High Optical Gain Research Articles

Novel Cascade Diode Lasers Based on Type-I Quantum Wells

A novel approach for cascade diode lasers was developed based on type-I quantum wells (QWs). This design adopted the leaky window effect in the GaSb / InAs band alignment for cascade pumping, and exploits the carriers recycling by the cascade pumping and the high optical gain of the type-I QWs. Two-stage cascade lasers were designed and fabricated for 2.45 and 3.0 μm, demonstrating twofold improvement in the internal efficiency. Record continuous-wave (CW) output power of 1.2 W for 2.45 um and 590 mW for 3.0 um were achieved in room temperature (RT), and the devices operate with higher power conversion efficiency at high power level, compared to conventional single-stage diode lasers.

Rare earth silicates as gain media for silicon photonics [Invited

ErxY2−xSiO5 and ErxYbyY2−x−ySiO5 crystalline thin films were investigated to apply to the high-gain media for silicon photonics. In addition to the sol–gel method, the directed self-assembly approach, using layer-by-layer deposition techniques, was also introduced to improve the crystallinity. The relaxation processes in Er ions were discussed to clarify the contribution of the energy transfer and cooperative upconversion. After optimization of the Er content, a Si photonic crystal slot ErxY2−xSiO5 waveguide amplifier was fabricated, and a 30 dB/cm modal gain was demonstrated. This achievement demonstrates the potential for compact and high optical gain devices on Si chips.

Investigation of gain characteristics in mixed crystals LiMeF4 (Me = Y, Lu, Yb) doped by Ce3+ ions

Differential gain spectra in the range 295–335 nm were measured in crystals of scheelite structure LiY1 − x Lu x F4 (x = 0–1), doped by Ce3+ ions. It is shown that variation of Lu3+ and Y3+ ions relative content in LiY1 − x Lu x F4 crystals allows to manipulate the spectral width of the amplification band. Cross-sections of excited-state absorption at the wavelengths of Ce3+ luminescence, probability ratios of formation and thermal destruction of color centers depending on the Y3+ ions content in LiY1 − x Lu x F4 crystals were estimated. Even better gain characteristics have been demonstrated by LiLuF4:Ce3+, doped by Yb3+ ions. The highest optical gain coefficient with a wide amplification band among studied samples was observed in LiLuF4:Ce3+ crystal, codoped by Yb3+ ions.

Design and optimization of tellurite hybrid microstructured optical fiber with high nonlinearity and low flattened chromatic dispersion for optical parametric amplification

The linear phase-mismatch and the optical signal gain in the highly nonlinear tellurite hybrid microstructured optical fiber based on the degenerate four-wave mixing are numerically simulated. The core and the cladding of this fiber are designed by TeO2–Li2O–WO3–MoO3–Nb2O5 and TeO2–ZnO–Na2CO3–P2O5 glass, respectively. This fiber has high nonlinearity and at the same time the chromatic dispersion is flattened and close to zero. High optical signal gain and broad band can be obtained by using a short length of this fiber with low pump power.

Naturally Assembled Excimers in Xanthenes as Singular and Highly Efficient Laser Dyes in Liquid and Solid Media

Herein, for the first time, outstanding laser performance is demonstrated in liquid solution and solid state by naturally assembling excimers of organic fluorophores (rosamine dyes). Highly efficient and photostable laser dye properties, with broadened tunability covering 80 nm in the red spectral region (590–670 nm), is attributed to the coexistence of monomers and excimers induced under high optical gain conditions. Amplified spontaneous emission measurements in rosamine‐doped polymer thin films show that the excimer exhibits a threshold lower and a gain higher than those corresponding to the monomer species. These laser properties make rosamines excellent candidates for biophotonic and spectroscopic applications, overcoming the main drawbacks exhibited by other long‐wavelength (>600 nm) laser dyes, including low absorption at the standard pump wavelength (532 nm), low laser efficiency, and poor chemo‐ and photostability.

Tensely strained GeSn alloys as optical gain media

This letter presents the epitaxial growth and characterization of a heterostructure for an electrically injected laser, based on a strained GeSn active well. The elastic strain within the GeSn well can be tuned from compressive to tensile by high quality large Sn content (Si)GeSn buffers. The optimum combination of tensile strain and Sn alloying softens the requirements upon indirect to direct bandgap transition. We theoretically discuss the strain-doping relation for maximum net gain in the GeSn active layer. Employing tensile strain of 0.5% enables reasonable high optical gain values for Ge0.94Sn0.06 and even without any n-type doping for Ge0.92Sn0.08.

