Dynamic optical limiting experiments on vanadiumdioxide and vanadium pentoxide thin filmsirradiated by a laser beam

Weiping Wang, Yongquan Luo, Dayong Zhang, and Fei Luo

Vanadium dioxide �VO2� and vanadium pentoxide �V2O5� thin films are irradiated by a near-infraredcontinuous-wave laser beam and the dynamic optical limiting performance is measured. The temperaturevarying with time of the films induced by a laser beam is also recorded by an IR thermal sensor. Under theirradiation of a laser beam with an intensity of 255 W�cm2 and a spot diameter of 2 mm, the laser beamtransmittance of the VO2 film decreases from 47% before phase transition to 28% after phase transition,and the response time is �200 ms; the laser beam transmittance of the V2O5 film decreases from 51%before phase transition to 24% after phase transition, and the response time is �40 ms. The opticallimiting is realized by this laser heating-induced phase transition. © 2006 Optical Society of America

OCIS codes: 140.3360, 310.6860, 120.6780, 140.6810.

1. Introduction

Since the first observation on a metal-insulator phasetransition in vanadium dioxide �VO2� by Morin,1 thevanadium oxides have been intensively studied bymany researchers.2–10 VO2 was found to undergo aphase transition from semiconductor to metal whenthe temperature increases from room temperatureto �68 °C. When this semiconductor-to-metal first-order phase transition occurs, the crystallographicstructure of VO2 changes from the low-temperaturemonoclinic form to the high-temperature tetragonalphase. The transformation is also accompanied bydramatic changes in electrical resistivity, optical re-flectivity, and transmission properties. In theory, thevanadium pentoxide �V2O5� thin film also has thisheat-induced phase transition behavior when thetemperature is higher than 257 °C. These interestingcharacteristics can make vanadium oxide thin filmsan optical limiter that is developed with the purposeof protecting sensors and eyes against damage pro-duced by high-power laser beams. The optical limit-er’s operating principle is based on a transmittance

that steeply decreases as the light energy increases,although it keeps a normal transmittance for lowinput levels.

The published papers about vanadium oxides weremainly on the research of the preparation, the crys-tallographic structure observation, and the electricalresistivity and optical properties before and afterphase transition. Most of the experiments were doneby a resistive heater rather than a laser beam. To ourknowledge there has been little research on the dy-namic optical limiting performance of V2O5 excited bya laser beam. In the experiments reported here, wehave measured the dynamic optical limiting perfor-mance and the semiconductor–metal phase transi-tion in VO2 and V2O5 thin films excited by a 1.06 and1.319 �m cw laser beam. The experimental resultsshow that there is little difference in the power-limiting property induced by these two beams, so weuse a near-infrared laser beam instead of a 1.06 and1.319 �m laser beam.

2. Sample Preparation and Measurements

The VO2 and V2O5 thin films used in this researchare provided by the Institute of Physics of the Chi-nese Academy of Science. The films are prepared by arf magnetron sputtering method in a vacuum cham-ber and deposited on a 9 mm � 9 mm glass substratewith a 1 mm thickness. The detailed descriptions ofthe sample preparation have been published.8 Weprovide here an example of the preparation conditionof a VO2 thin film: The distance between the target

The authors are with the Institute of Fluid Physics, ChineseAcademy of Engineering Physics, P.O. Box 919-113, Mianyang621900, China. W. Wang’s e-mail address is wwpwzc@yeah.net.

Received 16 September 2005; revised 3 November 2005; accepted2 December 2005; posted 7 December 2005 (Doc. ID 64817).

0003-6935/06/143378-04$15.00/0© 2006 Optical Society of America

3378 APPLIED OPTICS � Vol. 45, No. 14 � 10 May 2006

and the substrate is 60 mm, and the target is a va-nadium metal plate with the purity of 99.6%. Theinitial pressure in the chamber is smaller than 1.3� 10�3 Pa and the pressure during sputtering is1.0 Pa. The ratio of oxygen to argon is 1%, and thesubstrate temperature is set to 580 °C. After deposi-tion, the sample is cooled to room temperature natu-rally in the vacuum chamber. The difference in thepreparation conditions between VO2 and V2O5 thinfilms used in this research is mainly in the ratio ofoxygen to argon and the substrate temperature. Themeasured thickness of the films is �100–150 nm.The sample is also measured by x-ray diffractometry(XRD) with monochromatized Cu K� radiation. Theresult of the XRD measurement on a VO2 thin film isshown in Fig. 1.

