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Since the development of micro- and nanolasers, the question of laser threshold has been subject to debate. Different definitions have been used to try and establish its occurrence, often encountering major obstacles. We examine a set of common physical definitions which we apply to measurements taken in a micro-VCSEL. Their predictions not only clearly disagree, pointing to different pump values at which the laser should cross threshold, but they also correspond to autocorrelation values which demonstrate very low field coherence. A topological analysis of the rate equations, with average spontaneous emission added to the lasing mode, clearly identifies the contradictions and explains the origin of the discrepancies. Additional considerations help understanding the failure of the approach and highlight the path towards a unique and general definition of threshold in all lasers, irrespective of their sizes. A critical scrutiny of the assumptions made in the rate equations with spontaneous emission illustrates their strength and weaknesses and better defines the bounds within which their predictions hold. We remark in the conclusions how the main results of this paper could hold for other small systems., Comment: Main article: 29 pages, 24 figures Supplementary material: 16 pages, 25 figures

The molecule N,N′-bis(3-methylphenyl)-N,N′-dyphenylbenzidine (TPD) has been widely used in optoelectronic applications, mainly for its hole-transporting properties, but also for its capability to emit blue light and amplified spontaneous emission, which is important for the development of organic lasers. Here, we report deep-blue-emitting distributed feedback (DFB) lasers based on TPD dispersed in polystyrene (PS), as active media, and dichromated gelatin layers with holographically engraved relief gratings, as laser resonators. The effect of the device architecture (with the resonator located below or on top of the active layer) is investigated with a dye (TPD) that can be doped into PS at higher rates (up to 60 wt%), than with previously used dyes (&lt, 5 wt%). This has enabled changing the index contrast between film and resonator, which has an important effect on the laser performance. With regards to thresholds, both architectures behave similarly for TPD concentrations above 20 wt%, while for lower concentrations, top-layer resonator devices show lower values (around half). Remarkably, the operational durability of top-layer resonator devices is larger (in a factor of around 2), independently of the TPD concentration. This is a consequence of the protection offered by the resonator against dye photo-oxidation when the device is illuminated with pulsed UV light.

International audience; A Markov model of semiconductor nanolaser is constructed in order to describe finely the effects of quantum fluctuations in the dynamics of the laser, in particular by considering the transition to lasing. Nanolasers are expected to contain only a small number of emitters, whose semiconductor bands are simulated using true carrier energy states. The model takes into account carrier-carrier interactions in the conduction and valence bands, but the result is a huge Markov chain that is often too demanding for direct Monte-Carlo simulation. We introduce here a technique to split the whole chain into two subchains, one referring to thermalization events within the bands and the other to laser photonic events of interest. The model is applied to the analysis of laser transition and enlightens the coexistence of a pulse regime triggered by the quantum nature of the photon with the birth of the known coherent cw regime. This conclusion is highlighted by calculated time traces. We show that on the ultrasmall scale of nanolasers, we are unable to define perfectly the threshold.

We report the results of an experimental study of an ytterbium-doped fiber laser with low Q-factor cavity. We demonstrate that the laser operates in two randomly alternating sub-regimes, quasi-CW (QCW) and self-Q-switching (SQS), the latter ignited by stimulated Brillouin scattering (SBS). We show that probability of each sub-regime depends on pump power: QCW dominates slightly above the laser threshold while SQS pulsing prevails at higher pump powers. We also discuss the featuring details of QCW sub-regime and its role in instabilities (jittering) of SBS-boosted SQS pulsing as well as the statistical properties of the latter.

