Movie of Nicola Palombo Blascetta and James T Hugall describing experiments to study how materials respond to plasmonic excitations

Starting small and aiming big, ICFO began as part of the Universitat Politecnica de Catalunya in 2002 under the auspices of Lluis Torner, ICFO director and group leader in nonlinear optical phenomena research. Within just a few years the centre had already outgrown its original home and relocated to the coastal site at Castelldefells. Further expansions in 2008 and 2012 are visible like rings on a tree trunk as you walk round the centre, from the older to more recently added parts of the building. It is now home to 60 state-of-the-art labs used by 24 groups engaged in world-class research in all aspects of photonics from quantum information and energy, to 2D materials, sensing, medicine and biology.

World-class research needs world-class researchers, and from day one the centre hoped to not only attract back some of the young talent that had been lured to promising prospects overseas in the aftermath of Franco’s regime, but literally anyone from around the world bright and creative enough to make a notable contribution to the centre. Today more than 400 researchers and PhD students work at the centre, with competition for PhD positions at around 30 candidates per place.

Ultrafast nanophotonics and plasmonics

Niek van Hulst was among those attracted to the centre from overseas in its early days. “I was working in the Netherlands at the time and I came to give a talk,” he says. “There had been no real centre for Photonics in Europe before ICFO started and I was interested in what they were trying to do here.”

One of the main projects in his group is ultrafast molecular dynamics in single molecules. For this, PhD students Ion Hancu and Vikas Remesh bring the temporal sensitivity of pump-probe experiments to new extremes. They aim to study, at a molecular level, the energy transfer, Rabi oscillations and relaxation of the vibrational modes that give substances their distinctive Raman signatures. Studying these phenomena at the single-molecule level should provide important insights into processes such as photosynthesis for improving light-harvesting devices.

Meanwhile, PhD student Nicola Palombo Blascetta and post doc James T Hugall are looking at how materials respond to plasmonic excitations, the collective electron oscillations that enhance interactions with light. They bring plasmonic gold or aluminium nanoparticles to within a few nanometres of the surface. The plasmon resonance in the nanoparticle enhances the electromagnetic field – and hence interactions with incident light – in a very localized region. As a result, the material beneath can be imaged to a resolution of tens of nanometres.

To achieve the necessary distance control between the nanoparticle and the surface, they attach an optical fibre ending in the nanoparticle to a quartz tuning fork. The resonance of the tuning fork shifts with increasing interactions closer to the surface, allowing the distance to be monitored with nanoscale precision.

Graphene research – fundamental and applied

Plasmons also form the basis of studies in graphene, where patterning gives rise to plasmonic responses in the infrared that can be tuned by doping. With Frank Koppens from the Optoelectronics Work Package of the Graphene Flagship working as group leader at ICFO, there are a number of graphene investigations underway that are inching towards potential commercial returns on the research investment into graphene’s exotic properties. Some of the more developed projects are on display in the department, including heartbeat sensors that detect vein dilation with pulse from the effect on light transmission through the wrist or finger, and an infrared graphene camera.

Koppens lists bio and gas sensing, infrared imaging, wearables, low-power data communications and flexible displays as areas where graphene optoelectronics may have significant market potential. However, demonstrating CMOS compatibility is crucial for any application to be commercially viable. One of the challenges with building on CMOS chips is that the surface is far from flat – “it’s like the Manhattan skyline” says Ivan Nikitsky, one of the researchers in Koppens’s group. In fact, graphene’s flexibility makes this less of a problem, as shown in their graphene chip. Since other infrared-sensitive materials are not CMOS compatible, this may give graphene the edge over alternative materials both for infrared camera devices, as well as optical data transmission for faster lower-power circuits.

