4978
OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
Broadband electro-optical modulator based on transparent conducting oxide Kaifeng Shi, Riaz R. Haque, Bingyin Zhao, Runchen Zhao, and Zhaolin Lu* Microsystems Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, New York 14623, USA *Corresponding author: [email protected] Received April 17, 2014; revised July 17, 2014; accepted July 20, 2014; posted July 21, 2014 (Doc. ID 210008); published August 18, 2014 We report a broadband electro-optical (EO) modulator based on tunable plasmonic metamaterial. Transparent conducting oxides provide an excellent active plasmonic material for optoelectronic applications. By utilizing our indium-tin-oxide- (ITO) based multilayer structure, light absorption of the active ITO layer can be electrically modulated over a large spectrum range. Based on the attenuated total reflectance configuration, bias polaritydependent modulation up to 37% has been experimentally demonstrated. This EO modulator has advantages of simple design, easy fabrication, compact size, broadband performance, large modulation depth, as well as compatibility with existing silicon photonics platforms. © 2014 Optical Society of America OCIS codes: (160.2100) Electro-optical materials; (250.4110) Modulators; (230.2090) Electro-optical devices. http://dx.doi.org/10.1364/OL.39.004978
Ultracompact high-speed electro-optical (EO) modulators have become one of the critical technical bottlenecks impeding the wide applications of on-chip optical interconnects. Due to very poor EO properties of conventional materials, some phase modulators are on the order of millimeters [1,2]. Resonator modulators enhance the EO effect due to the large quality factor of the resonant cavity, so their dimensions can be shrunk to tens of micrometers [3]. However, resonator modulators usually suffer from bandwidth limitation, temperature fluctuation, as well as fabrication tolerance. A recently reported plasmonic phase modulator has a small dimension (29 μm) and high speed [4], but it requires a novel nonlinear material, which may add difficulty to its fabrication and integration. Some compact absorption modulators also rely on advanced materials [5,6]. In order to build EO modulators that can overcome both the footprint and bandwidth constrains, we need to (1) identify an efficient and low-cost active material and (2) greatly enhance lightactive medium interaction based on a novel platform. Recently, transparent conducting oxides (TCOs) have been widely investigated as promising plasmonic metameterials, with impressive advantages of low loss and fabrication compatibility [7–9]. The optical dielectric constant of TCOs can be approximated by the Drude model, ω2p 0 00 ; (1) ε ε jε ε∞ 1 − ωω jγ where ε∞ is the high-frequency dielectric constant, ω is the angular frequency of the light wave, and γ is the damping coefficient of free carriers related to optical losses. The plasma frequency is defined by s Ne2 ; (2) ωp ε∞ ε0 m which depends on carrier concentration N and the electron effective mass m . According to the above equations, the optical material’s dielectric constant shifts with different carrier concentrations. 0146-9592/14/174978-04$15.00/0
By manipulating the concentration of oxygen vacancies and interstitial metal dopants, the carrier concentration in ITO, a well-known representative of TCOs, can be controlled in the range of 1019 –1021 ∕cm3 . The high carrier concentration enables a guiding surface mode at the interface of ITO and dielectric materials. On the other hand, the carrier concentration in ITO can be engineered by an externally applied voltage [10,11]. In Ref. [10], Feigenbaum et al. have experimentally showed that a unity-order index change in ITO can be achieved in a metal-oxide-semiconductor- (MOS) like structure by a thin active layer (∼5 nm) of voltage-induced accumulation charge formed at the interface of the ITO and SiO2 . In this sense, ITO can work as a good EO material, which has a great potential in applications in EO modulators. Actually, some research work on electro-absorption modulators based on TCOs has already been presented [12–14]. Other EO modulators based on ferroelectric BaTiO3 (BTO) and BiFeO3 (BFO) have also been investigated [15,16]; however, these studies either lack experimental results or do not show a very strong EO response. In our previous work, we have investigated an EO modulator configuration based on ∼25 nm thick ITO (as the active medium) and ∼5 μm thick electrolyte gel (as gate “oxide”), where we observed the bias polaritydependent modulation effect: the absorption of the EO modulator could be increased by negative bias or decreased by positive bias. We applied the 5 nm active layer assumption and numerically fitted the measured results. We extracted the dielectric constant of the active ITO layer (5 nm) induced by the externally applied voltage and the corresponding carrier concentration at λ0 1520 nm [11]. However, the ITO-electrolyte gel modulator has a short lifetime and a very slow switching speed. In order to make more efficient modulators, we apply a high-k dielectric material in our designed EO modulator structure, as illustrated in Fig. 1. This is a simple multilayer MOS-like structure. BK7 glass serves as the substrate, and ITO is the tunable active material. High-k material HfO2 is utilized as the gate oxide for its ultrahigh permittivity as well as process stability [17]. Aluminum is © 2014 Optical Society of America
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS
Fig. 1.
