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High speed electrically-controlled terahertz modulator
Superlattices and Microstructures 79 (2015) 72–78
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
High speed electrically-controlled terahertz modulator Lili Feng, Xiong Zhang ⇑, Minliang Liao, Shuchang Wang, Jiawei Cong, Yiping Cui Advanced Photonics Center, School of Electronic Science and Engineering Southeast University, Nanjing 210096, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 October 2014 Received in revised form 3 December 2014 Accepted 8 December 2014 Available online 24 December 2014 Keywords: Terahertz modulator Split-ring resonator High electron mobility transistor Modulation speed
a b s t r a c t We have developed an electrically-controlled terahertz modulator which can be used to realize amplitude modulation of terahertz waves at an extremely high speed. The terahertz modulator is made on GaAs-based high electron mobility transistors (HEMTs) array on which numerous split-ring resonators (SRRs) are formed. The device exhibits the capability of dynamical response to incident terahertz waves under a fast time-varying voltage. Our measurement results reveal that an ultrafast modulation speed over 11 MHz under an applied AC gate voltage can be achieved. With the resistance and the capacitance of HEMTs further optimized, it was demonstrated that the HEMT/SRR-based modulator may operate at an even higher modulation frequency. Ó 2014 Elsevier Ltd. All rights reserved.
Recent progress in terahertz science and technology has greatly facilitated the development of terahertz devices in the areas of generation, detection and modulation. A terahertz modulator is one of the key components in constructing a high speed terahertz communication system. However, underutilization of the terahertz gap (0.1–10 THz) has been inhibiting the realization and development of these terahertz devices, especially terahertz modulators. During the past decade, terahertz modulators have been developed and their modulation speed has been enhanced from the order of KHz to MHz magnitude [1–11]. In particular, one-atomic layer-thick graphene in conjunction with metamaterial
⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.spmi.2014.12.009 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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was utilized to make THz modulator and a modulation ratio in the amplitude of the transmitted THz wave up to 47% at room temperature was achieved by S. Lee et al. [1]. Moreover, an electrically-tunable terahertz modulator consisted of graphene layer and metamaterial with capacitively-tunable layers of electrons was demonstrated to promise nearly 100% modulation depth and less than 15% attenuation [2]. On the other hand, Li and Yao showed an optically-controlled terahertz modulator based on a high resistivity silicon substrate [3]. Their device was stimulated by a modulated laser, resulting in a modulation of the density of the photon-generated carriers and thus a variation in the reflective index of the Si substrate for the incident terahertz waves to realize amplitude modulation of terahertz waves. Due to the relatively long lifetime of the photon-generated carriers in silicon, however, the modulation speed of this optically-controlled terahertz modulator turned out to be only 0.1 KHz. As a recent approach, a new type of terahertz modulator configuration where split-ring resonators (SRRs) have been incorporated onto semiconductor wafers was proposed and developed by Chen et al. [4]. According to their published results, an electrically-controlled composite terahertz modulator was consisted of an n-type GaAs epi-layer which was grown on a semi-insulating (SI) GaAs substrate and a metamaterial-made SRR array. With an AC voltage applied onto the device, a modulation speed of terahertz radiation greater than 2 MHz was obtained. More recently, it was reported by Shrekenhamer et al. that a modulation frequency of up to 10 MHz might be obtained with an electrically controlled terahertz modulator which was based on a combination of conventional HEMTs and metamaterial structure [5]. In spite of these recent progress in high speed terahertz modulators, further improvements in their performances, including device reliability, modulation speed, and modulation depth, are still necessary to meet the requirements for constructing a practical terahertz communication system. In this article, we demonstrate an ultrafast electrically-controlled amplitude modulation for terahertz waves realized by using our specially designed and fabricated HEMTs/SRR-based composite terahertz modulator. Our device is composed of a HEMT array, an array of SRRs made over HEMT array and metal electrode pads. HEMTs were utilized to switch on and off the transmitted terahertz waves incident onto the device by modulating the density of 2DEG inside the HEMTs with applied high frequency AC signal. As the mobility and response of the 2DEG in the HEMTs to the varied voltage bias are high enough, a modulation speed with the order of MHz magnitude for the