Optical investigation of InAs quantum dashes grown on InP(0 0 1) vicinal substrate

We investigate with photoluminescence (PL) measurements the optoelectronic properties of self-organized InAs quantum dots (QDs) grown on nominal InP(001) substrate. InAs/InP(001) QDs are grown by Molecular Beam Epitaxy (MBE) method with optimized conditions in Stranski-Krastanov regime. A lateral coupling behavior was shown by photoluminescence spectroscopy. This phenomena is considered as a degradation source of the optoelectronic properties of InAs/InP(001) QDs used in lasers applications. In order to overcome this disadvantage behavior, we have studied the optical properties of InAs quantum islands (QIs) grown on vicinal InP(001) with 2° off miscut angle toward the [110] direction. From Polarized Photoluminescence (PPL) measurements, we have deduced that InAs quantum nanostructures have quantum dashes (QDas) form elongated in [1–10] direction. From excitation density PL measurements, we have evidenced that the different observed PL peaks are attributed to the emission of InAs QDas of different size. The lateral coupling behavior is completely eliminated in the case of this sample. The temperature-dependent PL measurements show a good thermal stability and an emission wavelength at room temperature around 1.55μm of the vicinal sample. All these properties prove that this sample possess favorable characteristics for microlasers based devices functioning at room temperature and for optical telecommunication with long range weapon. The broad emission range observed at 300K of the vicinal sample gives the possibility to use it as an active zone in solar cells and in infrared photodectectors of high optical gain and excellent sensitivity on a wide energy range.

High upconversion optical gain of Er3+-doped tellurite glass

The upconversion technique of rare-earth-doped glasses or crystals is a potential route for achieving short-wavelength lasers (SWLs). The optical gain is an important parameter for evaluating lasing medium performance. Therefore, upconversion luminescence in glass with composition of 70TeO2–9PbF2–10AlF3–10BaF2–1Er2O3 was demonstrated, and the small-signal optical gain was measured via an amplified spontaneous emission technique when the sample was excited with a 980 nm laser. It was found that the optical gain was as large as 4.0 dB/cm, thus indicating that this glass may be a good medium for SWLs.

Dual ASE from the monomeric and excimeric states of a conjugated-polymer (PDHF) in solution

Abstract In this work, the spectral and amplified spontaneous emission (ASE) properties of a conjugated polymer poly [9, 9-di-(2′-ethylhexyl) fluorenyl-2, 7-diyl] (PDHF) in tetrahydrofuran (THF) have been studied. Our results showed that the absorption spectra of PDHF in THF have only one peak under wide range of concentrations (0.012 mol/m3 to 0.39 mol/m3), it could be seen that the shape of the absorption did not change; this indicates no dimer formation in these solutions for all concentrations mentioned above. On the other hand, the fluorescence spectra of PDHF in THF, at low concentration, showed two peaks, the primary at 415 nm and the secondary at 435 nm. The effects of concentration and the temperature on PDHF in THF under same conditions were studied. For example, the intensity of the peak at 435 nm became increasingly stronger for higher concentration and lower temperature. These are analogs to the excimeric behavior of organic molecules. Under high power pulsed laser excitation, we observed ASE at 418 nm and 437 nm. These ASE peaks could arise from the monomer and excimer states of the macromolecule respectively. The ASE spectra of PDHF were compared with the rhodamine 6 G (Rh 6 G). The most important and distinguishing features are that PDHF has a two times better photochemical stability than rhodamine 6 G and high optical gain compared with the conventional laser dyes.

