Active terahertz beam steering by photo-generated graded index gratings in thin semiconductor films

Steinbusch, T. P.; Tyagi, H. K.; Schaafsma, Martijn C.; Georgiou, Giorgos; Rivas, Jaime Gómez

doi:10.1364/oe.22.026559

Cited by 39 publications

(20 citation statements)

References 33 publications

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“…Because the light-matter interaction lengths are subwavelength and the quality factors modest ( Q ≈ 1−10), very large refractive index modulation is needed (Δ n ≥ 1). InSb [31][32][33] or InAs [ 27,34,35 ] support free-carrier index shifts of this magnitude due to their high mobility and low electron effective mass. In our previous theory work, [ 36 ] we demonstrated full 2π tuning of the transmission phase of semiconductor metasurface by modulating the carrier concentration in InSb resonators between 10 17 -10 18 cm −3 .…”

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confidence: 99%

Electrically Reconfigurable Metasurfaces Using Heterojunction Resonators

Iyer

Pendharkar

Schuller

2016

Advanced Optical Materials

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paradigm in reconfi gurable metasurfacebased optics. Here, we use electromagnetics and device simulations to design electrically pumped semiconductor heterostructure resonators capable of arbitrarily tuning the refl ection phase of infrared light with little change in amplitude.Previous approaches for tuning metasurfaces-using metallic resonators coupled to an index tunable background [ 8,[23][24][25][26][27][28] or mechanically changing the shape of metallic resonators [ 5,29,30 ] have been unable to achieve reconfi gurable 2π phase shifts. This inability to achieve 2π phase control stems fundamentally from the nature of the underlying optical antennas. Because the light-matter interaction lengths are subwavelength and the quality factors modest ( Q ≈ 1−10), very large refractive index modulation is needed (Δ n ≥ 1). InSb [31][32][33] or InAs [ 27,34,35 ] support free-carrier index shifts of this magnitude due to their high mobility and low electron effective mass. In our previous theory work, [ 36 ] we demonstrated full 2π tuning of the transmission phase of semiconductor metasurface by modulating the carrier concentration in InSb resonators between 10 17 -10 18 cm −3 . However, we did not consider how to achieve such modulation. Here, we use combined electromagnetic [ 37 ] and device simulations to demonstrate subwavelength 1D heterojunction device resonators with electrically reconfi gurable refl ection phase. We fi rst demonstrate a new resonator geometry that allows for integration of metal electrodes with dielectric-type Mie resonances. Subsequently, we incorporate an InSb based heterojunction device layer into the resonator architecture. This novel device design employs electron and hole blocking layers to achieve large modulations of carrier concentration over electrically large dimensions, enabling near-2π tuning of refl ection phase with minimal changes in refl ection amplitude. Finally, we demonstrate fully continuous and tunable steering of high-quality narrow, diffraction lobes in resonator arrays. Results and DiscussionGiven the need for integrating top and bottom electrical contacts, our basic metasurface element is a modifi ed version of a 1D "strip" resonator sitting atop a refl ecting back electrode shown in

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confidence: 99%

Electrically Reconfigurable Metasurfaces Using Heterojunction Resonators

Iyer

Pendharkar

Schuller

2016

Advanced Optical Materials

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“…Several modulation methods based on the opticallyinduced physical processes in semiconductors are proposed so far 7,[13][14][15][16][17][18] . In particular, pure Si and GaAs crystals have been used for this purpose 15 .…”

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Impurity-induced modulation of terahertz waves in optically excited GaAs

et al. 2017

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The effect of the photoinduced absorption of terahertz (THz) radiation in a semi-insulating GaAs crystal is studied by the pulsed THz transmission spectroscopy. We found that a broad-band modulation of THz radiation may be induced by a low-power optical excitation in the spectral range of the impurity absorption band in GaAs. The measured modulation factor achieves 80%. The amplitude and frequency characteristics of the resulting THz modulator are critically dependent on the carrier density and relaxation dynamics in the conduction band of GaAs. In semi-insulating GaAs crystals, the carrier density created by the impurity excitation is controlled by the rate of their relaxation to the impurity centers. The relaxation rate and, consequently, the frequency characteristics of the modulator can be optimized by an appropriate choice of the impurities and their concentrations. The modulation parameters can be also controlled by the crystal temperature and by the power and photon energy of the optical excitation. These experiments pave the way to the low-power fast optically-controlled THz modulation, imaging, and beam steering.

