Abstract—A terahertz Smith-Purcell backward-wave oscillator (BWO) with an inverse wet-etched grating based on silicon has been studied to enhance radiation intensity. Particle-in-cell simulation shows that the radiated power is increased up to 2.3 times higher at the radiated frequency of 0.66 THz for an electron-beam energy of 30 keV.

I. INTRODUCTION ITH the advent of micro-electro-mechanical system (MEMS) technology and high-current-density cathodes,

vacuum electron devices (VEDs) have been investigated as efficient terahertz (THz) sources. Among the VEDs, Smith-Purcell (SP) devices that radiate from electrons propagating along the grating surface are drawing significant attention as compact, tunable, and low-cost THz sources [1]. In this paper, a terahertz SP-BWO with an inverse wet-etched grating based on silicon is studied to enhance the radiation intensity [2].

II. RESULTS A schematic of the SP-BWO with a rectangular grating and

the inverse wet-etched grating modeled in the 2D MAGIC PIC code is shown in Fig. 1, where t is the beam thickness, g is the beam-grating distance, and L is the grating period. For the rectangular grating, h is the groove depth and p is the groove width. For the inverse wet-etched grating, s is the groove depth, d is the groove width at the top of the grating, and a is the wet-etched angle. The inverse wet-etched grating has an inversely triangular shape to enhance the intensity of the evanescent wave used for the interaction with the electron beam. The grating is modeled with a wet-etched angle of 54.7° based on silicon as shown in Fig. 1 (b). The wet chemical etching of silicon is characterized by the anisotropic etching, and the wet-etched profile in the 100-crystal plane is indicated by the sloped sidewall with an angle of 54.7°. The backward-wave oscillation occurs at the intersection point in the dispersion relation between the electron beam and the evanescent wave at 0.335 THz. Furthermore, the second harmonics of the evanescent wave with a frequency of 0.67 THz are propagated from the grating into the upper boundary.

The radiated power for beam currents from 300 A/m to 800 A/m is shown in Fig. 2 for an electron-beam energy of 30 keV and a grating with 35 periods. The radiated power is integrated over the roof for θ less than 90° at 6 mm from the top of the grating surface and is filtered by a frequency of 0.66 THz. In the case of a beam current of 500 A/m, the radiated power is 7 x 103 W/m and 20 x 103 W/m for the rectangular grating and the inverse wet-etched grating, respectively, and the enhancement

of the radiated power is more than two times higher than the conventional rectangular grating because of the reinforcement of the interactions between the electron beam and the evanescent wave due to the adjacent surface structure between the gratings.

Fig. 1. Schematic of the SP-BWO with (a) the rectangular grating and (b) the inverse wet-etched grating.

Fig. 2. Radiated power using the beam current from 300 A/m to 800 A/m when the electron beam energy is 30 keV.

III. CONCLUSIONS Radiated power is increased approximately twofold in THz

SP-BWO with an inverse wet-etched grating based on silicon by the enhanced evanescent wave.

REFERENCES [1] C. Prokop, P. Piot, M. C. Lin, and P. Stoltz, “Numerical modeling of a

table-tap tunable Smith-Purcell terahertz free-electron laser operating in the super-radiant regime”, Appl. Phys. Lett., 96, 2010, pp. 151502.

[2] Jung-Il Kim, Seok-Gy Jeon, Geun-Ju Kim, and Jaehong Kim, “Enhancement of terahertz radiation in a Smith-Purcell backward-wave oscillator by an inverse wet-etched grating”, Phys. Lett. A, 375, 2011, pp. 589-592.

Jung-Il Kim, Seok-Gy Jeon, Geun-Ju Kim, and Jaehong Kim Center for Pioneering Medical-Physics Research (CPMR),

Korea Electrotechnology Research Institute, Ansan 426-170, Korea

Enhanced Terahertz Radiation in a Smith-Purcell Backward-Wave Oscillator by an Inverse Wet-Etched Grating

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Abstract—A terahertz Smith-Purcell backward-wave oscillator (BWO) with an inverse wet-etched grating based on silicon has been studied to enhance radiation intensity. Particle-in-cell simulation shows that the radiated power is increased up to 2.3 times higher at the radiated frequency of 0.66 THz for an electron-beam energy of 30 keV.