HEMT/SRR-based terahertz modulator can be expected. In fact, it will be demonstrated that a modulation speed over 11 MHz could be achieved. The composite terahertz modulator in this study is consisted of an array of GaAs-based HEMTs and an array of metamaterial SRRs made over HEMTs array, as shown in Fig. 1(a). The GaAs-based HEMT epitaxial structure was grown by molecular beam epitaxy (MBE) on a 635 lm-thick semi-insulating (SI) (1 0 0)-oriented GaAs substrate, as shown in Fig. 1(b). The n-AlGaAs (20 nm)/InGaAs (7 nm)/nAlGaAs (20 nm) layers form a double hetero-structure which is more effective to enhance the density of 2DEG in InGaAs channel than the single AlGaAs/InGaAs hetero-structure used in a conventional HEMT as reported in Ref. [3] even for the same doping level in the AlGaAs layer. The 2DEG density and electron mobility of the epi-structure were measured with a Hall equipment to be 2.5 1012 cm2 and 5000 cm2/V s at room temperature, respectively. Ni/Ge/Au (15/40/150 nm) metal stacks were deposited using electron beam evaporation (EBE), and were then annealed at 350 °C for 90 s in nitrogen ambience to form the source and drain ohmic contacts. Ti/Pt/Au (25/25/300 nm) metal stacks were used to form the gate electrodes and to connect all the gate electrodes in the same row, forming an electrode pad with a size of 150 150 lm2. The source and drain ohmic contacts for all the HEMTs in each unit cell were shorted by a SRR. In this work, the array of SRRs was fabricated using conventional photolithography and electron beam evaporation technology. Both the line width and the split gap of the SRR are 5 lm. Each SRR unit cell has the dimensions of 42 lm in width and 30 lm in height. The overall SRR element size was 55 40 lm2, and thus there were 3200 elements in total for the SRR/HEMT-based composite terahertz modulator with a device area of 2.6 2.7 mm2. A single GaAs-HEMT lies underneath each of the split gaps in all the SRR unit cells and the schematic cross section view for a typical unit cell is shown in Fig. 1(b). The length and width of the gate are 1 lm and 5 lm for all the HEMTs, respectively. The same metal film was used to connect all the SRRs together with the source and drain contacts of
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Fig. 1. (a) The top-view of the electrically-driven HEMT/SRR-based composite terahertz modulator, and the dimensions of the SRR unit are: L = 5 lm, G = 5 lm, H = 30 lm and W = 42 lm; (b) the cross-section view of a HEMT/SRR element.
the HEMTs in the same row for constructing the metamaterial structure and forming another electrode pad with a size of 150 150 lm2. The interaction between terahertz wave and SRR leads to the realization of amplitude modulation for the incident terahertz wave, because SRR exhibits resonant response for terahertz wave at certain frequency [4–11]. The carefully engineered metamaterial structure can either absorb or reflect most parts of the incident terahertz waves. The geometry and dimensions of the SRR used in this study were designed and fabricated in such a way that its resonance frequency was set at 0.6 THz. Driven by the time-varying gate voltage, the width and thickness of the conducting channel in the HEMT are changed, which causes the change in the density of 2DEG. By applying a reverse gate voltage bias onto the HEMT/SRR-based composite THz modulator, the 2DEG in the channel of the HEMTs is depleted and the modulator is equivalent to an array of SRRs, resulting in low terahertz wave transmission. The formation and accumulation of 2DEG in the InGaAs channel of the HEMTs then gradually weakens the resonance phenomenon as the reverse gate voltage bias was decreased from 3 V to 0 V. In particular, when no voltage bias was applied onto the gate, the density of the 2DEG could reach its maximum value of 2.5 1012 cm2. In such a situation, the split gap of the SRR fabricated on the HEMT could be effectively shorted by the 2DEG. As a result, SRRs array or metamaterial structure shows no resonant response for THz transmission. Thus, high speed amplitude modulation of terahertz waves could be realized with the HEMT/SRR-based composite terahertz modulator by applying fast time-varying AC gate voltage bias onto the device as long as the mobility and the response of the 2DEG in HEMT to the AC bias are sufficiently high. The dependence of the drain to source current (IDS) on the reverse gate bias voltage (VGS) and IDS as a function of the drain to source voltage (VDS) for the HEMT in the fabricated HEMT/SRR-based composite THz modulator are shown in Fig. 2(a) and (b), respectively. As can be seen clearly in Fig. 2(a), with increasing VGS, IDS drops linearly and the device is depleted when VGS approaches 1 V. In other words, the density of 2DEG in the InGaAs channel in HEMT drops all the way to 0 as VGS is increased to
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Fig. 2. The dependence of the drain to source current (IDS) on the gate bias voltage (VGS) (a) and IDS as a function of the drain to source voltage (VDS) (b) for the HEMT in the fabricated HEMT/SRR-based composite THz modulator.
