Keshav M. Dani

The ability to manipulate ultrafast pulses has been at the heart of major technological advances, including chirp pulse amplification, pulse compression, and frequency conversion. At terahertz (THz) frequencies, precise control of pulse dispersion and frequency is key to next-generation high-speed wireless communication and sensing applications, yet such control remains difficult owing to inefficient nonlinear materials and the fundamental constraints of passive devices. Here, to overcome these limitations, we introduce a time-varying semiconductor waveguide platform with a cascade of temporal and spatial boundaries and demonstrate programmable control of THz pulse waveforms with efficient frequency conversion. By precisely timing optical pulses at multiple temporal boundaries to control the THz intra-pulse frequency shifts, we generate tailored positive, negative, and non-monotonic chirp pulses, as well as tunable frequency upconversion. These results open a way toward programmable THz pulse manipulation for time-varying photonics.

With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technologies. Optical techniques have previously observed dark excitons with a long-lived valley polarization. However, several aspects remain unknown, such as the populations and time evolution of the different valley-polarized dark excitons and the role of excitation conditions. Here, using time- and angle-resolved photoemission spectroscopy, we obtain a holistic view of the dynamics after valley-selective photoexcitation. By varying experimental conditions, we reconcile between the rapid valley depolarization previously reported in TR-ARPES, and the observation of long-lived valley polarized dark excitons in optical studies. For the latter, we find that momentum-dark excitons largely dominate at early times sustaining a 40% degree of valley polarization, while valley-polarized spin-dark states dominate at longer times. Our measurements provide the timescales and how the different dark excitons contribute to the previously observed long-lived valley polarization in optics.

High-order harmonic generation (HHG) is a nonlinear optical process with critical applications in ultrafast science, including attosecond pulse generation [1], coherent XUV imaging [2], and time-resolved photoemission spectroscopy [3]. Conventionally, HHG is driven by femtosecond laser pulses with mJ-level energies at kHz repetition rates, where the beam is focused to spot sizes on the order of hundreds of micrometers. Recent advancements have enabled scaling of the focusing geometry to tens of micrometers, allowing HHG with pulse energies in the tens of µJ [4]. However, efficient HHG using a µm-scale spot size has never been demonstrated. In this work, we demonstrate efficient HHG using a focal spot diameter down to 2 µm.

Two-dimensional (2D) semiconductors, such as monolayer (1L) transition metal dichalcogenides (TMDs), host tightly bound excitonic species. Recently, time-and angle-resolved photoemission spectroscopy (TR-ARPES) has shown great promise in studying these photoexcited states due to its ability to resolve momentum which has resulted in the measurement of the excitonic wavefunction [1,2] as well as the study of dark excitons [3]. These initial studies were done in nominally undoped samples, however, the ability to study momentum-resolved dynamics in doped samples will open new avenues of investigation. In this talk, we will discuss our progress in studying gated 2D semiconductors using TR-ARPES.

In this study, the frequency down‐conversion of terahertz waves is analytically and experimentally demonstrated at the temporal boundaries within a GaAs waveguide. The temporal boundary is established by photoexciting the top surface of the waveguide, thereby instantaneously increasing its electrical conductivity. This photoexcited waveguide supports a transverse electromagnetic (TEM) mode with a frequency lower than those of the transverse magnetic (TM) modes present in the original waveguide. At the temporal boundary, the incident TM mode couples with the TEM mode, resulting in frequency down‐conversion. Subtracting the propagation loss from the frequency‐converted components indicates that the frequency conversion occurs with an efficiency consistent with the analytical predictions. The propagation loss is primarily due to ohmic loss, caused by the finite electrical conductivity of the photoexcited region. Given that the frequency of transverse electric modes is up‐converted at the temporal boundary, our findings suggest that the direction of frequency conversion (upward or downward) can be controlled by manipulating the incident polarization. The polarization‐dependent frequency conversion in waveguides holds significant potential for applications in devices designed for the interconversion of terahertz signals across various frequency channels. This capability is instrumental in the development of frequency‐division‐multiplexed terahertz wave communication systems, thereby enabling high data transfer rates.

In this study, we demonstrate the management of terahertz pulse attenuation using a metallic structure in the frequency-conversion process with a temporal boundary. During the propagation of a terahertz pulse, rapid optical excitation of the waveguide surface induces a temporal boundary that can cause frequency conversion. The output amplitude of the terahertz pulse varies depending on the timing of the temporal boundary, which is a notable issue in this process. This is attributed to the difference in the attenuation constants before and after the temporal boundary. We fabricated a waveguide with a metal structure consisting of periodically aligned metal lines. This metallic structure decreases the area in which photoexcited carriers are generated and reduces the attenuation constant after the temporal boundary. On the other hand, the attenuation constant before the temporal boundary increased because of the Joule loss of the patterned metal. These effects balance the attenuation constants before and after the temporal boundary. Consequently, regardless of the location of the terahertz pulse within the waveguide at the timing of the temporal boundary, the amplitude of the output terahertz pulse remained constant.