High Optical Gain Research Articles

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.

Gain-enhanced optical coherence in a high optical gain semiconductor

The propagation of ultrashort pulses in a traveling wave semiconductor amplifier is considered. It is demonstrated that the effective polarization relaxation time, which determines the coherence of the interaction of pulses within the medium, strongly depends on its optical gain. As a result, it is shown that at large optical gains the coherence time can exceed the transverse relaxation time T2 by an order of magnitude, this accounting for the strong femtosecond superradiant pulse generation commonly observed in semiconductor laser structures.

Experimental investigation of wavelength-selective optical feedback for a high-power quantum dot superluminescent device with two-section structure

In this work, a high-power and broadband quantum dot superluminescent diode (QD-SLD) is achieved by using a two-section structure. The QD-SLD device consists of a tapered titled ridge waveguide section supplying for high optical gain and a straight titled ridge waveguide section to tune optical feedback from the rear facet of the device. The key point of our design is to achieve the wavelength-selective optical feedback to the emission of the QDs' ground state (GS) and 1st excited state (ES) by tuning the current densities injected in the straight titled section. With GS-dominant optical feedback under proper current-injection of the straight titled region, a high output power of 338 mW and a broad bandwidth of 65 nm is obtained simultaneously by the contribution associated to the QDs' GS and 1st ES emission.

Compressing light and sound

Using stimulated Brillouin scattering to achieve extremely high optical gain in silicon nanostructures may allow the realization of new integrated chip-scale photonic devices.

Design of three-well indirect pumping terahertz quantum cascade lasers for high optical gain based on nonequilibrium Green’s function analysis

The nonequilibrium Green’s function approach is applied to the design of three-well indirect pumping terahertz (THz) quantum cascade lasers (QCLs) based on a resonant phonon depopulation scheme. The effects of the anticrossing of the injector states and the dipole matrix element of the laser levels on the optical gain of THz QCLs are studied. The results show that a design that results in a more pronounced anticrossing of the injector states will achieve a higher optical gain in the indirect pumping scheme compared to the traditional resonant-tunneling injection scheme. This offers in general a more efficient coherent resonant-tunneling transport of electrons in the indirect pumping scheme. It is also shown that, for operating temperatures below 200 K and low lasing frequencies, larger dipole matrix elements, i.e., vertical optical transitions, offer a higher optical gain. In contrast, in the case of high lasing frequencies, smaller dipole matrix elements, i.e., diagonal optical transitions are better for achieving a higher optical gain.

Deep-tissue focal fluorescence imaging with digitally time-reversed ultrasound-encoded light

Fluorescence imaging is one of the most important research tools in biomedical sciences. However, scattering of light severely impedes imaging of thick biological samples beyond the ballistic regime. Here we directly show focusing and high-resolution fluorescence imaging deep inside biological tissues by digitally time-reversing ultrasound-tagged light with high optical gain (~5×105). We confirm the presence of a time-reversed optical focus along with a diffuse background—a corollary of partial phase conjugation—and develop an approach for dynamic background cancellation. To illustrate the potential of our method, we image complex fluorescent objects and tumour microtissues at an unprecedented depth of 2.5 mm in biological tissues at a lateral resolution of 36 μm×52 μm and an axial resolution of 657 μm. Our results set the stage for a range of deep-tissue imaging applications in biomedical research and medical diagnostics.

Multifunctional zinc oxide nanostructures for a new generation of devices

ZnO is a natural n-type widespread semiconductor with wide direct bandgap of 3.37eV, large exciton bending energy (60meV) and high optical gain (300cm−1). One dimensional ZnO nanomaterials such as nanowires or nanorods have focused much attention due to their multifunctionality in optoelectronic devices, gas sensing, piezoelectricity and thin film transistors for transparent and flexible electronics. Solution-phase chemical synthesis of nanomaterials has several important advantages, as low temperatures, high versatility, low cost, simple equipments and handling.The paper presents the chemical bath deposition synthesis and characterization of ZnO 1D nanostructures grown on glass substrates seeded with gold layer, pre-prepared ZnO nanoparticles or sol–gel derived ZnO layer. The obtained ZnO nanowires/nanorods were structurally and morphologically characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and the growth mechanism is discussed. The seed layer significantly affects the surface distribution and orientation of the grown 1D nanostructures. The diameter of the nanowires is mainly controlled by the concentration and temperature of the growth solution. The effect of growth conditions on the surface chemical composition and surface states/defects of semiconducting 1D nanostructures was investigated using X-ray photoelectron spectroscopy. The optical and electrical properties are presented.