I. INTRODUCTION ITH the advent of micro-electro-mechanical system (MEMS) technology and high-current-density cathodes,

vacuum electron devices (VEDs) have been investigated as efficient terahertz (THz) sources. Among the VEDs, Smith-Purcell (SP) devices that radiate from electrons propagating along the grating surface are drawing significant attention as compact, tunable, and low-cost THz sources [1]. In this paper, a terahertz SP-BWO with an inverse wet-etched grating based on silicon is studied to enhance the radiation intensity [2].

II. RESULTS A schematic of the SP-BWO with a rectangular grating and

the inverse wet-etched grating modeled in the 2D MAGIC PIC code is shown in Fig. 1, where t is the beam thickness, g is the beam-grating distance, and L is the grating period. For the rectangular grating, h is the groove depth and p is the groove width. For the inverse wet-etched grating, s is the groove depth, d is the groove width at the top of the grating, and a is the wet-etched angle. The inverse wet-etched grating has an inversely triangular shape to enhance the intensity of the evanescent wave used for the interaction with the electron beam. The grating is modeled with a wet-etched angle of 54.7° based on silicon as shown in Fig. 1 (b). The wet chemical etching of silicon is characterized by the anisotropic etching, and the wet-etched profile in the 100-crystal plane is indicated by the sloped sidewall with an angle of 54.7°. The backward-wave oscillation occurs at the intersection point in the dispersion relation between the electron beam and the evanescent wave at 0.335 THz. Furthermore, the second harmonics of the evanescent wave with a frequency of 0.67 THz are propagated from the grating into the upper boundary.

The radiated power for beam currents from 300 A/m to 800 A/m is shown in Fig. 2 for an electron-beam energy of 30 keV and a grating with 35 periods. The radiated power is integrated over the roof for θ less than 90° at 6 mm from the top of the grating surface and is filtered by a frequency of 0.66 THz. In the case of a beam current of 500 A/m, the radiated power is 7 x 103 W/m and 20 x 103 W/m for the rectangular grating and the inverse wet-etched grating, respectively, and the enhancement

of the radiated power is more than two times higher than the conventional rectangular grating because of the reinforcement of the interactions between the electron beam and the evanescent wave due to the adjacent surface structure between the gratings.

Fig. 1. Schematic of the SP-BWO with (a) the rectangular grating and (b) the inverse wet-etched grating.

Fig. 2. Radiated power using the beam current from 300 A/m to 800 A/m when the electron beam energy is 30 keV.

III. CONCLUSIONS Radiated power is increased approximately twofold in THz

SP-BWO with an inverse wet-etched grating based on silicon by the enhanced evanescent wave.

REFERENCES [1] C. Prokop, P. Piot, M. C. Lin, and P. Stoltz, “Numerical modeling of a

table-tap tunable Smith-Purcell terahertz free-electron laser operating in the super-radiant regime”, Appl. Phys. Lett., 96, 2010, pp. 151502.

[2] Jung-Il Kim, Seok-Gy Jeon, Geun-Ju Kim, and Jaehong Kim, “Enhancement of terahertz radiation in a Smith-Purcell backward-wave oscillator by an inverse wet-etched grating”, Phys. Lett. A, 375, 2011, pp. 589-592.

Jung-Il Kim, Seok-Gy Jeon, Geun-Ju Kim, and Jaehong Kim Center for Pioneering Medical-Physics Research (CPMR),

Korea Electrotechnology Research Institute, Ansan 426-170, Korea

Enhanced Terahertz Radiation in a Smith-Purcell Backward-Wave Oscillator by an Inverse Wet-Etched Grating

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