Sunay Turkdogan

A monolithic white laser

Monolithic semiconductor lasers capable of emitting over the full visible-colour spectrum have a wide range of important applications, such as solid-state lighting, full-colour displays, visible colour communications and multi-colour fluorescence sensing. The ultimate form of such a light source would be a monolithic white laser. However, realizing such a device has been challenging because of intrinsic difficulties in achieving epitaxial growth of the mismatched materials required for different colour emission. Here, we demonstrate a monolithic multi-segment semiconductor nanosheet based on a quaternary alloy of ZnCdSSe that simultaneously lases in the red, green and blue. This is made possible by a novel nanomaterial growth strategy that enables separate control of the composition, morphology and therefore bandgaps of the segments. Our nanolaser can be dynamically tuned to emit over the full visible-colour range, covering 70% more perceptible colours than the most commonly used illuminants.

Simultaneous two-color lasing in a single CdSSe heterostructure nanosheet

The ability of a single monolithic semiconductor structure to emit or lase in a broad spectrum range is of great importance for many applications such as solid-state lighting and multi-spectrum detection. But spectral range of a laser or light-emitting diode made of a given semiconductor is typically limited by its emission or gain bandwidth. Due to lattice mismatch, it is typically difficult to grow thin film or bulk materials with very different bandgaps in a monolithic fashion. But nanomaterials such as nanowires, nanobelts, nanosheets provide a unique opportunity. Here we report our experimental results demonstrating simultaneous lasing in two visible colors at 526 and 623 nm from a single CdSSe heterostructure nanosheet at room temperature. The 97 nm wavelength separation of the two colors is significantly larger than the gain bandwidth of a typical single II‐VI semiconductor material. Such lasing and light emission in a wide spectrum range from a single monolithic structure will have important applications mentioned above. (Some figures may appear in colour only in the online journal)

Preparation of Carbon Nanotube/TiO2 Mesoporous Hybrid Photoanode with Iron Pyrite (FeS2) Thin Films Counter Electrodes for Dye-Sensitized Solar Cell.

Multi-walled carbon nanotube (MWCNT)/TiO2 mesoporous networks can be employed as a new alternative photoanode in dye-sensitized solar cells (DSSCs). By using the MWCNT/TiO2 mesoporous as photoanodes in DSSC, we demonstrate that the MWCNT/TiO2 mesoporous photoanode is promising alternative to standard FTO/TiO2 mesoporous based DSSC due to larger specific surface area and high electrochemical activity. We also show that iron pyrite (FeS2) thin films can be used as an efficient counter electrode (CE), an alternative to the conventional high cost Pt based CE. We are able to synthesis FeS2 nanostructures utilizing a very cheap and easy hydrothermal growth route. MWCNT/TiO2 mesoporous based DSSCs with FeS2 CE achieved a high solar conversion efficiency of 7.27% under 100 mW cm−2 (AM 1.5G 1-Sun) simulated solar irradiance which is considerably (slightly) higher than that of A-CNT/TiO2 mesoporous based DSSCs with Pt CE. Outstanding performance of the FeS2 CE makes it a very promising choice among the various CE materials used in the conventional DSSC and it is expected to be used more often to achieve higher photon-to-electron conversion efficiencies.

CdxPb1–xS Alloy Nanowires and Heterostructures with Simultaneous Emission in Mid-Infrared and Visible Wavelengths

Alloying of CdS and PbS could potentially provide an important semiconductor with a wide range of bandgaps, with bandedge emission from mid-infrared to visible green, for various optoelectronic applications. We investigate the possibility of CdPbS alloy formation in nanowire and nanobelt forms, especially the dependence of alloy composition on two different cooling routes. Our results show that rapid cooling immediately after the growth phase can lead to a high-quality uniform alloy with Cd composition larger than possible at thermal equilibrium and by natural cooling. On the contrary, unassisted natural cooling leads to the formation of axial or core-shell heterostructures, containing segments with pure CdS and CdPbS alloys with lower Cd content than through rapid cooling. Such heterostructures with green and mid-infrared emission provide simultaneous access to two widely separated wavelengths from a single monolithic structure and can be important for many applications. Our results can help identify strategies for growing nanostructures with uniform alloy of high Cd incorporation, core-shell structures with shell serving as a passivating or protecting layer, or interesting longitudinal heterostructures. Both various heterostructures and uniform alloys of these materials could be important for high-efficiency solar cells, novel detectors, and nanolasing in wide spectral ranges.

Long lifetime, high density single-crystal erbium compound nanowires as a high optical gain material

Erbium-containing materials of long lifetime and high Er density are important for achieving strong luminescence and high optical gain in compact integrated photonics devices. We have systematically studied the lifetime and crystal quality as a function of growth conditions for an erbium compound that we recently reported, erbium chloride silicate (ECS). The lifetime for the best quality ECS nanowires can be as long as 540 μs, the longest for high-density Er-materials, representing a lifetime-density product as high as 8.7 × 1018 s cm−3. Such high density, long lifetime erbium materials can find many interesting applications such as compact lasers or amplifiers.

Multicolor and white lasers from semiconductor nanomaterials

We report on our recent research on demonstrating the simultaneous lasing in red, green and blue from a single monolithic nanosheet. The careful adjustment of relative output intensities of these elemental colors leads to an overall white appearance from a single laser structure. Larger tolerance to mismatch at nanoscale offers unique opportunity of growing monolithic semiconductor alloy structures of very dissimilar lattice constants and bandgaps. The wide range of available bandgaps from a single high quality nanostructure allow for simultaneous lasing in multiple colors such as red and green [1.2], or in all three elemental colors [3]. The flexibility of the materials and the structures allow the control of relative intensities of each individual colors, such that the overall color can appear as any visible colors in the triangular color gamut, especially as white. Such a monolithic multi-color or white laser can be potentially useful for many applications. In this talk, our recent efforts in growth and characterization of such unique nanostructures of alloy semiconductors to achieve lasing in white and multiple colors will be presented, together with demonstration of various lasing configurations. Some possible applications will be also discussed.

Semiconductor Nanolasers (A Tutorial)

This tutorial focuses on miniaturization of semiconductor lasers down to sub-micron scale based on both bottom-approach such as nanowires and nanosheets and top-down approach such as etched nano-pillars encased in metallic or plasmonic shells.

Mid-infrared lasing in a single lead sulfide subwavelength wire at 180 K

We report mid-infrared lasing around 3 μm from a single PbS subwavelength wire, with a cavity volume less than the wavelength cubed at 0.44 λ³. The maximal lasing temperature is 180 K under pulse operation.

Simultaneous green and red lasing in a single CdSSe heterostructure nanosheet at room temperature

We report simultaneous lasing in green and red colors at room temperature from a single CdSSe heterostructure nanosheet. The 51 nm wavelength separation exceeds the typical gain bandwidth of a single composition material.