Spontaneous photon bursts are observed in the output collected from a mesoscale semiconductor-based laser near the lasing threshold. Their appearence is compared to predictions obtained from Laser Rate Equations and from a Stochastic Laser Simulator. While the latter is capable of predicting the observed large photon bursts, the photon numbers computed by the former produces a noisy trace well below the experimentally detectable limit. We explain the discrepancy between the two approaches on the basis of an incorrect accounting of the onset of stimulated emission by the Rate Equations, which instead are capable of complementing the physical description through topological considerations., 12 pages, 7 figures, 76 references

In this study, high-order distributed-feedback (DFB) polymer lasers were comparatively investigated. Their performance relies on multiple lasing directions and their advantages include their high manufacturing tolerances due to the large grating periods. Nine laser cavities were fabricated by spin-coating the gain polymer films onto a grating structure, which was manufactured via interference lithography that operated at the 2nd, 3rd, and 4th DFB orders. Low threshold lasing and high slope efficiency were achieved in high-order DFB polymer lasers due to the large grating groove depth and the large gain layer thickness. A high-order DFB configuration shows possible advantages, including the ability to control the lasing direction and to achieve multiple-wavelength lasers. Furthermore, our investigation demonstrates that the increase in threshold and decrease in slope efficiency with an increase in the feedback order can be limited by controlling the structural parameters.

We report on a watt-level highly efficient europium laser operating at the 5 D 0 → 7 F 4 transition. It is based on the stoichiometric K E u ( W O 4 ) 2 crystal. Under pumping by a green laser at 532.1 nm, the K E u ( W O 4 ) 2 laser generated a maximum peak output power of 1.11 W at ∼ 703 n m with a slope efficiency of 43.2% and a linear polarization ( E ‖ N m ). A laser threshold as low as 64 mW was achieved. True continuous-wave operation was demonstrated. The polarized emission properties of monoclinic K E u ( W O 4 ) 2 were determined.

It is known (yet often ignored) from quantum mechanical or energetic considerations, that the threshold gain of the quasi-static spaser depends only on the dielectric functions of the metal and the gain material. Here, we derive this result from the purely classical electromagnetic scattering framework. This is of great importance, because electrodynamic modelling is far simpler than quantum mechanical one. The influence of the material dispersion and spaser geometry are clearly separated; the latter influences the threshold gain only indirectly, defining the resonant wavelength. We show that the threshold gain has a minimum as a function of wavelength. A variation of nanoparticle shape, composition, or spasing mode may shift the plasmonic resonance to this optimal wavelength, but it cannot overcome the material-imposed minimal gain. Furthermore, retardation is included straightforwardly into our framework; and the global spectral gain minimum persists beyond the quasi-static limit. We illustrate this with two examples of widely used geometries: Silver spheroids and spherical shells embedded in and filled with gain materials.

DOI: 10.1364/OME.7.000844 URL: https://www.osapublishing.org/ome/abstract.cfm?uri=ome-7-3-844 Filiació URV: SI Memòria We report on a detailed comparative study of the spectroscopic and thermo-optic properties of tetragonal Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals indicating their suitability for highly-efficient microchip lasers diode-pumped at ~791 nm and operating at ~1.91 ¿m. An a-cut 8 at.% Tm:LiYF4 micro-laser generated 3.1 W of linearly polarized output at 1904 nm with a slope efficiency of ¿ = 72% and a laser threshold of only 0.24 W. The internal loss for this crystal is as low as 0.0011 cm-1. For 8 at.% Tm:LiGdF4 and 12 at.% Tm:LiLuF4 lasers, the output power reached ~2 W and ¿ was 65% and 52%, respectively. The thermal lens in all Tm:LiLnF4 crystals is weak, positive and low-astigmatic. The potential for the Tm:LiLnF4 lasers to operate beyond ~2 ¿m due to a vibronic coupling has been proved. The Tm:LiYF4 vibronic laser generated 375 mW at 2026-2044 nm with ¿ = 31%. The Tm:LiLnF4 crystals are very promising for passively Q-switched microchip lasers.

