We report on laser-induced vaporization of mercury as a simple tool to increase the efficiency of a resonant 4-wave mixing process to produce VUV radiation near 125 nm. The specificity of this new nonlinear medium is discussed for the best experimental conditions giving a high density of ground state Hg atoms in a column-shape plume. Modification of phase-matching conditions is demonstrated in agreement with VUV profile simulations.

The research in the field of azopolymeric materials presents a great interest for their applications in different areas, like photonics and/or biology, due to their peculiar characteristics. In this idea we have studied the possibility to create nano-bi-dimensional structures on the surface of modified polysiloxane films under the action of ultraviolet light. We worked with polysiloxanes and linear poly (p-chloromethyl) styrene full modified with azophenols, which were synthesized and deposited on glass supports. Thickness of the films was in the 2-5 μm range. Under the action of a controlled distribution of monochromatic UV light on the film was induced a surface modulation which results in a structure (surface relief grating –SRG – in our case). As light source we have used laser radiation with wavelength of 193 nm or 355 nm. We have obtained structures with the pitch of 250 and 1000 nm as a function of the irradiation interference field pitch.

Highly coherent, tabletop, and reliable sources in the Vacuum UltraViolet (VUV) region (λ ≪ 180 nm) are desirable for both fundamental studies (e.g. laser cooling of (anti-)hydrogen atoms [1]) and nanopatterning applications. While excimer lasers are well suited for mask exposure in lithography or nanohole drilling, they are far from ideal for interference lithography (and holography), due to their poor coherence. An alternative solution consists in using nonlinear conversion of visible lasers in vapors down to the VUV. This makes sources with better coherence and beam quality but with very low energies (typically ≪ µJ). In order to reach energies allowing patterning standard polymers in a reasonable time (the threshold ablation fluence is ∼1 mJ/cm 2 in PMMA at 125 nm [2]), we recently developed an original concept in which the nonlinear medium is a dense, spatially localized, laser ablation plume of mercury atoms [3].

The efficiency of nonlinear processes in metallic vapours is limited by usually low atomic densities.One way to circumvent this limitation, while keeping simplicity (that is avoiding the use of complex and user-unfriendly heat pipes), is to use a laser-induced ablation plume of the metal as a nonlinear medium.Here we present a study of a Hg plume used to amplify 125-nm radiation created by a resonant 4-wave-mixing process.

We developed a reliable and convenient source of coherent pulsed radiation at 125 nm obtained by resonant four-wave-mixing in a mercury vapour. Recently, we improved the yield of the source by using a pulsed laser to create an ablation plume of mercury atoms, in which the nonlinear mixing takes place with increased efficiency. We discuss here the characteristics of the plume created by mercury vaporization, in relation to the energy source enhancement.

The efficiency of a coherent vacuum ultraviolet (VUV) source at 125 nm, based on two-photon resonant four-wave mixing in mercury vapor, has been enhanced by up to two orders of magnitude. This enhancement was obtained by locally heating a liquid mercury surface with a pulsed excimer laser, resulting in a high-density vapor plume in which the nonlinear interaction occurred. Energies up to 5 μJ (1 kW peak power) have been achieved while keeping the overall mercury cell at room temperature, avoiding the use of a complex heat pipe. We have observed a strong saturation of the VUV yield when peak power densities of the fundamental beams exceeded the GW/cm 2 range, as well as a large intensity-dependent broadening (up to ∼ 30 cm -1 ) of the two-photon resonance. The source has potential applications for high-resolution interference lithography and photochemistry.

Bright blue organic light-emitting diodes (OLEDs) based on 1, 4, 5, 8, N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole (N, N′-diethyl-3, 3′-bicarbazyl) (DEC) as emitting layers or as dopants in a 4, 4′-bis(2, 2′-diphenylvinyl)-1, 1′-biphenyl (DPVBi) matrix are described. Pure blue light with the CIE coordinates (x = 0.153, y = 0.100), electroluminescence efficiency ηEL of 0.4 cd A−1, external quantum efficiency ηext of 0.6% and luminance L of 236 cd m−2 (at 60 mA cm−2) were obtained with PMC as an emitter and the 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenantroline (BCP) as a hole-blocking material in five-layer emitting devices. The highest efficiencies ηEL of 4.7 cd A−1 and ηext = 3.3% were obtained with a four-layer structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158, y = 0.169, λpeak = 456 nm). The ηext value is one the highest reported at this wavelength for blue OLEDs and is related to an internal quantum efficiency up to 20%.

Bright-blue organic-light-emitting-diodes based on carbazolic-compounds as emitting layers or dopant in DPVBi matrix are described. Pure-blue light with CIE coordinates xy = (0.158;0.169) and external quantum efficiency as high as etaext = 3.3% were obtained.

