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Multiple stimulated emission fluorescence photoacoustic (MSEF-PA) phenomenon is demonstrated in this letter. Under simultaneous illumination of pumping light and stimulated emission light, the fluorescence emission process is speeded up by the stimulated emission effect. This leads to nonlinear enhancement of photoacoustic signal while the quantity of absorbed photons is more than that of fluorescent molecules illuminated by pumping light. The electronic states' specificity of fluorescent molecular can also be labelled by the MSEF-PA signals, which can potentially be used to obtain fluorescence excitation spectrum in deep scattering tissue with nonlinearly enhanced photoacoustic detection. In this preliminary study, the fluorescence excitation spectrum is reconstructed by MSEF-PA signals through sweeping the wavelength of exciting light, which confirms the theoretical derivation well. [ABSTRACT FROM AUTHOR]

By stepwise tapering, both the barrier heights and quantum-well depths in the active regions of 8.7-8.8 μm-emitting quantum-cascade-laser (QCL) structures, virtually complete carrier-leakage suppression is achieved. Such step-taper active-region-type QCLs possess, for 3 mm-long devices with high-reflectivity-coated back facets, threshold-current characteristic temperature coefficients, T0, as high as 283 K and slope-efficiency characteristic temperature coefficients, T1, as high as 561 K, over the 20-60 °C heatsink-temperature range. These high T0 and T1 values reflect at least a factor of four reduction in carrier-leakage current compared to conventional 8-9 μm-emitting QCLs. Room temperature, pulsed, threshold-current densities are 1.58 kA/cm²; values comparable to those for 35-period conventional QCLs of similar injector-region doping level. Superlinear behavior of the light-current curves is shown to be the result of the onset of resonant extraction from the lower laser level at a drive level of ~1.3× threshold. Maximum room-temperature slope efficiencies are 1.23 W/A; that is, slope efficiency per period values of 35 mW/A, which are 37%-40% higher than for same-geometry conventional 8-9 μm-emitting QCLs. Since the waveguide-loss coefficients are very similar, we estimate that the internal differential efficiency is at least 30% higher than in conventional QCLs. Such high internal differential efficiency values reflect the combined effect of nearly complete carrier-leakage suppression and high differential efficiency of the laser transition (~90%), due to resonant extraction from the lower laser level. [ABSTRACT FROM AUTHOR]

We report the results of relative intensity noise (RIN) measurement on the ground and first excited state transitions of a single quantum-well (QW) transistor laser (TL). Because of higher differential gain and faster recombination lifetime on the first excited state transition, a lower laser RIN is measured as compared with ground state laser operation. The minority carrier density in the base of QWTL extracted from the laser RIN shows a carrier density of 2.6-3.5 × 1016 cm-3, a more than 40× reduction from that of a conventional diode laser. [ABSTRACT FROM AUTHOR]

The gain spectrum of a quantum dot laser operating at 1300 nm is studied at high carrier densities, corresponding to dot occupancies of ∼8 e-h pairs per quantum dot. A reduction in peak gain with increasing carrier density is observed, attributed to the saturation of peak gain, yet the continuous increase in dephasing acting to broaden the individual quantum dot transitions. [ABSTRACT FROM AUTHOR]

In this paper we show that by applying a perpendicular magnetic field to a quantum cascade structure it is possible to enhance the gain of different optical transitions. The combination of magnetic confinement with a broadband, cutoff-free optical resonator allows the demonstration of laser action over a large bandwidth, from 733 GHz to 1.38 THz together with the emission at 3.2 THz. A different lasing scheme is revealed that does not rely on resonant tunneling as the main injection mechanism. In combination with the magnetically enhanced gain laser emission at 1 THz is observed up to a temperature of 115 K, which corresponds to a ratio kBT/hν=2.3 between the lattice thermal energy and the laser photon energy. [ABSTRACT FROM AUTHOR]

The authors report the analytical small-signal modeling of the distribution of minority carriers in the base of a transistor laser and of the high-speed dynamics of the device. The modeling is based on analytically solving the continuity equation and the rate equations, which incorporate the virtual states as a conversion mechanism. Wide-band operation of the transistor laser along with the suppression of the relaxation oscillation frequency peak is demonstrated. [ABSTRACT FROM AUTHOR]

The authors generate and compress short pulses via the ground and excited state transitions from a passively mode-locked grating coupled quantum dot two-section diode laser at a repetition rate of 2.7 GHz. The selection and isolation of either the ground state or excited state transition were performed by an angle tuning of the grating. The externally compressed pulse widths are 970 fs from the ground state and 1.2 ps from the excited state transition. The sign of the chirp is up chirped for both state transitions. [ABSTRACT FROM AUTHOR]

