</TEX>/KTP laser with respect to the change of wavelength and the polarizations of a tunable Ti:sapphire pump laser with a linewidth of 0.2 nm. In the case of S-polarization (E┴<TEX>$\pi$</TEX>) and P-polarization (E∥<TEX>$\pi$</TEX>) of a pump laser, the maximum absorption rate of the crystal was 82% at 809.4 nm and 98% at 808.8 nm, and slope efficiencies for the output power of the Nd:YVO<TEX>$_4$</TEX>laser (1064 nm) were 43% and 52%, respectively. The maximum Nd:YYO<TEX>$_4$</TEX>laser output power of 516 mW was obtained from the P-polarization pump laser of 1000 mW. As a result of an intracavity frequency-doubling, slope efficiency for the output power of the Nd:YVO<TEX>$_4$</TEX>/KTP green laker (532nm) was 23% and the maximum output power of 205 mW with the beam quality (M<TEX>$^2$</TEX>) of 1.42 was obtained from the P-polarization pump laser of 1000 mW.

Scanning Tunneling Optical Resonance Microscopy (STORM)

ABSTRACTThe ability to determine the in-situ optoelectronic behavior of semiconductor materials has become especially important as the size of device architectures is reduced and the development of complex microsystems has increased. Scanning Tunneling Optical Resonance Microscopy or STORM has the ability to interrogate the optical bandgap as a function of position within a semiconductor microstructure. This technique uses a tunable solid-state Ti sapphire laser whose output is “chopped” using a spatial light modulator and is coupled by a fiber optic to a scanning tunneling microscope in order to illuminate the tip-sample junction. The photoenhanced portion of the tunneling current is spectroscopically measured using a lock-in technique. The capabilities of this technique were verified using semiconductor microstructure calibration standards that were grown by organometallic vapor phase epitaxy (OMVPE) at the NASA Glenn Research Center. Bandgaps characterized by STORM measurements were found to be in good agreement with the bulk values determined by transmission spectroscopy, photoluminescence, and with the theoretical values that were based on x-ray diffraction results.

157-nm coherent light source as an inspection tool for F_2 laser lithography

We have developed a 157-nm coherent light source by two-photon resonant four-wave mixing in Xe, with two tunable single-mode 1-kHz Ti:sapphire laser systems at 768 and 681 nm. This light source has been developed to determine the instrumental function of a vacuum ultraviolet spectrometer and to evaluate optical designs for ultra-line-narrowed F(2) laser lithography. The spectral linewidth of the source was less than 0.008 pm (FWHM), with an average power of 0.6 mW.

Laser frequency converter for continuous-wave tunable Ti:sapphire lasers based on aperiodically poled LiNbO3:Nd3+

We present a laser device that shifts the frequencies contained in the 750–850 nm emission band interval of a continuous wave Ti:sapphire tunable laser and produces continuous-wave tunable laser radiation between 440 and 475 and 485 and 505 nm. The device is based on self-sum-frequency mixing of the tunable emission of a Ti:sapphire laser injected in the cavity of a diode-pumped aperiodically poled Nd3+:LiNbO3 laser oscillating in continuous-wave at 1084 or 1372 nm.

Pump-tuning KTP optical parametric oscillator with continuous output wavelength pumped by a pulsed tunable Ti:sapphire laser

We report on the implementation of a KTP optical parametric oscillator pumped by a pulsed tunable Ti:sapphire laser. Two major improvements were achieved, including the connection of the signal and idler tuning ranges and the high-output conversion efficiency through the signal and idler tuning ranges. Both in the signal and idler, the continuous output wavelength from 1.261 to 2.532µm was obtained by varying the pump wavelength from 0.7 to 0.98µm. The maximum output pulse energy was 27.2mJ and the maximum conversion efficiency was 35.7% at 1.311µm (signal).

Molecular-beam-epitaxy growth and luminescence properties of Nd3+-doped LaF3/CaF2 thin films

Nd 3+ -doped LaF3 heteroepitaxial waveguide films on two oriented CaF2 substrates were grown by the molecular-beam-epitaxy method. The spectroscopic properties of Nd3+ in the epitaxial films have been studied by using a tunable Ti:sapphire laser. Efficient IR emissions have been detected at 1.06, 0.86, and 1.32 nm ranges. The influences of substrate orientation, Nd3+ concentration, and temperature on the luminescence properties were investigated. By using a prism-coupling technique, waveguided luminescence has been measured at room temperature. Furthermore, laser emission at 1.06 μm was obtained at room temperature with a 790 nm laser pump.