Experimental observation of frequency up-conversion by flash ionization

Abstract

We have demonstrated frequency up-conversion in the terahertz (THz) region by flash ionization, which occurs due a terahertz wave interacting with a plasma created by an ultrashort laser pulse. The upshift frequency is controlled by the plasma density. We performed a proof-of-principle experiment with a plasma creation time scale much shorter than the period of the electromagnetic wave and a plasma length longer than its wavelength. We upshifted the frequency from 0.35 to 3.5 THz by irradiating a ZnSe crystal with a Ti:sapphire laser pulse.

Reflection and transmission of electromagnetic waves at a temporal boundary

“…These results were first obtained in 1975 in the context of plasma [2] and have been used in several works since then [5,8,10,11]. Clearly, only one set of these coefficients can be right as Eqs.…”

“…In addition to the generation of a backward-propagating wave, it was predicted that the frequency of the incident wave shifts linearly with changes in the refractive index [1][2][3][4][5][6][7][8]. Several experiments have confirmed such frequency shifts [9][10][11]. Recently, it was discovered that frequency shifts also occur in the optical regime when light is transmitted through dynamic optical structures in the form of photonic crystals or microring resonators [12][13][14][15][16].…”

“…One set was derived by Morgenthaler in his 1958 paper using specific boundary conditions [1] and has been used in many other studies since then [3,4,6,9,17,18]. The other set was presented in 1975 using different boundary conditions in the context of plasma [2] and has also been used by others since then [5,8,10,11]. So far, there has been little discussion on the connection between these two different sets of reflection and transmission coefficients.…”

Propagation of Optical Pulses in Dynamic Media: A Time Transformation Method

“…In this case, the reflection and transmission coefficients derived in Refs. [10,14,15,26,29,82] are the correct ones. 79 …”

“…One set was derived by Morgenthaler in his 1958 paper using specific boundary conditions [7] and has been used in many other studies since then [11,12,18,44,46]. The other set was presented in 1975 using different boundary conditions in the context of plasma [10] and has also been used by others [14,15,26,29,82]. So far, there has been little discussion of the connection between these two different sets of reflection and transmission coefficients.…”