Gap-size dependence of optical near fields in a variable nanoscale two-tip junction

Nanometer-sharp objects coupled with femtosecond laser pulses find much current interest because they allow us to reach high spatiotemporal control -- like a needle tip enables the scanning tunneling microscope (STM). With two needles facing each other, a special nanometric gap is formed, facilitating large optical near fields. Here, the authors experimentally retrieve the optical near fields of femtosecond laser pulses present in a nanogap as a function of the gap size, bearing direct ramifications for plasmonics and strong-field physics.