T. Sun

Optical properties and time-resolved photoluminescence of conjugated polymers with europium complex side chain as an emitter

Abstract We investigated four conjugated polymers of the same backbone with different lengths of side chains attached with europium (Eu 3+ ) complex (herein labeled P2, P4, P6 and P8). All samples exhibited strong luminescence belonging to f–f* electronic transitions of Eu 3+ . Sharp emission peaks of 5 D 0 – 7 F J ( J =0, 1, 2, 3) transitions were observed with the strongest peak at ∼612 nm (i.e. 5 D 0 – 7 F 2 transition). Ligand field splitting was seen for the 5 D 0 – 7 F J ( J =1, 2, 3) transitions. Weaker and very broad emission due to singlet S 0 →S 1 transition of the polymer was also observed. The temperature dependent fluorescence measurement indicating the energy transfer from polymer S 1 state to the Eu excited D levels is not a barrierless process; some kind of activation energy may be involved. The fluorescence rise-time indicates that the energy transfer process may take several hundred nanoseconds to be fully completed. Emission lifetimes at 612 nm were measured at 0.72, 0.37, 0.39 and 0.61 ms for P2, P4, P6 and P8, respectively. However, there was a fast decay component for the 612 nm emission, indicating some initial rapid relaxation process.

Molecular-beam-epitaxy-grown ZnMgS ultraviolet photodetectors

Epitaxial growth of Zn1−xMgxS alloy thin films on GaP(100) substrates was carried out using the molecular-beam-epitaxy technique. In situ reflection high-energy electron diffraction studies show that the alloys can be grown with a stable zinc-blende structure up to x around 30%. For x>30%, a structural transition will occur at a critical thickness which is sensitively dependent on the x composition. A near-band-edge peak with a full width at half maximum of about 10 nm was observed in room-temperature photoluminescence measurements made on as-grown alloy thin films. Several Zn1−xMgxS-based Schottky barrier photodetectors were fabricated. Room-temperature photoresponse measurements were performed on these detectors and abrupt long-wavelength cutoffs covering 325, 305, 295, and 270 nm were achieved for devices with Mg composition of 16%, 44%, 57%, and 75%, respectively. The response curve of the Zn0.43Mg0.57S device offers a close match to the erythemal action spectrum that describes human skin sensitivity ...

Two-photon absorption autocorrelation of visible to ultraviolet femtosecond laser pulses using ZnS-based photodetectors

In this letter, we demonstrated an autocorrelator based on two-photon absorption (TPA) of ZnSSe photodetectors, which is capable of measuring femtosecond laser pulses from 800 to 400 nm. This broad bandwidth autocorrelator has high sensitivity and is capable of measuring laser pulses as short as 27 fs and pulse energy of a few picojoules. These results could lead to cost-effective ultrafast autocorrelator with a single detector head to cover the whole visible to near ultraviolet spectral region. Our TPA measurement shows that the defect and impurity states below the bandgap (E/sub g/) have detrimental effect for excitation wavelengths near the bandedge, however, these states are also beneficial by extending the operation wavelengths of the detector beyond E/sub g//2. Using a ZnMgS photodetector with a larger E/sub g/ enables the autocorrelator to be more sensitive at 400 nm, and extends the responsivity further into the ultraviolet wavelengths.

MBE-GROWN ZnMgS ULTRA-VIOLET PHOTODETECTORS

Zn1−xMgxS alloy thin films covering Mg composition, x, from 0 to 1 were grown on GaP (100) substrates using molecular beamepitaxy. As observed by reflection high energy electron diffraction patterns, the alloys can be grown with stable zinc-blende structures up to x around 30%. For x>30%, a structural transition will occur at a critical thickness which is sensitively dependent on the composition x. Several Schottky barrier photodetectors using a Zn1−xMgxS layer with a thickness less than the critical thickness as the active layer were fabricated. The shortest cutoff wavelength achieved so far is 270 nm. Temporal response of the device shows that the photocurrent decays with a time constant as fast as 13 ns for a mesa area, of 1.5 mm2.