be larger than 1 V for any VDS. On the other hand, since IDS approaches 0 as VGS is larger than 1.4 V, the three IDS–VDS curves for VGS = 2.0, 1.7 and 1.4 V are overlapped together, and thus they cannot be identified in Fig. 2(b). A terahertz time domain spectrum (THz-TDS) system was used to characterize the static response property of the HEMT/SRR-based composite terahertz modulator. The sample was set in such a position that the direction of the incident THz electric field was parallel to the split-gap of the SRR elements. The transmitted electric field spectra as a function of applied reverse gate bias voltage, VGS were shown in Fig. 3. It was found from Fig. 3 that the THz transmission at certain frequency (0.58 THz) changed dramatically as the applied voltage was varied from 1.5 to 1 V. The mechanism responsible for this phenomenon is that when the VGS was 1.5 V or higher, the 2DEG in the channel of the HEMT is almost depleted and the modulator is equivalent to an array of SRRs. Therefore, an abrupt decrease in transmission for the incident THz radiation can be expected. However, as the VGS was 1.0 V or lower, the density of the 2DEG in the channel of the HEMT was still high, and the split gap of the SRR fabricated on the HEMT was effectively shorted by the 2DEG, resulting in an increase in transmission for the incident THz wave. At VGS = 0 V, the SRRs were shorted by the 2DEG to a certain extent and a relatively high THz transmittance at the resonant frequency of 0.58 THz was obtained. The modulation ratio in amplitude as a function of applied gate voltage at a frequency of 0.58 THz is shown in Fig. 4. From Figs. 3 and 4, a variation in transmittance amplitude as large as 28% at resonant frequency of 0.58 THz can be achieved as VGS was changed from 0 V to 3 V. More detailed results
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Fig. 3. The measurement results of THz wave transmission signals as a function of frequency for the fabricated HEMT/SRRbased composite THz modulator using a THz-TDS under varied reverse gate bias voltage.
Fig. 4. The modulation ratio in amplitude as a function of applied gate voltage at a frequency of 0.58 THz.
Fig. 5. The schematic circuit set-up utilized to characterize the dynamic frequency response of the HEMT/SRR-based composite terahertz modulator.