DOI: 10.1364/OME.7.000844 URL: https://www.osapublishing.org/ome/abstract.cfm?uri=ome-7-3-844 Filiació URV: SI Memòria We report on a detailed comparative study of the spectroscopic and thermo-optic properties of tetragonal Tm:LiLnF4 (Ln = Y, Gd, and Lu) crystals indicating their suitability for highly-efficient microchip lasers diode-pumped at ~791 nm and operating at ~1.91 ¿m. An a-cut 8 at.% Tm:LiYF4 micro-laser generated 3.1 W of linearly polarized output at 1904 nm with a slope efficiency of ¿ = 72% and a laser threshold of only 0.24 W. The internal loss for this crystal is as low as 0.0011 cm-1. For 8 at.% Tm:LiGdF4 and 12 at.% Tm:LiLuF4 lasers, the output power reached ~2 W and ¿ was 65% and 52%, respectively. The thermal lens in all Tm:LiLnF4 crystals is weak, positive and low-astigmatic. The potential for the Tm:LiLnF4 lasers to operate beyond ~2 ¿m due to a vibronic coupling has been proved. The Tm:LiYF4 vibronic laser generated 375 mW at 2026-2044 nm with ¿ = 31%. The Tm:LiLnF4 crystals are very promising for passively Q-switched microchip lasers.

This paper provides a review of advances in conjugated polymer lasers. High photoluminescence efficiencies and large stimulated emission cross-sections coupled with wavelength tunability and low-cost manufacturing processes make conjugated polymers ideal laser gain materials. In recent years, conjugated polymer lasers have become an attractive research direction in the field of organic lasers and numerous breakthroughs based on conjugated polymer lasers have been made in the last decade. This paper summarizes the recent progress of the subject of laser processes employing conjugated polymers, with a focus on the photoluminescence principle and excitation radiation mechanism of conjugated polymers. Furthermore, the effect of conjugated polymer structures on the laser threshold is discussed. The most common polymer laser materials are also introduced in detail. Apart from photo-pumped conjugated polymer lasers, a direction for the future development of electro-pumped conjugated polymer lasers is proposed.

Active laser arrays, of arbitrary pattern, were created by inkjet deposition of self-assembled photonic structures in the form of dye-doped chiral nematic liquid crystals. The print process itself achieves the requisite alignment for photonic band-edge lasing action normal to the substrate; laser threshold and linewidth characteristics are presented.

The role of temperature on the properties of quantum-cascade laser (QCL) waveguides is investigated. One-dimensional waveguide parameters are obtained using a transfer-matrix technique and the complex dielectric constants of the waveguide layers are calculated using a semi-classical Drude---Lorentz model. To model the effect of temperature on the waveguide parameters, a temperature dependent electron mobility is incorporated within the Drude---Lorentz framework. It is shown that by including the effect of temperature, a significant improvement in the agreement with experiment of the waveguide loss and hence the laser threshold current density can be achieved.

The laser was born on May 16, 1960 at Hughes Research Laboratories in Malibu, California. Instead of enjoying the view of the Pacific from his new office window, Theodore Maiman closeted himself in a windowless lab, where he slipped a spring-shaped photographic flashlamp over a stubby ruby rod, then put the assembly inside a reflective cylinder. Firing pulses with higher and higher voltage through the lamp, he carefully studied oscilloscope traces of the rise and fall of the ruby's red fluorescence. Once the voltage exceeded 950 volts, the lamp flashed bright enough to excite the ruby above laser threshold. The red light suddenly flashed so bright that Maiman's colorblind assistant Irnee D'Haenens saw red light for the first time [1]! The instruments recorded what Maiman had predicted would happen above laser threshold – the pulse rose and fell faster, and the range of red wavelengths narrowed. This article provides an overview of the first 50 years of laser development as well as a brief outlook what's coming next.

A kind of Faraday glass with a size of Φ40 mm × 14 mm and Φ100–300 mm × 25 mm, respectively, was prepared by the melting and casting process, which had neither a Pt particulate nor a bubble or striation. The measurement results showed that the surface homogeneity of the samples was up to ± 1 × 10−6, the laser threshold reached 10 J/cm2 (1ω, 3 ns), and the Verdet constants of the glasses were −0.273 min/Oe/cm at 632.8 nm and −0.076 min/Oe/cm at 1064 nm, respectively.

