The Influence of Excitation Density on Laser Induced White Lighting of Wide-Band-Gap Semiconductor ZnSe:Yb Polycrystallite Ceramics

The irradiation of ZnSe:Yb polycrystalline ceramics by focused beam of a CW 975 nm laser diode in vacuum conditions leads to generation intense white light in visible range. The emission band was centered at 630 nm. The intensity of white emission increased exponentially with the power density of incident laser light. The inﬂuence of excitation power density on generation of broadband emission was investigated. It was found that with increasing excitation power density the total intensity and the slope of exponential enhancement of white light signiﬁcantly increase. Moreover, for the highest excitation power density there appeared by excitation threshold. The impact of excitation density on white laser induced white emission (LIWE) was discussed in terms of multiphoton excitation and avalanche ionization The part of the JSS Focus Issue on Recent Advances in Wide Bandgap III-Nitride Devices and Solid State Lighting: A Tribute to Isamu Akasaki.

Semiconductors play an important role today as a high-efficiency light sources. The invention of the blue LED by Akasaki et al. marked the beginning of a new generation for general lighting. 1,2 In white-LEDs the blue light generated by GaN crystals is converted partially by phosphors to broadband white light emission. 3 In this paper the another method of generation of white light from semiconductors materials is presented.
The zinc selenide (ZnSe), the II-VI wide gap semiconductors, have been widely studied for their fundamental properties [4][5][6] and applications mostly as light emitting diodes and diode lasers to emits blue light. 7 The intentionally impurities are change significantly their properties. Activation by coper or manganese ions leads to red shifting and broadening of emission band. 8 ZnSe doped with chromium or iron have been used as infrared laser gain medium with emission respectively at 2.4 μm 9-11 and 4.4 μm. 12 Doping by tellurium ions caused its use as scintillator with emission at 640 nm. 13,14 The generation of intensive broadband white emission under illumination with focused beam of infrared laser in vacuum conditions is possible on oxide materials, 15-25 metals, 26 carbon structures such as graphene 27,28 and diamonds. 29,30 The phenomenon of laser induced white emission (LIWE) is characterized by nonlinear behavior characteristic for multiphoton absorption. It is important to highlight that the generation of white emission is accompanied by efficient photocurrent, which contributed to explain the mechanism of phenomenon. 18,31 The mechanism of LIWE was discussed in terms of inter valance charge transfer, 18 thermally stimulated black body emission, 22 trap emission 32 and resonance enhanced Raman process 33 related to the multiphoton absorption and electron avalanche processes responsible for ionization leading to white light continuum emission and efficient photocurrent. In this work we present the first time investigation of laser induced white emission (LIWE) in wide-band-gap ZnSe:Yb polycrystalline ceramic. Investigation of semiconductor material is related with expecting to obtain more efficient emission due to lower lying conductivity bands. Moreover, using Yb 3+ absorption band for excitation, could offer more efficient pumping.

Experimental
Zinc selenide doped with 2% mol of ytterbium was synthesized by solid-state reaction in high pressure conditions. The stoichiometric amount of ZnO, Se powder and ytterbium acetate 34 were mixed in agate mortar and press to green body pellet with 5 mm diameter under z E-mail: r.tomala@intibs.pl 50 MPa under hand press. The pellet prepared in this way was placed in toroid-type container from calcium carbonate with graphite as a heater and boron nitride layer as insulator. The pellet was press under 8GPa at 1000°C in 10 min (plus 1 min of rise time of temperature and 2 min of cooling time).
The X-ray patterns were recorded using PANalytical X'Pert Pro powder diffractometer (Cu Kα 1 :1.54060 Å). The emission spectra were recorded using the FLS980 Fluorescence Spectrometer from Edinburgh Instruments equipped with 50mW laser diode 266nm as excitation source. The luminescence spectra of laser induced white emission were measured at low pressure condition were performed using the vacuum cell supplied with Turbomolecular Drag Pump TMH071P and electronic drive unit TC 600 (Pfeiffer), 975 nm CW LD 1.6W as an excitation source and AVS-USB2000 Spectrometer from Avantes as a detection. The comparison of emission intensity was performed on the same emission setup with AVS-USB2000 as detector, changing the excitation source.
The power density experiment was performed with Thorlabs NRT100 linear positioning stage to control the distance between sample and lens. The area of laser spot in focal point was calculated by using a method of Saleh, 35 where the diameter of the focused spot is given by where f is the focal length of lens, λ is the wavelength of the laser and D is the beam diameter. The area of a laser beam spot The spot diameters in intermediate points were calculated using the cone method. 18

