Low Threshold Power Research Articles

Stimulated emission at 340 nm from AlGaN multiple quantum well grown using high temperature AlN buffer technologies on sapphire

It is necessary to further improve crystal quality of AlGaN multiple quantum well (MQW) structures on sapphire in order to achieve ultraviolet (UV) laser diodes. Two buffer technologies have been introduced based on our high temperature AlN buffer technology: modified “GaN interlayer” and “multiple porous AlN buffer.” The Al0.16Ga0.84N/Al0.05Ga0.95N MQWs have been grown on top of the two kinds of buffers on sapphire. High resolution x-ray diffraction measurements have confirmed that the crystal quality has been massively improved. As a result, an UV stimulated emission at 340 nm has been observed via optical pumping with a low threshold power of ∼6.6 kW/cm2 at room temperature. The developed approaches potentially provide a simple way for achieving electrical injection UV (including deep UV) laser.

ALD로 성장된 ZnO박막에 대한 질소이온 조사효과

ZnO는 육방정계결정구조의 물질로서 3.37 eV의 넓은 띠 간격과 60 meV의 큰 exciton 결합에너지에 따른 높은 효율의 자외선발광으로 짧은 파장의 빛 (녹, 청, 자외선)을 내는 LED (Light Emitting Diode) 분야에서 관심을 기울이고 있는 물질이다. LED제작을 위해서는 n형의 ZnO와 p형의 ZnO가 필요하지만 기본적으로 ZnO은 n형이므로 신뢰성 있는 p형 ZnO박막을 제작하기 위한 노력이 기울여지고 있다. 본 연구에서는 ALD (Atomic Layer Deposition)로 제작된 ZnO박막에 20 keV의 에너지를 갖는 질소이온을 <TEX>$10^{13}{\sim}10^{15}ions/cm^2$</TEX>로 조사한 후 Hall 효과 측정장치를 이용하여 질소이온 조사에 따른 전기적 특성변화를 조사하였다. ZnO, a wurtzite lattice structure, has attracted much attention as a promising material for light-emitting diodes (LEDs) due to highly efficient UV emission resulting from its large band gap of 3.37 eV, large exciton binding energy of 60 meV, and low power threshold for optical pumping at room temperature. For the realization of LEDs, both n-type ZnO and p-type ZnO are required. Now, n-type ZnO for practical applications is available; however, p-type ZnO still has many drawbacks. In this study, ZnO films were grown on glass substrates by using atomic layer deposition (ALD) and the ZnO films were irradiated by nitrogen ion beams (20 keV, <TEX>$10^{13}{\sim}10^{15}ions/cm^2$</TEX>). The effects of nitrogen-beam irradiation on the ZnO structure as well as the electrical property were investigated by using fieldemission scanning electron microscopy (FESEM) and Hall-effect measurement.

Low-threshold short-cavity diode laser for a miniature atomic clock

Short-cavity diode lasers (SCDLs) emitting at the 894-nm D1 line of caesium are developed. Low threshold currents and power consumption will make it possible to use these lasers in chip-size atomic clocks (CSACs) and magnetometers. The SCDL parameters are comparable with the parameters of surface-emitting lasers.

Supercontinuum generation in visible to mid-infrared region in square-lattice photonic crystal fiber made from highly nonlinear glasses

Square-lattice microstructure geometry of photonic crystal fibers made with high nonlinearity non-silica glass composition material as well as fused silica glass are studied here for modal propagation, dispersion analysis and supercontinuum generation. Our investigation of nonlinear propagation of low threshold power femtosecond pulse aims at generating supercontinuum in the near and mid infrared region using these highly nonlinear fibers. We performed a series of analyses through varying parameters of square-lattice geometry to map a desired broadband window. We also demonstrate a compact high power broadband source in visible region generated within few centimeter length of the fused silica square-lattice microstructure.

