Alexander Urich

Microcavity-Integrated Graphene Photodetector

There is an increasing interest in using graphene1,2 for optoelectronic applications.3−19 However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.

Intrinsic Response Time of Graphene Photodetectors

Graphene-based photodetectors are promising new devices for high-speed optoelectronic applications. However, despite recent efforts it is not clear what determines the ultimate speed limit of these devices. Here, we present measurements of the intrinsic response time of metal–graphene–metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses. We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers. This time translates into a bandwidth of ∼262 GHz. Moreover, we investigate the dependence of the response time on gate voltage and illumination laser power.

Silver nanoisland enhanced Raman interaction in graphene

Graphene shows great potential for optoelectronic applications but suffers from rather weak interaction with light due its single-atomic thickness. Here, we report the enhanced interaction of graphene and light for Raman transitions using localized surface plasmons. The plasmons are generated in silver nanoislands that we fabricate by simple means of metal deposition on top of graphene. Despite the broad size distribution of the nanoislands, we find a 100-fold enhancement of the Raman signal. We provide an analytical model for the description of the optical properties and obtain the scattering cross section as well as enhancement factors for the Raman transitions. In addition, we investigate, both optically and electrically, the doping that is introduced by the nanoislands.

Spectrally coded optical nanosectioning (SpecON) with biocompatible metal–dielectric-coated substrates

Significance In this paper we describe a high-resolution light microscopy technique that translates spatial (position) information of fluorescent markers into spectral (color) information for improved biological imaging. By designing a thin biocompatible nanostructure on a microscope slide, we show how the distance-dependent spectral “fingerprint” of fluorophores can be used to monitor their relative distance from the nanostructure with an accuracy far beyond the resolution power of a conventional light microscope. We demonstrate the technique by studying the positions and dynamics of key proteins that play a role in cell motility. Fluorescence nanosectioning within a submicron region above an interface is desirable for many disciplines in the life sciences. A drawback, however, to most current approaches is the a priori need to physically scan a sculptured point spread function in the axial dimension, which can be undesirable for optically sensitive or highly dynamic samples. Here we demonstrate a fluorescence imaging approach that can overcome the need for scanning by exploiting the position-dependent emission spectrum of fluorophores above a simple biocompatible nanostructure. To achieve this we have designed a thin metal–dielectric-coated substrate, where the spectral modification to the total measured fluorescence can be used to estimate the axial fluorophore distribution within distances of 10–150 nm above the substrate with an accuracy of up to 5–10 nm. The modeling and feasibility of the approach are verified and successfully applied to elucidate nanoscale adhesion protein and filopodia dynamics in migrating cells. It is likely that the general principle can find broader applications in, for example, single-molecule studies, biosensing, and studying fast dynamic processes.

Metal-graphene-metal photodetectors

We present photoconductivity studies of metal/graphene interfaces, discuss the origin of the photoconductive behavior, and present ultrafast photocurrent measurements. Conversion of surface plasmon polaritons into electrical current at metal/graphene interfaces will also be presented. Based on these findings we developed several concepts for graphenebased photodetectors. One of these concepts involves the deposition of inter-digitated metal electrodes on graphene to realize a metal-graphene-metal photodetector. We used this device to demonstrate the faithful detection of data streams at rates of 10 gigabits per second. Another concept relies on the monolithic integrating of graphene with a Fabry-Pérot microcavity. This device benefits from the large increase of the optical field inside a resonant cavity, giving rise to increased absorption. We demonstrate that the optical absorption can be 26-fold enhanced as compared to conventional devices.

Intrinsic speed limit of graphene-based photodetectors

In this contribution, we present measurements of the intrinsic speed limit of graphene photodetectors using ultrashort laser pulses. We obtain a bandwidth of 262 GHz, showing the great potential of graphene for high-speed optoelectronics.

Fluorescence enhancements and spectral modifications near the cut-off frequency of plasmonic structure

The effect of a population of fluorophores coupling to weakly bound surface plasmons in dielectric/metal/dielectric structures is investigated for the purpose of fluorescence enhancement near interfaces and live cell fluorescence surface imaging. We show theoretically and experimentally that for sufficient fluorophore concentrations near such SPP supporting structures significant enhancements in the radiative emission intensity can be observed, with a spectral modification that can be correlated to the average separation of the fluorophores from the substrate. We will discuss the theory behind the effect and some experimental results on imaging labeled proteins in the focal adhesion sites of cells.

Far -infrared power detector with optical readout

An electro-optic detection of the coherent free space terahertz waves has boosted the development of the terahertz technology [1]. Unlike other THz detectors, this detector type operates at room temperature and utilizes an electric field induced birefringence of the optical crystal. Induced birefringence can be monitored by an ultrashort near-infrared pulse and provides information about the instant vector of the THz electric field [2], so highly sensitive terahertz detection at room temperature is achievable in principle. In recently presented measurement of the terahertz quantum cascade laser output power in the CdTe electro-optic crystal is obtained from directly sampled electric field of the laser coherent radiation [3], but little attention was paid to responsivity of the used electro-optic crystal. Therefore, optimization of the detector concept has to be performed.