Gain evaluation of compact designing on Er 3+/Yb 3+ codoped germanate glass channel waveguide

Bent waveguide structures (U- and F-bend) based on UV-sensitive Er 3+/Yb 3+ codoped germanate glass substrates have been designed to achieve high-gain C-band amplification. Using simulated-bend method, the optimal radius for curved structure is offered to be 1.90 cm with loss coefficient of 0.0015 dB/cm, as the substrate size is minimally schemed. In the wavelength range of 1528–1559 nm, obvious gain enhancement for the bent structure waveguides is anticipated, while, for the F-bend waveguide, the internal gain at 1533.8 nm wavelength is derived to be 22.55 dB, which is much higher than the value of 14.06 dB in the U-bend waveguide, and over three times higher than that of the straight one, after compensating both the bend loss and the transition loss. The simulation results indicate that the bent structure designing is beneficial in attaining high optical gain in Er 3+/Yb 3+ codoped germanate glass substrates, which assures that long-period grating can be applied to implement practical C-band gain-flattened amplification.

Electrically pumped ultraviolet random lasing from ZnO films: Compensation between optical gain and light scattering

We have demonstrated the compensation between optical gain and light scattering in electrically pumped random lasing (RL) from ZnO films through investigating the electroluminescence of MgxZn1−xO/ZnO (x=0.25 and 1) heterostructured devices. As the active ZnO films are sol-gel derived, both heterostructured devices can be electrically pumped into RL at appropriate forward currents. While the active ZnO films are prepared by sputtering, only the MgO/ZnO heterostructured devices can generate RL pumped with sufficient forward currents. It is believed that the light scattering in the sputtered ZnO films is weaker than that in the sol-gel derived ones. However, the inferior light scattering capability of the sputtered ZnO films can be compensated by the MgO/ZnO-heterostructure-enabled higher optical gain in the electrically pumped RL.

Improved hole confinement in GaInAsN–GaAsSbN thin double-layer quantum-well structure for telecom-wavelength lasers

In this work we demonstrated increased hole confinement in a bilayer quantum well that consists of two thin layers of GaInAsN/GaAsSbN confined by GaAs barriers. Comparison between the temperature dependence of photoluminescence intensity of the bilayer and GaInAsN quantum wells indicated that electrons rather than holes are the less confined carriers in the bilayer structure. This structure enables independent control of the band gap energy, band offsets and reduces the temperature sensitivity of laser performance. The calculations showed that a bilayer based short-period superlattice would provide a high optical gain at 1.3–1.55 μm due to increased electron-hole wave functions overlap.

Enhancement of Optical Gain in Floating-Base InGaP–GaAs Heterojunction Phototransistors

Comparative studies on the performances of floating-base InGaP-GaAs heterojunction bipolar phototransistors (HPTs) with and without base contact metal were carried out. Both the fabricated HPTs with base contact metal (HPTs-WB) and without base contact metal (HPTs-WOB) exhibited collector dark current as low as 0.2 pA when they were operated in a floating-base configuration at VCE = 1 V. The HPTs-WOB demonstrated much higher optical gain (120 at an optical power of 1.1 μW irradiated by a 635-nm laser) than the HPTs-WB. The optical gain showed dependency on the illuminating optical power, increasing as the illuminating optical power was increased.

Optical gain characteristics of staggered InGaN quantum wells lasers

Staggered InGaN quantum wells (QWs) are analyzed as improved gain media for laser diodes (LDs) lasing at 440 and 500 nm. The calculation of band structure is based on a 6-band k⋅p method taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. Staggered InGaN QWs with two-layer and three-layer step-function like In-content InGaN QWs structures are investigated to enhance the optical gain as well as to reduce the threshold current density for LDs emitting at 440 and 500 nm. Our analysis shows that the optical gain is enhanced by 1.5–2.1 times by utilizing the staggered InGaN QW active region emitting at 440 nm, which leads to a reduction of the threshold current density up to 24% as compared to that of the conventional InGaN QW laser. Staggered InGaN QWs with enhanced optical gain shows significantly reduced blue-shift as carrier density increases, which enables nitride QWs with high optical gain in the green spectral regime. The use of green-emitting three-layer staggered InGaN QW is also expected to lead to reduction in the threshold carrier density by 30%.

Random lasing in nanocrystalline ZnO powders

AbstractWe investigate the properties of random lasing in nanocrystalline ZnO powders. The lowest threshold for lasing occurs for average particle diameters of about 260 nm. Reproducible lasing features are achieved for reduced ensemble sizes. Spatially resolved luminescence spectroscopy is used to probe directly the degree of localization of random laser mode. We find that strongly confined and extended modes can co‐exist in the same spatial area. However, localized modes appear for small optical gain while extended modes are only supported in the presence of large optical gain, as is expected from theory. Localized and extended random‐laser modes co‐exist in space but appear in spectral regions of low and high optical gain, respectively. magnified image

Fabrication Methods and Luminescent Properties of ZnO Materials for Light-Emitting Diodes

Zinc oxide (ZnO) is a potential candidate material for optoelectronic applications, especially for blue to ultraviolet light emitting devices, due to its fundamental advantages, such as direct wide band gap of 3.37 eV, large exciton binding energy of 60 meV, and high optical gain of 320 cm−1 at room temperature. Its luminescent properties have been intensively investigated for samples, in the form of bulk, thin film, or nanostructure, prepared by various methods and doped with different impurities. In this paper, we first review briefly the recent progress in this field. Then a comprehensive summary of the research carried out in our laboratory on ZnO preparation and its luminescent properties, will be presented, in which the involved samples include ZnO films and nanorods prepared with different methods and doped with n-type or p-type impurities. The results of ZnO based LEDs will also be discussed.