on the terahertz transmission spectrum for DC biased HEMT/SRR-based composite terahertz modulator has been published somewhere else [12]. The characterization of the dynamic frequency response of the terahertz modulator was performed indirectly by setting up a circuit as schematically shown in Fig. 5. The frequency of the AC electrical signal generator with an output impedance of 50 X was tunable up to 20 MHz and the monitored
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dynamic response signal voltage Vi was set to be 500 mV. An external series resistor with tunable resistance was loaded in the circuit. An oscilloscope was used to measure the voltage on the loaded tunable resistor. The voltage value on the external resistor, V0 as a function of the AC electrical signal frequency with a resistance varied from 3 to 50 X was measured. As shown in Fig. 6, with the frequency of the applied AC electrical signal increased from 100 KHz to 20 MHz, V0 increased monotonously before the frequency reaches certain critical point after which V0 tended to saturate at a maximum value of Vmax. We defined the value of Vmax – Vmin as DV. Here Vmin is the minimum value for V0, which is generally equal to the value of V0 obtained at a frequency of zero. Following the same process as described in Refs. [2,11], the maximum modulation frequency of thepdevice could be deterffiffiffi mined by finding out the frequency corresponding to the value of V 0 ¼ ð 2=2Þ DV þ V min . For instance, aspthe ffiffiffi loaded external resistor is 3 X, from Fig. 5 we can get Vmax = 72.8 mV, Vmin = 20 mV, and then ð 2=2Þ DV þ V min = 57.33 mV. Therefore, the corresponding maximum modulation frequency can be determined to be 11.3 MHz for the device. Similarly as the resistance of the external resistor was increased to 30 X, the maximum modulation frequency of the device was determined to be 11.9 MHz which is 0.6 MHz higher than the case where the resistance of the external resistor is 3 X. As the resistance of the external resistor was further increased to 50 X, however, V0 increased with increasing the frequency of the applied AC electrical signal, and was not saturated at the frequency of 20 MHz yet. Nevertheless, the maximum modulation frequencies for the HEMT/SRR-based composite terahertz modulator with other external resistance values, such as 8 X, 10 X and 20 X, were all determined to be exceeding 11 MHz. Although the introduction of an external resistor seems to be effective for increasing the maximum response frequency of the device, the practical modulation speed of the HEMT/SRR-based composite terahertz modulator with an external resistance of 0 X can be deduced to be over 11 MHz from Fig. 6, which is higher than any reported optically- or electricallycontrolled terahertz modulators in the literatures so far. Moreover, with the structure design and fabrication processes for HEMT and SRR further optimized, an even higher modulation speed could be expected for the HEMT/SRR-based composite terahertz modulator.
Fig. 6. The dynamic frequency response of the voltage on the loaded resistor with a tunable resistance from 3 to 50 X as a function of the AC electrical signal frequency.
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In conclusion, we have successfully developed an electrically-controlled HEMT/SRR-based composite terahertz modulator with high speed amplitude modulation of terahertz waves. According to the experimental results, the modulation speed of the device was estimated to be over 11 MHz for the terahertz wave with a frequency of 0.58 THz. With further optimization of the resistance and the capacitance for the HEMT/SRR composite structure, especially the epitaxial layer structure and doping level in the HEMTs located underneath the SRR array, the HEMT/SRR-based composite terahertz modulator is expected to be able to operate at an even higher modulation frequency with a larger modulation depth. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (‘‘863’’ Program) Grant No. 2011AA010204. The authors would like to thank Mr. H. Cao, Mr. H. Guo, Mr. P. Ding and Ms. L. Wang for their fruitful discussion and technical support throughout this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
