Pure optical phase control with vanadium dioxide thin films

Optics Communications 320 (2014) 151–155

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Pure optical phase control with vanadium dioxide thin films T.V. Son a, K. Zongo a, C. Ba b, G. Beydaghyan a, A. Haché a,n a b

Département de physique, Université de Moncton, Canada Centre optique photonique et laser (COPL), Université Laval, Canada

art ic l e i nf o

a b s t r a c t

Article history: Received 20 November 2013 Received in revised form 15 January 2014 Accepted 19 January 2014 Available online 3 February 2014

Under certain conditions, films made of vanadium dioxide exhibit wavelengths at which transmittance or reflectance do not change as the material undergoes insulator to metal phase transition, in spite of refractive index changes on the order of unity. Exploiting this effect, we demonstrate control of optical phase at 800 nm in transmission and at 1310 nm in reflection. With a 68 nm film, the optical phase is adjusted while leaving all other properties of light unchanged, including amplitude, polarization and frequency. The phase change per unit of propagated distance is Δk ¼107 rad/m, orders of magnitude higher than typically obtained with electro-optic effects. We discuss potential application to nano-sized phase devices or thin film lenses. & 2014 Elsevier B.V. All rights reserved.

Keywords: Vanadium dioxide Phase modulation Phase delay Thin films

Vanadium dioxide (VO2) is a correlated material that undergoes a reversible insulator-to-metal transition (IMT) above 68 1C, owing to a rearrangement of its crystalline structure [1–2]. This phase transition is accompanied by an increase in conductivity and a sharp drop in optical transmittance in the infrared. This IMT has been exploited in a wide range of photonic applications, including tunable metamaterials [3–5], spectrally selective filters [6–7], optical switches and limiters [8–11] and infrared imaging systems [12–13]. So far, much research has focussed on the amplitude modulation capabilities of this material, which are indeed remarkable in both range of amplitude (orders of magnitudes) and spectral width, spanning from the near infrared to the terahertz. While phase shifting of THz beams by VO2 films has been demonstrated [14], the capabilities of the material for phase and polarization modulation have been relatively unexplored. Yet, refractive index changes of VO2 during phase transition are on the order of unity, with the potential of phase control in thin films. While conventional techniques for phase shifting and modulation use piezoelectric actuators, the electro-optic effect and liquid crystals, the prospect of phase modulation with VO2 thin films is interesting for at least three reasons. First, conventional techniques are implemented in devices that are micrometers to centimeters in size, while VO2 thin films offer the possibility of nanometer-sized phase devices. Second, it is sometime difficult to disentangle phase control from polarization and amplitude effects. For example, in some configurations of Pockels cells or liquid crystals, the material

n

Corresponding author. E-mail address: [email protected] (A. Haché).

0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2014.01.037

anisotropy causes the polarization state of light to change, which in turn modifies the field amplitude when combined with polarization-dependant optical components. With VO2 thin films, as this paper demonstrates, it is possible to shift the optical phase of light beams without affecting its amplitude, frequency and polarization. Such “pure phase” control is made possible in both transmission and reflection at specific wavelengths where VO2 exhibits the same reflectivity or transmittance in its metallic and insulating states. Third, non-uniform phase shift profiles in the film could lead to beam focusing and nanometer-sized flat lenses. As above mentioned, when a VO2 film undergoes a phase transition from insulator to metal, a drop in transmittance is generally observed across all wavelengths in the infrared, while the reflectance increases accordingly. However, in some samples, typically with thicknesses of about 100 nm or less, there are specific wavelengths where transmittance or reflectance does not change. Fig. 1(a) shows an example of such effect. The transmittance, here measured at normal incidence, is plotted for the insulating and the metallic states of the VO2 film, with an arrow indicating where the two curves cross over (825 nm). A similar effect is observed in reflection at 1300 nm, as shown in Fig. 1(b). Both graphs were obtained with a 68 nm-thick layer of VO2 deposited on glass using a technique described in Refs. 15–16. In Fig. 2 are images taken by a scanning electron microscope and an atomic force microscope revealing the characteristic microstructure of the VO2 films deposited on glass. During the film deposition, only half of the substrate was exposed, and during post treatment, the whole film was treated, creating a uniform film to the edge (a necessary condition for experiments described later). A priori, there are two possible explanations for why transmittance or reflectance may remain the same during phase transition at some wavelengths: 1) the optical constants of the material