Gain bandwidth of 80 nm and 2 dB/cm peak gain in Al_2O_3:Er^3+ optical amplifiers on silicon

Erbium-doped aluminum oxide integrated optical amplifiers were fabricated on silicon substrates, and their characteristics were investigated for Er concentrations ranging from 0.27 to 4.2×1020 cm−3. Background losses below 0.3 dB/cm at 1320 nm were measured. For optimum Er concentrations in the range of 1 to 2×1020 cm−3, an internal net gain was obtained over a wavelength range of 80 nm(1500-1580 nm), and a peak gain of 2.0 dB/cm was measured at 1533 nm. The broadband and high peak gain are attributed to an optimized fabrication process, improved waveguide design, and pumping at 977 nm as opposed to 1480 nm. In a 5.4-cm-long amplifier, a total internal net gain of up to 9.3 dB was measured. By use of a rate-equation model, an internal net gain of 33 dB at the 1533 nm gain peak and more than 20 dB for all wavelengths within the telecom C-band (1525-1565 nm) are predicted for a launched signal power of 1 μW when launching 100 mW of pump power into a 24-cm-long amplifier. The high optical gain demonstrates that Al2O3:Er3+ is a competitive technology for active integrated optics.

Generation of tunable picosecond pulses by pulse stacking in an Yb-fiber gain-assisted pulse stacker

We report on the generation of tunable picosecond pulses of high repetition rate by pulse stacking in an Yb-fiber stacker that benefited from high optical gain, properly time delay and laser synchronization. The gain-assisted pulse stacker could be controlled for pulse shaping to produce tunable pulse duration from ps to sub-ns range by managing the intracavity dispersion and adjusting the time delay. The energy loss during the pulse shaping process was compensated by the gain of a 60-cm-long Yb-fiber pumped by a diode laser. The temporal profiles of the output pulses were measured by using a special cross-correlation technique. The duration of the stacked pulses could be tuned from 5 to 200 ps with a controllable time interval.

Correlation Between the Optical Performance of the Reflective Polarizer and the Structure of LCD Backlight

The optical performance of the reflective polarizer was investigated over three kinds of direct-lit backlights and/or different configurations of the optical sheets. The optical gain factor achieved by using the reflective polarizer increased as the diffuse nature of the optical sheet was enhanced, because of the more efficient polarization conversion. In addition, the gain factor decreased substantially in flat-lamp backlights compared to tubular-lamp backlights, which was ascribed to the high loss factor during the polarization recycling process due to the complex structure and large area of flat lamps. All these results suggested that reducing the loss factor of the backlight is very important for efficient polarization conversion and thus high optical gain of the reflective polarizer.

High-power and large-alignment-tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser

Butt-end fiber coupling is a compact and efficient scheme especially suitable for a surface-emitting photonic crystal (PC) band-edge laser. The honeycomb-lattice PC offers a relatively large intact area without air-holes, and therefore, a high optical gain. The use of a Γ-point band-edge mode allows vertical laser emission, yielding a fiber-coupled output power of more than 30 μW. The system is virtually alignment-free in the lateral directions; however, it exhibits oscillatory decay in the vertical direction, which can be significantly suppressed by using an angle-cleaved fiber tip.

Spectral properties and broadband optical amplification of Yb-Bi codoped MgO-Al_2O_3-ZnO-SiO_2 glasses

We reported on the spectral properties of Yb-Bi codoped MgO-Al2O3-ZnO-SiO2 glasses. The near-infrared luminescence intensity in Yb-Bi codoped glass was ~31 times stronger than that of Bi-doped glass with 980 nm laser diode excitation. No optical amplification was observed in Bi-doped glass, while apparent broadband optical amplification between 1270 and 1340 nm was observed from Yb-Bi codoped glass with 980 nm laser diode excitation. The highest optical gain and gain coefficient at 1270 nm of Yb-Bi codoped silicate glass reached to 1.57 dB and 7.84 dB⋅cm−1, respectively. Yb-Bi codoped silicate glass is a promising candidate for broadband optical amplification.

Punchthrough Enhanced Phototransistor Fabricated in Standard CMOS Process

A simple lateral structure phototransistor, combined with a normal phototransistor and a punchthrough transistor, has been successfully designed and fabricated in standard commercial CSMC 0.5- mum CMOS process. The proposed punchthrough enhancement mechanism provides a high optical gain of close to 107 for a low-level optical power of 7.0 times10-15W at a wavelength of 650 nm. Compared with conventional punchthrough phototransistors, a lower dark current of around 1 muA is obtained at a 2.0-V operating voltage.