Results and Discussion
The structure of ZnSe:Yb of polycrystalline ceramics was investigated using the X-ray diffraction method. In Fig. 1 comparison the experimental diffraction pattern and pattern simulated from reference PDF#00-037-1463 was presented. The peaks observed in the diffractogram well corresponds with the F-43m cubic structure of ZnSe. The average grain size of crystals calculated from Rietveld analysis is 122 nm. The peaks marked with asterisk are derived from BN which acts as an insulator in sintering process.
The photoluminescence spectrum of ZnSe:Yb under 266 nm excitation is shown in Fig. 2a. The broadband emission centered at 515 nm was observed. The irregular shape of emission band show that observed emission is a result of overlapping of transitions with different origin. Following Radevici et al. 36 the observed emission can be attributed to exciton region (∼446 nm), recombination of carriers bound to donor-acceptor pairs (∼ 458 nm), self-activated PL band (∼545 nm) and resonant energy transfer between Yb and a deep impurity center (∼635 nm).
The anti-Stokes emission of ZnSe:Yb was measured using 975 nm laser diode as an excitation line under 10 −5 mbar pressure conditions (Fig. 2a). Broadband Gaussian-type peak is centered at 650 nm. The intensity of LIWE band is about 200 times greater than the intensity of the Stokes emission observed under 266 nm excitation.
The dependence of LIWE intensity of ZnSe:Yb on the excitation power density (P exd ) is illustrated in Fig. 3 and Fig. 4. The measurements were performed using a lens with a focal length of 4 cm. The excitation power density was controlled by lengthening the distance between sample and lens. It was observed that with decreasing excitation density the slope of LIWE intensity plotted in log-log scale significantly decreased. Moreover, the plots were composed from two parts. The first was characterized by higher value of slope for lower excitation density, where the second part was almost two time smaller value of slope. The inflection point is dependent on power density, so its location shifts in higher power direction with increasing of spot size (Figs. 3b-3e). Similar results were reported earlier for Y 2 O 3 :Yb microcrystalline powder by Wu et al. 33 by pumping by 940 and 980 nm laser diodes, however the range of investigated power density experiments was significantly lower (30-370 W/cm 2 ) than in our measurements (100-7000 W/cm 2 ).
The intensity of LIWE is well scaled by exponential power formula I LIWE ∝ P N , where P is the excitation power and N is the scaling order parameter that usually is treated as a number of photons involved in excitation process to the conductivity band characterized by energy gap E CB . Due to the similar behavior of emission like in graphene foam the mechanism of emission can be discussed like in, 31 based on generation of free electrons due to multiphoton (MP) and avalanche ionization. 37 The transition rate of multiphoton ionization may be related simply to the order of multiphoton absorption (N MP = E CB /ћω), whereas for avalanche ionization the free electron density N e grows exponentially with laser field due to sufficiently high kinetic energy of emitted electrons (hot electrons) initiated by laser field 38 where N 0 is the number of starting electrons and α(E) is the rate of ionization in electric field E of laser beam. The hot electrons have sufficiently high energy to proliferate more electrons in conduction band, so the avalanche ionization is more efficient process than multiphoton ionization at high excitation power density. 39 Since both the multiphoton absorption and avalanche ionization contribute to LIWE the total intensity is described by the product of two exponential formula I LIWE ∝ P N P α ≈ P N+α . In our experiment the order parameter N increased from 3.52 to 6.09 for maximally focused laser beam.

Conclusions
In the present work the laser induced white light emission of the wide-band-gap semiconductor ZnSe:Yb polycrystalline ceramics was reported. This system exhibits under UV excitation the broadband emission centered at 515 nm assigned to the emission from matrix, however under irradiation in infrared by the focused beam of laser diode operating on 975 nm this system generates the efficient anti-Stokes white emission. That emission increased exponentially with excitation power density. The influence of excitation power density on generation of white emission was investigated. It was found that   the slope of LIWE decreased with increasing the spot of focused beam. It appeared with linear decreasing the total white intensity. The mechanism of LIWE was discussed in terms of multiphoton excited ionization and avalanche ionization. The second process contributes in dominant way to enhancement of white emission.