MXenes are 2D transition metal carbides and nitrides with electronic properties that can be tuned by their chemistry and structure. Three members of MXene family, Ti3C2Tz , Mo2Ti2C3Tz and Mo2TiC2Tz are all intrinsically metallic, with high intrinsic free carrier densities and high carrier mobility within individual nanosheets. However, they respond to photoexcitation in dramatically different ways: while photoexcitation suppresses conductivity in Ti3C2Tz, it results in a long-lived positive photoconductivity in both Mo2Ti2C3Tz and Mo2TiC2Tz. Those responses suggest applications of MXenes in a variety of electro-optical and THz devices.
The growing experimental evidence suggests that broadband, picosecond-duration THz pulses may influence biological systems and functions. While the mechanisms by which THz pulse-induced biological effects are not yet known, experiments using in vitro cell cultures, tissue models, as well as recent in vivo studies have demonstrated that THz pulses can elicit cellular and molecular changes in exposed cells and tissues in the absence of thermal effects. Recently, we demonstrated that intense, picosecond THz pulses induce phosphorylation of H2AX, indicative of DNA damage, and at the same time activate DNA damage response in human skin tissues. We also find that intense THz pulses have a profound impact on global gene expression in human skin. Many of the affected genes have important functions in epidermal differentiation and have been implicated in skin cancer and inflammatory skin conditions. The observed THzinduced changes in expression of these genes are in many cases opposite to disease-related changes, suggesting possible therapeutic applications of intense THz pulses.
We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can
image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient
conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage
transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage
(I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse
induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The
nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.
We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales [1]. In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.
Pulsed terahertz (THz) imaging has been suggested as a novel high resolution, noninvasive medical diagnostic tool.
However, little is known about the influence of pulsed THz radiation on human tissue, i.e., its genotoxicity and effects on
cell activity and cell integrity. We have carried out a comprehensive investigation of the biological effects of THz
radiation on human skin tissue using a high power THz pulse source and an in vivo full-thickness human skin tissue
model. We have observed that exposure to intense THz pulses causes DNA damage and changes in the global gene
expression profile in the exposed skin tissue. Several of the affected genes are known to play major roles in human
cancer. While the changes in the expression levels of some of them suggest possible oncogenic effects of pulsed THz
radiation, changes in the expression of the other cancer-related genes might have a protective influence. This study may
serve as a roadmap for future investigations aimed at elucidating the exact roles that all the affected genes play in skin
carcinogenesis and in response to pulsed THz radiation.
Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using
intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to
explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such
interactions is found to be intervalley scattering.
We studied the wavelength-, time-, and intensity-dependence of the 3rd-order nonlinear optical response of As2Se3 chalcogenide glass. Bulk samples were characterized using a wavelength-tunable z-scan system, over the range 1200-1600 nm. Thin film samples were characterized using an ultrafast time-resolved differential optical Kerr effect (DOKE) experiment, fed by 125 fs pulses centered at 1425 nm. The z-scans revealed only slight variation in the optical Kerr coefficient n2 over the wavelength range studied. The DOKE experiment confirmed that the nonlinear response is predominately electronic, with response time limited by the experimental setup. For the same beam intensity, DOKE and z-scan measurements were in good agreement. The optical Kerr coefficient extracted from DOKE measurements at varying pump beam intensity showed intensity-dependent behavior, which can be attributed to fifth and higher order nonlinearities.
Differential optical Kerr effect (DOKE) detection is a powerful tool for studying the ultrafast time-resolved dynamics of 3rd-order nonlinear processes. In this study, DOKE was used to measure the nonlinear absorption properties of tetraethynylphenylene (TEP) solutions in THF using 800 nm, 80 fs laser pulses. These two-dimensional, highly-conjugated chromophores (also known as TPEBs) show high instantaneous two-photon absorption (TPA) for relatively small chromophores. The TPA cross section is strongly dependent on the donor-acceptor geometry in these materials: a quadrupolar, all-donor TEP shows the smallest TPA, with a cross-section of σ(2)= 90 ± 15 GM. ortho-TEP, for which the donors (and acceptors) are conjugated via the ortho position across the central phenyl ring, is dipolar and displays the largest cross-section, of σ(2)= 260 ± 30 GM. para-TEP, which is quadrupolar, and meta-TEP, which is dipolar, display similar cross-sections of σ(2)= 160 ± 10 GM and σ(2)= 150 ± 10 GM, respectively. In addition to an instantaneous TPA response, these isomers show unique two-photon assisted excited-state absorption (ESA), with the ortho- and meta-TEP displaying a clear 3-10 ps rise to an ESA peak, and subsequent decay. The differences in the nonlinear absorption behaviour of these materials may be partially explained by selection rules and UV-vis spectroscopy. In addition, the polar geometries, coupled to the various in-plane conjugation paths, may further influence their optical nonlinearities. Understanding these trends impacts both the design of materials with desirable nonlinear absorption properties and our understanding of the electronic landscape in functionalized organic materials.