In Fig. 1 the corresponding diffraction angles ofpeaks 1, 3, and 4 are �14.3°, 28.9°, and 44.0°. Theycoincide with the (001), (002), and (003) reflections ofthe VO2, which indicates that the measured film ismainly a VO2 film.

The spectral transmittance T of the sample is mea-sured with a visible–IR spectrophotometer before la-ser irradiation; the result is shown in Fig. 2. It isobvious that VO2 and V2O5 thin films have a near-flattransmittance from 1000 to 2000 nm, and V2O5 thinfilms have a much higher transmittance than VO2 ata visible wavelength. Figure 2 shows that, at a wave-length of more than 1.1 �m, the transmittances ofVO2 and V2O5 thin films are �47% and 51%, respec-tively. The transmittance of a glass substrate is in-cluded in the results.

3. Experiment Setup

A near-IR cw laser beam was used to irradiate thesurface of the VO2 and V2O5 thin films, and the dy-namic optical limiting of these vanadium oxides wasmeasured. Figure 3 shows a schematic of the experi-mental setup. In Figure 3 the laser beam is separatedinto two parts by beam splitter 1; the transmitted partis attenuated and irradiated on photoelectric detector1 and used to record the laser waveform. The reflectedpart is again separated into two parts by beam splitter2, and the reflected beam from beam splitter 2 is usedto measure the laser beam power; the transmittedbeam from beam splitter 2 is focused on the vanadiumoxide thin-film sample through a lens �f � 200 mm�.We dynamically measured the transmitted laserwaveform through a sample by photoelectric detector

2 during laser irradiation. An IR thermal sensor wasused to measure the laser-induced transient temper-ature on the samples. The laser, oscillograph, and IRthermal sensor were controlled synchronously by atrigger. The beam size on the sample was measuredby a laser beam profiler and set to 2 mm in diameterby changing the distance between the lens and thesample.

4. Results and Discussion

The typical voltage signals (V) from photoelectric de-tectors 1 and 2 are shown in Figs. 4(a) and 4(b),respectively. Figure 4(a) gives the typical incidentlaser waveform on the samples and Fig. 4(b) gives thetransmitted laser waveform through the V2O5 thinfilm. The incident laser beam is delayed �60 ms dueto a mechanical shutter in the laser cavity. The inci-dent laser waveform is stable during the output time.Here the incident laser power on the V2O5 thin filmis 10 W, and the diameter of the laser spot is set to2 mm; thus the laser intensity on the sample is318 W�cm2. The first steep decrease of the transmit-ted waveform in Fig. 4(b) as the laser irradiation timeor energy increases corresponds to the laser heating-induced semiconductor–metal phase transition, andthe second steep increase of the transmitted wave-form in Fig. 4(b) corresponds to the laser-induced

Fig. 1. XRD pattern of a VO2 thin-film sample.

Fig. 2. Transmittance of VO2 and V2O5 thin films fabricated onglass at visible and IR wavelengths.

Fig. 3. Experimental setup.

10 May 2006 � Vol. 45, No. 14 � APPLIED OPTICS 3379

damage on the film. We verified the film damage afterlaser irradiation and decreased the laser outputpower in subsequent experiments.

Figure 5 shows the average temperature in a laser-irradiated spot and the transmitted laser wavethrough vanadium oxide thin films varying with timeduring laser irradiation. The black boxes in Fig. 5 showthe temperature data from the IR thermal sensor, andthe polynominal fitting is shown by the smooth curve.The emissivity ε of the sample is assumed to be 0.7during temperature measurement, and the emissivityε varying with temperature is not considered here. Theincident laser power on the sample is 8 W, and thediameter of the laser spot is set to 2 mm; thus thelaser intensity on the sample is 255 W�cm2, and thefilms were not damaged by the laser beam with theseparameters.