S. Lee, M. Choi, T. Kim, S. Lee, M. Liu, X. Yin, Nat. Mater. 11 (2012) 936–941. R. Yan, B. Sensale-Rodriguez, L. Liu, D. Jena, H. Xing, Opt. Express 20 (2012) 28664–28671. J. Li, J. Yao, Opt. Commun. 281 (2008) 5697–5700. H. Chen, S. Palit, T. Tyler, C.M. Bingham, J.M.O. Zide, J.F. O’Hara, D.R. Smith, A.C. Gossard, R.D. Averitt, W.J. Padilla, N.M. Jokerst, A.J. Taylor, Appl. Phys. Lett. 93 (2008) 091117–091117-3. D. Shrekenhamer, S. Rout, A.C. Strikwerda, C. Bingham, R.D. Averitt, S. Sonkusale, W.J. Padilla, Opt. Express 19 (2011) 9968– 9975. H. Chen, W.J. Padilla, J.M.O. Zide, A.C. Gossard, A.J. Taylor, R.D. Averitt, Nature 444 (2006) 597–600. Q. Zhou, Y. Shi, A. Wang, L. Li, D. Zhao, J. Liu, H. Sun, C. Zhang, J. Opt. 13 (2011) 235305–235305-5. T. Kleine-Ostmann, K. Pierz, G. Hein, P. Dawson, M. Marso, J. Appl. Phys. 105 (2009) 093707–093707-5. X. Zhang, R. Tan, Z. Zheng, X. Li, Z. Zhang, J. Appl. Phys. 113 (2013) 014504–014504-4. H. Chen, Willie.J. Padilla, M.J. Cich, A.K. Azad, R.D. Averitt, A.J. Taylor, Nat. Photon. 3 (2009) 148–151. A. Gabbay, J. Reno, J.R. Wendt, A. Gin, M.C. Wanke, M.B. Sinclair, E. Shaner, I. Brener, Appl. Phys. Lett. 98 (2011) 203103– 203103-3. M. Liao, J. Cong, X. Zhang, Y. Cui, J. Mod. Opt. 60 (2013) 1690–1695.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
High speed electrically-controlled terahertz modulator Lili Feng, Xiong Zhang ⇑, Minliang Liao, Shuchang Wang, Jiawei Cong, Yiping Cui Advanced Photonics Center, School of Electronic Science and Engineering Southeast University, Nanjing 210096, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 18 October 2014 Received in revised form 3 December 2014 Accepted 8 December 2014 Available online 24 December 2014 Keywords: Terahertz modulator Split-ring resonator High electron mobility transistor Modulation speed
a b s t r a c t We have developed an electrically-controlled terahertz modulator which can be used to realize amplitude modulation of terahertz waves at an extremely high speed. The terahertz modulator is made on GaAs-based high electron mobility transistors (HEMTs) array on which numerous split-ring resonators (SRRs) are formed. The device exhibits the capability of dynamical response to incident terahertz waves under a fast time-varying voltage. Our measurement results reveal that an ultrafast modulation speed over 11 MHz under an applied AC gate voltage can be achieved. With the resistance and the capacitance of HEMTs further optimized, it was demonstrated that the HEMT/SRR-based modulator may operate at an even higher modulation frequency. Ó 2014 Elsevier Ltd. All rights reserved.
Recent progress in terahertz science and technology has greatly facilitated the development of terahertz devices in the areas of generation, detection and modulation. A terahertz modulator is one of the key components in constructing a high speed terahertz communication system. However, underutilization of the terahertz gap (0.1–10 THz) has been inhibiting the realization and development of these terahertz devices, especially terahertz modulators. During the past decade, terahertz modulators have been developed and their modulation speed has been enhanced from the order of KHz to MHz magnitude [1–11]. In particular, one-atomic layer-thick graphene in conjunction with metamaterial
⇑ Corresponding author. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.spmi.2014.12.009 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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was utilized to make THz modulator and a modulation ratio in the amplitude of the transmitted THz wave up to 47% at room temperature was achieved by S. Lee et al. [1]. Moreover, an electrically-tunable terahertz modulator consisted of graphene layer and metamaterial with capacitively-tunable layers of electrons was demonstrated to promise nearly 100% modulation depth and less than 15% attenuation [2]. On the other hand, Li and Yao showed an optically-controlled terahertz modulator based on a high resistivity silicon substrate [3]. Their device was stimulated by a modulated laser, resulting in a modulation of the density of the photon-generated carriers and thus a variation in the reflective index of the Si substrate for the incident terahertz waves to realize amplitude modulation of terahertz waves. Due to the relatively long lifetime of the photon-generated carriers in silicon, however, the modulation speed of this