In this report, we theoretically show that intense multicycle terahertz (THz) pulses can be generated by optical rectification in an artificial periodically poled lithium niobate (PPLN) structure formed by placing a phase-shift mask in front of a large aperture stoichiometric lithium niobate (SLN) crystal. In contrast to the common scheme with a triangular prism-shaped congruent LN crystal, THz generation is studied for a rectangular trapezoid SLN sample having a small angle (≈ 26°) of the inclined surface. A matching Si-prism is attached to the trapezoid base to guide the generated THz wave into free space. It is shown that the number of field oscillations (from nearly single-cycle to many cycles) can be varied by the changing of the pump beam linear size in the crystal. Also, there is a possibility of tuning the generation frequency (in the range of 0.4 - 0.8 THz) by building a mask image in the SLN with various demagnification. According to estimates, the energy of narrowband THz pulses at a frequency of 0.5 THz in SLN crystal at temperature 100 K is 265 μJ at a pump pulse energy of 220 mJ. This corresponds to a pump-to-THz conversion efficiency of 0.12 %.
In this paper, we demonstrate significant enhancement of electro-optic (EO) sampling signal in the detection of pulsed terahertz (THz) waves by using a technique we call “polarization filtering”.
In the EO sampling of pulsed THz waves, a linearly polarized probe optical pulse is phase-modulated by THz electric field through the linear EO effect and, as the result, it becomes slightly elliptically polarized after passing through the EO crystal. The phase-retardation of the probe optical pulse is then detected as an optical intensity modulation (EO signal), dI/I (the ratio of the intensity change, dI, and the original intensity, I) with an appropriate optical detection system.
In “polarization filtering,” the EO sampling signal, dI/I, is enhanced by suppressing the main polarization component of the probe beam, resulting in a reduced probe beam intensity I’ = b^2*I, after the interaction with THz field in the EO crystal. Since the intensity modulation, dI, also reduces to dI’= b*dI, as the result of the polarization filtering, the THz EO sampling signal is enhanced by a factor of 1/b: dI’/I’ =(1/b)*dI/I. This “polarization filtering” is applicable not only to the conventional ellipsometric EO sampling but also to the heterodyne EO sampling. Firstly, we explain the principle of the polarization filtering, and then show the results of the proof-of-principle experiment for the standard and the heterodyne EO sampling, respectively.
We develop a theory of terahertz emission from a femtosecond laser pulse with tilted intensity front propagating through
a prism-shaped electro-optic crystal. The theory accounts for transient effects at the entrance boundary of the crystal and
allows us to explore the dynamics of terahertz generation in the crystal. In particular, transverse walk-off length is
introduced as an important parameter of the terahertz field formation process. Two typical experimental situations -
LiNbO3 excited with Ti:sapphire laser (0.8 μm wavelength) at room and cryogenic temperatures - are considered, and
new schemes, in which GaAs is excited at 1.8 and 3.5 μm, are proposed and analyzed. The parameters of the laser pulse
(transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield are calculated.
We propose to excite surface plasmon-polaritons using the moving spot of nonlinear polarization created by a laser pulse. Two perspective excitation schemes - with superluminal and subluminal spots - are considered and their efficiencies are compared. These techniques can be used for surface spectroscopy at terahertz frequencies.
The dynamics of electromagnetic fields in open waveguiding structures whose dielectric properties are changed in time due to rapid plasma creation is considered. The phenomenon of transient coupling between bulk and surface waves is found. The applicability of this phenomenon for input/output of electromagnetic energy is discussed for simple waveguiding structures with both semiconductor and gaseous plasmas. In particular, a component for transient input of an electromagnetic wave into a planar dielectric waveguide covered with a nonstationary semiconductor film is proposed. Using the example of standing waves we show that the transient dynamics is sensitive to the values of the electromagnetic fields at the moment of the plasma density growth. The knowledge of the transient dynamics in rapidly ionized material can be applied for the control of guided modes in open waveguiding structures of integrated optics and (sub)millimeter wave electronics, rapid manipulation by coupling between electromagnetic radiation and guided modes, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
The transformation of a circularly polarized electromagnetic wave in a magnetoplasma medium with increasing plasma density is considered. The wave propagates along the static magnetic field. Complete analysis, including ion motion, is given both for slow (compared to the wave frequency) and rapid ionization rate. In the case of slow temporal variation of the plasma density, a relation between the energy of the wave and its frequency, which is conserved during the plasma creation process (adiabatic invariant), is found. The existence of significant energy losses follows from the invariant. The dissipative mechanism is explained via consideration of the case of a sudden growth of plasma density in time from one value to another. It is shown that energy transforms into the kinetic energy of carriers and preionization of the medium plays a principal role in the dissipation process. In the special case of a whistler wave, up to 50% of the energy may be transformed into ion-cyclotron wave when dense plasma is rapidly created.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.