Figure 5(a) shows that, 200 ms after laser irradia-tion on VO2 thin film, the average temperature of thelaser-irradiated spot rises to �70 °C, and the ampli-tude of the transmitted laser wave begins to decrease,which shows where the phase transition from thesemiconductor to metal begins. Then 480 ms afterlaser irradiation on the VO2 thin film, the average

temperature of the laser-irradiated spot rises to�100 °C, and the amplitude of the transmitted laserwave decreases to 60% of the value before the phasetransition, which indicates that the transmittance ofthe laser beam through VO2 thin film decreases from47% before the phase transition (see Fig. 2) to 28%after the phase transition; the subsequent trans-mitted laser energy can be reduced, and thus adynamic power-limiting function can be realized. Fig-ure 5(b) shows that, 40 ms after laser irradiation onV2O5 thin film, the temperature of the film rises to�120 °C, and the phase transition from the semicon-ductor to metal occurs. Then 200 ms after laser irra-diation on V2O5 thin film, the average temperature ofthe laser-irradiated spot rises to �170 °C, and theamplitude of the transmitted laser wave decreases to47% of the value before the phase transition, whichindicates that the transmittance of the laser beamthrough V2O5 thin film decreases from 51% beforephase transition to 24% after phase transition; thesubsequent transmitted laser energy can be reduced,and thus a dynamic power-limiting function can alsobe realized.

Fig. 4. (a) Typical incident laser waveform and (b) transmitted laser waveform through a V2O5 thin film.

Fig. 5. Transmitted laser waveform and temperature of vanadium oxide thin films during laser irradiation.

3380 APPLIED OPTICS � Vol. 45, No. 14 � 10 May 2006

Analyzing the results from Fig. 5, we can see that,under the irradiation of a near-IR cw laser beam withan intensity of 255 W�cm2 and a spot diameter of2 mm, the phase transition response times of VO2and V2O5 thin films are 200 and 40 ms, respectively.After the phase transition, the changes in transmit-tance are 19% and 27%, and the corresponding rela-tive changes in transmittance are 40% and 53%,respectively. The results show that the average tem-peratures needed by a semiconductor–metal phasetransition of VO2 and V2O5 thin films are 70° and120 °C, respectively, in this experiment. The temper-ature needed by a semiconductor–metal phase tran-sition of V2O5 thin film in our experiments is differentfrom that in theory; this can be explained as follows:(1) It is difficult to prepare pure V2O5 film by the rfmagnetron sputtering method; the XRD results showthat there is little VO2 contained in the sample, whichdecreases the semiconductor–metal phase transitiontemperature. (2) The measurement error of the risein the laser-induced temperature on the film is an-other possible reason. The exact temperature mea-surement by the IR thermal sensor needs the exactIR emissivity ε of the sample, whereas � � 0.7 is onlyan estimated value.

From the optical limiting mechanism of vanadiumoxides, we can see that the response time of dynamicoptical limiting in VO2 and V2O5 thin films is depen-dent on the time needed to heat the films to the phasetransition temperature. To a definite film sample, thetime needed to heat the film to the phase transitiontemperature is dependent on the absorbed laserintensity and thermal physical parameters of thefilm. Our experimental results show that, under thesame incident laser parameters, the response time ofdynamic optical limiting in V2O5 thin film is shorterthan that in VO2 thin film, which is due to theirdifferences in laser absorbance and thermal physicalparameters. We are sure that the laser absorbance ofV2O5 thin film used in our experiments is higher thanthat of VO2 thin film used in our experiments bymeasuring their laser reflectance and transmittance.But we do not know the exact difference in theirthermal physical parameters (thermal conductivity,specific heat, etc.) because there is little in the liter-ature about them; further research into the thermalproperties of vanadium oxides needs to be done.