Illustration of an ITO-based multilayer EO modulator.
chosen as the metal for its excellent conductivity, low absorption in the near-infrared (NIR) regime, as well as low cost. Another advantage of using an aluminum layer is that light absorption can be directly measured by 1 − R (R is the power reflectance) in the device illustrated in Fig. 1. Assume light is incident into ITO with angle θ1 to the normal. For a transverse magnetic (TM) mode, where the magnetic field is parallel to the multilayers, Maxwell’s equations determine that the normal electric flux density D is continuous at the ITO-HfO2 interface so E-field intensity is reversely proportional to the dielectric constant. Thus, the dissipation power density is 1 1 1 pd σE 2 ∝ ε00 E 2 ∝ ε00 ∕jεj2 ; 2 2 2
(3)
which can be enhanced when jεITO j decreases [12]. As a result, the EO modulator is at a low-absorption (highreflection) state under positive applied voltage and at a high-absorption (low-reflection) state under negative applied voltage, due to the change of the dielectric constant. The power reflectance R as a function of incident angle θ1 can be calculated based on the transfer matrix method (TMM) [18]. We found even though only a 5 nm active ITO layer has been electrically tuned, the power reflectance or the absorption of the modulator could be significantly changed at certain incident angles. It is worth mentioning that if aluminum is replaced by doped silicon, very little modulation between can be observed. This is because most power gets transmitted through the structure due to the transparency of silicon in the NIR regime. The cross section of the ITO-based EO modulator is shown in Fig. 2(a). The fabrication of the modulators starts from ITO film deposition on transparent glass, by the method of physical vapor deposition (PVD) process using a In2 O3 0.9 SnO2 0.1 weight percentage target at room temperature and 7.3 mTorr pressure within the chamber. During deposition, argon is the only gas
VP
BS BK7 Prism
θ1
(a) (c)
10mm
(b)
SiO2
Al
ITO
HfO2
Fig. 2. (a) Cross section of the proposed multilayer ITO modulator. (b) Picture of the fabricated modulator. (c) Illustration of the setup for ATR measurement.
4979
used. The thickness of the ITO film is measured in the range of 10–12 nm with a 5 min deposition time. After a 4 h postannealing process, the sheet resistance of the ITO film is measured around 400–600 Ω/square. After that, 50 nm thick HfO2 is deposited on the ITO layer by atomic layer deposition (ALD). Finally, a 200 nm thick layer of aluminum is deposited on top by E-beam evaporation. General photolithography and lift-off processes are applied to define the pattern of HfO2 and Al, where AZ nLOF 2020 negative photoresist and Shipley 1165 remover are used, respectively. The picture of the fabricated sample is shown in Fig. 2(b). We made the samples very large in order to easily focus our light sources on the multilayers. Theoretically, the modulator can be made within microscale. In order to test the performance of the ITO-based modulator, we built an attenuated total reflectance (ATR) setup in the Kretschmann configuration [19], as illustrated in Fig. 2(c). During the experiment, a collimated laser beam propagates through a broadband polarizer (P1) to choose either TE- or TM-polarization, and is then split into two by a beam splitter (BS). One beam is fed into a photodiode (PD2) and used for collecting source power fluctuation; another is incident at the angle θ1 into a BK7 glass hemicylindrical (Ø100 mm) lens, which functions as a coupling prism. The reflected light is then measured by another photodiode (PD1). The power ratio between PD1 and PD2 can well define the power reflectance R. The modulator is mounted at the back of the prism. In order to avoid any air gap between the prism and the sample, a BK7 index matching liquid is applied. The rotation of the prism (together with the modulator) and PD1 is in a θ1 − 2θ1 configuration (i.e., PD1 rotates twice the angle of the prism), which is precisely controlled by two motorized rotation stages. The increment of θ1 is set as 0.25°. The measurement result is recorded by a LabVIEW program, thus, the reflectance R as a function of θ1 can be plotted [20]. In the experiments, we first measured the reflectance of the ITO modulator at different wavelengths ranging from λ0 1260–1620 nm without externally applied voltage. The measured reflectance is then numerically fitted by calculating the reflectance through the multilayer structure based on the TMM. The fitting parameters of the dielectric constant of ITO are shown in Fig. 3(a) (blue markers), which are further fitted based on the Drude model (colored lines). The parameters in Eq. (1) are ε∞ 4.4, ωp 2.51 × 1015 rad∕s, and γ 1 × 1015 rad∕s. The corresponding carrier concentration