Stress in single and periodic shallow-trench-isolated Si structures was examined by 364 nm excitation confocal resonance Raman microscopy, laser penetration being restricted to the near-surface region. Using a 1.3 numerical aperture microobjective lens with a theoretical ∼140 nm spatial resolution, the authors show that the configuration with both incident and scattered lights polarized parallel to each other and perpendicular to Si stripes is favorable for stress detection in the middle of the stripes, suppressing contributions from their edges. The stresses located in different areas of the structures were identified and analyzed. [ABSTRACT FROM AUTHOR]

Intersubband relaxation dynamics in InGaAs/AlAsSb multiquantum wells (QWs) is studied by single-color femtosecond pump-probe measurements. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of the order of a picosecond. The relaxation dynamics at later delay times strongly depend on both QW thickness and doping location. A non-single-exponential decay behavior indicates extra competing relaxation channels, as further confirmed by solving three-level rate equations. It is shown that slowly decaying components are due to electron transfer to states related to indirect valleys in the wells or in the barriers. [ABSTRACT FROM AUTHOR]

Oxidation kinetics of Al-rich layers in ambient room conditions is investigated on cleaved mirrors of GaSb/Ga1-xAlxAsSb (x=0.9–0.93) laser heterostructure. The measurements performed by atomic force microscopy demonstrate intensive growth of native oxide during the first two months with further saturation of the oxide thickness to about 1 μm. In studies of the oxide cross section using additional cleavages perpendicular to the laser mirror, the volume of the oxidized part of the Al-rich layer is found to expand through elevations on the heterostructure mirror so that the height of the elevations reaches one-third of the total thickness of the oxide. [ABSTRACT FROM AUTHOR]

Zn1-xMgxO nanoneedles were prepared by an ion-beam technique on Zn1-xMgxO thin films with Mg contents of up to 21 at. %. The photoluminescence emission energies of the Zn1-xMgxO nanoneedles measured at room temperature increased monotonically with Mg contents and it reached 3.6 eV when x=0.21. Random laser action was observed in the Zn1-xMgxO nanoneedles with x≤=0.1 at temperature ranging from 300 to 470 K under 355 nm optical excitation. The characteristic temperature of the Zn1-xMgxO nanoneedles was determined to be 84 K. The high-temperature lasing of the Zn1-xMgxO nanoneedles are attributed to the high crystal quality of the nanoneedles, enhancement of oscillator strength in nanostructures, and a self-compensation mechanism in random laser cavities [ABSTRACT FROM AUTHOR]

The authors demonstrate stable mode locking that involves transitions within either the ground state (1260 nm) or the excited state (1190 nm) in a two-section quantum-dot laser, at repetition frequencies of 21 and 20.5 GHz, respectively. The average power of the mode-locked output was in excess of 35 mW for operation in the ground state and 25 mW in the excited state. The selection of pulse generation between these states in the mode-locking regime is controlled by the electrical biasing conditions. [ABSTRACT FROM AUTHOR]

Infrared laser has been used to irradiate carbon nanotube (CNT) powders and the performance of CNT emitter was enhanced. The CNTs were prepared by conventional electric arc discharge method and laser irradiation was processed by Nd:YAG infrared laser with a wavelength of 1064 nm. It was found that the turn-on electric field was significantly decreased for laser treated sample. As compared to the sample of no laser treatment, the emission current density was enhanced by a magnitude of 16.56 for 225 J/cm2 treated plus annealing one. Possible mechanism is discussed here. [ABSTRACT FROM AUTHOR]

Microstructures and electrical properties of lead zirconate titanate (PZT) thick films made by using a screen-printing laser transfer method have been studied. The PZT thick-film elements were first screen printed on a sapphire substrate and sintered at 1150 to 1250 °C. Then they were bonded to a target substrate such as silicon and released from the sapphire by means of an excimer laser exposure from the backside of the sapphire substrate. This approach makes it possible to obtain highly densified films because there is less limitation on sintering conditions, and allows integrating patterned PZT thick films onto many kinds of substrates. The thick films have dielectric constants of 1397 to 1675, remnant polarization of 32 to 35 μC/cm2, a piezoelectric constant d31 of about -124 pm/V, and Young’s modulus Y11E(59.4 GPa) of almost the same as the corresponding bulk ceramic. [ABSTRACT FROM AUTHOR]

Fourier-transform infrared absorption measurements have been carried out on nitrogen-doped Czochralski-grown silicon crystals after thermal annealing at 600 °C. The strength of the electronic transitions due to N–O related shallow donors shows a square root dependence on the nitrogen concentration and donor-specific power dependences on the interstitial oxygen concentration. Based on mass action laws, chemical compositions are discussed. It is found that the N–O complexes investigated all involve one nitrogen atom but three different numbers of oxygen atoms. [ABSTRACT FROM AUTHOR]