The threshold properties of photonic crystal quantum dot lasers operating in the slow-light regime are investigated experimentally and theoretically. Measurements show that, in contrast to conventional lasers, the threshold gain attains a minimum value for a specific cavity length. The experimental results are explained by an analytical theory for the laser threshold that takes into account the effects of slow-light and random disorder due to unavoidable fabrication imperfections. Longer lasers are found to operate deeper into the slow-light region, leading to a trade-off between slow-light induced reduction of the mirror loss and slow-light enhancement of disorder-induced losses., 5 pages, 7 figures

We demonstrate how exactly bound cavity modes can be realized in dielectric structures other than 3d photonic crystals. For a microcavity consisting of crossed anisotropic layers, we derive the cavity resonance frequencies, and spontaneous emission rates. For a dielectric structure with dissipative loss and central layer with gain, the beta factor of direct spontaneous emission into a cavity mode and the laser threshold is calculated., Comment: 5 pages, 3 figures

On the base of experimental spontaneous emission spectra measured at laser threshold the temperature dependencies of the radiative Rsp and nonradiative Q recombination rates as well as the spontaneous emission internal quantum efficiency ηsp for optically pumped wurtzite GaN/Al2O3 epilayers have been calculated. The recombination rate induced by the amplified luminescence Rlum was taken into account. It is shown that the main reason of the increasing threshold pump rate Rp with temperature T for the GaN-based compounds is a substantial rise of the nonradiative recombination rate. The portion of Q in Rp exceeds 80–90%. The contributions of Rsp and Rlum in Rp amount to several percents. The ηsp value is of 0.12–0.18 within the temperature interval of 300–450 K. Further increase in T leads to an abrupt decrease in ηsp down to 0.06 at 517 K.

We study theoretically the non linear polarization dynamics of Vertical-Cavity Surface-Emitting Lasers in the presence of an external cavity providing delayed optical feedback and cross polarization re-injection. We show that far from the laser threshold, the dynamics remains confined close to the equatorial plane of a Stokes sphere of a given radius and we reduce the dynamics to a dynamical system composed of two phases: the orientation phase of the quasi-linear polarization and the optical phase of the field. We explore the complex modal structure given by the double feedback configuration and recovers as particular cases the Lang-Kobayashi modes and the modes founds by Giudici et al. [1]. We also re-interpret the square waves switching dynamics as phase kinks.

Room-Temperature Stimulated Emission in ZnO: Doubts on Excitonic Lasing Claus Klingshirn, Johannes Fallert, Robert Hauschild, Mario Hauser, Heinz Kalt and Huijuan Zhou Institut f ur Angewandte Physik, Universit at Karlsruhe, 76131 Karlsruhe, Germany (Received 10 September 2007) Concerning stimulated emission, we discuss for ZnO the roles of excitonic processes and of recombination in an inverted electron-hole plasma. While excitonic processes are well justi ed at lower temperatures and densities, doubts arise concerning the concept of excitonic lasing in ZnO at or even above room temperature. The densities at laser threshold and at room temperature are frequently at or above the Mott density, but below the density at which population inversion is reached. We suggest alternative processes in a non-inverted plasma. PACS numbers: 78.55.Et, 78.45.+h

LuLiF4 single crystals codoped with thulium (Tm) and holmium (Ho) were successfully grown by the Czochralski method. The excellent laser performance, in terms of highest slope efficiency, and highest optical-to-optical efficiency and lower laser threshold, was studied in comparison with 5% Tm, 0.5% Ho-codoped YLiF4, and Lu3Al5O12 under the same conditions.

When AIGalnAs laser structures are grown by molecular beam epitaxy on InP, the substrate deoxidation procedure is found to influence the quality of the structure: the laser threshold current densities are found to be low and reproducible when P2 is used for the substrate deoxidation. On the other hand, the reproducibility is not achieved with a deoxidation under As2. In order to interpret this result, very thin AlInAs overlayers have been deposited on InP substrates which were preliminarily deoxidized under P2 or As2 flux. The AlInAs morphology is observed by atomic force microscopy. The InP anneal under As2 leads to a rough surface as compared to the InP anneal under P2. These observations suggest that a systematic use of P2 for the substrate deoxidation can definitely improve the reliability of AIGalnAs laser structures on InP.