is N 0 3.05 × 1021 cm−3 . As a nonstoichiometric compound, the optical properties of ITO largely depend on the deposition processes and postannealing conditions. The ε∞ is slightly different from those obtained in other groups [9,10,13] due to the nonidentical fabrication conditions. After that, an external voltage was applied. During the experiment, we found that the reflectance (or 1−absorption) of the modulator could either be increased or decreased by applying positive or negative voltage, respectively. We conducted measurements with an externally applied voltage of different polarities but the same magnitude V P . For different wavelengths, the measured reflectance of the modulator with different applied voltages as a function of θ1 with a TM-polarized incident light beam is shown in Figs. 3(b)–3(e), respectively. Here, we
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OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
Fig. 3. (a) Fitting of the dielectric constant (real and imaginary part) of the active ITO layer (the rest of the ITO layer remains untuned with the value of the blue markers). Blue, green, and red markers correspond to parameters with zero bias, positive bias, and negative bias, respectively; solid and dashed lines are from further fitting by the Drude model. (b)–(e) Reflectance as a function of incident angle for the modulator under different applied voltages for different wavelengths (solid lines) and the fitting curves (dashed lines).
assume that there is a 5 nm thick depletion layer (under positive voltage V P 10 V, or accumulation layer under negative voltage −V P −10 V) formed at ITO-HfO2 interface. The modulation depth, Mθ1 , as a function of angle θ1 at a given wavelength can be defined as: Mθ1 jRV p − R−V p j∕R0 , where R0 is the experimentally measured reflectance without applied voltage; jRV p − R−V p j is the magnitude of the difference of the reflectance under applied voltages. The measured reflectance of the ITO modulator under applied voltages is also numerically fitted to find the change of optical constant of the active ITO layer. In order to simplify the fitting, we used the permittivity of the depletion layer or accumulation layer in ITO (both real and imaginary parts), and the thickness of the ITO layer as the variables to model the measured reflectance data. The fitting parameters of the active ITO layer, together with further Drude model fitting curves, are also shown in Fig. 3(a). Table 1 shows the specific fitting parameters as well as the maximum modulation depth for different wavelengths. From Fig. 3 and the table, broadband EO modulation has been achieved and the largest modulation depth obtained at a specific angle, θ1 78°, is M max 78° 37.42% for λ0 1620 nm. We call the angle where the maximum modulation depth is achieved, the modulation angle. By comparing the fitting parameters, we can see the dielectric constant of 5 nm active ITO layer can indeed be tuned to smaller magnitude or larger magnitude by bias with different polarities, which corresponds to higher absorption or lower absorption. We attribute the modulation mainly to the change of the free
carrier concentration in the 5 nm thick voltage-induced active layer in ITO at the interface. For instance, when negative bias is applied, excess positive carriers will be induced at the ITO-HfO2 interface, which results in major carrier (holes) accumulation in the active ITO layer; on the contrary, the active layer will be depleted under positive bias. The carrier concentrations for the accumulation ITO and depletion ITO are calculated as N a 3.23 × 1021 cm−3 and N d 1.57 × 1021 cm−3 , respectively. By comparison with the original value N 0 , we can see the free carriers are more easily depleted than accumulated. However, we find it difficult to fit the dielectric constant of the active ITO layer quite well simply by the Drude model equation [see Fig. 3(a)], so we also believe there is a more complicated mechanism existing for the EO modulation. Besides, we examined the variation of modulation depth with regard to applied voltages of different magnitude. Starting from 0 V, we measured the power reflectance at the modulation angle at a 2 V step. The measured results for λ0 1550 nm are shown in Fig. 4. From the figure, when 2 V voltages are applied, nearly no modulation can be observed, which indicates that there may exist a threshold voltage for the modulator to start working. After that, modulation depth and applied voltage almost follows a linear relation. It is true that the higher voltage we apply, the larger modulation depth we can achieve; however, higher voltage may induce large current and heat which would burn or break down the sample. The voltage limit may vary due to different sheet resistances of the samples.
