密度調製的電子束可用於產生在其調製頻率上的同調性自發輻射。本次實驗使用了多色雷射產生近兆赫茲的拍頻。利用此光源產生在keV的電子腔內產生光電流。利用調製過的電子束來驅動Smith-Purcell 自由電子雷射來產生兆赫茲同調性輻射。首先，將於Nd:YVO4雷射腔內放置一塊薄的etalon用來產生雙色的紅外光雷射。雙色雷射的頻率差為124.6 GHz。利用非線性晶體產生雙色雷射的四倍頻UV光，估計此UV光的頻率差也為12.4 GHz。利用此UV光照射於光陰極電子腔內，可產生頻率調製的光電流。取其中一個設計為例：將光源照射在一個可以產生10~25 keV的光陰極電子槍內，產生出近毫安培的電子從上方傳遞過一個長30 mm、週期240 um的光柵，用此產生Smith-Purcell輻射。我們使用電腦模擬軟體Magic在模擬中證實在與光柵夾角56度的方向產生了124.6 GHz的第三階輻射。其結果可清晰的在模擬結果中觀察到同調性的Smith-Purcell輻射。

A density-modulated electron beam can generate coherent spontaneous radiation at the harmonics of the modulating frequency. This study employs a multicolor laser beating at sub-THz frequencies to modulate the photocurrent from a keV electron gun. The modulated electron beam drives a Smith–Purcell FEL to generate THz coherent radiation. We first built an infrared two-color laser containing a thin etalon in an Nd:YVO4 laser cavity to generate two laser spectral components separated by 124.6 GHz. Quadrupling the laser frequency in nonlinear optical materials generates a UV frequency with adjacent components separated by 124.6 GHz. This UV laser is useful to modulate the photocurrent from a photoinjector at the comb frequencies. As a design example, the photocathode of a 10–25 keV DC electron gun is illuminated with this multicolor laser and the keV electron beam is propagated above a 30 mm-long Smith–Purcell grating with a groove period of 240 um. With 18.2 keV beam energy, our simulation using the MAGIC code confirms coherent radiation at the third harmonic of the 124.6 GHz at a 56-degree radiation angle from the Smith–Purcell grating. Coherent radiation can be observed in simulation with a sub-mA beam current.

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