Reduction of gain depletion and saturation on a Brillouin-erbium fiber laser utilizing a Brillouin pump preamplification technique

This paper presents the characteristics of a multiwavelength L-band Brillouin-erbium comb fiber laser with a preamplified Brillouin pump (BP) power technique at low pumping powers. The issue of erbium-doped fiber gain depletion and Brillouin gain saturation are resolved by the proposed structure. For long single-mode fiber length, the Stokes line emission occurs at low pumping powers because of the high strength of spontaneous Brillouin scattering, which provides a strong seed for coherent regenerative amplification of the Stokes line in the laser cavity. The laser structure achieves a low threshold power of 17 mW and is able to produce high number of output channels at low pumping powers. We experimentally show that the fiber laser structure can produce up to 37 channels at 55 and 0.045 mW of 1480 nm pump and BP powers, respectively.

Room temperature continuous wave operation and characterization of photonic crystal nanolaser on a sapphire substrate

A compact photonic crystal defect nanocavity was fabricated on a sapphire substrate. Continuous wave lasing at room temperature was achieved with a low threshold power. We characterized the operated mode, quality factor and radiative efficiency for the compact laser. The thermal properties of the nanolaser were also investigated.

Interband Cascade Lasers with Wavelengths Spanning 3.2–4.2 μm

We report a detailed experimental investigation of five interband cascade lasers with five active stages each and emitting at wavelengths between 3.2 μm and 4.2 μm at room temperature. Pulsed threshold current densities as low as 394 A/cm2 and voltage efficiencies as high as 76% are obtained at 300 K. The low pulsed threshold power densities (0.9–1.6 kW/cm2 at 300 K) imply that ambient-temperature cw operation should be possible over the entire spectral band once optimized narrow ridges can be fabricated.

Low-Power Super-resolution Readout with Antimony Bismuth Alloy Film as Mask layer

Sb–Bi alloy films are proposed as a new kind of super-resolution mask layer with low readout threshold power. Using the Sb–Bi alloy film as a mask layer and SiN as a protective layer in a read-only memory disc, the super-resolution pits with diameters of 380 nm are read out by a dynamic setup, the laser wavelength is 780 nm and the numerical aperture of pickup lens is 0.45. The effects of the Sb–Bi thin film thickness, laser readout power and disc rotating velocity on the readout signal are investigated. The results show that the threshold laser power of super-resolution readout of the Sb–Bi mask layer is about 0.5 mW, and the corresponding carrier-to-noise ratio is about 20 dB at the film thickness of 50 nm. The super-resolution mechanism of the Sb–Bi alloy mask layer is discussed based on its temperature dependence of reflection.

Flexible compact microdisk lasers on a polydimethylsiloxane (PDMS) substrate

Compact microdisk cavities were fabricated on a polydimethylsiloxane substrate. The lasing of the flexible compact cavity was achieved with a low threshold power. The whispering-gallery mode of the microdisk was also characterized with three-dimensional finite-difference time-domain simulation. The curvature dependence in output power and threshold was also demonstrated by bending the microdisk cavity.

Nonlinear propagation of circularly polarized light in magneto-optic fiber Bragg gratings

The theoretical model of magneto-optic fiber Bragg gratings （MFBG） is proposed and the nonlinear coupled-mode equations for circularly polarized light in MFBG is derived for the first time. Our analysis shows that the coupling between right- and left-handed circularly polarized light along with the magnetically circular birefringence （the Faraday effect） resulting from the nonlinear effects and the grating structure is responsible for the coupling of forward and backward guided optical waves with identical states of polarization. In the MFBGs, the reversal of transmission characteristics and lower power threshold of nonlinear bistable switching can be achieved under appropriate magneto-optic bias. The MFBG-based pulse reshaping scheme by utilizing cross-phase modulation （XPM） of right- and left-handed circularly polarized light has a great advantage in magnetic adjustability over that based on conventional fiber gratings and is useful for the development of dynamically controllable all-optical 3R regenerators.

Non-ionic photo-acid generators for applications in two-photon lithography

Non-ionic photoacid generators (PAGs) have been designed and synthesized for use in two-photon lithography (TPL). The chromophores in these new PAGs are covalently linked to the photocleavable group by a flexible joint. Their thermal stability, solubility and efficiency to produce acid under both one- and two-photon excitation were characterized. The potential of these PAGs for TPL was tested in two negative-tone resist systems relying on different mechanisms: free-radical/cationic polymerization or a cationically initiated cross-linking reaction. These PAGs needed lower threshold power for polymerization compared to a commercially available photoinitiator, isopropylthioxanthone, and a photoacid generator, N-hydroxynaphthalimide triflate. Microstructures with a resolution of 0.6 µm were fabricated and the threshold power for polymerization was found to be below 2 mW.