optically-controlled terahertz modulator turned out to be only 0.1 KHz. As a recent approach, a new type of terahertz modulator configuration where split-ring resonators (SRRs) have been incorporated onto semiconductor wafers was proposed and developed by Chen et al. [4]. According to their published results, an electrically-controlled composite terahertz modulator was consisted of an n-type GaAs epi-layer which was grown on a semi-insulating (SI) GaAs substrate and a metamaterial-made SRR array. With an AC voltage applied onto the device, a modulation speed of terahertz radiation greater than 2 MHz was obtained. More recently, it was reported by Shrekenhamer et al. that a modulation frequency of up to 10 MHz might be obtained with an electrically controlled terahertz modulator which was based on a combination of conventional HEMTs and metamaterial structure [5]. In spite of these recent progress in high speed terahertz modulators, further improvements in their performances, including device reliability, modulation speed, and modulation depth, are still necessary to meet the requirements for constructing a practical terahertz communication system. In this article, we demonstrate an ultrafast electrically-controlled amplitude modulation for terahertz waves realized by using our specially designed and fabricated HEMTs/SRR-based composite terahertz modulator. Our device is composed of a HEMT array, an array of SRRs made over HEMT array and metal electrode pads. HEMTs were utilized to switch on and off the transmitted terahertz waves incident onto the device by modulating the density of 2DEG inside the HEMTs with applied high frequency AC signal. As the mobility and response of the 2DEG in the HEMTs to the varied voltage bias are high enough, a modulation speed with the order of MHz magnitude for the HEMT/SRR-based terahertz modulator can be expected. In fact, it will be demonstrated that a modulation speed over 11 MHz could be achieved. The composite terahertz modulator in this study is consisted of an array of GaAs-based HEMTs and an array of metamaterial SRRs made over HEMTs array, as shown in Fig. 1(a). The GaAs-based HEMT epitaxial structure was grown by molecular beam epitaxy (MBE) on a 635 lm-thick semi-insulating (SI) (1 0 0)-oriented GaAs substrate, as shown in Fig. 1(b). The n-AlGaAs (20 nm)/InGaAs (7 nm)/nAlGaAs (20 nm) layers form a double hetero-structure which is more effective to enhance the density of 2DEG in InGaAs channel than the single AlGaAs/InGaAs hetero-structure used in a conventional HEMT as reported in Ref. [3] even for the same doping level in the AlGaAs layer. The 2DEG density and electron mobility of the epi-structure were measured with a Hall equipment to be 2.5 1012 cm2 and 5000 cm2/V s at room temperature, respectively. Ni/Ge/Au (15/40/150 nm) metal stacks were deposited using electron beam evaporation (EBE), and were then annealed at 350 °C for 90 s in nitrogen ambience to form the source and drain ohmic contacts. Ti/Pt/Au (25/25/300 nm) metal stacks were used to form the gate electrodes and to connect all the gate electrodes in the same row, forming an electrode pad with a size of 150 150 lm2. The source and drain ohmic contacts for all the HEMTs in each unit cell were shorted by a SRR. In this work, the array of SRRs was fabricated using conventional photolithography and electron beam evaporation technology. Both the line width and the split gap of the SRR are 5 lm. Each SRR unit cell has the dimensions of 42 lm in width and 30 lm in height. The overall SRR element size was 55 40 lm2, and thus there were 3200 elements in total for the SRR/HEMT-based composite terahertz modulator with a device area of 2.6 2.7 mm2. A single GaAs-HEMT lies underneath each of the split gaps in all the SRR unit cells and the schematic cross section view for a typical unit cell is shown in Fig. 1(b). The length and width of the gate are 1 lm and 5 lm for all the HEMTs, respectively. The same metal film was used to connect all the SRRs together with the source and drain contacts of
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Fig. 1. (a) The top-view of the electrically-driven HEMT/SRR-based composite terahertz modulator, and the dimensions of the SRR unit are: L = 5 lm, G = 5 lm, H = 30 lm and W = 42 lm; (b) the cross-section view of a HEMT/SRR element.