Non-doped white organic light-emitting diodes using (i) a yellow-emitting ultrathin layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on the either side of the interface of a hole-transporting alpha-NPB (4,4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl) layer and a blue-emitting DPVBi (4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl) layer, (ii) a hole-injecting layer of CuPc (copper pthalocyanine), (iii) an electron-transporting layer of Al(Q)3 (aluminium tris[8-hydroxyquinolinate]), are described. Both the thickness and the position of the rubrene layer within the hole transporting layer of alpha-NPB and the blue-emitting film of DPVBi allow to achieve the desired color from deep blue with CIE coordinates (x= 0.17, y= 0.15) and an external quantum efficiency etaext= 3.4%, (at 30 mA/cm 2), to pure yellow with CIE (0.51, 0.48) and etaext =1.3%, via a bright white (WOLED) with excellent CIE coordinates (x= 0.33, y= 0.32), a etaext of 1.9%, a color rendering index (CRI) of 70 and a luminance of 2230 cd/m2 at 60 mA/cm 2. Such a single-well-like structure provides a fine tuning of the emission color by means of the adjustment of the exciton diffusion zone via the exciton-confinement layer of rubrene

Nous presentons une technique simple pour ameliorer d'un a deux ordres de grandeur l'energie d'un rayonnement coherent a 125nm, obtenu par somme de frequence resonante dans une vapeur de mercure a temperature ambiante. En focalisant sur la surface de mercure un laser ArF a 193nm, le signal VUV est multiplie par 6, facteur qui peut s'elever jusqu'a ∼60 lorsque l'intensite des faisceaux fondamentaux est reduite. Cette experience de validation ouvre de nombreuses opportunites d'amelioration pour une source VUV de forte brillance, applicable a la nanostructuration de materiaux pour l'optique.

A blue fluorescence of N-ethylcarbazole (EtCz) and of its dimer N,N′-diethyl-3,3′-bicarbazyl ((EtCz)2) as dopants of poly(methyl methacrylate) (PMMA) matrices, under laser pulsed UV irradiation, was observed. The fluorescence lifetimes were found to be 16.1 and 11.6ns for EtCz and (EtCz)2, respectively. Concomitantly, a green phosphorescence occurred at room temperature and peaking at 444nm for EtCz and 486nm for (EtCz)2. From both luminescence spectra, singlet–triplet energy transition was estimated to be 0.56eV for EtCz and 0.45eV for (EtCz)2. Moreover, under UV irradiation, photoluminescence spectra exhibited a strong degradation with exposure time, accompanied by the appearance of new bands in the 400–500nm range. Species associated to these new signals exhibited shorter lifetimes than those of the main peaks. Photodegradation of the doped PMMA matrices were not observed under nitrogen atmosphere, whereas in the presence of oxygen gas, they readily appeared. In the same way, thin film of pure (EtCz)2 did not show any modification under vacuum, whereas in air the photochemical evolution clearly appears. Possible causes of the oxygen-sensitive photodegradation of both the EtCz and (EtCz)2-doped PMMA matrices, were discussed.

The photoresponse of different photoconductive diamond sensors to excimer laser pulses is investigated as a function of the pulse energy E over three order of magnitude. The integrated photoresponse pulse intensity changes according to the power law I∝Eβ. Initially, the pulse intensity increases linearly with the pulse energy up to a turning point, which depends on the electronic quality of the diamond film, where a sublinear behavior (β=0.5) is found. The photoresponse pulse shape also changes accordingly, largely broadening at high pulse energies. Such behavior has been interpreted in terms of monomolecular and bimolecular recombination mechanisms acting in different pulse intensity ranges. A further reduction of β is also observed at very high intensity in less defected samples, where the induced carrier density is high enough to cause carrier–carrier scattering and mobility reduction.

Laser induced cleaning of silicon wafer covered with photoresist thin film was studied. We have used ultraviolet laser beams with wavelength 248 nm and 313 nm. Experimental results have evidenced that a good cleaning of the surface can be obtained with a proper choice of the irradiation conditions (intensity, number of pulses). The removal of the photoresist in the irradiated region was obtained for laser intensities of the order of 10(7) W/cm(2) and multipulse laser irradiation. Also photoresist removal from the silicon surface can be obtained for one pulse or for a low number of laser pulses, but for incident intensity greater than the melting threshold of the silicon substrate. In this case the very accurate selection of the incident energy is more important to avoid substrate surface damage.

