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Terahertz wave modulator based on optically controllable metamaterial
Optics & Laser Technology 43 (2011) 102–105
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Terahertz wave modulator based on optically controllable metamaterial Jiu-Sheng Li n Centre for THz Research, China Jiliang University, Hangzhou 310018, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 November 2009 Received in revised form 22 May 2010 Accepted 22 May 2010
We demonstrated experimentally a terahertz wave modulator based on optically controlled metamaterial. The signal modulation mechanism of the presented terahertz wave modulator was based on the resonance characteristic of metamaterial controlled without or with light excitation. A modulated semiconductor laser with 808 nm wavelength was employed to light the substrate. The interaction between the metamaterial and terahertz wave was strengthened and yielded an appreciable modulation of the terahertz output beam. The modulation speed is 0.1 Kb/s and the modulation depth of the proposed terahertz modulator is about 57% at a frequency of 0.32 THz. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Terahertz communications Terahertz wave modulator Metamaterial
1. Introduction The terahertz (THz) frequency range 0.1–10 THz, located midway between microwaves and infrared light, presents a new frontier containing numerous technical applications and fundamental research problems. With the realization of the terahertz generator and detector, terahertz wave has attracted significant attention and has been extensively investigated. Due to their special properties, within the past few years, many potential applications of terahertz waves have been dramatically explored in many fields such as biomedical diagnostics, security screening, military detection, radio astronomy, atmospheric studies, high speed communication, quality control of packaged goods, and moisture analysis for agriculture [1,2]. With wide bandwidth and high data transmission bit rates, terahertz wave wireless communications have great potential in future short range wireless communications [3]. As an important device in terahertz communication for signal processing, terahertz wave modulator has been attracting significant attention. In 2004, Ostmann et al. [4,5] designed an electrically driven terahertz modulator. In 2006, Chen et al. [6] proposed a terahertz modulator based on metamaterial. In 2007, using a one dimensional photonic crystal with a GaAs defect, Fekete et al. [7,8] demonstrated the possibility of ultrafast modulation of THz radiation. This year, Li [9] analyzed a terahertz wave modulator using photonic crystals. However, they are still relatively undeveloped with only a few examples of cryogenically cooled and room temperature modulators. Robust terahertz wave modulators are still needed that can be easily implemented and integrated into a chip-scale platform. Therefore, it is valuable to n
Tel.: +86 57186875673; fax: +86 57186875618. E-mail address: [email protected]
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.05.011
investigate the design of modulator in the terahertz range. Metamaterials consisting of periodically patterned split ring resonators (SRRs) display a strong resonant electromagnetic response. Recently, it has been shown that the resonance strength, and hence the transmission, can be controlled via external stimulation for planar metamaterials fabricated on semiconducting substrates [6,10]. This has resulted in efficient switching and modulation of freely propagating THz radiation, which is essential for many potential applications such as secure short-range communication. Modulation of THz radiation with an applied voltage was recently demonstrated using a hybrid structure consisting of a Schottky diode and a planar metamaterial array [10]. Owing to the large capacitance and series resistance, the device operates at a low modulation rate. In this study, we have proposed and demonstrated a novel terahertz wave modulator using metamaterial. The terahertz wave modulation mechanism of the proposed terahertz wave modulator is based on the resonance characteristic of metamaterial that is controlled using a modulated laser. Experimental results show that the presented terahertz wave modulator has a modulation speed of 0.1 Kb/s, a modulation depth of 57%, and simplicity.
2. Device fabrication The size of the metamaterial unit is optimized and obtained with CST Microwave Studio. The dimensions of the resonator unit are as follows: a ¼40, b ¼30, c¼ 2, and d ¼4 mm. We transferred metamaterial pattern (100 100 periods) to mask. A 210 nm thickness of gold was deposited on a 500-mm-thick intrinsic silicon wafer (10 K O cm) using a radio frequency magnetron
J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
sputtering method. About 1 mm AZ-601 photoresist was coated on the wafer by a GKF-411 gluing purifying machine. To evaporate the solvent and densify the AZ-601 film, the wafer was put in hotplate at 70 1C persistence 5 min, and was cooled for 5 min. The mask and the wafer were mounted onto the mask aligner. Then the AZ-601 film was exposed to URE-2000S deep ultraviolet
103
lithography exposure plane. The AZ-601 was baked on a hotplate at 70 1C persistence 5 min for acid-initiated, thermally driven epoxy cross-linking. Then it was developed with an RZX-3038 positive photoresists developer with agitation until the pattern was clear and then rinsed with deionized water. To evaporate deionized water and densify the AZ-601 film, the wafer was put in hotplate at 70 1C persistence 5 min, followed by 120 1C oven for 10 min, and was then cooled for 15 min. The wafer was etched by MNL/DIII reactive ion etcher, rinsed with deionized water and baked in a 120 1C oven for 10 min. Strip photoresist with ORNIII5532 plasma stripping photoresist system. The wafer is sawed to 1 1 cm2 samples by an HP-603 automatic precision dicing saw. The sample is evaluated by a scanning electron microscope (SEM) and is shown in Fig.1. The electric field distribution for metamaterial SRR in the x, y plane at the corresponding resonances was simulated by finite element method, as shown in Fig. 2. The electric field is strongly concentrated at the split gaps at the resonant frequency.