Thin film formation of EuGa2S4 by rf sputtering and improvement of film transmittance

AbstractEuxCa1–xGa2S4 is one of the phosphors with high luminescence quantum efficiency. It has high optical gain due to less concentration quenching for Eu centre. Production of EuGa2S4 film with smooth surface would lead to a possible construction of surface emitting lasers. We prepared films by sputtering with EuGa2S4 target. The EuGa2S4 films deposited on the substrates heated at 550–640 °C had the surfaces smoother than that of films post‐annealed at 800–900 °C. The Eu3Ga5O12 films were deposited on the substrates heated at 660–800 °C. The Eu3Ga5O12 films, which were transparent and colourless, showed no photoluminescence and had smooth surfaces. The films were deposited on the cooled substrate for 30 min and sequentially deposited on the substrate heated at 640–700 °C for 10–30 min. The films, which consisted of two layers, EuGa2S4 and Eu3Ga5O12, show high transmittances and exhibited yellow green emission. The films have the possibility to be applied to the laser. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stimulated emission from ZnO thin films with high optical gain and low loss

Stimulated surface- and edge-emissions were investigated for ZnO thin films grown epitaxially by pulsed laser deposition. The lasing threshold was 0.32 MW/cm2 for surface pumping and 0.5 MW/cm2 for edge pumping, which is significantly lower than thresholds observed previously. A modified variable stripe length method was used to measure the gain, which was 1369 cm−1 for the N-band emission. Losses were measured using the shifting excitation spot method and values of 6.2 cm−1 and 6.3 cm−1 were found for the N-band and P-band, respectively. The measured gain and loss were the highest and lowest (respectively) ever reported for ZnO films.

Nonresonant tunneling phonon depopulated GaN based terahertz quantum cascade structures

GaN based terahertz quantum cascade structures are theoretically studied. Since the Fröhlich interaction is ∼15 times higher in GaN than in GaAs, level broadening makes obtaining appreciable optical gain difficult even with a large population inversion. A density matrix Monte Carlo method is used to calculate the broadening of the optical gain spectra as a function of lattice temperature. We find by using a proposed method of nonresonant tunneling and electron-longitudinal-optical phonon scattering for depopulation of the lower lasing state, that it is possible to sufficiently isolate the upper lasing state and control the lower lasing state lifetime to obtain high optical gain in GaN. The results predict lasing out to 300 K which is significantly higher than for GaAs based structures.

High optical gain of I–VII semiconductor quantum wells for efficient light-emitting devices

In this work, we propose I–VII semiconductor hetero-structure quantum wells as active layers, which have a large exciton binding energy suitable for strong optical transitions in light-emitting devices. They are also closely lattice-matched to Si substrates, therefore free from defects. A multiband effective mass approach and non-Markovian model including the excitonic effects are used to study the optical gain. It is found that the optical gain of our design would be much higher than that of III–V nitride layers due to the inherent strong excitonic effects and negligible electrostatic fields within the active layers leading to highly efficient optical devices.

Enhancement of optical gain and amplified spontaneous emission due to waveguide geometry in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene

We report enhanced amplified spontaneous emission (ASE) and optical gain performance in a conjugated polymer (CP)-based thin film waveguide (WG) Si(100)/SiO2/poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) by encapsulating the active layer with a transparent dielectric film of poly(methyl methacrylate) (PMMA). With index matched SiO2 and PMMA claddings, symmetric WGs are formed that exhibit increased mode confinement and reduced propagation loss enabling lower ASE threshold (40%) and higher optical gain (50%) compared to Si(100)/SiO2/MEH-PPV/air asymmetric WGs. An extremely large net gain coefficient of 500 cm−1 is achieved under picosecond pulse excitation, which is >4× larger than values previously reported in the literature. Fabrication of symmetric WGs requires no complex processing techniques, thus offering a simple, low-cost approach for effectively controlling the ASE behavior of CP-based WGs and related optical devices.

Design and analysis of high-temperature operating 795 nm VCSELs for chip-scale atomic clocks

We propose a new structure design and analysis of high-temperature (>350 K) operating 795 nm vertical-cavity surface-emitting lasers (VCSELs) for 87Rb based chip-scale atomic clocks (CSACs). Compositions and thicknesses of the InAlGaAs/AlGaAs multiple quantum wells (MQWs) are optimized for high optical gain at elevated temperatures. The temperature sensitivity of the threshold current is estimated by calculating the temperature dependence of the cavity-mode gain. A self-consistent VCSEL model based on quasi-3D finite element analysis is employed to investigate the temperature distribution and output of the proposed structure. An output of 1 mW with a 3 dB bandwidth of 6 GHz is obtained from a 3 μm aperture VCSEL under 2.4 mA current at temperature higher than 340 K. These findings indicate that high-gain QWs and gain-offset consideration are especially crucial to make high-performance VCSELs at elevated temperatures.

