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Observation of a laser-induced transmittance modification in Ni2+doped CsCdCl3 |
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PhysChemComm,
Volume 2,
Issue 13,
1999,
Page 67-69
Ueli Oetliker,
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摘要:
Observation of a laser-induced transmittance modification in Ni2+ doped CsCdCl3 and unique electronic structure as an attractive starting point for experiments involving the control of optical properties by using excited states. We examine one of the simplest cases, the tuning of the intensity of a weak electronic transition. The left hand panel (a) of Fig. 1 shows a schematic view of the laser-induced transmittance modification. The transition used to detect the transmittance modification from the ground state to the excited states is depicted in green. The red double arrow indicates the coupling transition from the excited state to a third state. On the right hand panel (b), a more detailed view of the actual states in our material is shown. The modified transmittance is detected at the 3A Ueli Oetliker and Christian Reber Département de Chimie, Université de Montréal, Montréal QC H3C 3J7, Canada. E-mail: oetliker@chiphy.unige.ch, reber@chimie.umontreal.ca; Fax: +1 (514) 343-7586; Tel.: +1 (514) 343-7332 Received 1st October 1999, Accepted 4th November 1999, Published 9th November 1999 1A The absorbance of the narrow 3A2® 1A1 (O point group symmetry approximation) crystal-field transition in CsCdCl3 doped with Ni2+ ions can be influenced by a laser tuned to a strong inter-excited state transition reported earlier for this material.The intensity of the band maximum at 525.2 nm decreases to nearly zero for the duration of a laser pulse in resonance with the intense 1 « 3T2 inter-excited state transition at 701.7 nm which couples the 1A1 and 3T2 states.Detuning the coupling laser wavelength reduces the effect considerably and thus confirms the importance of this particular inter-excited state transition for the control of a conventional transition from the electronic ground state. 6 Introduction The quantitative investigation of optical effects beyond conventional transitions observed with low-intensity radiation has become a very active domain of research. A wide variety of nonlinear effects, from multiphoton spectroscopy to different techniques of upconversion and to coherent control have been described.1–10 The inspiration for the work presented in the following was the experimental demonstration of "electromagneticallyinduced transparency" for isolated atoms in the gas phase.In these experiments, transition intensities from the electronic ground state to an excited state are influenced by interference with another transition to a third state, induced by a strong electromagnetic field tuned to the energy difference between the third state and the ground state or the excited state. It has also recently been shown that interference effects can arise from pulses with arbitrary phase relationships in systems with at least one avoided crossing.11,12 The main emphasis of these detailed studies is again aimed at atoms or small polyatomic molecules. Recently, "electromagnetically-induced transparency" has been reported for ruby.13 To our knowledge this was the first study involving a solid material with transition metal chromophores.In this communication, we present experimental evidence for an enhancement of the transmittance in another crystalline solid, but we do not attempt to identify the detailed physical mechanism of the effect. Materials containing transition metal centers have a long history of applications in optical technology, from a variety of phosphor materials to the first active laser medium and most recently to promising materials for photon avalanches and near-infrared to visible upconversion.3,4 We demonstrate a novel approach to the tuning of the optical properties of transition metal based materials in the following, using CsCdCl 4– 3 doped with Ni2+ ions, a system containing discrete NiCl units with the metal ion in a trigonally distorted octahedral coordination. A molecular picture of the electronic states localized on this unit is shown in Fig.1 and will be used to discuss the experimental results. All electronic states of the title material have been studied in detail from the near-infrared to the ultraviolet spectral region,14–16 