the HEMTs in the same row for constructing the metamaterial structure and forming another electrode pad with a size of 150 150 lm2. The interaction between terahertz wave and SRR leads to the realization of amplitude modulation for the incident terahertz wave, because SRR exhibits resonant response for terahertz wave at certain frequency [4–11]. The carefully engineered metamaterial structure can either absorb or reflect most parts of the incident terahertz waves. The geometry and dimensions of the SRR used in this study were designed and fabricated in such a way that its resonance frequency was set at 0.6 THz. Driven by the time-varying gate voltage, the width and thickness of the conducting channel in the HEMT are changed, which causes the change in the density of 2DEG. By applying a reverse gate voltage bias onto the HEMT/SRR-based composite THz modulator, the 2DEG in the channel of the HEMTs is depleted and the modulator is equivalent to an array of SRRs, resulting in low terahertz wave transmission. The formation and accumulation of 2DEG in the InGaAs channel of the HEMTs then gradually weakens the resonance phenomenon as the reverse gate voltage bias was decreased from 3 V to 0 V. In particular, when no voltage bias was applied onto the gate, the density of the 2DEG could reach its maximum value of 2.5 1012 cm2. In such a situation, the split gap of the SRR fabricated on the HEMT could be effectively shorted by the 2DEG. As a result, SRRs array or metamaterial structure shows no resonant response for THz transmission. Thus, high speed amplitude modulation of terahertz waves could be realized with the HEMT/SRR-based composite terahertz modulator by applying fast time-varying AC gate voltage bias onto the device as long as the mobility and the response of the 2DEG in HEMT to the AC bias are sufficiently high. The dependence of the drain to source current (IDS) on the reverse gate bias voltage (VGS) and IDS as a function of the drain to source voltage (VDS) for the HEMT in the fabricated HEMT/SRR-based composite THz modulator are shown in Fig. 2(a) and (b), respectively. As can be seen clearly in Fig. 2(a), with increasing VGS, IDS drops linearly and the device is depleted when VGS approaches 1 V. In other words, the density of 2DEG in the InGaAs channel in HEMT drops all the way to 0 as VGS is increased to
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Fig. 2. The dependence of the drain to source current (IDS) on the gate bias voltage (VGS) (a) and IDS as a function of the drain to source voltage (VDS) (b) for the HEMT in the fabricated HEMT/SRR-based composite THz modulator.
be larger than 1 V for any VDS. On the other hand, since IDS approaches 0 as VGS is larger than 1.4 V, the three IDS–VDS curves for VGS = 2.0, 1.7 and 1.4 V are overlapped together, and thus they cannot be identified in Fig. 2(b). A terahertz time domain spectrum (THz-TDS) system was used to characterize the static response property of the HEMT/SRR-based composite terahertz modulator. The sample was set in such a position that the direction of the incident THz electric field was parallel to the split-gap of the SRR elements. The transmitted electric field spectra as a function of applied reverse gate bias voltage, VGS were shown in Fig. 3. It was found from Fig. 3 that the THz transmission at certain frequency (0.58 THz) changed dramatically as the applied voltage was varied from 1.5 to 1 V. The mechanism responsible for this phenomenon is that when the VGS was 1.5 V or higher, the 2DEG in the channel of the HEMT is almost depleted and the modulator is equivalent to an array of SRRs. Therefore, an abrupt decrease in transmission for the incident THz radiation can be expected. However, as the VGS was 1.0 V or lower, the density of the 2DEG in the channel of the HEMT was still high, and the split gap of the SRR fabricated on the HEMT was effectively shorted by the 2DEG, resulting in an increase in transmission for the incident THz wave. At VGS = 0 V, the SRRs were shorted by the 2DEG to a certain extent and a relatively high THz transmittance at the resonant frequency of 0.58 THz was obtained. The modulation ratio in amplitude as a function of applied gate voltage at a frequency of 0.58 THz is shown in Fig. 4. From Figs. 3 and 4, a variation in transmittance amplitude as large as 28% at resonant frequency of 0.58 THz can be achieved as VGS was changed from 0 V to 3 V. More detailed results
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Fig. 3. The measurement results of THz wave transmission signals as a function of frequency for the fabricated HEMT/SRRbased composite THz modulator using a THz-TDS under varied reverse gate bias voltage.