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happen to be the same in both material phases at these wavelengths, or 2) the optical constants do change, but interferences effects lead to unchanged optical properties. The first hypothesis can be verified by measuring the film's complex refractive index (given as nþ ik) by ellipsometry. We used an ellipsometer adapted to measure the film reflectance and transmittance simultaneously at temperatures ranging from 20 to 90 1C. The home-built instrument uses silicon and InGaAs detectors covering the 400 to 2000 nm spectral range and measures for incidence angles from 10 to 801. Once the field amplitudes of the s and p polarizations are measured, the optical constants are extracted by fitting to a fourth-order polynomial, appropriate for non-dielectric media. The model assumes a single,

uniform layer of VO2 on glass. The refractive indices shown in Fig. 3 were obtained. While keeping in mind that optical constants of VO2 are sensitive to stoichiometry and microstructure, we compared our ellipsometry results with that of Refs. 17 and 18 and found many similarities. During phase transition, the real part of the refractive index drops considerably, namely Δn ¼  0:82 at 825 nm and Δn ¼  1:11 at 1300 nm. Meanwhile, the imaginary part varies by Δk ¼ 0.28 and Δk ¼ 1.63 at the same respective wavelengths. We can therefore conclude that interference effects are responsible for the constant transmission and reflection. From ellipsometry data, we verify theoretically a possible optical phase shift associated to these refractive index changes by calculating the phase of the complex field amplitudes in transmission t and in reflection r in a thin film r ¼ r 12 

t¼

  t 12 t 21 1  1 r 21 1  r 12 r 23 e2iϕ

ð1Þ

t 12 t 23 eiϕ 1  r 23 r 21 e2iϕ

ð2Þ

where ϕ is the phase through a film of thickness d

ϕ¼

2π d

λ

ðn þ ikÞ

ð3Þ

and the following Fresnel coefficients are used: r 12 ¼ r 21 ¼

r 23 ¼

1n 1þn

ð4Þ

n  nt n þ nt

ð5Þ

3 n (20 °C) n (80°C)

n,k

2

k (80°C) 1 k (20°C)

Fig. 1. (a). Transmittance of a VO2 film at temperatures of 23 1C (blue) and 80 1C (red). (b). Reflectance of film of VO2 at temperatures of 23 1C (blue) and 80 1C (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0 500

1000

1500

Wavelength (nm)

Fig. 3. Optical constants of a 68 nm VO2 film measured by ellipsometry.

Fig. 2. SEM (left) and AFM (right) images showing the microstructure of the VO2 films deposited on glass.

T.V. Son et al. / Optics Communications 320 (2014) 151–155

t 12 ¼

2 1 þn

ð6Þ

t 21 ¼

2n 1 þn

ð7Þ

t 23 ¼

2n n þnt

ð8Þ

In these equations we used air (no ¼1) as the incident medium, and n and nt as the refractive indices of the film (VO2) and the substrate (glass), respectively. Fig. 4 shows the calculated optical phase change Δϕ of the complex field amplitudes as the VO2 film switches from insulator to metal. From 20 1C to 80 1C, Δϕ ¼  0:47 rads in transmission at 825 nm while Δϕ ¼ 0:61 rads in reflection at 1300 nm. To directly and experimentally verify the optical phase adjustability with VO2 thin films, we monitored the film by laser interferometry during the material phase transition. This was done centering a laser beam at the edge of the film, so that the beam equally overlaps the bare substrate and the film, while measuring in the far field the resulting diffraction profile. This method effectively creates a halfplane phase shift on the Gaussian beam. The concept, outlined in

Phase difference (rad)

1.0

0.5

0

-0.5

-1.0 400

600

800

1000

1200

1400

1600

Wavelength (nm)

Fig. 4. Theoretical optical phase change during insulator to metal transition of a 68 nm-thick VO2 film for a beam at normal incidence. The black and red curves are for transmission and reflection, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