Dynamics of random laser and coherent backscattering of light from ZnO amplifying random medium

We investigated the dynamics of coherent backscattering cone and random laser in a disordered gain medium of ZnO powder. Theoretical fit to the coherent backscattering cone yields a transport mean free path l approximately equal to the probe wavelength λ, indicating that the scattering process is in the strong scattering regime. Supernarrow emissions from the laser are accompanied by enhancement and sharpening of the coherent backscattering cone. This enhanced and sharpened backscattering cone returns to a normal profile within 10–20ps, indicating an ultrafast gain lifetime in the medium, which is in good agreement with the time-resolved measurement of the random laser emissions.

Triplet state dynamics in poly(2,5-pyridine diyl)

Pulse radiolysis was used to determine the triplet state energy (2.3 eV) of Poly(2,5-pyridine diyl) (PPY) film which was found to be coincident with the emission energy. Detailed time-resolved photoluminescence (PL) and pump-probe measurements have been applied to study the photoexcited state relaxation dynamics. In films, a very large spectral red-shift (0.35 eV) for the PL occurred within the first 100 ps whereas no spectral red-shift was observed for the PPY in solution. This result shows clear evidence for the evolution of short-lived singlet emission (S1! S0 )a t450 nm to long-lived triplet emission (T1! S0 )a t520 nm for the PPY thin film. Streak camera measurement indicates the long live component has a decay time constant of several ns. The picosecond photo-induced triplet state absorption (T1! T2 transition) peaks at600 nm as measured by pump-probe which is consistent with both the radiolysis and cw photoinduced absorption measurements. A triplet lifetime of6 ns is measured which is again consistent with the streak camera measurement. These results lead us to believe that the long live component of the emission from PPY film is in fact phosphorescence. Furthermore, there is evidence that oxygen plays a very important role in the fast triplet radiative lifetime in PPY films. # 2001 Elsevier Science B.V. All rights reserved.

Dynamics of stimulated emission and coherent backscattering of light from amplifying random medium

We investigated random scattering of light in a disorder gain medium of ZnO powder using the pump-probe technique. Using a probe beam at (lambda) =390nm, the width ((theta) ) of the coherent backscattering peak from the ZnO powder is measured to be ~7.5 degree(s), thus the coherent scattering length l is approximately 1.2(lambda) ((theta) =(lambda) /2(pi) l) which is close to the strong scattering regime. When a pump beam ((lambda) =267nm) exceeds a certain excitation threshold, supernarrow emission peaks (bandwidth less than 1nm) emerged from the ZnO broad photoluminescence background. Concurrently, we also observed enhancement and sharpening of the coherent backscattering cone. Since light from the center of the backscatter cone experience the largest number of scatterings (i.e. longest gain length), this result is thus consistent with the random laser model that the supernarrow peak is due to amplification and stimulated emission of photon in the random gain medium. The time-resolved pump-probe measurement shows that the lifetime of the emission state above the lasing threshold is only a few picoseconds which is consistent with the interpretation that the supernarrow peaks are due to stimulated emission.

Spectra super-narrowing and random lasing dynamics in neat and TiO 2 -doped polymer films

We investigated the dynamics of random lasing in 2,5-dioctyloxy poly(p-phenylene-vinylene) (DOO-PPV) neat and TiO2–doped films. We observed laser-like sharp emission (i.e., spectral width less than 1nm) in the films under certain threshold excitation condition. Fig.1 shows the random lasing time-integrated spectra in neat DOO-PPV film at various excitation intensities using excitation wavelength of 532nm (35ps,10Hz). Near the threshold intensity Io,ceven single mode emission can be achieved. At excitation below the threshold (i.e., I=0.5Io), only the broad photoluminescence (PL) background can be seen, although here the intensity scale is too large to see this PL clearly. At threshold intensity Io and just above it, three super-narrow emission peaks emerge. Increasing excitation intensity will increase number of super-narrow emission peaks. At very high excitation (i.e., 5 to 6 time above Io), all the peaks merge together into a single stimulated emission peak (with spectral width 5-10nm) commonly seen in lasing action experiment with an emission lifetime of a few picoseconds [1]. Fig 2 shows the time-resolved emission under the excitation condition where only a single super-narrow peak emerges. This time-resolved PL shows laser-like sharp emission lifetime at 622 nm was much less than the broad spontaneous emission lifetime and only limited by the time-resolution (~20ps) of our streak camera system. Similar random lasing dynamics and spectra super-narrowing were also measured in TiO2-doped (1×109cm−3) DOO-PPV polymer films. This ultrafast emission decay strongly indicates the super-narrow spectral peak is due to laser action.