4.1. Nanomachining with Scanning Probes 1831 4.2. Soft Lithography 1832 4.3. Embossing with Rigid Masters 1835 4.4. Near-Field Phase-Shifting Photolithography 1835 4.5. Topographically Directed Photolithography 1837 4.6. Topographically Directed Etching 1837 4.7. Lithography with Neutral Metastable Atoms 1838 4.8. Approaches to Size Reduction 1839 5. Techniques for Making Regular or Simple Patterns 1839

https://www.engr.colostate.edu/ECE581/fall07/xia%20chem%20rev%2099.pdf

Unconventional Methods for Fabricating and Patterning Nanostructures.

Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages

Carrier transport can significantly affect the high-speed properties of quantum-well lasers. The authors have developed a model and derived analytical expressions for the modulation response, resonance frequency, damping rate, and K factor to include these effects. They show theoretically and experimentally that carrier transport can lead to significant low-frequency parasitic-like rolloff that reduces the modulation response by as much as a factor of six in quantum-well lasers. They also show that, in addition, it leads to a reduction in the effective differential gain and thus the resonance frequency, while the nonlinear gain compression factor remains largely unaffected by it. The authors present the temperature dependence data for the K factor as further evidence for the effects of carrier transport. >

High speed quantum-well lasers and carrier transport effects

This paper is divided into two major parts. Following a brief introduction that establishes some definitions and assumptions, Section II updates our earlier study on the limits of the RF performance of optical links. Section III reviews progress since our 1997 review paper in the development of devices enabling link performance closer to these limits, including (but not limited to): 1) cascade lasers that permit broad-band direct modulation links with gain >0 dB; 2) injection-locked edge- and surface-emitting lasers at 1300 and 1550 nm with modulation frequency responses as great as 40 GHz; 3) modulators with improved performance, especially electroabsorption modulators that now have switching voltages as low as 0.36 V, or handle optical powers as great as 60 mW, or have bandwidths as great as 50 GHz (but not all three of these in one device yet); and 4) high-speed photodetectors with high saturation currents, e.g., a 20-GHz device with a saturation current of 90 mA and a 55-GHz device with saturation at 50 mA. We conclude in Section IV by summarizing the component developments necessary for higher performance RF-over-fiber links, i.e.: 1) semiconductor lasers (for direct modulation) that have higher slope efficiency and bandwidth and lower relative intensity noise (RIN) at reasonable bias current levels; 2) continuous wave (CW) lasers (for external modulation) with higher fiber-coupled power and lower RIN; 3) higher frequency lower loss external modulators with more linear transfer functions and lower V/sub /spl pi// that can withstand larger CW optical powers; and 4) photodetectors with higher responsivity and bandwidth that respond linearly even when illuminated by greater average optical powers.

Limits on the performance of RF-over-fiber links and their impact on device design

It is a well-known phenomenon that two solids with sufficiently flat surfaces can stick to each other when brought into intimate contact in ambient air at room temperature. The attraction between the two bodies is primarily mediated through van der Waals forces or hydrogen bonding. Without a subsequent heating step, that type of bonding is reversible. Annealing may increase the energy of adhesion up to the cohesive strength of the materials concerned. The wafer bonding phenomena in brittle materials systems, especially in silicon, is reviewed in the experiment. The focus is on low temperature bonding techniques. The pivotal influence chemical species on the surfaces have on the subsequent type of bonding (van der Waals, hydrogen, covalent bonding, mechanical interlocking) is discussed. Methods of modifying the surface chemistry for tailoring bonding properties are addressed. The paper is aimed at providing an overview of the current understanding of the factors determining the bondability and strength of the bonding obtainable. The authors assess the present state of the experimental methods for determining basic parameters governing the adhesion. A number of examples illustrate the applicability of fusion bonding for as diverse fields as opto-electronics, microsystems technology, and fabrication of advanced substrates like silicon-on-insulator wafers.

