A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like SrTiO_3 (STO) and LaTiO_3 (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of 10^12 cm^2 and 1000 cm^2V^-1s^-1, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 m_e.

Self-assembled plasmonic metasurfaces are promising optical platforms to achieve accessible flat optics, due to their strong light-matter interaction, nanometer length scale precision, large area, light weight, and high-throughput fabrication. Here, using photothermal continuous wave laser lithography, we show the spectral and spatial tuning of metasurfaces comprised of a monolayer of ligand capped hexagonally packed gold nanospheres. To tune the spectral response of the metasurfaces, we show that by controlling the intensity of a laser focused onto the metasurface that the absorption peak can be reconfigured from the visible to near-infrared wavelength. The irreversible spectral tuning mechanism is attributed to photothermal modification of the surface morphology. Combining self-assembled metasurfaces with laser lithography, we demonstrate an optically thin (λ/42), spectrally selective plasmonic Fresnel zone plate. This work establishes a new pathway for creating flat, large area, frequency selective optical elements using self-assembled plasmonic metasurfaces and laser lithography.

Transient absorption (TA) experiments were performed to determine the mechanisms controlling the welding process of plasmonic nanorod dimers using ultrafast light pulses. The TA signals during the self-assembly and welding of the nanorod dimers were temporally and spectrally resolved, enabling each potential mechanism to be isolated by their associated time scales. For all the TA measurements collected between 5 and 1000 ps, no abrupt or anomalous TA signals were observed that would indicate a welding event had occurred during this temporal regime. A cumulative multipulse welding mechanism beyond 1000 ps is unlikely since the nanorods return to their ground state before the next laser pulse arrives. To understand the dynamics below 5 ps, we fit the welding kinetics data to a rate equation and determined a threshold fluence of 79 mu J/cm(2) was needed to initiate welding. To correlate the threshold fluence to a physical mechanism, finite element calculations were used to show the nanorods reached temperatures of 635 K, sufficient to cause surface melting when irradiated with a 75 mu J/cm(2) pulse. We then demonstrate the optical forces in the nanojunction are attractive and sufficiently large to displace the molecules linking the unwelded dimer structures. Accordingly, these experiments suggest the welding mechanism is governed by a threshold fluence from a single laser pulse, resulting in the surface of the individual nanorods melting, which are then pulled together by attractive optical forces forming a welded dimer structure. Globally, these results set an upper bound on the rate these materials can be produced.

Black phosphorus possesses several attractive properties for optoelectronics, notably a direct and layer dependent bandgap that varies from the visible to mid-infrared and the ability to transfer the material to nearly arbitrary substrates. A less utilized property of black phosphorus for optoelectronics is the nonlinear photoresponse. The photocarrier lifetime in black phosphorus exhibits a strong nonlinear dependence on the excitation density that is utilized in the present work for optoelectronic mixing. In this scheme, two telecommunications-band lasers are intensity-modulated by a radio frequency (RF) and local oscillator (LO) frequency and focused onto a black phosphorus photoconductive detector. Above the saturation carrier density, the photocurrent is proportional to the square root of the optical power which produces photocurrents at the sum and difference frequencies of the input beams. The bandwidth of the mixing process increases from 10 to 100 MHz for incident powers of 0.01 to 1 mW, respectively. An excess carrier model accurately describes the power dependence of the cutoff frequency and mixing conversion, which are both limited by photocarrier recombination. Optimizing our device geometry to support larger bias fields and decreased carrier transit times could increase the maximum RF/LO frequency beyond a GHz by reducing the excess carrier lifetime. Frequency mixing based on the photocarrier nonlinearity in multilayer black phosphorus demonstrated here can be readily extended to mid-infrared wavelengths as long as 4 µm.

The combination of magnetostrictive thin films with surface acoustic wave (SAW) devices enables the magnetic sensors that exploit the Delta E-effect (magnetically induced change in modulus) without the need for free-standing structures that are susceptible to vibration and damage. The performance of these sensors is strongly influenced by magnetic anisotropy and, therefore, the state of stress in the magnetostrictive film. We report the fabrication and characterization of magnetic SAW devices comprised of a magnetostrictive layer and interdigital transducers on a piezoelectric substrate. The state of stress in the magnetostrictive layer is varied by annealing. Vibrating-sample magnetometer measurements indicate that the annealed devices show a strong uniaxial anisotropy with the easy axis parallel to the direction of acoustic propagation. The induced magnetic anisotropy is attributed to the anisotropic thermal expansion in the substrate. The change in frequency of one-port resonators and two-port SAW oscillators is measured as a function of dc bias field and ambient temperature. The normalized change in frequency (Delta f / f(sat)) is on the order of 10(-4). These results demonstrate that the film stress can be used to optimize the performance of magnetic SAW devices and the SAW devices can be used to characterize the state of stress and magnetic anisotropy of magnetostrictive films.

Tunable metamaterials and THz devices based on vanadium dioxide films are demonstrated. The effects of strain engineering during film growth on the temperature and dynamics of the metal-insulator transition will also be discussed.

Suspensions of isolated and molecularly linked gold nanorods are investigated using time-domain spectroscopy. Unique dynamics are observed for each configuration, and indicate laser-induced concatenation of the linked nanorods occurs on picosecond timescales.

An optoelectronic mixer based on the nonlinear photoconductivity of black phosphorus is presented. We demonstrate mixing with a maximum conversion loss of 31 dB at local oscillator frequencies up to 640 MHz.