and we use its rich PhysChemComm, 1999, 13 2 ® 1A1 transition (green arrow) which is a weak and narrow ground state absorption at 19040 cm–1 with a half width of 25 cm–1.16 The coupling transition 3T2 « 1A1 (red arrow) on the other hand is a very strong and narrow inter-excited state absorption at 14251 cm–1 with a half width of 5 cm–1.16 As an exceptional case among the first row transition metal ions, Ni2+ in CsCdCl3 shows emission from a higher excited Fig.1 (a) Schematic representation of the transitions used to induce and observe the transmittance modification. (b) The electronic states involved in the experiments on CsCdCl3 doped with Ni2+ ions. Symmetry labels are given in the idealized O point group.state (1T2), leading to a rich luminescence spectroscopy15 and enabling us to investigate the inter-exited state interactions in detail. Experimental The intense and sharp 3T2«1A1 transition of Ni2+ doped into CsCdCl3 (see Fig. 1b) provides an excellent handle to probe the laser-induced coupling of these excited states and does not require prohibitively high laser powers. An important experimental aspect is the alignment of the coupling and probe beams, shown as red and green arrows in Fig.1. The beams can be adjusted by monitoring the intensity of the 3A2 � 1T2 upconversion luminescence.16 This unique property allows us to verify the optical alignment with high precision, but it is not intrinsically necessary for the observation of transmittance modifications due to transitions between excited states. The conventional transmission spectrum was recorded using a tungsten lamp (50 W) in conjunction with a Spex 500M monochromator equipped with a 600 grooves mm–1 grating (1 µm blaze). The transmission spectrum over this narrow wavelength range is in exact agreement with the literature absorption data.16 The sample was cooled to 13 K in an Oxford Instruments CF 1204 Helium gas continuousflow cryostat.The transient transmission traces were measured with the same instrumentation, with coupling pulses provided by a Lumonics EPD 330 dye laser with Pyridin 1 (Exciton, Inc.) laser dye. The laser wavelength range between 698 and 704 nm was explored. These wavelengths do not correspond to ground state absorption transitions.16 A Lumonics HE-420-SM-B XeCl excimer laser was used for pumping the dye laser, whose beam was focused to a spot size of less than 100 µm diameter inside the sample crystal. The average pulse intensity was 3 mJ integrated over the pulse width of 10 ns. The transmittance through this illuminated volume was measured and is reported in Fig.2 and 3. Residual stray light of the laser was eliminated by a combination of Schott BG18 and BG38 filters in front of the entrance slit of the monochromator. A Hamamatsu R928 photomultiplier was used to detect the time-dependent transmittance, and for some experiments a Stanford Research SR440 preamplifier (one 5� stage) was added. Traces were recorded with a Hewlett Packard HP 54503A 500 MHz digital oscilloscope using accumulation times of up to 1 h per wavelength. The laser intensity was measured with a Thorlabs Det2-Si (1 ns rise time) diode detector. Results The green upper trace in Fig. 2 shows the change in transmittance at 525.2 nm (13 K) during the laser pulse (lower trace). Before and after the pulse, the signal is proportional to the conventional transmittance.During the pulse, a higher transmittance is observed, corresponding to a reduced absorbance and therefore a weaker 3A2 ® 1A1 transition. The integrated light intensity in the "signal" and "pre-signal" gates will be used to analyze the transmittance modification. The transient enhancement of the transmittance disappears within less than a few ns after the laser pulse. This observation allows us to exclude effects such as the population of an excited state from the ground state, since at 13 K the 3T2 and 1T2 metastable states have lifetimes of 5.3 ms and 70 µrespectively,15 longer by several orders of magnitude than our transient enhancement. Fig. 2 Green upper trace: transient enhancement of the transmittance at 525.2 nm during the coupling laser pulse.Lower trace: time-dependent intensity of the coupling laser. The time gates used for the data points in Fig. 3 are indicated by blue and violet double arrows. Fig. 3 Violet squares: intensity during the "signal" gate from 0 to 15 ns on the time scale of Fig. 2. Blue diamonds: transmitted