Laser power and energy have been measured for the transitions at 1.73 and 2.03 /spl mu/m in atomic xenon, and 1.27 and 1.79 /spl mu/m in atomic argon, for gas temperatures between 290-585 K. Nuclear excitation using /sup 235/U fission-fragments provided pump powers between 2-10 W/cm/sup 3/. Peak laser output in xenon was independent of gas temperature below 400 K. Laser output at 1.79 /spl mu/m in argon decreased monotonically with increasing gas temperature while the 1.27 /spl mu/m laser output was independent of gas temperature below 380 K. In all cases, the laser threshold increased as the gas temperature increased. >

Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-27T11:25:30Z No. of bitstreams: 0Bitstream added on 2014-05-27T14:47:10Z : No. of bitstreams: 1 2-s2.0-79952615742.pdf: 1081751 bytes, checksum: 686fe3218ec563e5b0d170077dd5dbe5 (MD5) Made available in DSpace on 2014-05-27T11:25:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-03-14 An upconversion random laser (RL) operating in the ultraviolet is reported for Nd 3+ doped fluoroindate glass powder pumped at 575 nm. The RL is obtained by the resonant excitation of the Nd 3+ state 2G 7/2 followed by energy transfer among two excited ions such that one ion in the pair decays to a lower energy state and the other is promoted to state 4D 7/2 from where it decays emitting light at 381 nm. The RL threshold of 30 kW/cm 2 was determined by monitoring the photoluminescence intensity as a function of the pump laser intensity. The RL pulses have time duration of 29 ns that is 50 times smaller than the decay time of the upconversion signal when the sample is pumped with intensities below the RL laser threshold. © 2011 Optical Society of America. Departamento de Física Universidade Federal de Pernambuco, 50670-901 Recife, PE Instituto de Química Universidade do Estado de São Paulo, 14801-970 Araraquara, SP

The authors examine the power efficiency of the fission-fragment-excited atomic argon laser operating on the 1.27- mu m (3d'(3/2)/sub 1/-4'/sub p/(3/2)/sub 1/) argon transition as a function of pump power, gas mixture, and pressure. The maximum measured power efficiency was 1.1+or-0.3% for a gas pressure of 1300 torr and a He/Ar ratio of 99.88/0.12. Neon addition to the He/Ar gas mixture increased both the energy deposited in the gas and the energy output without decreasing efficiency for a neon gas fraction of less than 0.5. Small-signal gain and saturation intensity are between 0.15-0.27%/cm and 25-200 W/cm/sup 2/ for pump rates of 7.5-30 W/cm/sup 3/ in He/Ar and He/Ne/Ar gas mixtures. The laser threshold as a function of total pressure and argon concentration is presented. >

Antiresonant reflecting optical waveguide (ARROW-type) vertical-cavity surface-emitting lasers are designed for high-power single-mode operation. A detailed full-vector finite-difference time-domain (FDTD) study shows strong modal discrimination in favor of the fundamental mode for large aperture (>6 /spl mu/m), large index-step (/spl Delta/n>0.025) simplified ARROW-type devices. The FDTD model identifies the polarization-dependent radiation losses of the higher order modes, which prevent them from reaching laser threshold.

The increase in slope efficiency and the reduction of the laser threshold of a flash-lamp-pumped Ho,Cr,Tm:YAG laser that are due to increased flash-lamp efficiency when the current of the dc simmer is increased are described. Investigations of the electrical characteristics of the flash lamp as a function of the simmer current prove that the flash-lamp impedance decreases to a constant value when the dc simmer current is increased. As a consequence an increase of the peak current through the flash lamp is observed, which improves the flash-lamp efficiency.