Development of semipolar laser diode

AbstractIn this paper, we discuss experimental results and the potential for improvement of blue and green laser diode (LD) performance by using semipolar substrates. We show that the InGaN quantum well (QW) grown on a semipolar plane allows higher characteristic temperature, T0, and higher optical gain, which are important for improving laser efficiency, thanks to its unique electronic spectrum properties. Epitaxial structure growth benefits from the wider process window for certain orientations. At the same time it is associated with a risk of strain relaxation, which can be addressed by appropriate strain relaxation management and/or strain balancing of waveguide core.

Molecular beam epitaxial growths of high-optical-gain InAs quantum dots on GaAs for long-wavelength emission

Uniform InAs quantum dots (QDs) have been realized on GaAs by molecular beam epitaxy, which can increase maximum optical gain in laser diodes without sacrificing original high-density and high optical quality characteristics. This improvement enables laser operation under higher environment temperature. In order to improve QD uniformity, we searched for a proper growth condition and sequence for suppressing In out-diffusion during QD coverage. Although the same initial InAs QD structure on GaAs was used, PL linewidth was reduced after coverage under such improved conditions. In QD lasers, higher optical gain as large as 54cm−1 at room temperature which is about 10cm−1 larger than our previous data was realized. Those improved QDs are attractive for realizing novel optical devices.

Cleaved-coupled nanowire lasers

The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed toward controlling their size, shape, and composition, most nanowire lasers currently suffer from emitting at multiple frequencies simultaneously, arising from the longitudinal modes native to simple Fabry-Pérot cavities. Cleaved-coupled cavities, two Fabry-Pérot cavities that are axially coupled through an air gap, are a promising architecture to produce single-frequency emission. The miniaturization of this concept, however, imposes a restriction on the dimensions of the intercavity gaps because severe optical losses are incurred when the cross-sectional dimensions of cavities become comparable to the lasing wavelength. Here we theoretically investigate and experimentally demonstrate spectral manipulation of lasing modes by creating cleaved-coupled cavities in gallium nitride (GaN) nanowires. Lasing operation at a single UV wavelength at room temperature was achieved using nanoscale gaps to create the smallest cleaved-coupled cavities to date. Besides the reduced number of lasing modes, the cleaved-coupled nanowires also operate with a lower threshold gain than that of the individual component nanowires. Good agreement was found between the measured lasing spectra and the predicted spectral modes obtained by simulating optical coupling properties. This agreement between theory and experiment presents design principles to rationally control the lasing modes in cleaved-coupled nanowire lasers.

A neutron radiation-hardened superluminescent diode

We present a novel superluminescent diode (SLD) with high optical performances for hardened neutron irradiation. The degradation of the light output from the SLDs is caused by a reduction of the minority carrier lifetime resulting from displacement damage after high-energy neutron irradiation. The SLDs with a higher pre-irradiation light output will be less sensitive to radiation. We have selected an InGaAsP/InP multi-quantum well (MQW) as the active region structure for its performance, its high optical gain and minute active region. Graded-index separate-confinement-heterostructure (GRIN-SCH) has been applied for the waveguide structure. A specific absorbing region and anti-reflective coatings have been designed and optimized. Moreover, the radiation test results indicate that the SLD with an InGaAsP/InP MQW structure has better neutron hardening ability than the SLD with DH structures after a 6 × 1013 − 1 × 1014 n/cm2 1 MeV neutron irradiation.

Fluorescence imaging beyond the ballistic regime by ultrasound pulse guided digital phase conjugation.

Fluorescence imaging has revolutionized biomedical research over the past three decades. Its high molecular specificity and unrivaled single molecule level sensitivity have enabled breakthroughs in a variety of research fields. For in vivo applications, its major limitation is the superficial imaging depth as random scattering in biological tissues causes exponential attenuation of the ballistic component of a light wave. Here we present fluorescence imaging beyond the ballistic regime by combining single cycle pulsed ultrasound modulation and digital optical phase conjugation. We demonstrate a near isotropic 3D localized sound-light interaction zone. With the exceptionally high optical gain provided by the digital optical phase conjugation system, we can deliver sufficient optical power to a focus inside highly scattering media for not only fluorescence imaging but also a variety of linear and nonlinear spectroscopy measurements. This technology paves the way for many important applications in both fundamental biology research and clinical studies.