Table 1. Fitting of the Dielectric Constant of the Active ITO Layer and Maximum Modulation Depth Wavelength (nm)
ε0 jε00 0 V
ε0 jε00 10 V
ε0 jε00 −10 V
M max (%)
1260 1360 1460 1520 1550 1620
2.49 j1.32 2.25 j1.66 2.10 j1.90 2.01 j1.88 1.81 j2.12 1.62 j2.32
3.4 j0.45 3.3 j0.68 3.1 j0.74 2.9 j0.82 2.6 j0.95 2.4 j1.04
1.53 j0.65 1.24 j0.88 1.06 j0.91 0.84 j1.09 0.52 j1.24 0.26 j1.36
6.94 13.70 22.94 27.93 32.31 37.42
Reflectance
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS
0.65
0=1550nm
0.55 Fitting curve (VP>0) Fitting curve (VP0) Measured data (VP
OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
Broadband electro-optical modulator based on transparent conducting oxide Kaifeng Shi, Riaz R. Haque, Bingyin Zhao, Runchen Zhao, and Zhaolin Lu* Microsystems Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, New York 14623, USA *Corresponding author: [email protected] Received April 17, 2014; revised July 17, 2014; accepted July 20, 2014; posted July 21, 2014 (Doc. ID 210008); published August 18, 2014 We report a broadband electro-optical (EO) modulator based on tunable plasmonic metamaterial. Transparent conducting oxides provide an excellent active plasmonic material for optoelectronic applications. By utilizing our indium-tin-oxide- (ITO) based multilayer structure, light absorption of the active ITO layer can be electrically modulated over a large spectrum range. Based on the attenuated total reflectance configuration, bias polaritydependent modulation up to 37% has been experimentally demonstrated. This EO modulator has advantages of simple design, easy fabrication, compact size, broadband performance, large modulation depth, as well as compatibility with existing silicon photonics platforms. © 2014 Optical Society of America OCIS codes: (160.2100) Electro-optical materials; (250.4110) Modulators; (230.2090) Electro-optical devices. http://dx.doi.org/10.1364/OL.39.004978
Ultracompact high-speed electro-optical (EO) modulators have become one of the critical technical bottlenecks impeding the wide applications of on-chip optical interconnects. Due to very poor EO properties of conventional materials, some phase modulators are on the order of millimeters [1,2]. Resonator modulators enhance the EO effect due to the large quality factor of the resonant cavity, so their dimensions can be shrunk to tens of micrometers [3]. However, resonator modulators usually suffer from bandwidth limitation, temperature fluctuation, as well as fabrication tolerance. A recently reported plasmonic phase modulator has a small dimension (29 μm) and high speed [4], but it requires a novel nonlinear material, which may add difficulty to its fabrication and integration. Some compact absorption modulators also rely on advanced materials [5,6]. In order to build EO modulators that can overcome both the footprint and bandwidth constrains, we need to (1) identify an efficient and low-cost active material and (2) greatly enhance lightactive medium interaction based on a novel platform. Recently, transparent conducting oxides (TCOs) have been widely investigated as promising plasmonic metameterials, with impressive advantages of low loss and fabrication compatibility [7–9]. The optical dielectric constant of TCOs can be approximated by the Drude model, ω2p 0 00 ; (1) ε ε jε ε∞ 1 − ωω jγ where ε∞ is the high-frequency dielectric constant, ω is the angular frequency of the light wave, and γ is the damping coefficient of free carriers related to optical losses. The plasma frequency is defined by s Ne2 ; (2) ωp ε∞ ε0 m which depends on carrier concentration N and the electron effective mass m . According to the above equations, the optical material’s dielectric constant shifts with different carrier concentrations. 0146-9592/14/174978-04$15.00/0
