The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Optical properties of metallic films for vertical-cavity optoelectronic devices.

The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

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Quantum dot solar cells

We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

Epitaxial liftoff is an alternative to lattice‐mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin‐film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self‐bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin‐film atomic lattices can be simultaneously imaged, showing only a thin (20–100 A) amorphous layer in between.

Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 mu m are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different. >

Dynamic, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers

Vertical-cavity electrically driven lasers with three GaInAs quantum wells and diameters of several μm exhibit roomtemperature pulsed current thresholds as low as 1·3mA with 958 nm output wavelength.

Low-Threshold Electrically Pumped Vertical Cavity Surface-Emitting Microlasers

Reflectance-difference spectroscopy (RDS) is employed to study in situ the (4\ifmmode\times\else\texttimes\fi{}2), (1\ifmmode\times\else\texttimes\fi{}6), (4\ifmmode\times\else\texttimes\fi{}6), (3\ifmmode\times\else\texttimes\fi{}1), (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\alpha}, (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\beta}, (2\ifmmode\times\else\texttimes\fi{}4)-\ensuremath{\gamma}, c(4\ifmmode\times\else\texttimes\fi{}4), and d(4\ifmmode\times\else\texttimes\fi{}4) reconstructions of (001) GaAs surfaces prepared in ultrahigh vacuum (UHV) by molecular-beam epitaxy and simultaneously characterized by reflection high-energy electron diffraction (RHEED). Reproducibility of the data is excellent. With the aid of previous theoretical calculations, we interpret characteristic spectral features at 1.9, 2.6, and 4.2 eV in terms of electronic excitations involving surface dimers of Ga, As, and As, respectively. Because RD couples to local electronic structure rather than to long-range order, RD spectra not only determine surface reconstructions but also provide details not accessible by RHEED, such as the existence of As dimers in the (1\ifmmode\times\else\texttimes\fi{}6), (4\ifmmode\times\else\texttimes\fi{}6), and (3\ifmmode\times\else\texttimes\fi{}1) reconstructions and of the fractional coverage within a given reconstruction. Our data show that the (3\ifmmode\times\else\texttimes\fi{}1), (1\ifmmode\times\else\texttimes\fi{}6), and (4\ifmmode\times\else\texttimes\fi{}6) reconstructions are at least partly determined by kinetics, since they can only be obtained by following specific heating or cooling procedures under very low ${\mathrm{As}}_{4}$ flux. More generally, it is possible to employ this optical technique to determine surface atomic and electronic structure. Because RD spectra can be obtained with the surface in any transparent ambient, the database that we have established here provides a new approach for elucidating surface reconstructions of (001) GaAs and hence the dynamics of surface reactions in non-UHV environments.

Reflectance-difference spectroscopy of (001) GaAs surfaces in ultrahigh vacuum.

Room-temperature continuous and pulsed lasing of vertical-cavity, single-quantum-well, surface-emitting microlasers is achieved at ~983nm. The active Ga[sub][0-8]In[sub][0-2]As single quantum well is 100 [angstroms] thick. These microlasers have the smallest gain medium volumes among lasers ever built. The entire laser structure is grown by molecular beam epitaxy and the microlasers are formed by chemically assisted ion-beam etching. The microlasers are 3-50-μm across. The minimum threshold currents are 1.1 mA (pulsed) and 1.5 mA (CW).

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L. T. Florez

Papers

Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates

Dynamic, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers

Low-Threshold Electrically Pumped Vertical Cavity Surface-Emitting Microlasers

Reflectance-difference spectroscopy of (001) GaAs surfaces in ultrahigh vacuum.

Room-Temperature Continuous-Wave Vertical-Cavity Single-Quantum-Well Microlaser Diodes