3. Experimental results and discussion Fig. 3 shows the experimental setup for backward-wave oscillator (BWO) used to test the proposed terahertz-wave modulator. The backward-wave oscillator system is used as a terahertz wave source. As depicted, the metamaterial device is placed at the focal point of the terahertz radiation. Both excitation and terahertz wave spot sizes and also the carrier versus terahertz spot size overlap were determined by sweeping the optical
Fig. 1. Scanning electron microscope (SEM) image of (a) a square metamaterial and (b) a square metamaterial array.
Fig. 3. Sketch of the experiment setup.
Fig. 2. The simulated electric field distribution for metamaterial in the x, y plane at the corresponding resonances.
Fig. 4. Measured terahertz wave transmittance characteristic of the metamaterial.
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J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
Fig. 5. Measured modulation characteristic of the proposed terahertz wave modulator: (a) applied signal and (b) detected signal.
excitation. The silicon substrate has indirect bandgap energy (1.12 eV), and light with a wavelength less than 1.1 mm should be able to excite free carriers. Fig. 4 shows the terahertz wave transmittance characteristic of metamaterial based on silicon at the frequency from 0.23 to 0.36 THz with and without light excitation. Carrier injection into the silicon substrate is achieved by using a continuous wave semiconductor laser at 808 nm with incident optical intensities 100 mW. At this time, the metamaterial is not in resonance and the terahertz wave attenuation is larger than that of without optical excitation. From the figure, one sees that the change of the terahertz wave transmission intensity at the frequency of 0.32 THz is larger than any other measured terahertz wave frequency band. Therefore, in order to achieve the largest modulation depth, the backward-wave oscillator is set to be 0.32 THz and used as the continuous terahertz wave source. A pyroelectric detector was used to detect the modulated terahertz wave signal. A modulated semiconductor laser at 808 nm with incident optical intensity of 100 mW is employed to light the metamaterial based on intrinsic silicon. The resonance of the metamaterial is controlled by the photo-excited carriers. Measured terahertz wave transmittance characteristic of the novel terahertz wave modulator with the modulated laser is shown in Fig. 5. Applied square wave signal with 0.1 Kb/s for laser modulation is shown in Fig. 5(a). Fig. 5(b) shows the detected signal from a terahertz wave pyroelectric detector with 0.1 Kb/s. Thereby, the modulation speed of the proposed modulator is 0.1 Kb/s with a modulated semiconductor laser of 100 mW in experiment. We achieved a modulation depth (DT/T) of up to 57% at a terahertz frequency of 0.32 THz. The modulation speed of the device is limited by the lifetime of photocarriers. It is well known that the lifetime of silicon photocarriers is about several milliseconds, and the proposed modulator can obtain the modulation speed of MHz. Because the response time is the limitation of the terahertz wave pyroelectric detector, we cannot measure the MHz modulation speed of the modulator (experimentally limited). Here, the signal modulation mechanism of the presented terahertz wave modulator is based on the resonance
characteristic of metamaterial controlled without or with light excitation. That is to say, when the silicon substrate is illuminated by the pumping light, photocarriers are induced and the resonance characteristic of metamaterial is changed. The modulation speed is limited by the lifetime of photocarriers. Further studies will be conducted with another semiconductor substrate (e.g. GaAs) having a short lifetime of photocarriers. 4. Conclusion We experimentally demonstrated whether the designed optically controlled terahertz wave modulator based on metamaterial is able to achieve terahertz wave signal modulation. The terahertz wave modulator presented here is based on the modulated semiconductor laser excitation at 100 mW average power at 808 nm to illuminate the metamaterial, demonstrating terahertz wave transmission modulation that has a modulation speed of 0.1 Kb/s. The modulation depth of the presented terahertz wave modulator is more than 57% at a frequency of 0.32 THz. The size of the modulator is about 4 4 mm2. We believe that the proposed terahertz wave modulator can be useful for future terahertz wave communication systems. Acknowledgments The author thanks Prof. Yao for valuable discussions during this study and Prof. Wen for his technical assistance. This research was partially supported by the National Natural Science Foundation of China (no. 60971027), the National Basic Research Program of China (2007CB310403), and China Postdoctoral Science Foundation. References [1] Li J, Li X. Determination principal component content of seed oils by THz-TDS. Chem Phys Lett 2009;476:92–6.
