Ultrashort terahertz (THz) pulses are a powerful tool for both probing and controlling novel phenomena in quantum materials. This is particularly useful in Dirac materials, since these materials exhibit novel magnetic and lattice excitations that can potentially be used to control their properties. Here, the use of ultrashort THz pulses to reveal the circular photogalvanic effect in the Weyl semimetal TaAs and probe magnetoplasmon modes in graphene nanoribbons will be examined.
We performed THz emission spectroscopy on the (112) and (001) surfaces of the Weyl semimetal TaAs. Our data enables us to clearly distinguish between helicity-dependent photocurrents generated within the ab-plane and polarization-independent photocurrents flowing along the non-centrosymmetric c-axis. Such findings are in excellent agreement with previous static photocurrent measurements. However, by considering both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay, we can attribute these transient photocurrents to the underlying crystal symmetry of these materials.
We also used terahertz (THz) magneto-optical spectroscopy to demonstrate how a periodic array of graphene micro-ribbons can be used to control the transmission spectrum and polarization state of a THz pulse whose electric field is oriented along the pattern’s axis of periodicity (perpendicular to the long axis of the ribbons). Our results demonstrate that graphene micro-ribbon arrays are a powerful system for controlling the coupling between light and magnetoplasmonic modes. This enables the tailoring of THz transmission profiles and polarization states using applied magnetic fields.
We investigate polarization-dependent ultrafast photocurrents in theWeyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the non-centrosymmetric c-axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab-plane. Such findings are consistent with earlier static photocurrent experiments, and demonstrate on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry. Such generality in the microscopic origin of photocurrent generation in the transition metal monopnictide family of Weyl semimetals makes these materials promising candidates as next generation sources or detectors in the mid-IR and terahertz frequency ranges.
Nonlinear terahertz (THz) spectroscopy gives insight into high-field charge transport in semiconductors. Strong
THz transients with field amplitudes of up to megavolts/cm serve as a driving field for free carriers and the
resulting transport behavior is directly inferred from the field radiated by the moving charges. We study the
transition from a ballistic to a drift-like transport regime of electrons in bulk GaAs. While electrons in the lowest
conduction band of an n-type sample display ballistic transport, a transition to a drift-like behavior is found
in an optically generated electron-hole plasma. Time-resolved measurements reveal the onset of friction on a
time scale of a few picoseconds, mainly due to interactions of electrons with the hole distribution heated by the
intense THz driving field. Experiments in which photoexcited electrons undergo intervalley scattering from the
Γ to the L valley reveal characteristic changes of the transport behavior due to the picosecond backscattering
to the Γ valley. The experimental results are in agreement with theoretical calculations of the time-dependent
friction including both electron-hole scattering and local-field effects.
We study the diffraction of Gaussian pulses and beams within the framework of boundary diffraction wave theory. For the first time the boundary diffraction wave theory is applied to pulsed Gaussian beams, and it is shown that the diffracted field of a pulsed Gaussian beam on a circularly symmetric aperture can be evaluated by a single 1D integration along the diffractive aperture of every point of interest. We compare the theoretical simulations to experimental measurements of ultrashort pulses diffracted off a circular aperture, an opaque disc, an annular aperture, and a system of four concentric annular apertures. Using the recently developed SEA TADPOLE measurement technique, we obtain micron spatial and femtosecond temporal resolutions in the spatio-temporal measurements of the diffracted fields.
It is relatively straightforward to completely measure both long (>10ns) and very short (<100ps)
laser pulses in time. But intermediate pulse lengths-that of the most common laser pulses-
remain nearly immeasurable and, not coincidentally, correspond to the least stable of all lasers.
True, ultrahigh-bandwidth oscilloscopes and streak cameras can now resolve such pulses, but such
exotic electronic devices are expensive and fragile and only yield the temporal intensity and not
the temporal phase. Here we describe a simple, elegant, accurate, complete, compact, all-optical,
entirely passive, and single-shot FROG device that solves the problem. It simultaneously achieves
a very large delay range of ~10ns and very high spectral resolution of <1pm. It accomplishes both
feats using high-efficiency, high-finesse etalons, the first to tilt the pulse by 8⪅9, .t9hat is, by
several meters over a centimeter beam, and another to generate massive angular dispersion for a
high-resolution spectrometer. We demonstrate this device for measuring pulses 100ps to several
ns long from a fiber-amplified micro-disk laser.
We measure the complete electric field of extremely complex ultrafast waveforms using the simple linear-optical,
interferometric pulse-measurement technique, MUD TADPOLE. In its scanning variation, we measured waveforms with
time-bandwidth products exceeding 65,000 with ~40 fs temporal resolution over a temporal range of ~3.5ns. In the
single-shot variation we measured complex waveforms time-bandwidth products exceeding 65,000. The approach is
general and could allow the measurement of arbitrary optical waveforms.