Carbyne is a hypothetical carbon allotrope that consists of sp-hybridized carbon atoms in an infinitely-long, one-dimensional (1-D) linear chain. Polyynes, the oligomeric cousins of carbyne, with a dense delocalized-electron framework, could offer groundbreaking electronic properties. We have studied the linear and third-order nonlinear optical properties of both triisopropylsilyl end-capped (TIPS-PY) and phenyl end-capped (p-PY) polyynes containing pure sp-hybridized carbon backbones. Analysis of the TIPS-polyyne UV-vis absorption spectra shows that the absorption gap, Eg, in these materials scales very precisely as a power-law with increasing oligomer length, n, with Eg~n-0.379±0.002. The phenylated polyynes show a similar trend of Eg~n-0.36±0.01. Ultrafast molecular second-hyperpolarizabilities, γ, were obtained in solution using 800nm, 100fs pulses in a differential optical Kerr effect (DOKE) setup. Polyyne second-hyperpolarizabilities also scale with a power-law, and, surprisingly, with exponents higher than that of any other reported oligomer system, yielding a behavior of γ~n4.3±0.1 and γ~n3.8±0.1 for the TIPS-polyynes and phenylated-polyynes, respectively. These findings contrast direct theoretical predictions that increases in gamma with increasing conjugation length for polyynes should be considerably lower than those of polyenes and polyenynes. Furthermore, the combined linear and nonlinear optical results agree with recent theoretical studies on ideal 1-D conjugated systems, suggesting that polyynes display true 1-D behavior.
We present a comprehensive study of ultrafast time-resolved photconductivity in pentacene and functionalized pentacene single crystals and thin films measured using optical pump-terahertz probe technique. By investigating the wavelength and temperature dependence of the transient photoconductivity, we reveal a sub-picosecond wavelength-independent charge carrier photogeneration and band-like charge transport in both single crystal and thin film samples. The amplitude and decay dynamics of the photoconductivity transients are correleated with the morphology of the films, assessed by atomic force and electron microscopy.
We examine ultrafast photoconductivity in functionalized pentacene single crystals using optical-pump terahertz-probe techniques. The 0.5 ps rise time observed in the photoconductive transients, which is limited by the response time of the terahertz pulse setup, suggests that mobile charge carriers are a primary photoexcitation. The peak of the photoconductive signal increases as the temperature decreases due to higher carrier mobilities at lower temperatures. A lower limit for the carrier mobility of 1.6 cm2/Vs at 10 K and 0.2 cm2/Vs at room temperature is obtained. We further show that the absorption edge near the pump excitation wavelength of 800 nm remains temperature independent, and is therefore not a contributing factor in our observation of larger transient signals at lower temperatures. After an initial fast decay, a power-law decay is observed in the tail of the transient response from 2 to 600 ps. The dependence of the photoconductive response on the pump fluence and the electric field amplitude of the terahertz pulse are examined.
Finally, we show some preliminary results of transient photocurrent measurements on contact-biased samples using a fast oscilloscope with a system rise time of about 50 ps.
We use an optical pump - terahertz probe technique to study relaxation dynamics of photoexcited carriers in semiconductors. The optical pump pulse (400/800 nm, 100 fs) from an amplified Ti:sapphire laser generates free carriers within the optical penetration depth of the semiconductor surface, and the transmission of the terahertz (THz) probe pulse is monitored as a function of delay time between pump and probe. In particular, we investigate carrier relaxation dynamics in radiation-damaged silicon-on-sapphire (RD-SOS). We measure pump-induced changes in the transmission of the amplitude of the THz pulse, which proves to be a valid technique for these low-mobility samples due to negligible phase shifts in the transmitted THz pulse. Using a simple thin-film Drude model, single-exponential relaxation times of 4 to 6 ps are observed and transient mobilities of about 420 cm2/Vs are obtained for moderately damaged RD-SOS> Picosecond carrier relaxation dynamics in high-implant-dose RD-SOS and sub-picosecond transients in amorphous silicon thin films are also observed.
A source of high-intensity, ultra-short terahertz pulses has been developed. The operation and performance of a terahertz pulse-slicing system for use with the UCSB free-electron lasers are discussed. Short pulses are sliced from the microsecond long output of the free-electron laser using laser-activated semiconductor switches; the pulse length may be freely varied from a few picoseconds up to four nanoseconds. The temporal response of a heavily compensated gallium-doped germanium photoconductor has been investigated. At low excitation intensity, a recombination time of 2 +/- 0.1 ns is found. At higher THz pulse powers non-exponential relaxation is observed; the data is well modeled using a rate equation approach and including impact- ionization impact-ionization effects due to the terahertz- heated free holes.
A source of high-intensity, picosecond-to-nanosecond far-IR pulses has been developed at the UCSB Center for Free- Electron Laser Studies. The microsecond-long, far-IR output of the UCSB free-electron laser is sliced into shorter pulses using laser-activated semiconductor reflection switches. We have observed pulse durations as short as 20 ps at 22 cm-1. An overview of the design and performance of the pulse-slicer system is given.
A general trend is observed in the photoresponse of current biased epitaxial YBa2Cu3O7-(delta ) thin film bridge structures exposed to picosecond laser pulses. Both fast and slow components are seen in the photoresponse near the transition region. The slow component, which has a decay over several nanoseconds, is a resistive bolometric response due to heating of the film by the laser pulse in the resistive transition region. At lower temperatures, only the fast component is observed with an amplitude which is linear with bias current. The fast component has been observed in films ranging in thickness from 30 nm to 260 nm. Using 100 ps, 532 nm laser pulses, the origin of the fast component can be explained by a kinetic inductive bolometric response where the superfluid fraction is rapidly decreased by the laser pulse heating the bridge. Recent results using 5 ps, 820 nm laser pulses on samples maintained at liquid nitrogen temperature (77.4 K) in a high speed measurement setup have revealed fast components in the photoresponse as short as 16 ps full width at half maximum. To our knowledge, this is the fastest photoresponse signal observed to date from YBa2Cu3O7-(delta ) thin films. A large portion of this fast response can be attributed to a kinetic inductive bolometric response. The possibility of a nonbolometric component over this short time scale is discussed.
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