J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
[2] Kersting R, Strasser G, Unterrainer K. Terahertz phase modulator. Electron Lett 2000;36:1156–7. [3] Piesiewicz R, Ostmann TK, Krumbholz N, Mittleman D, Koch M. Short-range ultra-broadband terahertz communications: concept and perspectives. IEEE Antennas Propag Mag 2007;49(6):24–39. [4] Ostmann TK, Pierz K, Hein G, Dawson P, Koch M. Audio signal transmission over THz communication channel using semiconductor modulator. Electron Lett 2004;40(2):124–6. [5] Ostmann TK, Dawson P, Pierz K, Koch M. Room temperature operation of an electrically driven terahertz modulator. Appl Phys Lett 2004;84:3555.
105
[6] Chen HT, Padilla WJ, Zide JM, Gossard AC, Taylor AJ, Averitt RD. Active terahertz metamaterials devices. Nature 2006;444:597. ˇ emec H. Ultrafast opto-terahertz photonic crystal ˇ zel P, N [7] Fekete L, Kadlec F, Ku modulator. Opt Lett 2007;32:680. ˇ emec H, Ku ˇ zel P. Fast one-dimensional photonic crystal [8] Fekete L, Kadlec F, N modulators for the terahertz range. Opt Express 2007;15(14):88–98. [9] Li JS. Terahertz modulator using photonic crystals. Opt Commun 2007;269:98. [10] Padilla WJ, Taylor AJ, Highstrete C, Lee M, Averitt RD. Dynamical electric and magnetic metamaterial response at terahertz frequencies. Phys Rev Lett 2006;96:107401.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Terahertz wave modulator based on optically controllable metamaterial Jiu-Sheng Li n Centre for THz Research, China Jiliang University, Hangzhou 310018, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 28 November 2009 Received in revised form 22 May 2010 Accepted 22 May 2010
We demonstrated experimentally a terahertz wave modulator based on optically controlled metamaterial. The signal modulation mechanism of the presented terahertz wave modulator was based on the resonance characteristic of metamaterial controlled without or with light excitation. A modulated semiconductor laser with 808 nm wavelength was employed to light the substrate. The interaction between the metamaterial and terahertz wave was strengthened and yielded an appreciable modulation of the terahertz output beam. The modulation speed is 0.1 Kb/s and the modulation depth of the proposed terahertz modulator is about 57% at a frequency of 0.32 THz. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Terahertz communications Terahertz wave modulator Metamaterial
1. Introduction The terahertz (THz) frequency range 0.1–10 THz, located midway between microwaves and infrared light, presents a new frontier containing numerous technical applications and fundamental research problems. With the realization of the terahertz generator and detector, terahertz wave has attracted significant attention and has been extensively investigated. Due to their special properties, within the past few years, many potential applications of terahertz waves have been dramatically explored in many fields such as biomedical diagnostics, security screening, military detection, radio astronomy, atmospheric studies, high speed communication, quality control of packaged goods, and moisture analysis for agriculture [1,2]. With wide bandwidth and high data transmission bit rates, terahertz wave wireless communications have great potential in future short range wireless communications [3]. As an important device in terahertz communication for signal processing, terahertz wave modulator has been attracting significant attention. In 2004, Ostmann et al. [4,5] designed an electrically driven terahertz modulator. In 2006, Chen et al. [6] proposed a terahertz modulator based on metamaterial. In 2007, using a one dimensional photonic crystal with a GaAs defect, Fekete et al. [7,8] demonstrated the possibility of ultrafast modulation of THz radiation. This year, Li [9] analyzed a terahertz wave modulator using photonic crystals. However, they are still relatively undeveloped with only a few examples of cryogenically cooled and room temperature modulators. Robust terahertz wave modulators are still needed that can be easily implemented and integrated into a chip-scale platform. Therefore, it is valuable to n
Tel.: +86 57186875673; fax: +86 57186875618. E-mail address: [email protected]
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.05.011
investigate the design of modulator in the terahertz range. Metamaterials consisting of periodically patterned split ring resonators (SRRs) display a strong resonant electromagnetic response. Recently, it has been shown that the resonance strength, and hence the transmission, can be controlled via external stimulation for planar metamaterials fabricated on semiconducting substrates [6,10]. This has resulted in efficient switching and modulation of freely propagating THz radiation, which is essential for many potential applications such as secure short-range communication. Modulation of THz radiation with an applied voltage was recently demonstrated using a hybrid structure consisting of a Schottky diode and a planar metamaterial array [10]. Owing to the large capacitance and series resistance, the device operates at a low modulation rate. In this study, we have proposed and demonstrated a novel terahertz wave modulator using metamaterial. The terahertz wave modulation mechanism of the proposed terahertz wave modulator is based on the resonance characteristic of metamaterial that is controlled using a modulated laser. Experimental results show that the presented terahertz wave modulator has a modulation speed of 0.1 Kb/s, a modulation depth of 57%, and simplicity.
