N. Amer

Generation of terahertz pulses with arbitrary elliptical polarization

We employ two different methods to generate controllable elliptical polarization of teraherz (THz) pulses. First, THz pulses are generated via optical rectification in nonlinear crystals using a pair of temporally separated and perpendicularly polarized optical pulses. The THz ellipticity is controlled by adjusting the relative time delay and polarization of the two optical pulses. We generate mixed polarization states of single-cycle THz pulses using ZnTe, and elliptically polarized multicycle THz pulses in periodically poled lithium niobate crystals. Second, we generate elliptically polarized THz pulses by making a THz “wave plate” using a combination of a wire-grid THz polarizer and a mirror to transform linearly polarized multicycle THz pulses into elliptical polarization.

Generation of arbitrary terahertz wave forms in fanned-out periodically poled lithium niobate

The authors demonstrate a flexible terahertz pulse-shaping technique, manipulating spatially dispersed multifrequency components generated by optical rectification in a fanned-out periodically poled lithium niobate crystal. Spatial masks of low pass, high pass, and double slit in front of the crystal manipulate the spatial pattern of the optical excitation beam on the crystal, which is mapped onto the intensity profile of the terahertz spectrum. The spatial dispersion of the terahertz spectrum is removed by the line-to-point imaging of a spherical mirror.

Terahertz wave propagation in one-dimensional periodic dielectrics

We demonstrate the temporal evolution of terahertz (THz) wave propagation in one-dimensional periodic dielectrics. Distributed Bragg reflectors and a resonant cavity are investigated: The structures involve air gaps interleaved between polymer films. Transmitted and reflected broadband THz waves are measured by means of THz time-domain spectroscopy. The experimental results agree well with transfer matrix calculations.

Multi-Cycle THz Pulse Generation and Manipulation in Poled Lithium Niobate

We have developed novel schemes to generate and manipulate multi-cycle THz pulses via optical rectification in orientation inverted nonlinear crystals such as poled lithium niobate. Exploiting the methods, we have demonstrated generation of tunable narrow-band THz pulses, adaptive THz pulse shaping, and arbitrary control of THz ellipticity.

Arbitrary terahertz pulse shaping via optical rectification in fanned-out periodically-poled lithium niobate

We demonstrate a novel terahertz (THz) pulse shaping technique, which guarantees ultimate flexibility for arbitrary THz pulse generation. The THz pulse shaper consists of a fanned-out periodically-poled lithium niobate (FO-PPLN) crystal-the domain width of the FO-PPLN crystal varies continuously across the lateral direction-, a spatial mask, and a spherical mirror. Optical pulses are line-focused on the FO-PPLN crystal to generate spatially separated multi-frequency components of THz pulses. The spatial mask is placed in front of the FO-PPLN crystal in order to manipulate the spatial pattern of the incident optical beam, thus to control the amplitudes of the spatially dispersed THz frequency components. Spectral resolution of this method is determined by FO-PPLN bandwidth and mask resolution: estimated practical resolution is ≈0.01 THz for 1 THz bandwidth. After the spherical mirror assembles the various frequencies into a single collimated beam, a shaped THz pulse can be obtained, with the pulse shape determined by the Fourier transform of the pattern transferred by the mask. As a proof-of-principle experiment, we measured THz waveforms using metal masks. The experiment was performed using 800-nm, 100-fs pulses from a 1-kHz Ti:sapphire regenerative amplifier. We used a 5-mm long FO-PPLN sample (width = 10 mm, height = 0.5 mm) continuously tunable from 0.6 to 1.5 THz. We tested the metal masks of three different spatial patterns: low-pass filter, high-pass filter, and double slit. The experimental results show that the THz waveforms are determined by the spatial patterns of the masks.

Generation of arbitrary THz waveforms via optical rectification in fanned-out periodically-poled lithium niobate

We demonstrate THz-pulse shaping of ultimate flexibility, manipulating spatially dispersed multi-frequency components generated by optical rectification in a fanned-out PPLN. The modulated THz-spectrum maps out the spatial pattern of optical excitation beam on the crystal.

THz pulse shaping and ellipticity control via optical rectification in nonlinear optical crystals

Pulse-shape and ellipticity of continuously tunable multi-cycle THz pulses are controlled by adjusting the relative time delay, intensity, and polarization between two femtosecond pulses. A THz wave plate using wire-grid polarizer also converts THz ellipticity.

Generation and manipulation of multi-cycle terahertz pulses via optical rectification in poled lithium niobate

We demonstrate control of terahertz (THz) waves developing novel devices in the THz regime: THz pulse shapers. THz technology is a relatively unexplored subject, yet the importance of THz wave manipulation cannot be emphasized enough considering its potential application to THz imaging systems, ultrafast optical signal processing, ultrahigh-speed computing, quantum information science, nanotechnology, and chemical reaction dynamics among other areas. THz time-domain spectroscopy (THz-TDS) can assess the performance of the THz pulse shapers monitoring time-dependent THz wave propagation. THz-TDS permits precise measurements not only of the amplitude but also of the phase of THz waves, thus a comprehensive assessment of the THz devices can be achieved. The phase sensitivity is also vital to many applications such as high-contrast THz imaging and quantum control of semiconductor nanostructures. We develop arbitrary THz pulse generators synthesizing THz waveforms via optical rectification in pre-engineered domain structures of poled nonlinear crystals using femtosecond lasers. The terahertz waveforms coincide with the crystal domain structures. The one dimensional nonlinear wave equation simulates the experimental results with a good qualitative agreement. The ratio of the domain length to the optical pulse length in the crystal turns out to be the crucial limiting factor to generating optimum terahertz fields and preventing waveform distortion. Optical pulse shaping techniques is integrated into the THz pulse generators to extend the scope of THz pulse shaping control. Continuously tunable narrow-band THz pulses are generated in a fanned-out periodically-poled lithium niobate crystal. We measure the free induction decay of rotational transitions in gas-phase HCl molecules using the narrow-band THz pulses. The shape of the multi-cycle THz pulses is controlled by adjusting the relative time delay and intensity between the two optical pulses.

Terahertz pulse shaping via optical rectification in poled lithium niobate

We demonstrate a novel technique for terahertz pulse shaping via optical rectification in the pre-engineered domain structure of poled lithium niobate crystals. The terahertz waveforms map out the corresponding crystal domain structures.