153

Fig. 5, was recently developed as a mean to measure phase shifts caused by a film [19]. Because this single beam method probes a small area of the sample (o1 mm), it minimizes phase shifts introduced in the substrate by non-uniform temperature distributions. For experiments done in transmission, a Ti:sapphire laser operating in continuous mode at 800 nm was used and measured with a silicon CCD camera (Thorlabs DCU224C-CCD Camera, 1280 pixels). In reflection mode, a diode laser at 1310 nm was used and monitored with linear array of 512 InGaAs detectors (Jobin Yvon Spectrum One). As Fig. 6 shows, the sample is flat against an aluminum block with only a small hole to let the laser beam through. Thermistors heat the block and sample in a uniform way. The sample temperature was controlled to within 0.2 1C and slowly ramped from from 23 to 90 1C to observe the VO2 phase transition. All laser beams were aligned to within 101 of normal incidence. Such small angles yield transmittances and reflectances that are very close to that of normal incidence, especially given the large refractive index of VO2 which further reduces the internal angle. Sample heating by the laser beams was deemed negligible compared to that generated by the heater. Indeed, with powers of 2.5 mW at 1310 nm and 0.5 mW at 800 nm and with beam diameters of 1 mm, the intensities are 0.3 W/cm2 and 0.1 W/cm2, respectively, much lower than the 5 W/cm2 required to reach the onset of self-switching with strongly absorbed beams [20]. From previous work [21], we estimate the surface temperature rise by the weakly absorbed laser beams to be on the order of 1 1C and 0.2 1C for 1310 nm and 800 nm, respectively. Fig. 7 shows examples of laser beam profiles diffracted by the edge of the film at room temperature. When the beam is positioned entirely on the substrate or the film, the beam profile is smooth and Gaussian, but when it is centered between the film and the substrate, interference fringes appear, which move when the optical thickness of the film changes. The optical phase shift caused by the film was measured by counting fringe displacement as the VO2 sample was heated. The resulting shifts in transmission at 800 nm and in reflection at 1310 nm are presented in Fig. 8(a) and (b), respectively. In transmission, the optical phase drops by a total 0.76 70.1 rads. Between 25 and 80 1C (corresponding to ellipsometry measurements temperatures) the shift is  0.46 70.1 rads, close to the theoretically predicted value. Reflection has a shift of þ0.8 7 0.1 rads, also in agreement with theory. Measurements were

Fig. 5. Experimental setup to measure optical phase shifts during the material phase transition of a VO2 film. The beam is centered at the edge of the film.

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Fig. 6. Geometry of a heater used to uniformly control the temperature of a sample.

Fig. 7. Self-interference of a Gaussian laser beam probing the edge of a thin film. Insets show beam profile when positioned entirely on the VO2 film or the glass substrate.

repeated several times to confirm reproducibility and consistency with theory. As observed in other optical and electrical properties of VO2, there is hysteresis, as the heating and cooling cycles have different transition temperatures. The total insertion loss of the device is 4.5 dB in reflection for an operating bandwidth of 200 nm with the criterion that the amplitude changes by less than 10% during the switch. In transmission, the insertion loss is 4 dB with a bandwidth of 160 nm. Compared to conventional means for modulating optical phases, VO2 films offer significantly larger phase shift per unit of distance propagated. In this experiment, Δϕ=Δx ¼ Δk C 107 rad/ m. With Pockels cells using materials like KDP and LBO, ¼ Δk C 102  103 rad/m. Liquid crystals offer greater modulation, but with a birefringence of 0.25 or less and layers typically of several micrometers, the rate of phase shift is still much less than with VO2 film. Other potential applications to this phase control at constant field amplitude include nanometer-sized flat lenses. By putting the film under a temperature gradient, a variable phase shift profile would be imposed on an incident beam, thereby focusing or defocussing the beam. With the measured phase shift

Fig. 8. (a). Measured phase shift on a laser beam at 800 nm in transmission through a VO2 film undergoing phase transition. Red and blue curves are for heating and cooling, respectively. (b). Measured phase shift on a laser beam at 1310 nm in reflection from a VO2 film undergoing phase transition. Red and blue curves are for heating and cooling, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

magnitudes, we estimate the focal length on a 1 mm-wide laser beam to be on the order of 50 cm, enabling the focusing of the beam by a factor of 10.

T.V. Son et al. / Optics Communications 320 (2014) 151–155

In conclusion, we have theoretically and experimentally investigated the optical phase control of a beam by exploiting the large refractive index changes of a VO2 film during phase transition. The concept was demonstrated at wavelengths where the reflected and transmitted field amplitudes do not change, thereby offering the possibility of pure phase control. Many other aspects need to be investigated, however. Future work includes investigating the influence of parameters like film thickness, beam polarization state, to explore the range of adjustability in wavelength and phase shift magnitude. Acknowledgments The authors wish to thank Kris Bulmer and George Bader for help with ellipsometry measurements. References [1] [2] [3] [4]

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