Progressive carrier-envelope phase control of THz pulses via near-field reflection in photoconductive antennas

The carrier-envelope phase (CEP) of terahertz (THz) pulses is a critical parameter in ultrafast THz science, as it directly governs the temporal waveform and peak field strength of the pulse-quantities that are particularly important in THz scanning tunneling microscopy, where the pulse shape determines the transient potential at the nanotip, and in nonlinear THz experiments that require reproducible peak fields. Here, we report the demonstration of progressive CEP control achieved through pure near-field reflection by simply tuning the electrode width of a photoconductive antenna (PCA), without the need for complex metasurfaces or additional optical components in the beam path. By varying the electrode width, a frequency-independent universal phase shift is induced through near-field reflection of THz waves from the electrode surface, enabling a progressive transformation of the THz waveform from half-cycle (symmetric) to single-cycle (antisymmetric). Fourier analysis confirms that the relative phase between carrier waves is tunable up to 0.3π and remains consistent across the 0.2-2 THz frequency range. Crucially, both half-cycle and single-cycle pulses maintain nearly identical peak amplitudes, overcoming a key limitation of prior CEP control methods. The near-field reflection origin of the phase shift is corroborated by finite-difference time-domain (FDTD) simulations and independently confirmed by a control experiment using metallic reflectors physically separated from the active electrode gap, which reproduces the same waveform evolution. Our approach requires no complex fabrication beyond standard photolithography, making it directly compatible with existing THz time-domain spectroscopy systems and offering a practical route toward waveform-tailored THz sources for ultrafast spectroscopy, THz-STM, and nonlinear THz studies.