Crucially, as well as research for potential commercial devices, there is avid activity in fundamental studies of graphene. As well as graphene plasmonics, researchers at ICFO are investigating other properties, such as the material’s mechanical responses when stretched like a drum over holes. In this system, light is enough to activate mechanical motion in the graphene. Monitoring this motion with piezo-resistive detectors can provide a mechanical measure of the optical activity of fluorophores with an electrical read out. Pump-probe experiments are also investigating the relaxation dynamics of graphene with time-resolved photocurrent measurements. Previous attempts to monitor these dynamics have been limited by the photocurrent detection time, but according to Fabien Vialla – a post-doc in the group – intrinsic speeds above 100GHz have been measured at ICFO.

Quantum information and communication

In terms of harnessing exotic material properties for mainstream applications, quantum research is a prime forerunner, with protocols for quantum computing, and quantum cryptography and communication long attracting interest in both academia and industry. These protocols largely rely on “spooky action at a distance” whereby measurement of one of a pair of entangled particles in a superposition of states instantaneously affects the state of the other. This phenomenon has been investigated in increasingly rigorous “Bell tests”, and ICFO will be among the groups participating in one – “the Big Bell Test” – on 30 November this year. One of the things the researchers want to ensure is the unpredictability of the questions they ask the particles, which translates in practice as the measurements they make on them. To tackle this, the Big Bell Test plans to harness human randomness, whereby participants around the world will play the Big Bell test games online to generate random numbers for the experiment.

Hugues de Riedmatten describes methods for encrypted quantum communication

“It will be interesting to see whether people are random enough,” says Hugues de Riedmatten, who heads one of the groups at ICFO taking part. They will be using a cloud of laser-cooled atoms for the test, whereby photons emitted by the cloud will be entangled with the atoms. They chose this set up for the Big Bell Test as it is a well understood system, but there are other solid-state approaches to quantum communication also under investigation in the group.

De Riedmatten describes how Y2SiO5 doped with Pr3+ ions can be used for mapping photon information for quantum memory. “Pr3+ is a good optical absorber, is efficient and has a good coherence because it is well isolated from phonons and so on in the system,” he explains. The coherence properties are a huge advantage for using these systems in quantum memory. Pr3+ ions exist in ground and excited states with a level of hyperfine spin splitting, and while the coherence of superposition between the excited and ground states lasts around 100 microseconds, that between the ground and hyperfine states lasts for up to a second. In fact there are experimental reports of light (bright classical light, not single photons) stored for around 1 minute using techniques that decouple the spins from the environment.

“People have even proposed physically transporting the crystals in vehicles in experiments as opposed to photon transmission because the coherence time is so long,” says de Riedmatten. As well as attempting to put this protocol into practice, the team also plan to look at how confinement effects from nanosized crystals affect their behaviour.

Photovoltaics

The development of alternatives to fossil-fuel sources of energy is another sector that equals if not exceeds secure communication requirements as a pressing global concern. One of the researchers who have been at the centre from the beginning is Jordi Martorell, who is focused on solar cells. His group has produced photovoltaic “glass” using ITO as one electrode and silver electrodes 10 nm thin – thin enough that silver is transparent – separated by organic active photovoltaic material. Low carrier mobility prohibits the use of thick layers of organic active materials with the advantage that these solar cell devices can be transparent enough to use in windows while achieving 6% solar to electrical energy conversion efficiency.

Another of the group’s solar cell designs is shaped like merged cylinders so that the cell forms a plane that is corrugated on both sides. Incident light bounces chaotically within the structure like a whispering gallery mode, which enhances interaction with the active material, with the additional benefit from the curved corrugations of performing well at a range of incident angles.

ICFO certainly seems to be achieving its goal as a hub for state-of-the-art photonics research in Europe. Without doubt the success of the centre can be largely attributed to the great wealth of talent attracted from all over the world, in which there is a good deal of pride, as typified by the name of the centre’s newsletter – ICFOnians. The centre’s independence from some of the bureaucracy hampering the universities and larger institutions in the region has also been a huge advantage for responding quickly to emerging and changing needs. One of the challenges for the future of the centre will certainly be to maintain this agility while accommodating the administrative requirements of an institution of its current and future size.