The changes in optical transmittance of V2O5 andVO2 thin films during laser irradiation are induced bythe changes in the crystallographic transition in thematerial.11,12 In a VO2 crystal with a low-temperature monoclinic structure, the bond lengthsof V–O �dV�O� between a V4� ion and an O2� ion are0.126, 0.186, 0.187, 0.201, 0.203, and 0.205 nm. Po-larization occurs when dV�O is less than the sum ofthe radii of the V4� ion and O2� ion �RV�O � rV4�

� rO2� � 0.200 nm�. The greater the difference be-tween dV�O and RV�O, the larger the polarizability.However, in the case of VO2 with a high-temperaturetetragonal structure, dV�O � 0.194 nm, which is closeto RV�O. Therefore the polarizability of the monoclinic

VO2 is larger than that of the tetragonal VO2, whichcauses the differences in optical properties before andafter phase transition.

5. Conclusion

The dynamic optical limiting performance of VO2 andV2O5 thin films irradiated by a near-IR cw laser isinvestigated in this paper. This dynamic optical lim-iting is due to the laser heating-induced phase tran-sition of vanadium oxide thin films and acorresponding laser transmittance decrease. The in-cident laser waveform on these films, the transmittedlaser waveform through these films, and the tran-sient temperature rise of the films during laser irra-diation are all dynamically measured synchronously.The results show that these films may be used toprotect eyes and sensors against damage by near-IRcw intense laser beams, but a faster response timeand a better optical limiting performance of vana-dium oxides are still needed to achieve practicability.After this work, we will undertake research to de-crease the response time and improve the optical lim-iting performance by optimizing the preparationconditions and incident laser parameters.

We gratefully acknowledge K. A. Feng at the Insti-tute of Physics of the Chinese Academy of Science forhis help with the sample preparations and measure-ments.

References1. F. J. Morin, “Oxides which show a metal-to-insulator transi-

tion at the Neel temperature,” Phys. Rev. Lett. 3, 34–36 (1959).2. W. R. Roach, “Holographic storage in VO2,” Appl. Phys. Lett.

19, 453–455 (1971).3. F. C. Case, “Modifications in the phase transition properties of

predeposited VO2 films,” J. Vac. Sci. Technol. A 2, 1509–1512(1984).

4. E. E. Chain, “Optical properties of vanadium dioxide and vana-dium pentoxide thin films,” Appl. Opt. 30, 2782–2787 (1991).

5. M. F. Becker, A. B. Buckman, and R. M. Walser, “Femtosecondlaser excitation of the semiconductor-metal phase transition inVO2,” Appl. Phys. Lett. 65, 1507–1509 (1994).

6. D. C. Yin, N. K. Xu, J. Y. Zhang, and X. L. Zheng, “Vanadiumdioxide films with good electrical switching property,” J. Phys.D 29, 1051–1057 (1996).

7. M. A. Sobhan, M. R. Islam, and K. A. Khan, “Preparation andproperties of V2O5 thin films for energy-efficient selective-surface applications,” Appl. Energy 64, 345–351 (1999).

8. X. J. Wang, H. D. Li, Y. J. Fei, X. Wang, Y. Y. Xiong, Y. X. Nie,and K. A. Feng, “XRD and Raman study of vanadium oxidethin films deposited on fused silica substrates by RF magne-tron sputtering,” Appl. Surf. Sci. 177, 8–14 (2001).

9. S. H. Chen, H. Ma, X. J. Yi, H. C. Wang, X. Tao, M. X. Chen,X. W. Li, and C. J. Ke, “Optical switch based on vanadiumdioxide thin films,” Infrared Phys. Technol. 45, 239–242 (2004).

10. S. H. Chen, H. Ma, X. J. Yi, X. Tao, H. C. Wang, and C. J. Ke,“Smart VO2 thin film for protection of sensitive infrared de-tectors from strong laser radiation,” Sens. Actuators A 115,28–31 (2004).

11. J. B. Goodenough, “The two components of the crystallographictransition in VO2,” J. Solid State Chem. 3, 490–500 (1971).

12. S. W. Lu, L. S. Hou, and F. X. Gan, “Preparation and opticalproperties of phase-change VO2 thin films,” J. Mater. Sci. 28,2169–2177 (1993).

10 May 2006 � Vol. 45, No. 14 � APPLIED OPTICS 3381