By manipulating the concentration of oxygen vacancies and interstitial metal dopants, the carrier concentration in ITO, a well-known representative of TCOs, can be controlled in the range of 1019 –1021 ∕cm3 . The high carrier concentration enables a guiding surface mode at the interface of ITO and dielectric materials. On the other hand, the carrier concentration in ITO can be engineered by an externally applied voltage [10,11]. In Ref. [10], Feigenbaum et al. have experimentally showed that a unity-order index change in ITO can be achieved in a metal-oxide-semiconductor- (MOS) like structure by a thin active layer (∼5 nm) of voltage-induced accumulation charge formed at the interface of the ITO and SiO2 . In this sense, ITO can work as a good EO material, which has a great potential in applications in EO modulators. Actually, some research work on electro-absorption modulators based on TCOs has already been presented [12–14]. Other EO modulators based on ferroelectric BaTiO3 (BTO) and BiFeO3 (BFO) have also been investigated [15,16]; however, these studies either lack experimental results or do not show a very strong EO response. In our previous work, we have investigated an EO modulator configuration based on ∼25 nm thick ITO (as the active medium) and ∼5 μm thick electrolyte gel (as gate “oxide”), where we observed the bias polaritydependent modulation effect: the absorption of the EO modulator could be increased by negative bias or decreased by positive bias. We applied the 5 nm active layer assumption and numerically fitted the measured results. We extracted the dielectric constant of the active ITO layer (5 nm) induced by the externally applied voltage and the corresponding carrier concentration at λ0 1520 nm [11]. However, the ITO-electrolyte gel modulator has a short lifetime and a very slow switching speed. In order to make more efficient modulators, we apply a high-k dielectric material in our designed EO modulator structure, as illustrated in Fig. 1. This is a simple multilayer MOS-like structure. BK7 glass serves as the substrate, and ITO is the tunable active material. High-k material HfO2 is utilized as the gate oxide for its ultrahigh permittivity as well as process stability [17]. Aluminum is © 2014 Optical Society of America
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS
Fig. 1.
Illustration of an ITO-based multilayer EO modulator.
chosen as the metal for its excellent conductivity, low absorption in the near-infrared (NIR) regime, as well as low cost. Another advantage of using an aluminum layer is that light absorption can be directly measured by 1 − R (R is the power reflectance) in the device illustrated in Fig. 1. Assume light is incident into ITO with angle θ1 to the normal. For a transverse magnetic (TM) mode, where the magnetic field is parallel to the multilayers, Maxwell’s equations determine that the normal electric flux density D is continuous at the ITO-HfO2 interface so E-field intensity is reversely proportional to the dielectric constant. Thus, the dissipation power density is 1 1 1 pd σE 2 ∝ ε00 E 2 ∝ ε00 ∕jεj2 ; 2 2 2
(3)
which can be enhanced when jεITO j decreases [12]. As a result, the EO modulator is at a low-absorption (highreflection) state under positive applied voltage and at a high-absorption (low-reflection) state under negative applied voltage, due to the change of the dielectric constant. The power reflectance R as a function of incident angle θ1 can be calculated based on the transfer matrix method (TMM) [18]. We found even though only a 5 nm active ITO layer has been electrically tuned, the power reflectance or the absorption of the modulator could be significantly changed at certain incident angles. It is worth mentioning that if aluminum is replaced by doped silicon, very little modulation between can be observed. This is because most power gets transmitted through the structure due to the transparency of silicon in the NIR regime. The cross section of the ITO-based EO modulator is shown in Fig. 2(a). The fabrication of the modulators starts from ITO film deposition on transparent glass, by the method of physical vapor deposition (PVD) process using a In2 O3 0.9 SnO2 0.1 weight percentage target at room temperature and 7.3 mTorr pressure within the chamber. During deposition, argon is the only gas
VP
BS BK7 Prism
θ1
(a) (c)
10mm
(b)
SiO2
Al
ITO
HfO2
Fig. 2. (a) Cross section of the proposed multilayer ITO modulator. (b) Picture of the fabricated modulator. (c) Illustration of the setup for ATR measurement.
