Threshold Laser Power Density Research Articles

Switching of exciton character in double InGaN/GaN quantum wells

The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6--13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells.

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, T_{C}=4.8 K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density-temperature phase diagram over the temperature range 0.1-4.8K. We find that the latent heat increases linearly with temperature at T≲1.1 K, similarly to a Bose-Einstein condensate transition, and becomes constant at 1.1≲T<4.8 K. Resonant Rayleigh scattering measurements reveal that the disorder in the sample is strongly suppressed and the diffusion coefficient sharply increases with decreasing temperature at T<T_{C}, allowing the liquid to spread over large distances away from the excitation region. We suggest that our findings are manifestations of a quantum liquid behavior.

低于阈值激光功率密度下Mg原子的非序列双电离

低于阈值激光功率密度下Mg原子的非序列双电离

Maskless laser processing of graphene

Graphene on a SiO2/Si substrate was removed by ultraviolet pulsed laser irradiation. Threshold laser power density to remove graphene depended on the graphene thickness. The mechanism is discussed using kinetic energy of thermal expansion of the substrate surface. Utilizing the thickness dependence, thickness (or layer-number) selective process for graphene is demonstrated. Maskless patterning of graphene using laser irradiation in the air is also demonstrated.

Diagnostics of VHF Argon Plasmas by Laser Thomson Scattering

The laser Thomson scattering (LTS) method has been applied to measure the electron density ne and electron temperature Te of very-high-frequency (VHF) argon plasmas. When the probing laser wavelength was 532 nm and the laser power density was ∼1015 W/m2, the Thomson scattering spectrum was obviously deformed by the effect of the photo-ionization of metastable argon atoms. The threshold laser power density at which the scattered light intensity from electrons in the plasma and that from electrons produced by photo-ionization are equivalent was found to be unexpectedly low (4× 1013 W/m2). To avoid the photo-ionization of metastable argon atoms, the laser power density was decreased to around 1×1013 W/m2 by using a cylindrical lens as the focusing lens. Then, the ne and Te values measured by LTS and the probe method were compared for a VHF plasma using argon gas at a pressure of 100 mTorr. This comparison confirmed that the LTS method gave reasonable ne and Te values.

Threshold laser power density for regime transition of a laser absorption wave in a reduced-density air atmosphere

Shadowgraph visualization experiments provide measurements of the threshold laser power density (Sth) for the regime transition of a laser absorption wave generated using a transversely excited atmospheric CO2 pulse laser with various pulse shapes. Results revealed a great influence of the plasma expansion in the direction lateral to the wave propagation on the regime transition by showing that the threshold increased proportionally to the inverse of the beam cross-sectional radius at which the transition occurred (rf,tr): Sth=Cth∕rf,tr. The proportionality constant, Cth, was insensitive to air densities of 0.2–1.3kg∕m3.

Comparison of nanosecond laser ablation at 1064 and 308 nm wavelength

To study the solid Cu ablation in vacuum, two different laser sources operating at 1064 and 308 nm wavelength are employed at similar values of laser fluences. The infrared laser is a Q-switched Nd:Yag having 9 ns pulse width (INFN-LNS, Catania), while the ultraviolet one is a XeCl excimer having 20 ns pulse width (INFN-LEA, Lecce). Both experiments produced a narrow angular distribution of the ejected material along the normal to the target surface. The ablation showed a threshold laser power density, of about 7 and 3 J/cm 2 at 1064 and 308 nm, respectively, below which the ablation effect was negligible. The laser interaction produces a plasma at the target surface, which expands very fast in the vacuum chamber. Time-of-flight (TOF) measurements of the ion emission indicated an average ion velocity of the order of 4.7×10 4 and 2.3×10 4 m/s for the infrared and ultraviolet radiation, respectively. We also estimated approximately the corresponding temperature of the plasma from which ions originated, i.e. about 10 6 and 10 5 K for IR and UV wavelength, respectively. A discussion of the analysis of the ablation mechanism is presented. At the used laser power densities the produced Cu ions showed ionisation states between 1+ and 5+ in both cases.