The occupation of the laser levels in an undoped GaAs/(Al,Ga)As quantum-cascade structure is directly detected by interband photoluminescence spectroscopy. The upper laser level and the injector state exhibit an anticrossing behavior with a splitting of about 10 meV, which is attributed to a strong coupling between them. The energy splitting matches well the oscillation period observed in recent, ultrafast, coherent electron-transport experiments by F. Eickemeyer et al. [Phys. Rev. Lett. 89, 047402 (2002)]. A comparison with the calculated transition energies qualitatively confirms the anticrossing behavior in the vicinity of the threshold field strength. Furthermore, the measured energy difference between the upper and lower laser level agrees with the lasing energies of a set of complete quantum-cascade lasers. At the same time, the variation of the lasing energies corresponds to the energy splitting between the upper laser level and the injector state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discrete transitions in the position of a particle optically trapped in the vicinity of an interface are observed using a nanometer position sensing system. These discrete transitions in the position with distance from the interface are attributed to interference effects due to reflection and scattering of the trapping laser beam from the interface and the particle. The discrete transitions occurred at uniform intervals of ∼400 nm, corresponding to the spacing of standing wave nodes of the trapping laser beam. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report in this letter an elastic force analysis of two-photon-polymerized submicron oscillators with spiral radius of 150 nm. A laser trapping force was first utilized to mechanically drive the spring of this small size. An overdamped oscillation was observed, according to which, the spring constant was measured to approximately 10[sup -8] N/m. This value was 5-order smaller than the one calculated using oscillator parameters. Both deviated from a more reliable result, 10[sup -6] N/m, as was estimated from the laser trapping force. The significant discrepancies were attributed to scaling effects and to the difference of condensed states of the polymer related to exposure processes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report that a nanoscale laser-induced structural phase transition involving just a few nanometers of gallium at an interface with silica can drive reversible changes in the optical properties of the interface in a very broad spectral range from 440 to 680 nm and beyond. At temperatures up to 15° below the melting point of gallium (30 °C) 3 ns excitation pulses, with fluences of just a few mJ/cm2, are sufficient to induce reflectivity increases of up to 40%. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report direct gain measurements on the 351-nm XeF(B–X) transition in mixtures of 3He/Xe/NF3, pumped with moderated neutrons produced in a fast-burst reactor. The peak small-signal gain was 0.7%/cm at a deposited power of 5 kW/cc in the gas. [ABSTRACT FROM AUTHOR]

The role of strain in modifying lasing transition energy and gain coefficient in photopumped GaAs/GaAs1-xPx strained-layer superlattices is investigated by examining photoluminescence, excitation, and lasing spectra for samples with x in the range 0.1–0.5. Over this range, the lasing transition energy increases 50 meV, independent of quantum size effects. The compressive biaxial strain present in the GaAs layers is expected to significantly increase the gain coefficient for the |3/2,±1/2> ‘‘light’’ hole transitions. However, polarization measurements of the lasing spectra show no evidence for lasing on the TM mode |3/2,±1/2> transitions and show lasing only on the TE mode |3/2,±3/2> ‘‘heavy’’ hole transitions. [ABSTRACT FROM AUTHOR]

A procedure is described by which the temperature-dependent upper critical field Hc2(T) in highly disordered thin-film superconductors with broad resistive transitions is determined. Resistance versus perpendicular magnetic field isotherms are used in conjunction with the mean field transition temperature Tc 0, derived from Aslamazov-Larkin fits to the zero-field resistive transition, to determine Hc2(T). The sensitivity of the slope parameter (dHc2/dT)|Tc 0 with respect to the details of the procedure is discussed and the conditions under which meaningful comparisons with theory can be made are elaborated. [ABSTRACT FROM AUTHOR]

Demonstrates the blue upconversion laser action in Tm:YLF on two different transitions. Fabrication of the crystal; Combination of the dyer laser and Ti:sapphire laser beams; Observation of a blue laser emission; Illustration of the upconversion excitation of G[sub 4]; Estimation of the threshold inversion.

Direct measurements of the gain profile of optically pumped far-infrared lasers show that large shifts in the laser frequency can be caused by the absorption from thermal molecules on the laser transition. The absorption shifting greatly exacerbates pump frequency deviations, resulting in an extreme sensitivity to pump offsets and drifts. This pressure-dependent shifting mechanism is not present in transversely pumped lasers, which explains their superior frequency reproducibility compared to longitudinally pumped lasers, and reconciles two apparently conflicting results regarding laser stability. [ABSTRACT FROM AUTHOR]