4979
used. The thickness of the ITO film is measured in the range of 10–12 nm with a 5 min deposition time. After a 4 h postannealing process, the sheet resistance of the ITO film is measured around 400–600 Ω/square. After that, 50 nm thick HfO2 is deposited on the ITO layer by atomic layer deposition (ALD). Finally, a 200 nm thick layer of aluminum is deposited on top by E-beam evaporation. General photolithography and lift-off processes are applied to define the pattern of HfO2 and Al, where AZ nLOF 2020 negative photoresist and Shipley 1165 remover are used, respectively. The picture of the fabricated sample is shown in Fig. 2(b). We made the samples very large in order to easily focus our light sources on the multilayers. Theoretically, the modulator can be made within microscale. In order to test the performance of the ITO-based modulator, we built an attenuated total reflectance (ATR) setup in the Kretschmann configuration [19], as illustrated in Fig. 2(c). During the experiment, a collimated laser beam propagates through a broadband polarizer (P1) to choose either TE- or TM-polarization, and is then split into two by a beam splitter (BS). One beam is fed into a photodiode (PD2) and used for collecting source power fluctuation; another is incident at the angle θ1 into a BK7 glass hemicylindrical (Ø100 mm) lens, which functions as a coupling prism. The reflected light is then measured by another photodiode (PD1). The power ratio between PD1 and PD2 can well define the power reflectance R. The modulator is mounted at the back of the prism. In order to avoid any air gap between the prism and the sample, a BK7 index matching liquid is applied. The rotation of the prism (together with the modulator) and PD1 is in a θ1 − 2θ1 configuration (i.e., PD1 rotates twice the angle of the prism), which is precisely controlled by two motorized rotation stages. The increment of θ1 is set as 0.25°. The measurement result is recorded by a LabVIEW program, thus, the reflectance R as a function of θ1 can be plotted [20]. In the experiments, we first measured the reflectance of the ITO modulator at different wavelengths ranging from λ0 1260–1620 nm without externally applied voltage. The measured reflectance is then numerically fitted by calculating the reflectance through the multilayer structure based on the TMM. The fitting parameters of the dielectric constant of ITO are shown in Fig. 3(a) (blue markers), which are further fitted based on the Drude model (colored lines). The parameters in Eq. (1) are ε∞ 4.4, ωp 2.51 × 1015 rad∕s, and γ 1 × 1015 rad∕s. The corresponding carrier concentration is N 0 3.05 × 1021 cm−3 . As a nonstoichiometric compound, the optical properties of ITO largely depend on the deposition processes and postannealing conditions. The ε∞ is slightly different from those obtained in other groups [9,10,13] due to the nonidentical fabrication conditions. After that, an external voltage was applied. During the experiment, we found that the reflectance (or 1−absorption) of the modulator could either be increased or decreased by applying positive or negative voltage, respectively. We conducted measurements with an externally applied voltage of different polarities but the same magnitude V P . For different wavelengths, the measured reflectance of the modulator with different applied voltages as a function of θ1 with a TM-polarized incident light beam is shown in Figs. 3(b)–3(e), respectively. Here, we
4980
OPTICS LETTERS / Vol. 39, No. 17 / September 1, 2014
Fig. 3. (a) Fitting of the dielectric constant (real and imaginary part) of the active ITO layer (the rest of the ITO layer remains untuned with the value of the blue markers). Blue, green, and red markers correspond to parameters with zero bias, positive bias, and negative bias, respectively; solid and dashed lines are from further fitting by the Drude model. (b)–(e) Reflectance as a function of incident angle for the modulator under different applied voltages for different wavelengths (solid lines) and the fitting curves (dashed lines).