Fig. 4. The modulation ratio in amplitude as a function of applied gate voltage at a frequency of 0.58 THz.
Fig. 5. The schematic circuit set-up utilized to characterize the dynamic frequency response of the HEMT/SRR-based composite terahertz modulator.
on the terahertz transmission spectrum for DC biased HEMT/SRR-based composite terahertz modulator has been published somewhere else [12]. The characterization of the dynamic frequency response of the terahertz modulator was performed indirectly by setting up a circuit as schematically shown in Fig. 5. The frequency of the AC electrical signal generator with an output impedance of 50 X was tunable up to 20 MHz and the monitored
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dynamic response signal voltage Vi was set to be 500 mV. An external series resistor with tunable resistance was loaded in the circuit. An oscilloscope was used to measure the voltage on the loaded tunable resistor. The voltage value on the external resistor, V0 as a function of the AC electrical signal frequency with a resistance varied from 3 to 50 X was measured. As shown in Fig. 6, with the frequency of the applied AC electrical signal increased from 100 KHz to 20 MHz, V0 increased monotonously before the frequency reaches certain critical point after which V0 tended to saturate at a maximum value of Vmax. We defined the value of Vmax – Vmin as DV. Here Vmin is the minimum value for V0, which is generally equal to the value of V0 obtained at a frequency of zero. Following the same process as described in Refs. [2,11], the maximum modulation frequency of thepdevice could be deterffiffiffi mined by finding out the frequency corresponding to the value of V 0 ¼ ð 2=2Þ DV þ V min . For instance, aspthe ffiffiffi loaded external resistor is 3 X, from Fig. 5 we can get Vmax = 72.8 mV, Vmin = 20 mV, and then ð 2=2Þ DV þ V min = 57.33 mV. Therefore, the corresponding maximum modulation frequency can be determined to be 11.3 MHz for the device. Similarly as the resistance of the external resistor was increased to 30 X, the maximum modulation frequency of the device was determined to be 11.9 MHz which is 0.6 MHz higher than the case where the resistance of the external resistor is 3 X. As the resistance of the external resistor was further increased to 50 X, however, V0 increased with increasing the frequency of the applied AC electrical signal, and was not saturated at the frequency of 20 MHz yet. Nevertheless, the maximum modulation frequencies for the HEMT/SRR-based composite terahertz modulator with other external resistance values, such as 8 X, 10 X and 20 X, were all determined to be exceeding 11 MHz. Although the introduction of an external resistor seems to be effective for increasing the maximum response frequency of the device, the practical modulation speed of the HEMT/SRR-based composite terahertz modulator with an external resistance of 0 X can be deduced to be over 11 MHz from Fig. 6, which is higher than any reported optically- or electricallycontrolled terahertz modulators in the literatures so far. Moreover, with the structure design and fabrication processes for HEMT and SRR further optimized, an even higher modulation speed could be expected for the HEMT/SRR-based composite terahertz modulator.
Fig. 6. The dynamic frequency response of the voltage on the loaded resistor with a tunable resistance from 3 to 50 X as a function of the AC electrical signal frequency.
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In conclusion, we have successfully developed an electrically-controlled HEMT/SRR-based composite terahertz modulator with high speed amplitude modulation of terahertz waves. According to the experimental results, the modulation speed of the device was estimated to be over 11 MHz for the terahertz wave with a frequency of 0.58 THz. With further optimization of the resistance and the capacitance for the HEMT/SRR composite structure, especially the epitaxial layer structure and doping level in the HEMTs located underneath the SRR array, the HEMT/SRR-based composite terahertz modulator is expected to be able to operate at an even higher modulation frequency with a larger modulation depth. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (‘‘863’’ Program) Grant No. 2011AA010204. The authors would like to thank Mr. H. Cao, Mr. H. Guo, Mr. P. Ding and Ms. L. Wang for their fruitful discussion and technical support throughout this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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