Two new laser transitions are reported for the Er3+ ion in YLiF4: in the blue at 469.7 nm and in the green at 560.6 nm. These are pumped with red and near-infrared laser radiation using nonlinear excitation processes in which more than one upconversion step occurs, yet the threshold pump powers are as low as 10 mW. Single longitudinal mode cw operation was observed at temperatures below 40 K. This is the first report of a blue laser using the Er3+ ion, and the first report of an infrared-pumped blue laser. [ABSTRACT FROM AUTHOR]

Sharp spectral lines are obtained for the Cu i 511 nm and the Au i 628 nm transitions from a sputtering-based laser operating at room temperature. The Au i line exhibits a single peak with a linewidth of 360 MHz and the Cu i line consists of three well-resolved peaks with linewidths of 800, 700, and 700 MHz in order of increasing frequency. The spectral lines result from saturated emission on the strongest hyperfine components. [ABSTRACT FROM AUTHOR]

We have observed cw laser action on a new transition (4D3/2→4I11/2) of the Nd3+ ion at 380 nm. This was demonstrated in Nd:LaF3 using an upconversion pumping scheme in which near infrared (788 nm) and visible (591 nm) pump photons from cw dye lasers produce stepwise excitation of the Nd3+ ion. In addition, lasing was observed with a single pump source at wavelengths around 578 nm using doubly resonant, sequential absorption of two yellow photons. Single-mode, cw operation with an output power of 12 mW was measured at 20 K with 1% output coupling and pump powers of several hundred mW. At 77 K the maximum power dropped to 4 mW. [ABSTRACT FROM AUTHOR]

Examines the 946 nanometer laser operation of an epitaxially grown ND:Y[sub 3]Al[sub 5]O[sub 12] planar waveguide. Comparison of the spectroscopic characteristics between gallium-doped YAG and Nd:YAG epitaxial layers; Termination of laser transition in Yb:YAG in the ground state; Fabrication of Nd:YAG waveguides by liquid phase epitaxial growth of Nd:YAG layers.

Examines semiconductor electron wave Fabry-Perot interference filter with two quasibound states for optical transitions. Absorption measurements at multiple temperature; Basis for room-temperature infrared semiconductor laser; Creation of large dipole matrix elements.

Free-running Fabry-Perot lasers normally operate in a single-mode regime until the pumping current is increased beyond the single-mode instability threshold, above which they evolve into a multimode state. As a result of this instability, the single-mode operation of these lasers is typically constrained to few percents of their output power range, this being an undesired limitation in spectroscopy applications. In order to expand the span of single-mode operation, we use an optical injection seed generated by an external-cavity single-mode laser source to force the Fabry-Perot quantum cascade laser into a single-mode state in the high current range, where it would otherwise operate in a multimode regime. Utilizing this approach, we achieve single-mode emission at room temperature with a tuning range of 36 cm−1 and stable continuous-wave output power exceeding 1 W at 4.5 μm. Far-field measurements show that a single transverse mode is emitted up to the highest optical power, indicating that the beam properties of the seeded Fabry-Perot laser remain unchanged as compared to free-running operation. [ABSTRACT FROM AUTHOR]

The small signal modulation response of semiconductor lasers is commonly used to predict large signal modulation capabilities. Recent experiments suggest that this prediction may fail in some quantum dot (QD) lasers. We present a model supported by experiments, which shows that when the small signal modulation response is limited by gain compression and the gain is large, the laser can be modulated at very high bit rates. This effect is inherent to dynamics governing all semiconductor lasers but the conditions needed for high bit rate modulation in the presence of narrow small signal bandwidths are only obtainable in QD lasers. [ABSTRACT FROM AUTHOR]

Data are presented on the lasing transitions from InGaAs/GaAs quantum dots that exhibit four well-resolved energy transitions, with the electronic state density of a two-dimensional harmonic oscillator. Lasing has been obtained on the second and third transitions, depending on the cavity (gain) length, with the longest lasing wavelength measured to be 1.19 μm. The temperature dependence of threshold is studied and regions of nearly temperature-independent threshold are found. Interesting aspects of the unique electronic state density for lasers are discussed. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Al[sub x]Ga[sub 1-x]As–GaAs–In[sub y]Ga[sub 1-y]As quantum well heterostructure lasers with thin sharply defined cavities that operate longitudinally or vertically are demonstrated. Longitudinally, the cavity is defined by cleaving (edge emission) while vertical definition is provided by a lower Al[sub x]O[sub y]/GaAs distributed Bragg reflector (DBR) and an upper SiO[sub 2]/Si DBR that form a resonant structure. A reverse-biased tunnel contact junction provides lateral electron current to support hole injection through a native-oxide-defined aperture. This ultrathin cavity configuration produces low threshold high efficiency (η∼91%, L=140 μm) edge emission (longitudinal operation) in the case of long lasers (L>=90 μm), and vertical emission for shorter lasers (L≲65 μm). The transition from longitudinal to vertical laser operation as a function of cavity length, L, is demonstrated. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]