Solar ultraviolet imaging instruments in space pose most demanding requirements on their detectors in terms of dynamic range, low noise, high speed, and high resolution. Yet UV detectors used on missions presently in space have major drawbacks limiting their performance and stability. In view of future solar space missions we have started the development of new imaging array devices based on wide band gap materials (WBGM), for which the expected benefits of the new sensors - primarily visible blindness and radiation hardness - will be highly valuable. Within this initiative, called "Blind to Optical Light Detectors (BOLD)", we have investigated devices made of AlGa-nitrides and diamond. We present results of the responsivity measurements extending from the visible down to extreme UV wavelengths. We discuss the possible benefits of these new devices and point out ways to build new imaging arrays for future space missions.

According to a previously developed pure photochemical model of VUV laser ablation of polymers, the velocity of ablation front is proportional to surface intensity, and a stationary value of the surface temperature does not depend on laser intensity. Previous estimations show, however, that this stationary surface temperature could be too high to be relevant to the photochemical mechanism. This raises a question of whether the stationary value of the surface temperature can be achieved for a given time shape of light intensity coming to the surface irradiated by a laser pulse of high enough fluence. The intensity time shape is connected not only with the time shape of a laser pulse but also with screening of laser radiation by the plume. This problem is discussed in the present communication. Specifically, it is shown that with a hyperbolic surface intensity time shape, heat diffusion can successfully compete with laser heating decreasing maximum surface temperature compared to its stationary value. The hyperbolic surface laser intensity corresponds to a rectangular laser pulse screened by plume during the photochemical ablation. This allows one to estimate that the photochemical model for a multiple-pulse VUV laser ablation with a high plume extinction coefficient is self-consistent even for a high value of stationary temperature and for high enough laser fluences.

Depending on purity, diamond could exhibit very high breakdown threshold voltages, high free carrier mobilities and relatively high free carrier lifetimes. For these reasons, diamond has been considered to be well suited for radiation induced high power switching applications in continuous operation mode. Excitation using deep UV sources has been studied but it suffers from their intrinsic low conversion efficiency (below 0.1%). The other problem encountered with deep UV excitation, especially in a bulk configuration, is the short penetration length leading to poor collection efficiencies, polarization and space charge effects. Depending on energy, electron beam excitation should be more adapted for this kind of application. In the present study we report on the conductivity modulation induced by continuous electron beam excitation in CVD diamond samples as a function of electron beam energy and current. The current gain is found to depend strongly on the energy and current of the electron beam. At low electron beam currents continuous gains up to 180 have been measured for 30 keV electrons, believed to be the highest continuous gain ever reported.

We suggest the use of deep UV photons as a probe to investigate the photoelectrical properties of different depths in the diamond films. The penetration depth of UV photons decreases rapidly towards shorter wavelengths in the region of and just below the band gap energy, achieving ∼2 nm at wavelengths approximately 100 nm. Therefore, they are suitable for probing the bulk properties and the surface proximity of diamond layers. We have irradiated different CVD diamond samples with UV laser pulses at 193, 213 and 266 nm as well as continuous radiation emitted by a UV source in the 140–250 nm wavelength range. We have analysed the photoconductivity at different depths as a function of material thickness, electric field value and polarity, radiation intensity, time response to laser pulses and to illumination transients. We have observed different behaviours above and below the band gap energy that could be due to polarization effects and a photo-response quenching above the band gap that is enhanced by reversing the electric field.

Two carbazole molecular derivatives have been used as blue emitters in multilayer light-emitting-diodes i.e. the dimer of N-ethylcarbazole (bicarbazyl, DEC) and the 1,4,5,8,9-pentamethylcarbazole (PMC). Various devices with transporting layers for both electrons and holes and hole blocking material, have been achieved and tested. The influence of the device configuration and of the electronic structure of the organic materials on electroluminescent performance are presented and discussed. It has been found that a five-layer configuration with pentamethylcarbazole as an active layer associated to bathocuproine (BCP) as a hole blocking layer exhibits a very pure and bright-blue light (/spl lambda//sub peak/=456 nm). A luminance-to-current efficiency of 4.7 cd/A, a power conversion efficiency of 1.3 lm/W and an external quantum optical-to-electrical conversion efficiency as high as 3.3% have been obtained for DPVBi doped with PMC. Moreover, all the designed devices were working under ambient atmosphere and exhibited a satisfactory long-lived emission.

The interest to use laser surface processing in microtechnology as a friendly method from the technologic and environmental point of view leaded our studies about laser radiation interaction with photo-resist and metallic thin films. In this view we have tried in our experiments to process metallic thin films deposited on silicon substrate by using laser radiation.To obtain a good quality of the metallic thin film removal from the silicon surface a careful selection of the incident laser intensity, number of pulses and irradiation geometry is needed. The threshold value for the laser cleaning intensity depends on the number of incident laser pulses. A careful experimental estimation of the cleaning conditions from the point of view of incident laser energy, fluence, intensity and irradiation geometry was realized for aluminum, copper, and chromium thin films.