We demonstrate a photonic crystal nanocavity laser essentially driven by a self-assembled InAs/GaAs single quantum dot gain. The investigated nanocavities contain only 0.4 quantum dots on an average; an ultra-low density quantum dot sample (1.5 x 108 cm-2) is used so that a single quantum dot can be isolated from the surrounding quantum dots. Laser oscillation begins at a pump power of 42 nW under resonant condition, while the far-detuning conditions require ~145 nW for lasing. This spectral detuning dependence of laser threshold indicates substantial contribution of the single quantum dot to the total gain. Moreover, photon correlation measurements show a distinct transition from anti-bunching to Poissonian via bunching with the increase of the excitation power, which is also an evidence of laser oscillation with the single quantum dot., Comment: 11 pages, 5 figures

The two-dimensional theory of a distributed feedback (DFB) laser (which was previously presented and applied to the analysis of the laser threshold conditions for the transverse-electric (TE) mode in a simple three-layer waveguide structure) is developed to treat both TE and transverse-magnetic (TM) modes in a four-layer waveguide structure with a thin grating layer, which more closely reflects actual DFB laser structure. The differences between TE and TM modes for the dispersion relations and the laser threshold conditions are clarified. The effects of the waveguide structure (including grating layer thickness, refractive indexes of layers, coupling constant, and corrugation period) on the threshold gains and the gain differences between the two longitudinal modes on both sides of the Bragg frequencies are studied in detail for both TE and TM modes. >

Radiation damage in 1310 nm InGaAsP/InP multi-quantum-well lasers caused by 0.8 MeV neutrons is compared with the damage from other radiation sources, in terms of the increase in laser threshold current. The annealing behavior is then presented both in terms of both temperature and forward-bias current dependence. The annealing can be described by a model where radiation induced defects have a uniform distribution of activation energies for annealing. This model can then be used to predict the long-term damage expected for lasers operating inside the CMS tracker. (19 refs).

We studied the topographical distribution of laser sensory thresholds on the human hairy skin, using a small laser beam for pinprick and a large beam for warmth sensations. The threshold for pinprick sensation correlated positively with the distance from the brain, suggesting that A delta nociceptors, the fibers which convey pinprick sensation, are more dense at proximal than at distal body sites. This finding adds information to skin biopsy studies of epidermal free nerve endings which showed a similar gradient, but could not differentiate small myelinated from unmyelinated fiber afferents. Possibly because of a diffuse low density of warmth receptors, laser warmth thresholds showed no trend. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.

In this paper we investigated cavity soliton switching in a VCSEL type semiconductor laser which has been driven by homogeneous holding beam. Using quadratic fitting of the gain curve we studied the incoherent switching technique for the controlling of cavity solitons in a VCSEL-type laser and we show the cavity soliton spontaneous mobility in this type of injection.

Basing on measuring the luminescence spectra at room temperature pumped by 975nm light, the action mechanism of Na+ in the novel Yb3+,Na+:CaF2 crystal was systemically investigated. The relation between the concentration of doping Na+ and the laser operation threshold was analyzed. The optimum match concentration for Yb3+,Na+:CaF2 with low laser threshold was obtained. In the experiment, the lowest laser operation threshold at only 70mW of absorbing pumped power for 2%Yb3+, 3%Na+:CaF2 crystal is realized, which is in good agreement with the analysis of luminescence properties.

Experimental results on the realisation and spectral characterisation of Er-ZBLAN microspherical lasers at 1.56 µm are presented. The laser effect is obtained using the properties of whispering gallery modes and pumping at 1.48 µm. Multimode operation and a laser threshold as low as 600 µW have been observed.

In this paper, a threshold model for an end pumped Ho3+ laser is developed by using quasi-three-level laser model. The relations between threshold and design parameters are analyzed. A general principle for design low threshold Ho3+ laser is given. The results are also valid for other quasi-three-level laser systems.

The first theoretical evaluation is reported of the effect of biaxial compression on the quantum-well Ga/sub x/In/sub 1-x/As/Ga/sub 0.20/InAs/sub 0.45/P and Ga/sub 0.20/In/sub 0.80/As/sub y/P/sub 1-y//Ga/sub 0.20/In/sub 0.80/As/sub 0/ /sub .45/P/sub 0.55/ laser threshold current density. Reference is made to known experimental results, and a comparison carried out of the potentialities of the two types of laser.