Two-dimensional atomically thin materials, most notably graphene and transition metal dichalcogenides (TMDs), have generated tremendous interest among researchers. The high electron mobility and strong light absorption exhibited by these materials make them attractive for opto-electronic applications. We will present our recent experimental and theoretical work on the ultrafast dynamics of collective excitations, such as excitons, phonons, and plasmons, in these materials for electronic and photonic device applications.
We study the dynamics of excitons in 2D materials and optoelectronic devices using ultrafast optical/terahertz pump-probe and correlation spectroscopy. Our experimental work on metal dichalcogenide materials and devices (such as photodetectors) as well as our theoretical results show that defect assisted recombination involving capture of excitons and carriers by Auger scattering is the fastest mechanism for the non-radiative recombination of photoexcited electrons and holes. In particular, the very Coulomb interaction that resulted in the strongly bound excitons in these materials, causes extremely fast capture of the excitons by defects resulting in extremely poor quantum efficiencies in optoelectronic devices. The large sensitivity of device performance to defects is thus fundamental to 2D TMD materials. Defect-passivated 2D materials have demonstrated quantum efficiencies approaching ten percent. Our ultrafast two-pulse photovoltage correlation experiments show that the photoresponse of TMD photodetectors can be very fast making them useful for operation at frequencies in the hundreds of gigahertz range.
Our recent experimental work has shown that 2D materials could be very promising for high frequency phononic devices. Our work has shown that mechanical oscillations in these atomically thin membranes can reach terahertz frequencies and are tunable from few tens of gigahertz to almost one terahertz. 2D material membranes can therefore enable MEMs resonator structures with record frequency-quality factor products at these high frequencies.
Our ultrafast work in graphene plasmonic structures has revealed enormous potential for graphene based VLSI interconnects in which electrical signals are carried by plasmonic waves with much reduced propagation delays, losses, signal distortions, and cross-talk compared to conventional metal interconnects like copper.
We discuss a family of nanoscale cavities for electrically-pumped surface-emitting semiconductor lasers that use
surface plasmons to provide optical mode confinement in cavities which have dimensions in the 100-300 nm range.
The proposed laser cavities are in many ways nanoscale optical versions of micropatch antennas that are commonly
used at microwave/RF frequencies. Surface plasmons are not only used for mode confinement but also for output
beam shaping to realize single-lobe far-field radiation patterns with narrow beam waists from subwavelength size
cavities. We identify the cavity modes with the largest quality factors and modal gain, and show that in the near-IR
wavelength range (1.0-1.6 μm) cavity losses (including surface plasmon losses) can be compensated by the strong
mode confinement in the gain region provided by the surface plasmons themselves and the required material threshold
gain values can be smaller than 700 cm<sup>-1</sup>.
Theoretical and experimental results on ultra-fast all-optical switches based on intersubband transitions for Tb/s operation are presented. Designs for engineering intersubband transitions (ISBT) in GaN/AlN quantum wells near communication wavelengths (~1.55 μm) and for realizing all-optical switches requiring small pulse energies are discussed. Optimized designs show all-optical switching at Tb/s data rates with pulse energies as small as 200 fJ. Experimental realization of narrow line-width ISBT in GaN/AlN superlattices is also demonstrated.
Semiconductor cascade lasers have larger photon noise than conventional single stage semiconductor lasers as a result of positive correlations in photon emission in different gain stages which are connected electrically in series. The photon noise of
cascade lasers can be related to the photon noise of single stage lasers with scaled external circuit impedances. This scaling relation for the photon noise holds for bipolar as well as unipolar cascade lasers.
This paper explores the development of cascade semiconductor lasers for communications applications. Both interband and intersubband cascade emission devices are examined theoretically and experimentally. The motivation for cascade sources in both high fidelity and high bandwidth applications is presented. The ability to transmit signals with lower signal loss and improved noise performance is verified by measurements on a model systems consisting of series coupled DFB lasers.
Conference Committee Involvement (1)
Fluctuations and Noise in Photonics and Quantum Optics