We demonstrate a powerful and practical spectral interferometer with near-field scanning microscopy (NSOM) probes
for measuring the spatiotemporal electric field of tightly focused ultrashort pulses with sub-micron spatial resolution and
high spectral resolution. To make these measurements we use SEA TADPOLE which is a high spectral resolution,
experimentally simplified version of spectral interferometry that uses fiber optics to introduce the pulse into the device.
To measure the spatiotemporal field of focusing pulses, the entrance fiber is scanned around the focus and a
measurement at each fiber position is made, so that E(ω) is found at many positions along the beam's cross section so
that we can reconstruct E(x,y,z,ω). The make these measurements we require that the fiber's mode size be smaller than
the focused spot size. In the past using optical fibers we were limited to measuring foci with NA's less than 0.1, and
here by replacing the fiber with an NSOM fiber, we measure the spatiotemporal field of focused pulses with NAs as high
as 0.44. To demonstrate this technique we measured pulses that were focused with two different microscope objectives
to verify that we achieved the expected results. We also measured foci that had severe distortions, such as the Bessel-like
X-shaped pulse caused by spherical aberrations and the "fore-runner pulse" due to chromatic aberrations and we
verified these results with non-paraxial simulations. In our measurements we observed spatial features smaller than 1μm.
We introduce a simple, compact, and automatically distortion-free single-grism pulse compressor that can compensate
for large amounts of material dispersion in ultrashort pulses, which increases the pulse duration and decreases the peak
intensity. Diffraction-grating pulse compressors can compensate for high dispersion, but they do not compensate for
higher-order dispersion (important when GDD is large). Worse, all previous general-purpose grating designs have
involved multiple gratings and so are also difficult to align and prone to distortions: small misalignments cause
unwanted spatio-temporal pulse distortions. A compressor based on grisms solves the higher-order-dispersion problem
because grisms allow the ratio of third-order to second-order dispersion to be tuned to match that of the material that
introduced the GDD. A grism can also compensate for large amounts of dispersion. Unfortunately, previous grism
compressors used multiple grisms and so are difficult to align and prone to spatio-temporal distortions. To overcome this
problem, we introduce a single-grism compressor. It comprises only three elements: a reflection grism, a corner cube,
and a roof mirror. SEA TADPOLE measured the compressor GDD and third-order dispersion, verifying its operation.
This convenient device should be a valuable general tool.
We describe two techniques for measuring the complete spatio-temporal intensity and phase, E(x,y,z,t), of an ultrashort
pulse. The first technique is an experimentally simple and high-spectral resolution version of spectral interferometry,
which uses fiber optics to introduce the pulse that is to be characterized into the device. By scanning the fiber around the
focus, this device can be used to measure the spatio-temporal field of a focusing ultrashort pulse. We illustrate this technique by measuring the spatio-temporal filed for several different focused pulses. The other technique measures the complete spatio-temporal field of a pulse using a very simple experimental setup. While this technique will not work at the focus, it is single shot and requires only a single camera frame to reconstruct the complete filed versus space and time. This technique involves measuring multiple holograms, each at a different wavelength, and all in a single camera frame. To test this technique we show that it can accurately measure the spectral phase. We also illustrate this technique by measuring, E(x,y,t) of a single laser pulse.
We present two complementary interferometric techniques for measuring the complete spatio-temporal intensity and
phase, E(x,y,z,t), of ultrashort pulses. The first technique, called SEA TADPOLE, allows for the first time the complete
measurement of pulses near a focus, while the second technique, called STRIPED FISH, allows the complete
measurement of mostly collimated pulses, but on a single-shot basis.
We describe two novel, practical ultrashort laser pulse measurement devices, which are also experimentally very simple. The first one is an "ultra-broadband" pulse characterization device that is based on FROG, but uses transient grating (TG) process. TG FROG involves forming an induced grating in a piece of glass by crossing two pulses in space and time and then diffracting a third pulse off it to create a fourth diffracted pulse. The TG process is inherently very broadband and automatically phasematched. We have implemented an ultrasimple TG FROG device, which can also operate single-shot. First, three beams are created using a simple mask. Then, a cylindrical beams line-focuses the beams horizontally, where the induced grating is generated. The variation of the relative delay is achieved by crossing the two grating-creation beams at an angle using a Fresnel biprism. Then, by detecting the diffracted pulse with spatial resolution, the TG FROG trace is captured. The second device that we present aims to measure ultrashort pulses with complex spectral and temporal structure. Spectral interferometry (SI) works perfectly for this purpose. SI simply involves measuring the spectrum of the sum of the unknown (shaped) and known (reference) light waves. Unfortunately, SI is very difficult to align and maintain aligned, as it requires that the two beams be nearly perfectly collinear. We solved this problem by utilizing optical fibers. Spectral resolution is also significantly improved by using spatial fringes, avoiding time-domain filtering.
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