2. Device fabrication The size of the metamaterial unit is optimized and obtained with CST Microwave Studio. The dimensions of the resonator unit are as follows: a ¼40, b ¼30, c¼ 2, and d ¼4 mm. We transferred metamaterial pattern (100 100 periods) to mask. A 210 nm thickness of gold was deposited on a 500-mm-thick intrinsic silicon wafer (10 K O cm) using a radio frequency magnetron
J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
sputtering method. About 1 mm AZ-601 photoresist was coated on the wafer by a GKF-411 gluing purifying machine. To evaporate the solvent and densify the AZ-601 film, the wafer was put in hotplate at 70 1C persistence 5 min, and was cooled for 5 min. The mask and the wafer were mounted onto the mask aligner. Then the AZ-601 film was exposed to URE-2000S deep ultraviolet
103
lithography exposure plane. The AZ-601 was baked on a hotplate at 70 1C persistence 5 min for acid-initiated, thermally driven epoxy cross-linking. Then it was developed with an RZX-3038 positive photoresists developer with agitation until the pattern was clear and then rinsed with deionized water. To evaporate deionized water and densify the AZ-601 film, the wafer was put in hotplate at 70 1C persistence 5 min, followed by 120 1C oven for 10 min, and was then cooled for 15 min. The wafer was etched by MNL/DIII reactive ion etcher, rinsed with deionized water and baked in a 120 1C oven for 10 min. Strip photoresist with ORNIII5532 plasma stripping photoresist system. The wafer is sawed to 1 1 cm2 samples by an HP-603 automatic precision dicing saw. The sample is evaluated by a scanning electron microscope (SEM) and is shown in Fig.1. The electric field distribution for metamaterial SRR in the x, y plane at the corresponding resonances was simulated by finite element method, as shown in Fig. 2. The electric field is strongly concentrated at the split gaps at the resonant frequency.
3. Experimental results and discussion Fig. 3 shows the experimental setup for backward-wave oscillator (BWO) used to test the proposed terahertz-wave modulator. The backward-wave oscillator system is used as a terahertz wave source. As depicted, the metamaterial device is placed at the focal point of the terahertz radiation. Both excitation and terahertz wave spot sizes and also the carrier versus terahertz spot size overlap were determined by sweeping the optical
Fig. 1. Scanning electron microscope (SEM) image of (a) a square metamaterial and (b) a square metamaterial array.
Fig. 3. Sketch of the experiment setup.
Fig. 2. The simulated electric field distribution for metamaterial in the x, y plane at the corresponding resonances.
Fig. 4. Measured terahertz wave transmittance characteristic of the metamaterial.
104
J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
Fig. 5. Measured modulation characteristic of the proposed terahertz wave modulator: (a) applied signal and (b) detected signal.