Photoconductive antennas are widely used for time-resolved detection of terahertz (THz) pulses. In contrast to photothermoelectric or bolometric THz detection, the coherent detection allows direct measurement of the electric field transient of a THz pulse, which contains both spectral and phase information. In this Letter, we demonstrate for the first time photoconductive detection of free-space propagating THz radiation with thin flakes of a van der Waals material. Mechanically exfoliated flakes of black phosphorus are combined with an antenna that concentrates the THz fields to the small flake (∼10 μm). Similar performance is reached at gating wavelengths of 800 and 1550 nm, which suggests that the narrow bandgap of black phosphorus could allow operation at wavelengths as long as 4 μm. The detected spectrum peaks at 60 GHz, where the signal-to-noise ratio is of the order of 40 dB, and the detectable signal extends to 0.2 THz. The measured signal strongly depends on the polarization of the THz field and the gating pulse, which is explained by the role of the antenna and the anisotropy of the black phosphorus flake, respectively. We analyze the limitations of the device and show potential improvements that could significantly increase the efficiency and bandwidth.

The control of light-matter interaction through the use of subwavelength structures known as metamaterials has facilitated the ability to control electromagnetic radiation in ways not previously achievable. A plethora of passive metamaterials as well as examples of active or tunable metamaterials have been realized in recent years. However, the development of tunable metamaterials is still met with challenges due to lack of materials choices. To this end, materials that exhibit a metal-insulator transition are being explored as the active element for future metamaterials because of their characteristic abrupt change in electrical conductivity across their phase transition. The fast switching times (▵t < 100 fs) and a change in resistivity of four orders or more make vanadium dioxide (VO2) an ideal candidate for active metamaterials. It is known that the properties associated with thin film metal-insulator transition materials are strongly dependent on the growth conditions. For this work, we have studied how growth conditions (such as gas partial pressure) influence the metalinsulator transition in VO2 thin films made by pulsed laser deposition. In addition, strain engineering during the growth process has been investigated as a method to tune the metal-insulator transition temperature. Examples of both the optical and electrical transient dynamics facilitating the metal-insulator transition will be presented together with specific examples of thin film metamaterial devices.

An ultrafast insulator-metal-insulator phase transition cycle in epitaxially strained vanadium dioxide films is observed. The films are characterized by optoelectronic autocorrelation measurements that reveal a 400 fs transient change in response spanning two orders in magnitude. These findings suggest a predominantly electronic mechanism and demonstrate the promise of this material for optoelectronic applications requiring fast, high-contrast switching.

Two-dimensional black phosphorus is a new material that has gained widespread interest as an active material for optoelectronic applications. It features high carrier mobility that allows for efficient free-carrier absorption of terahertz radiation, even though the photon energy is far below the bandgap energy. Here we present an efficient and ultrafast terahertz detector, based on exfoliated multilayer flakes of black phosphorus. The device responsivity is about 1 mV/W for a 2.5 THz beam with a diameter of 200 μm, and is primarily limited by the small active area of the device in comparison to the incident beam area. The intrinsic responsivity is determined by Joule heating experiments to be about 44 V/W, which is in agreement with predictions from the Drude conductivity model. Time resolved measurements at a frequency of 0.5 THz reveal an ultrafast response time of 20 ps, making black phosphorus a candidate for high performance THz detection at room temperature.

Strained VO2 films are characterized by optoelectronic autocorrelation measurements that reveal a material response of 400 fs. This suggests the insulator-metal transition in strained VO2 can be activated by electronic, rather than thermal, means.

We present three THz detection mechanisms in black phosphorus: bolometric detection, photothermoelectric detection, and photoconductive sampling of THz fields. Photothermal detection exhibits an internal responsivity of 44V/W and 20ps response time.

The intrinsic speed of a black phosphorus photoconductive detector is determined using pulsed autocorrelation photocurrent measurements. The data reveals two timescales of 125 ps and 775 ps, indicating fast photodetection is possible with black phosphorus photoconductive detectors.

Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral-point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage -- an effect that is unique to graphene.

Black phosphorus has attracted interest as a material for use in optoelectronic devices due to many favorable properties such as a high carrier mobility, field-effect, and a direct bandgap that can range from 0.3 eV in its bulk crystalline form to 1.7–2.0 eV for a single atomic layer. The low bandgap energy for multilayer black phosphorus consisting of more than approximately five atomic layers allows for direct transition photoabsorption that enables detection of light out to mid-infrared frequencies. In this work, we characterize the room temperature optical response of a black phosphorus photoconductive detector at wavelengths ranging from 1.56 to 3.75 μm. Pulsed autocorrelation measurements in the near-infrared regime reveal a strong, sub-linear photocurrent nonlinearity with a response time of 1 ns, indicating that gigahertz electrical bandwidth is feasible. Time resolved photoconduction measurements covering near- and mid-infrared frequencies show a fast 65 ps rise time, followed by a carrier relaxation with a time scale that matches the intrinsic limit determined by autocorrelation. The sublinear photoresponse is shown to be caused by a reduction in the carrier relaxation time as more energy is absorbed in the black phosphorus flake and is well described by a carrier recombination model that is nonlinear with excess carrier density. The device exhibits a measured noise-equivalent power of 530 pW Hz−1/2, which is the value expected for Johnson noise limited performance. The fast and sensitive room temperature photoresponse demonstrates that black phosphorus is a promising new material for mid-infrared optoelectronics.