light intensity during the "pre-signal" gate from –100 to –20 ns on the time scale of Fig. 2. Bottom trace: conventional transmission spectrum of CsCdCl3:Ni2+ at 10 K. Fig. 3 shows a transient transmission spectrum (violet squares) obtained from the integrated transmittance during the laser pulse, a 15 ns time range denoted as the "signal" gate in Fig. 2. This time-resolved spectrum is obtained from traces as given in Fig.2 at a series of different detection wavelengths. The blue diamonds denote the signal intensity during the 80 ns "pre-signal" gate before the laser pulse. The error bars for data measured during the "signal" gate are larger because of the shorter total integration time than for the "pre-signal" gate. Electronic noise during the pump laser discharge could be an additional source of noise during the "signal" time range. The "pre-signal" curve follows exactly the conventional transmission spectrum shown at the bottom of Fig. 3. The effect reported in Fig. 3 depends strongly on the coupling laser wavelength. Using laser wavelengths of 699.8 and 702.7 nm instead of 701.7 nm, the maximum of the 3T2«1A1 transition, has lead to transmittance changesof only 50 and 30%, respectively, of the effect illustrated in Fig.3 for the optimized coupling wavelength. This important variation traces the experimental excited-state excitation spectrum and shows that this particular transition between two excited states does in fact influence the transmittance. It is unlikely that transitions involving other excited states could lead to the same profile over this narrow wavelength range. Laser power variations have shown a threshold below which no transmission enhancement could be observed, but our results do not allow us to determine a quantitative power dependence and mechanism for the observed effect. We do not attempt to give a model to quantitatively rationalize the transmittance enhancement.In a qualitative, time-dependent view17,18 the coupling laser leads to a faster decrease of the autocorrelation function of the molecular unit in its 1A1 excited state, resulting in a broader spectrum with higher transmittance at the band maximum, in agreement with our experimental observation. Conclusion Our experiments on Ni2+ ions doped into CsCdCl3 show a striking reduction of the transmittance of the 3A2 ® 1A1 transition. This new approach to the control of a spectroscopic property uses the laser-induced interaction with a second excited state. Detailed studies into its mechanism are now needed to further explore these new phenomena in transition metal chromophores. Acknowledgements This work was made possible by research grants from the Natural Sciences and Engineering Research Council (Canada) and the Swiss National Science Foundation.References 1 Molecules in Laser Fields, ed. A. D. Bandrauk, M. Dekker, New York (NY), USA, 1993. 2 U. Oetliker, M. J. Riley, P. S. May and H. U. Güdel, J. Lumin., 1992, 53, 553. 3 M. P. Hehlen, K. Kraemer, H. U. Güdel, R. A. McFarlane and R. N. Schwartz, Phys. Rev. B: Condens. Matter, 1994, 49, 12475. 4 M. Wermuth, T. Riedener and H. U. Güdel, Phys. Rev. B: Condens. Matter, 1998, 57, 4369. 5 S. E. Harris, Phys. Rev. Lett., 1989, 62, 1033. 6 M. O. Scully, S.-Y. Zhu and A. Gavrielides, Phys. Rev. Lett., 1989, 62, 2813. 7 A. Nottelmann, C. Peters and W. Lange, Phys. Rev. Lett., 1993, 70, 1783. 8 E. S. Fry, X. Li, D. Nikonov, G. G. Padmabandu, M. O. Scully, A. V. Smith, F. K. Tittel, C. Wang and S. R. Wilkinson, Phys. Rev. Lett., 1993, 70, 3235. 9 W. E. van der Veer, R. J. J. van Diest, A. Dönszelmann and H. B. van Linden van den Heuvell, Phys. Rev. Lett., 1993, 70, 3243. 10 M. O. Scully and M. Fleischhauer, Science, 1994, 263, 337. 11 G. Granucci and M. Persico, Chem. Phys. Lett., 1995, 245, 228. 12 D. Romstad, G. Granucci and M. Persico, Chem. Phys., 1997, 219, 21. 13 Y. Zhao, C. Wu, B.-S. Ham, M. K. Kim and E. Awad, Phys. Rev. Lett., 1997, 79, 641. 14 P. S. May and H. U. Güdel, Chem. Phys. Lett., 1989, 164, 612. 15 P. S. May and H. U. Güdel, J. Lumin., 1990, 46, 277. 16 U. Oetliker, M. J. Riley and H. U. Güdel, J. Lumin., 1995, 63, 63. 17 E. J. Heller, Acc. Chem. Res., 1981, 14, 368. 18 J. I. Zink and K.-S. Kim Shin, Adv. Photochem., 1991, 16, 119. Paper 9/07907F PhysChemComm © The Royal Society of Chemistry 1999
ISSN:1460-2733
DOI:10.1039/a907907f
出版商:RSC
年代:1999
数据来源: RSC
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