excitation. The silicon substrate has indirect bandgap energy (1.12 eV), and light with a wavelength less than 1.1 mm should be able to excite free carriers. Fig. 4 shows the terahertz wave transmittance characteristic of metamaterial based on silicon at the frequency from 0.23 to 0.36 THz with and without light excitation. Carrier injection into the silicon substrate is achieved by using a continuous wave semiconductor laser at 808 nm with incident optical intensities 100 mW. At this time, the metamaterial is not in resonance and the terahertz wave attenuation is larger than that of without optical excitation. From the figure, one sees that the change of the terahertz wave transmission intensity at the frequency of 0.32 THz is larger than any other measured terahertz wave frequency band. Therefore, in order to achieve the largest modulation depth, the backward-wave oscillator is set to be 0.32 THz and used as the continuous terahertz wave source. A pyroelectric detector was used to detect the modulated terahertz wave signal. A modulated semiconductor laser at 808 nm with incident optical intensity of 100 mW is employed to light the metamaterial based on intrinsic silicon. The resonance of the metamaterial is controlled by the photo-excited carriers. Measured terahertz wave transmittance characteristic of the novel terahertz wave modulator with the modulated laser is shown in Fig. 5. Applied square wave signal with 0.1 Kb/s for laser modulation is shown in Fig. 5(a). Fig. 5(b) shows the detected signal from a terahertz wave pyroelectric detector with 0.1 Kb/s. Thereby, the modulation speed of the proposed modulator is 0.1 Kb/s with a modulated semiconductor laser of 100 mW in experiment. We achieved a modulation depth (DT/T) of up to 57% at a terahertz frequency of 0.32 THz. The modulation speed of the device is limited by the lifetime of photocarriers. It is well known that the lifetime of silicon photocarriers is about several milliseconds, and the proposed modulator can obtain the modulation speed of MHz. Because the response time is the limitation of the terahertz wave pyroelectric detector, we cannot measure the MHz modulation speed of the modulator (experimentally limited). Here, the signal modulation mechanism of the presented terahertz wave modulator is based on the resonance
characteristic of metamaterial controlled without or with light excitation. That is to say, when the silicon substrate is illuminated by the pumping light, photocarriers are induced and the resonance characteristic of metamaterial is changed. The modulation speed is limited by the lifetime of photocarriers. Further studies will be conducted with another semiconductor substrate (e.g. GaAs) having a short lifetime of photocarriers. 4. Conclusion We experimentally demonstrated whether the designed optically controlled terahertz wave modulator based on metamaterial is able to achieve terahertz wave signal modulation. The terahertz wave modulator presented here is based on the modulated semiconductor laser excitation at 100 mW average power at 808 nm to illuminate the metamaterial, demonstrating terahertz wave transmission modulation that has a modulation speed of 0.1 Kb/s. The modulation depth of the presented terahertz wave modulator is more than 57% at a frequency of 0.32 THz. The size of the modulator is about 4 4 mm2. We believe that the proposed terahertz wave modulator can be useful for future terahertz wave communication systems. Acknowledgments The author thanks Prof. Yao for valuable discussions during this study and Prof. Wen for his technical assistance. This research was partially supported by the National Natural Science Foundation of China (no. 60971027), the National Basic Research Program of China (2007CB310403), and China Postdoctoral Science Foundation. References [1] Li J, Li X. Determination principal component content of seed oils by THz-TDS. Chem Phys Lett 2009;476:92–6.
J.-S. Li / Optics & Laser Technology 43 (2011) 102–105
[2] Kersting R, Strasser G, Unterrainer K. Terahertz phase modulator. Electron Lett 2000;36:1156–7. [3] Piesiewicz R, Ostmann TK, Krumbholz N, Mittleman D, Koch M. Short-range ultra-broadband terahertz communications: concept and perspectives. IEEE Antennas Propag Mag 2007;49(6):24–39. [4] Ostmann TK, Pierz K, Hein G, Dawson P, Koch M. Audio signal transmission over THz communication channel using semiconductor modulator. Electron Lett 2004;40(2):124–6. [5] Ostmann TK, Dawson P, Pierz K, Koch M. Room temperature operation of an electrically driven terahertz modulator. Appl Phys Lett 2004;84:3555.
105
[6] Chen HT, Padilla WJ, Zide JM, Gossard AC, Taylor AJ, Averitt RD. Active terahertz metamaterials devices. Nature 2006;444:597. ˇ emec H. Ultrafast opto-terahertz photonic crystal ˇ zel P, N [7] Fekete L, Kadlec F, Ku modulator. Opt Lett 2007;32:680. ˇ emec H, Ku ˇ zel P. Fast one-dimensional photonic crystal [8] Fekete L, Kadlec F, N modulators for the terahertz range. Opt Express 2007;15(14):88–98. [9] Li JS. Terahertz modulator using photonic crystals. Opt Commun 2007;269:98. [10] Padilla WJ, Taylor AJ, Highstrete C, Lee M, Averitt RD. Dynamical electric and magnetic metamaterial response at terahertz frequencies. Phys Rev Lett 2006;96:107401.