assume that there is a 5 nm thick depletion layer (under positive voltage V P 10 V, or accumulation layer under negative voltage −V P −10 V) formed at ITO-HfO2 interface. The modulation depth, Mθ1 , as a function of angle θ1 at a given wavelength can be defined as: Mθ1 jRV p − R−V p j∕R0 , where R0 is the experimentally measured reflectance without applied voltage; jRV p − R−V p j is the magnitude of the difference of the reflectance under applied voltages. The measured reflectance of the ITO modulator under applied voltages is also numerically fitted to find the change of optical constant of the active ITO layer. In order to simplify the fitting, we used the permittivity of the depletion layer or accumulation layer in ITO (both real and imaginary parts), and the thickness of the ITO layer as the variables to model the measured reflectance data. The fitting parameters of the active ITO layer, together with further Drude model fitting curves, are also shown in Fig. 3(a). Table 1 shows the specific fitting parameters as well as the maximum modulation depth for different wavelengths. From Fig. 3 and the table, broadband EO modulation has been achieved and the largest modulation depth obtained at a specific angle, θ1 78°, is M max 78° 37.42% for λ0 1620 nm. We call the angle where the maximum modulation depth is achieved, the modulation angle. By comparing the fitting parameters, we can see the dielectric constant of 5 nm active ITO layer can indeed be tuned to smaller magnitude or larger magnitude by bias with different polarities, which corresponds to higher absorption or lower absorption. We attribute the modulation mainly to the change of the free
carrier concentration in the 5 nm thick voltage-induced active layer in ITO at the interface. For instance, when negative bias is applied, excess positive carriers will be induced at the ITO-HfO2 interface, which results in major carrier (holes) accumulation in the active ITO layer; on the contrary, the active layer will be depleted under positive bias. The carrier concentrations for the accumulation ITO and depletion ITO are calculated as N a 3.23 × 1021 cm−3 and N d 1.57 × 1021 cm−3 , respectively. By comparison with the original value N 0 , we can see the free carriers are more easily depleted than accumulated. However, we find it difficult to fit the dielectric constant of the active ITO layer quite well simply by the Drude model equation [see Fig. 3(a)], so we also believe there is a more complicated mechanism existing for the EO modulation. Besides, we examined the variation of modulation depth with regard to applied voltages of different magnitude. Starting from 0 V, we measured the power reflectance at the modulation angle at a 2 V step. The measured results for λ0 1550 nm are shown in Fig. 4. From the figure, when 2 V voltages are applied, nearly no modulation can be observed, which indicates that there may exist a threshold voltage for the modulator to start working. After that, modulation depth and applied voltage almost follows a linear relation. It is true that the higher voltage we apply, the larger modulation depth we can achieve; however, higher voltage may induce large current and heat which would burn or break down the sample. The voltage limit may vary due to different sheet resistances of the samples.
Table 1. Fitting of the Dielectric Constant of the Active ITO Layer and Maximum Modulation Depth Wavelength (nm)
ε0 jε00 0 V
ε0 jε00 10 V
ε0 jε00 −10 V
M max (%)
1260 1360 1460 1520 1550 1620
2.49 j1.32 2.25 j1.66 2.10 j1.90 2.01 j1.88 1.81 j2.12 1.62 j2.32
3.4 j0.45 3.3 j0.68 3.1 j0.74 2.9 j0.82 2.6 j0.95 2.4 j1.04
1.53 j0.65 1.24 j0.88 1.06 j0.91 0.84 j1.09 0.52 j1.24 0.26 j1.36
6.94 13.70 22.94 27.93 32.31 37.42
Reflectance
September 1, 2014 / Vol. 39, No. 17 / OPTICS LETTERS
0.65
0=1550nm
0.55 Fitting curve (VP>0) Fitting curve (VP0) Measured data (VP