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“…While a few spatial THz intensity modulators have already been proposed, a pure phase spatial modulation can greatly increase the immunity to noise, therefore is more robust and applicable in THz systems where the intensity of THz radiation is typically low. Steinbusch et al used the photogenerated graded index grating to actively steer the THz beam; the phase modulation was considered . However, the phase modulation is caused by the change of reflection index of the thin semiconductor film pumped by the visible light, which is quite small.…”

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confidence: 99%

Reconfigurable Terahertz Metasurface Pure Phase Holograms

Guo

Wang

Zhao

et al. 2019

Advanced Optical Materials

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of applications. Spatial light modulators, which can impose programmable modification of the spatial intensity and/or phase distribution of an optical beam, are commonly used in applications, such as optical information processing, imaging, and communications. However, in the THz frequency range, the development of spatial light modulators remains a great challenge due to the lack of electrooptical materials, although they are urgently requested with the ongoing advancement of THz technology and the identification of a variety of potential applications. [5] While a few spatial THz intensity modulators have already been proposed, [6][7][8][9] a pure phase spatial modulation can greatly increase the immunity to noise, therefore is more robust and applicable in THz systems where the intensity of THz radiation is typically low. Steinbusch et al. used the photogenerated graded index grating to actively steer the THz beam; the phase modulation was considered. [10] However, the phase modulation is caused by the change of reflection index of the thin semiconductor film pumped by the visible light, which is quite small. Other attempts used semiconductor quantum well structures at cryogenic temperatures and liquid crystals with low modulation speed. [11,12] More recently, metamaterials/ metasurfaces integrated with semiconductor and graphene have allowed more efficient modulation of THz phase. [13,14] However, the range of THz phase modulation needs further improvement, and one should also address the drawback of accompanied intensity modulation associated with the resonant response.Despite these limitations to overcome, the emergence of metasurfaces has provided an excellent opportunity to accomplish a variety of functional THz devices. [15][16][17] Metasurfaces are constructed from carefully designed discrete metallic or dielectric elements and have spatially varying characteristics across its surface, [18] enabling effective manipulation of the amplitude, phase, and polarization state of electromagnetic waves with continuously broadband achromatism for ultrathin planar lenses, [19,20] complex light field generation, [21] spin orbital angular momentum coupling, [22,23] and metasurface holograms. [24,25] Activating these functionalities will no doubt expand the application scope and create a greater societal impact of metasurfaces, which necessitates the integration of functional materials within the metasurface structures, including most widely used semiconductors, phase transition materials, and Terahertz (THz) radiation has many potential applications. However, comparing with the rapid development of THz sources and detectors, functional devices for THz modulation, especially the spatial modulation devices, are still insufficient. Here, a novel approach for generating arbitrary wavefronts of a THz beam is presented. By dynamically creating metasurface structures through illuminating a thin silicon wafer with femtosecond laser, which is spatially modulated, an array of reconfigurable subwavelength resonators is generate...

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Active terahertz beam steering by photo-generated graded index gratings in thin semiconductor films

Cited by 39 publications

References 33 publications

Electrically Reconfigurable Metasurfaces Using Heterojunction Resonators

Electrically Reconfigurable Metasurfaces Using Heterojunction Resonators

Impurity-induced modulation of terahertz waves in optically excited GaAs

Reconfigurable Terahertz Metasurface Pure Phase Holograms

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