/pdf/wafer-direct-bonding-tailoring-adhesion-between-brittle-zsnabx6cpy.pdf

Wafer direct bonding: tailoring adhesion between brittle materials

The fabrication and basic characteristics of a InGaAs/InGaAsP multi-quantum-well (MQW) electroabsorption modulator with a novel structure integrated with a distributed-feedback (DFB) laser are presented. A fundamental study was performed on the applicability of the InGaAs/InGaAsP MQW structure to an electroabsorption-type modulator. Efficient attenuation small hole pileup and small chirp characteristics of a discrete modulator based on this MQW structure were demonstrated experimentally. A study of the controllability of in-plane band-gap energy by the use of selective-area metal-organic chemical vapor deposition (MOCVD) was also demonstrated. The modulator was monolithically integrated with a MQW DFB laser of the same material. Using a low-capacitance semi-insulating buried heterostructure, over 14 GHz modulation under high-light-output operations up to +10 dBm was achieved. Modulation at 10 Gb/s with a modulation voltage swing of only 1 V/sub pp/ demonstrates the potential value of this system for 1.55- mu m lightwave communications. >

InGaAs/InGaAsP MQW electroabsorption modulator integrated with a DFB laser fabricated by band-gap energy control selective area MOCVD

A novel structure electroabsorption modulator/DFB laser integrated device is proposed and demonstrated. Both functional devices consist of MQW structures with different quantum energy levels, which are automatically formed in the same MOCVD run by using a selective area growth technique. A fundamental modulation with a 12.6dB extinction ratio is demonstrated.

Novel structure MQW electroabsorption modulator/DFB-laser integrated device fabricated by selective area MOCVD growth

For the first time, the oscillation wavelength and the laser structure dependence of the K factor and of the nonlinear gain suppression coefficient e in semiconductor lasers were systematically investigated. As a result, we suggest that the nonlinear gain effect can be explained by the spectral hole burning theory.

Oscillation wavelength and laser structure dependence of nonlinear damping effect in semiconductor lasers

This paper describes the use of direct wafer bonding technique to implement the novel concept of "free-material and free-orientation integration" which we propose. The technique is applied for various wafer combinations of an InGaAsP material system, and the properties of the bonded structures are studied in terms of the crystalline and electrical characterization through transmission electron microscope, X-ray diffraction, and so on. This technique's advantage for use in the fabrication of lattice-mismatched structures is confirmed by the crystalline characterization, together with its second advantage of enabling bonded structures with an orientation mismatch, is investigated. The high crystalline quality of the bonded structures with both lattice and orientation mismatches is proved, and the electrical property of the bonded interface is examined for some of them. We show a practicability in a laser fabricated on a lattice- and orientation-mismatched structure by direct bonding. The results demonstrate the remarkable feasibility of using the direct wafer bonding technique to obtain integrated structures of material- and orientation-mismatched wafers with satisfactory quality.

Direct wafer bonding of III-V compound semiconductors for free-material and free-orientation integration

A novel structure for an InP-based ridge-waveguide (RWG) laser is demonstrated. It has a reverse-trapezoid-ridge shape which offers reduced threshold current and smaller electrical and thermal resistances over the conventional vertical-mesa (VM) RWG devices. We found this new RWG laser quite suitable for wide-temperature-range and high-power operations. 1.3-1.55-/spl mu/m wavelength strained InGaAsP-InP multiple quantum-well (MQW) lasers were demonstrated with the new ridge structure, which showed sufficient lasing behavior such as high-temperature lasing up to 165/spl deg/C and a high light output over 300 mW. This simple and high-performance laser structure was also applied for integrated lasers with other functional elements such as an absorption-type modulator or a beam-expander waveguide lens. To achieve low-threshold current operation in RWG lasers, suppression of the lateral-current spreading is important. We proposed a new method to effectively reduce this lateral diffusion current by using a doped active region with n-type dopant. Combined by the reduced transparency carrier density in the n-doped active region, this leads to a very low-threshold current of less than 2.2 mA for a coated 200-/spl mu/m-long cavity device, which is comparable to those of recent advanced same-material buried-heterostructure (BH) lasers. These excellent lasing properties, along with the sufficient device reliability under strict environmental conditions, make this reversed-mesa (RM) RWG a promising candidate for the widespread use of low-cost long-wavelength light sources.

Masahiro Aoki

Papers

InGaAs/InGaAsP MQW electroabsorption modulator integrated with a DFB laser fabricated by band-gap energy control selective area MOCVD

Novel structure MQW electroabsorption modulator/DFB-laser integrated device fabricated by selective area MOCVD growth

Oscillation wavelength and laser structure dependence of nonlinear damping effect in semiconductor lasers

Direct wafer bonding of III-V compound semiconductors for free-material and free-orientation integration

InP-based reversed-mesa ridge-waveguide structure for high-performance long-wavelength laser diodes