Exploring the frontiers of light-speed electronics, utilizing unique sources of extremely-short pulses and innovative measurement infrastructure
All electronic devices are based on the coupled dynamics of charge carriers and electromagnetic fields – this synchronous motion has enabled many of the great advances in technology that have appeared in the last century, from computing to telecommunications.
Have we reached the speed limit? After all, in the past two decades we have seen minimal progress in computer clock rates, which seem to have stagnated at just a few gigahertz.
In the same timeframe, however, optical technology has accelerated by leaps in bounds, enabled first by modelocked lasers and more recently by phase-controlled optical waveforms and the generation of attosecond pulses. These pulses are enabling us to explore light-matter interaction taking place within a fraction of an oscillation cycle of a visible light field: the domain of petahertz electronics.
Our research is centered upon this fundamental question: how do intense light fields interact with and influence the behavior of solid-state systems?
We have already seen that dielectric materials behave in unexpected ways when exposed to intense, ultrashort electric fields (far above the intensities that would break down the material entirely if they weren’t turned on and off so quickly).
Near-perfect insulators turn briefly into conductors, and carrier concentrations change in a stepwise manner within half an oscillation of the laser field. We have seen that optical fields can both drive currents inside of materials, and simultaneously be used to make precise observations of the temporal evolution of the electronic motion they induce. We are now using these methods to perform the first studies of light-field-controlled, petahertz electronics and the coherent nature of electronic motion on the femtosecond-to-attosecond time scale.
Tools, Techniques & Labs
Experimental approaches: In order to replicate the precision and clarity with which conventional electronics can be characterized and understood, direct analogues for some of the fundamental tools in electronics were required:
Function generators: Before the measurement can begin, we need to be able to control the electric field that will interact with the target material. This field should be both repeatable and controllable; our laser sources provide just that capability. Using »phase-stable« sources, which provide trains of identical electric field waveforms, and field synthesis techniques to manipulate them into the forms best suited to the experiment, we exert precise control over the interactions we wish to study. Using multiple, high-energy sources in the visible, near-infrared, and mid-infrared spectral regions, plus nonlinear conversion to cover the ultraviolet and extreme ultraviolet, we are able to trigger and study dynamics across a wide spectrum of materials and types of interaction.
Oscilloscopes: Some of the most precise information regarding the interactions we study is contained within the electromagnetic fields they emit — from Maxwell’s equations we can see that the electric field is directly coupled to the motion of charged particles, allowing us to reconstruct the full history of the interaction. But how does one capture a laser field? That is, what can serve as an oscilloscope for infrared and visible radiation?
We make use of a set of techniques: attosecond streaking, electro-optic sampling and nonlinear polarization sampling.
Attosecond streaking makes use of attosecond bursts of x-ray light as short as 70 attoseconds. Such a pulse can then be used to directly measure petahertz laser fields inside of an attosecond streak camera. This is done by directing the attosecond pulse onto a noble gas atom, causing an electron to be ejected via the photoelectric effect — the electron will emerge into the vacuum and travel to an electron spectrometer. If we apply a laser field to the atom at the same time as the attosecond pulse, the electron will be accelerated in the field, causing a shift of its final energy when it reaches the spectrometer. Thus, by controlling precisely when the attosecond pulse reaches the atom, we can sample the force applied by the laser to the electron, and thereby map the electric field as a function of time.
Made popular in the terahertz spectral domain, electro-optical sampling provides an alternative route to electric-field information, making use of nonlinear optical crystals to measure a laser field by its influence on the polarization of another laser pulse as they pass through the detection medium. Using optical pulses with ultrashort durations at the current technological limits, we are able to directly access the field oscillations of even near-infrared light.
Using extremely short laser pulses to excite, with extreme temporal precision, the active region of a small metal-dielectric-metal junction, we can use solid-state optoelectronic devices to measure electric field transients based on the laser-field-induced generation of electric current. Driving these circuits with a longer-wavelength field provides not only access to the electric field waveform from a single, solid-state device, but also gives new insight into the dynamics of charge currents switched at petahertz rates.
Nonlinear polarization sampling
Adopting a versatile scheme of microwave and terahertz spectroscopy we measure the laser electric field waveform after passage through a nonlinear medium. This measurement is done twice, once with the peak intensity of the laser set high enough to accumulate significant nonlinear modifications, and once at an intensity level low enough to ensure that the light-matter interaction is virtually free of nonlinearities. By comparison of the two recorded waveforms normalized for their difference in peak amplitude, we can for the first time extract the temporal evolution of the nonlinear polarization wave inside the sample. This allows sensitive quantitative investigation of the energy exchange dynamics between laser field and sample, the response time of the underlying nonlinearities and the electron kinetics responsible for the optical response.
Of course, different experiments demand different observations to provide a full physical picture. For this reason, we utilize three different attosecond beamlines for advanced experiments, including time-resolved photoelectron spectroscopy, x-ray and extreme ultraviolet time-resolved spectroscopy and more. Our toolbox includes electro-optical sampling and photoconductive switching at frequencies ranging from the microwave to near-infrared, photoelectron time-of-flight spectroscopy from gases and surfaces and attosecond transient absorption measurements in gaseous and solid systems. Nonlinear polarization sampling relies on the capability of a modified gas-phase photoelectron streaking set-up to sample the electric field of light.