Optical response of materials to intense terahertz electric fields has become a new frontier in optics in the recent decade. We focus on the nonlinear optical responses that are quadratic in terahertz electric field, which can arise in second and third orders of nonlinearity. The second order nonlinear polarizability can lead to terahertz second harmonic generation, a phenomenon that has not been experimentally observed yet. The difficulty in detecting terahertz second harmonic generation stems from vanishingly small conversion efficiencies in this region of spectrum. However, an additional experimental difficulty results from a significant overlap of the fundamental and second-harmonic terahertz pulses both in the time and frequency domain. This makes it hard to distinguish the second harmonic from the fundamental terahertz wave. The third order nonlinear polarizability results in terahertz Kerr effect, which has been observed as an induced gating-beam birefringence that is quadratic in terahertz electric field. In noncentrosymmetric materials, terahertz Kerr effect may coexist with terahertz Pockels effect that is commonly used for time-domain terahertz detection via electro-optic sampling. Distinguishing the terahertz Kerr effect from the Pockels effect can also be difficult if the latter is significantly stronger. In this paper, we will present a method for measuring quadratic terahertz nonlinearities based on the second-harmonic lock-in detection in a time-domain electro-optic sampling experiment. We illustrate our method using a measurement of terahertz Kerr effect in a zinc blende semiconductor in geometry where both terahertz Kerr and Pockels effects are present. We will also discuss the possibility of measuring terahertz second harmonic generation in metamaterials.
When a light beam traveling in one direction in a crystal experiences a different absorption and refractive index compared to the beam traveling in the opposite direction, it is called nonreciprocal directional anisotropy, or simply nonreciprocity. This phenomenon is governed by the fundamental symmetries of crystals under spatial inversion and time reversal symmetries. We will discuss the necessary symmetry conditions for the nonreciprocity of light propagation and of other excitations in solids. Among specific examples, we will consider light propagation, in polar magnetic materials along and opposite the toroidal vector. In this case, a crystal can be completely transparent in one direction and completely opaque in the opposite one – an optical diode. We report a giant optical diode effect in the polar material FeZnMo3O8, where we find more than a 100-fold difference in intensity of light transmitted in the two opposite directions. In addition to the high magnitude of the effect, we show that the effect exists at high temperature in the magnetically disordered state. We will also present a study of the nonreciprocal reflectance of magneto-plasma in semiconductor InSb. This material can be used for the construction of high-performance terahertz isolation devices, as no dominant technology has emerged yet for this application. Room-temperature operation, moderate applied magnetic field, and an unmatched simplicity of design make this material a good candidate for practical terahertz isolators.
Detection and identification of molecular materials based on their THz frequency vibrational resonances remains an open technological challenge. The need for such technology is illustrated by its potential uses in explosives detection (e.g., RDX) or identification of large biomolecules based on their THz-frequency vibrational fingerprints. The prevailing approaches to THz sensing often rely on a form of waveguide spectroscopy, either utilizing geometric waveguides, such as metallic parallel plate, or plasmonic waveguides made of structured metallic surfaces with sub-wavelength corrugation. The sensitivity of waveguide-based sensing devices is derived from the long (1 cm or longer) propagation and interaction distance of the THz wave with the analyte. We have demonstrated that thin InSb layers with metallic gratings can support high quality factor “true” surface plasmon (SP) resonances that can be used for THz plasmonic sensing. We find two strong SP absorption resonances in normal-incidence transmission and investigate their dispersion relations, dependence on InSb thickness, and the spatial distribution of the electric field. The sensitivity of this approach relies on the frequency shift of the SP resonance when the dielectric function changes in the immediate vicinity of the sensor, in the region of deeply sub-wavelength thickness. Our computational modeling indicates that the sensor sensitivity can exceed 0.25 THz per refractive index unit. One of the SP resonances also exhibits a splitting when tuned in resonance with a vibrational mode of an analyte, which could lead to new sensing modalities for the detection of THz vibrational features of the analyte.
The novel properties of semiconductor nanowires, along with their potential for device applications in areas including
nanoscale lasers and thermoelectrics, have led to a resurgence of interest in their growth and characterization over the
past decade. However, the further development and optimization of nanowire-based devices will depend critically on an
understanding of carrier relaxation in these nanostructures. For example, the operation of GaN-based photonic devices is
often influenced by the presence of a large defect state concentration. Ultrafast optical spectroscopy can address this
problem by measuring carrier transfer into and out of these states, which will be important in optimizing device
In this work, we use ultrafast wavelength-tunable optical spectroscopy to temporally resolve carrier dynamics in
semiconductor nanowires. Wavelength-tunable optical pump-probe measurements enable us to independently measure
electron and hole dynamics in Ge nanowires, revealing that the lifetime of both electrons and holes decreases with
decreasing nanowire diameter. Similar measurements on CdSe nanostructures reveal that the surface-to-volume ratio
strongly influences carrier relaxation. Finally, ultrafast optical experiments on GaN nanowires probe carrier dynamics in
the defect states that influence device operation. These experiments provide fundamental insight into carrier relaxation in
these nanosystems and reveal information critical to optimizing their performance for applications.
Conference Committee Involvement (1)
Next-Generation Spectroscopic Technologies XIII
26 April 2020 | Anaheim, California, United States