Multifrequency tapered plasmonic nanoantennas

We suggest a multifrequency Yagi–Uda-type nanoantenna loaded with an array of tapered plasmonic nanorods. The arrays of director nanorods can be used for the excitation of the nanoantenna by emitters matched spectrally with their resonant frequency and placed in their near-field region. An overlap of multifrequency operating bands of the nanoantenna provides novel opportunities for broadband operation, and the same nanoantenna architecture can be employed both as a transmitter and/or as a receiver, thus being useful for broadband wireless communication.     

Design of plasmonic nanoantennae for enhancing spontaneous emission

We apply two- and three-dimensional numerical calculations to study optical nanoantennae made of two coupled gold nanostructures, enclosing a single emitter in their gap. We show that, using structures manufacturable with today's nanotechnology, it is possible to increase the radiative decay rate by three orders of magnitude while keeping a quantum efficiency larger than 80% in the near-infrared regime. We examine the competition between the radiative and nonradiative processes in the presence of the antennae as a function of wavelength and antenna geometry. Our results hold great promise for improving the quantum efficiency of poor emitters such as silicon nanocrystals or carbon nanotubes.     

Picosecond pulse shortening by travelling wave amplified spontaneous emission

A new scheme for travelling-wave excitation of amplified spontaneous emission (ASE) employing transversal pumping is presented. ASE pulses emitted in the forward direction had a duration of 6 ps, corresponding to one half of the pump pulse duration. The spectrum was strongly structured, with individual components having a width of 0.1–1.0 Å. Essential characteristics of longitudinal and transversal excitation of travelling wave ASE are compared.     

Fabrication of electrically contacted plasmonic Schottky nanoantennas on silicon

We fabricate Schottky contact photodetectors based on electrically contacted Au nanoantennas on p-Si for the plasmonic detection of sub-bandgap photons in the optical communications wavelength range. Based on a physical model for the internal photoemission of hot carriers, photons coupled onto the Au nanoantennas excite resonant plasmons, which decay into energetic "hot" holes emitted over the Schottky barrier at the Au/p-Si interface, resulting in a photocurrent. In our device, the active Schottky area consists of Au/p-Si contact and is very small, whereas the probing pad for external electrical interconnection is larger but consists of Au/Ti/p-Si contact having a comparatively higher Schottky barrier, thus producing negligible photo and dark currents.We describe fabrication that involves an electron-beam lithography step overlaid with photolithography. This highly compact component is very promising for applications in high-density Si photonics.     

Wideband omnidirectional infrared absorber with a patchwork of plasmonic nanoantennas

In this Letter, we demonstrate experimentally that a patchwork of four metal-insulator-metal patches leads to an unpolarized wideband omnidirectional infrared absorption. Our structure absorbs 70% of the incident light on a 2.5 μm bandwidth at 8.5 μm. It paves the way to the design of wideband efficient plasmonic absorbers in the infrared spectrum.     

Calculated plasmonic enhancement of spontaneous emission from silicon nanocrystals with metallic gratings

The spontaneous emission rate (SER) enhancement due to the surface plasmon polariton (SPP) band gap effect of metallic gratings was evaluated by calculating the dispersion relation and electromagnetic field distribution simultaneously. Within the luminescence range of silicon nanocrystals (Si-NC) (?ω = 1.9 eV-1.6 eV), the calculated peak Purcell factors for Au-, Ag-, and Al-Si₃N₄ gratings are 266.9-30.1, 80.2-22.6, and 78.7-20.3, respectively. These results indicate that the common metals such as Au, Ag and Al, whose SPP resonance frequencies/energies are rather higher, can be adopted to enhance the SER of Si-NCs with the help of metallic gratings.     

Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness

We have investigated the light scattering properties of smooth and roughened nanoshells with dipolar and quadrupolar plasmon resonances tuned to 830 nm. In the dipole resonant case small but measurable variations in the angle dependent light scattering (ADLS) due to the introduction of surface roughness are observed. In the quadrupole case, the distinctive side lobe scattering characteristic of quadrupolar emission is strongly quenched for roughened nanoshells. PACS 81.07.-b; 78.67.-n     

Picosecond time scale imaging of mechanical contacts

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates using a ceramic ball bearing. Acoustic pulses at ∼40 GHz returning from the film-indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas ∼25 μm in radius. The deformation of the films during loading is thereby revealed. Furthermore, thermal wave imaging of the contact at megahertz frequencies is simultaneously achieved.     

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ～10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.     

Picosecond time resolved luminescence change upon saturation

A new sensitive method is reported for the measurement of picosecond duration luminescence change upon saturation from photoexcited samples. Examples of 600 ps and 20 ps lifetimes are given.     

Mapping the near fields of plasmonic nanoantennas by scattering‐type scanning near‐field optical microscopy

Near‐field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization‐resolved scattering‐type scanning near‐field optical microscopy (s‐SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S‐ and P‐polarized maps obtained in s‐SNOM as directly proportional to the in‐plane ( or ) and out‐of‐plane () near‐field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple‐scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s‐SNOM near‐field maps: the model reveals that the fields detected by polarization‐resolved interferometric s‐SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local near‐field components at each point (, and ). Furthermore, depending on the detection scheme (S‐ or P‐polarization), a different scaling of the scattered fields as a function of the local near‐field enhancement is obtained. The theoretical findings are corroborated by s‐SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s‐SNOM maps as a complex‐valued combination of vectorial local near fields is crucial to correctly understand scattering‐type near‐field microscopy measurements as well as to interpret the signals obtained in field‐enhanced spectroscopy.

     

Picosecond time resolved reflectivity of direct gap semiconductors

The relaxation of optically excited hot carrier distributions in GaAs and CdSe has been measured by time resolved reflectivity measurements with sub-picosecond optical pulses from a mode-locked cw dye laser. A characteristic transient in the reflectivity has been interpreted by an analysis of the perturbed optical dielectric function. Estimated energy loss rates are 0.4 eV/ps for electrons and holes having mean energies in the range of 1 to 2 eV for GaAs at room temperature.     

Interference and spontaneous emission

The field structure of the spontaneously emitted radiation from a single two-level atom is calculated in terms of atomic source variables and used to discuss the coherence properties of spontaneous emission.     

Picosecond time-resolved laser emission of coumarin 102: Solvent relaxation

The time-resolved laser emission of coumarin 102 was investigated in various aprotic and protic solvents at picosecond resolution by frequency upconversion technique. The spectral shift of the transient emission spectrum is attributed to solvent reorientation and the results are discussed.     

On the response time of plasmonic terahertz detectors

The response time of a plasmonic terahertz detector is investigated using two independent experimental techniques relying on time-resolved and heterodyne measurements, respectively. Both methods demonstrate that the detector response time does not exceed 110 ps. The results obtained suggest that the designed terahertz detector can be used as a component of high-speed telecommunication systems of a new generation.