LASER-INDUCED CRYSTALLIZATION IN Sb 2S 3 FILMS

Crystallization in vacuum evaporated amorphous Sb 2S 3 films induced by a continuous wave argon laser is studied systematically as a function of laser power, irradiation time, film thickness etc. Results show that the observed amorphous-to-crystalline transformation is irreversible and is mainly due to the thermal process during the laser interaction. The threshold laser power density required to induce crystallization is found to be 98 ± 5 W/cm 2. Analyses indicate Sb 2S 3 films as a potential material for WORM kind of storage applications. © 1997 Elsevier Science Ltd

Carbon growth by thermal laser-assisted chemical vapour deposition

The heated area in thermal laser-assisted chemical vapour deposition (LCVD) is much smaller than in large area CVD. This affects the growth conditions drastically. One important growth aspect is that CVD processes requiring homogeneous reactions in the vapour are not effective for thermal LCVD. In this work LCVD of carbon from a non-adsorbable precursor (CH 4) and an adsorbable precursor (C 2H 4) in combination with cold wall and hot wall reactors was investigated. The combination with the hot wall reactor also means that adsorbable carbon-carrying species can be generated homogeneously in the vapour before deposition. For laser CVD of carbon from CH 4 a high deposition temperature was needed. The threshold laser power density was 9.0 × 10 4W cm −2 (3500 K) for a hot wall reactor temperature of 1270 K. For carbon LCVD from C 2H 4 carbon lines could be deposited already under cold wall conditions at the same laser power density as was used in the CH 4 case. The thickness profile perpendicular to the grown lines was close to gaussian for deposition temperatures below 3600 K. Bimodal profiles, which were observed for higher temperatures, indicate evaporation or surface diffusion of carbon. The deposited lines were fine grained with a nodular appearance. Crystalline carbon was found to grow in a layer structure parallel to the substrate surface. The lines deposited from CH 4 were considerably wider than the laser spot diameter. This may be due to the thermal anisotropy of the material. The temperature profile and also the line will be broadened since the heat conduction is much better along the graphitic planes than perpendicular to them. The lateral growth direction of the lines as well as the orientation of the graphite layers indicate that the edge atoms of the graphitic basal planes are very active during the growth.

Experimental determination of thermal and nonthermal mechanisms for laser desorption from thin metal films

The mechanism of laser desorption of peptides as negative ions from Au thin films with 193, 248 and 351 nm laser radiation has been studied. Variation of the threshold laser power density with metal film thickness is used to distinguish between thermal and nonthermal mechanisms. The influence of laser wavelength on the desorption of peptides with different optical absorption spectra has been studied. Thermal desorption is observed when 351 nm laser radiation is utilized. However, both 248 and 193 nm radiation result in nonthermal desorption processes. At 248 nm, the threshold power density is observed to be independent of the optical absorption of the peptide adsorbate, supporting suggestions that a mechanism involving excitation of hot electrons in the metal is important.

Improvements on the electrical resistivity of chemical bath deposited CdS films by laser annealing

The dark resistivity of cw Ar+ ion laser annealed chemical bath deposited CdS films has been measured. A two-decade decrement of the electrical resistivity, as compared to the unannealed samples, is reported. A threshold laser power density for reproducible annealing has been found to be ≳50 W/cm2. Log ρ vs 1/T plots were used to calculate the mobility ratio b = μn/μp. This ratio shows a slight increment on laser annealed but it is not sufficient to explain the resistivity changes.

Laser Annealing of Chemical Bath Deposited CdS Films

AbstractChemical Bath Deposited (CBD) CdS films have been exposed to a cw Ar+ laser annealing. Measurementes on the dark electrical resistivity have shown a two decade decrement in the resistivity. A trend of improving this resistivity is shown upon atmospheric pressure increase. The annealing mechanisms of the samples show a threshold laser power density of 50 W/cm2. This power threshold accounts for some heating mechanisms and Cd ion evaporation from the samples; thus a gaseous Cd atmosphere annealing process is suggested. This laser annealing mechanism parallels thermal annealing of polycrystalline CdS films. Assessment of this mechanisms is supported by the fact that the mobility ratio, b = μn/μp, (greatly influenced by crystallite size and intergranular potential barrier in thin film semiconductors) does not show major changes to account for a two decade decrement of CBD laser annealed samples.

TWO-PHOTON IONIZATION PROCESS IN OPTICAL BREAKDOWN OF CESIUM VAPOR

Optical breakdown of cesium vapor by Q-switched laser light is studied. The threshold laser power density for the breakdown is observed over fairly wide range of the cesium atoms density. A pair of probes are inserted in the vicinity of the focal point for the purpose of detecting the occurrence of breakdown. The magnitude of the threshold laser power density and its dependence on the atom density measured by us are different from those reported by Rizzo and Klewe. The present result confirms the validity of the two-photon ionization mechanism suggested by Bebb and Hall.