Optics & Laser Technology 43 (2011) 102–105

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Optics & Laser Technology

0030-39

doi:10.1

n Tel.:

E-m

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 i n f o

Article history:

Received 28 November 2009

Received in revised form

22 May 2010

Accepted 22 May 2010

Keywords:

Terahertz communications

Terahertz wave modulator

Metamaterial

92/$ - see front matter & 2010 Elsevier Ltd. A

016/j.optlastec.2010.05.011

+86 57186875673; fax: +86 57186875618.

ail address: forever-li@126.com

a b s t r a c t

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.

1. Introduction

The terahertz (THz) frequency range 0.1–10 THz, locatedmidway between microwaves and infrared light, presents a newfrontier containing numerous technical applications and funda-mental research problems. With the realization of the terahertzgenerator and detector, terahertz wave has attracted significantattention and has been extensively investigated. Due to theirspecial properties, within the past few years, many potentialapplications of terahertz waves have been dramatically exploredin many fields such as biomedical diagnostics, security screening,military detection, radio astronomy, atmospheric studies, highspeed communication, quality control of packaged goods, andmoisture analysis for agriculture [1,2]. With wide bandwidth andhigh data transmission bit rates, terahertz wave wireless com-munications have great potential in future short range wirelesscommunications [3]. As an important device in terahertz com-munication for signal processing, terahertz wave modulator hasbeen attracting significant attention. In 2004, Ostmann et al. [4,5]designed an electrically driven terahertz modulator. In 2006, Chenet al. [6] proposed a terahertz modulator based on metamaterial.In 2007, using a one dimensional photonic crystal with a GaAsdefect, Fekete et al. [7,8] demonstrated the possibility of ultrafastmodulation of THz radiation. This year, Li [9] analyzed a terahertzwave modulator using photonic crystals. However, they are stillrelatively undeveloped with only a few examples of cryogenicallycooled and room temperature modulators. Robust terahertz wavemodulators are still needed that can be easily implemented andintegrated into a chip-scale platform. Therefore, it is valuable to

ll rights reserved.

investigate the design of modulator in the terahertz range.Metamaterials consisting of periodically patterned split ringresonators (SRRs) display a strong resonant electromagneticresponse. Recently, it has been shown that the resonancestrength, and hence the transmission, can be controlled viaexternal stimulation for planar metamaterials fabricated onsemiconducting substrates [6,10]. This has resulted in efficientswitching and modulation of freely propagating THz radiation,which is essential for many potential applications such as secureshort-range communication. Modulation of THz radiation with anapplied voltage was recently demonstrated using a hybridstructure consisting of a Schottky diode and a planar metamater-ial array [10]. Owing to the large capacitance and seriesresistance, the device operates at a low modulation rate.

In this study, we have proposed and demonstrated a novelterahertz wave modulator using metamaterial. The terahertzwave modulation mechanism of the proposed terahertz wavemodulator is based on the resonance characteristic of metama-terial that is controlled using a modulated laser. Experimentalresults show that the presented terahertz wave modulator has amodulation speed of 0.1 Kb/s, a modulation depth of 57%, andsimplicity.

2. Device fabrication

The size of the metamaterial unit is optimized and obtainedwith CST Microwave Studio. The dimensions of the resonator unitare as follows: a¼40, b¼30, c¼2, and d¼4 mm. We transferredmetamaterial pattern (100�100 periods) to mask. A 210 nmthickness of gold was deposited on a 500-mm-thick intrinsicsilicon wafer (10 K O cm) using a radio frequency magnetron

J.-S. Li / Optics & Laser Technology 43 (2011) 102–105 103

sputtering method. About 1 mm AZ-601 photoresist was coated onthe wafer by a GKF-411 gluing purifying machine. To evaporatethe solvent and densify the AZ-601 film, the wafer was put inhotplate at 70 1C persistence 5 min, and was cooled for 5 min. Themask and the wafer were mounted onto the mask aligner. Thenthe AZ-601 film was exposed to URE-2000S deep ultraviolet

Fig. 1. Scanning electron microscope (SEM) image of (a) a square metamaterial

and (b) a square metamaterial array.

Fig. 2. The simulated electric field distribution for metamaterial in the x, y plane at

the corresponding resonances.

lithography exposure plane. The AZ-601 was baked on a hotplateat 70 1C persistence 5 min for acid-initiated, thermally drivenepoxy cross-linking. Then it was developed with an RZX-3038positive photoresists developer with agitation until the patternwas clear and then rinsed with deionized water. To evaporatedeionized water and densify the AZ-601 film, the wafer was put inhotplate at 70 1C persistence 5 min, followed by 120 1C oven for10 min, and was then cooled for 15 min. The wafer was etchedby MNL/DIII reactive ion etcher, rinsed with deionized waterand baked in a 120 1C oven for 10 min. Strip photoresist withORNIII5532 plasma stripping photoresist system. The wafer issawed to 1�1 cm2 samples by an HP-603 automatic precisiondicing saw. The sample is evaluated by a scanning electron micro-scope (SEM) and is shown in Fig.1. The electric field distributionfor metamaterial SRR in the x, y plane at the correspondingresonances was simulated by finite element method, as shown inFig. 2. The electric field is strongly concentrated at the split gapsat the resonant frequency.

3. Experimental results and discussion

Fig. 3 shows the experimental setup for backward-waveoscillator (BWO) used to test the proposed terahertz-wavemodulator. The backward-wave oscillator system is used as aterahertz wave source. As depicted, the metamaterial device isplaced at the focal point of the terahertz radiation. Both excitationand terahertz wave spot sizes and also the carrier versus terahertzspot size overlap were determined by sweeping the optical

Fig. 3. Sketch of the experiment setup.

Fig. 4. Measured terahertz wave transmittance characteristic of the metamaterial.

Fig. 5. Measured modulation characteristic of the proposed terahertz wave modulator: (a) applied signal and (b) detected signal.

J.-S. Li / Optics & Laser Technology 43 (2011) 102–105104

excitation. The silicon substrate has indirect bandgap energy(1.12 eV), and light with a wavelength less than 1.1 mm should beable to excite free carriers. Fig. 4 shows the terahertz wavetransmittance characteristic of metamaterial based on silicon atthe frequency from 0.23 to 0.36 THz with and without lightexcitation. Carrier injection into the silicon substrate is achievedby using a continuous wave semiconductor laser at 808 nm withincident optical intensities 100 mW. At this time, the meta-material is not in resonance and the terahertz wave attenuation islarger than that of without optical excitation. From the figure, onesees that the change of the terahertz wave transmission intensityat the frequency of 0.32 THz is larger than any other measuredterahertz wave frequency band. Therefore, in order to achieve thelargest modulation depth, the backward-wave oscillator is set tobe 0.32 THz and used as the continuous terahertz wave source.A pyroelectric detector was used to detect the modulatedterahertz wave signal. A modulated semiconductor laser at808 nm with incident optical intensity of 100 mW is employedto light the metamaterial based on intrinsic silicon. The resonanceof the metamaterial is controlled by the photo-excited carriers.Measured terahertz wave transmittance characteristic of thenovel terahertz wave modulator with the modulated laser isshown in Fig. 5. Applied square wave signal with 0.1 Kb/s for lasermodulation is shown in Fig. 5(a). Fig. 5(b) shows the detectedsignal from a terahertz wave pyroelectric detector with 0.1 Kb/s.Thereby, the modulation speed of the proposed modulator is0.1 Kb/s with a modulated semiconductor laser of 100 mW inexperiment. We achieved a modulation depth (DT/T) of up to 57%at a terahertz frequency of 0.32 THz. The modulation speed ofthe device is limited by the lifetime of photocarriers. It is wellknown that the lifetime of silicon photocarriers is about severalmilliseconds, and the proposed modulator can obtain the modula-tion speed of MHz. Because the response time is the limitationof the terahertz wave pyroelectric detector, we cannot measurethe MHz modulation speed of the modulator (experimentallylimited). Here, the signal modulation mechanism of the presentedterahertz wave modulator is based on the resonance

characteristic of metamaterial controlled without or with lightexcitation. That is to say, when the silicon substrate is illumi-nated by the pumping light, photocarriers are induced and theresonance characteristic of metamaterial is changed. Themodulation speed is limited by the lifetime of photocarriers.Further studies will be conducted with another semiconductorsubstrate (e.g. GaAs) having a short lifetime of photocarriers.

4. Conclusion

We experimentally demonstrated whether the designedoptically controlled terahertz wave modulator based on metama-terial is able to achieve terahertz wave signal modulation. Theterahertz wave modulator presented here is based on themodulated semiconductor laser excitation at 100 mW averagepower at 808 nm to illuminate the metamaterial, demonstratingterahertz wave transmission modulation that has a modulationspeed of 0.1 Kb/s. The modulation depth of the presentedterahertz wave modulator is more than 57% at a frequency of0.32 THz. The size of the modulator is about 4�4 mm2. Webelieve that the proposed terahertz wave modulator can be usefulfor future terahertz wave communication systems.

Acknowledgments

The author thanks Prof. Yao for valuable discussions duringthis study and Prof. Wen for his technical assistance. This researchwas partially supported by the National Natural Science Founda-tion of China (no. 60971027), the National Basic Research Programof China (2007CB310403), and China Postdoctoral ScienceFoundation.

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