Novel Multiwavelength L-Band Brillouin–Erbium Fiber Laser Utilizing Double-Pass Brillouin Pump Preamplified Technique

We experimentally demonstrate successful operation of an enhanced multiwavelength L-band Brillouin-erbium fiber laser and analyze its performance under various operating conditions. This scheme utilizes double-pass amplification technique to preamplify the Brillouin pump (BP) power within the laser cavity before entering the single-mode fiber. Owing to this double-pass pre-amplification within the erbium gain medium, the proposed laser structure is able to operate at low pumping power and exhibits a low-threshold power of 15.9 mW. Moreover, the double-pass pre-amplification of BP is able to shift the unstable operation of the laser to a higher pump power, enabling us to generate high-power laser signals. We experimentally show that the proposed novel setup can produce up to 30 channels at 40 and 0.035 mW of 1480 nm pump and BP powers, respectively. An obvious suppressant for unstable self-lasing cavity modes because of the effect of homogenous saturation of the erbium-doped fiber gain due to the high intensity of BP is observed in the laser cavity configuration.

Theory and experimental studies of all-optical transmission switching in a twist-alignment dye-doped nematic liquid crystal

We present a theoretical model and experimental demonstration of an all-optical switching process occurring in twist alignment dye-doped nematic liquid crystal cells. A Landau-deGennes theory is applied together with the thermal buildup and diffusion equation to analyze the laser-induced temperature, the order parameter, and the birefringence changes in the liquid crystal cell. The resulting transmission switching processes are calculated using a modified Jones matrix method. Experimental observations of the switching processes in cells of varying thicknesses and their dependence on the input optical power are in good agreement with theoretical predictions. We have also studied the dynamics of the switching processes and observed microseconds switching times. These studies demonstrate the possibility of low-threshold power and microseconds speed switching applications.

Tm:Ho co-doped single mode optical fibre laser pumped by a 1600 nm Er fibre laser

An all-fibre Tm:Ho laser system is reported, using a 1600nm Er fibre pump laser and 0.3m length of the fibre through a two-stage optimization of both the pump source and laser configuration to achieve a low threshold operation. As a result a low threshold power of 33mW and a slope efficiency of 0.6% have been achieved, in laser operation at a wavelength of 1870nm and a cross-comparison with 785nm laser diode pumping has been made.

Ultra-low threshold photonic crystal nanocavity laser

We demonstrate that continuous-wave (CW) photonic crystal (PhC) nanocavity laser operates at room temperature with a very low effective threshold power of ∼375 nW. The CW lasing was achieved at 1.3 μm with InAs/GaAs self-assembled quantum dots (QDs) and high-quality PhC nanocavity with a quality factor of 87,000. The light-in versus light-out curve shows no sharp threshold unlike conventional lasers with pronounced kinks around the thresholds. This near-thresholdless behavior with smooth transition from thermal to coherent light region indicates that this nanocavity laser has a very high spontaneous emission coupling efficiency. We also discuss the characteristics of background noise of QD-based PhC nanocavity lasers compared with quantum well based PhC nanocavity lasers.

Dependence of lasing threshold power on excitation wavelength in dye-doped cholesteric liquid crystals

Relationship between threshold power and excitation wavelength has been investigated in distributed feedback (DFB) lasers using dye-doped cholesteric liquid crystals (CLCs). We found that the threshold shows strong dependence on the excitation wavelength. The excitation wavelength was varied over the whole absorption band of a commercial DCM dye, resulting in almost one-order of magnitude lower threshold power by the excitation at the higher energy tail than at the lower energy tail of the absorption band. We confirmed the existence of an optimum wavelength giving the lowest threshold power due to the competition between this effect and decreasing absorption at the higer-energy tail of the absorption band using a different dye, pyromethene 580.

A microwave detector based on an MCT photodiode for subthermonuclear plasma research

A microwave detector including a single-element infrared (IR) photodiode mounted in a cooled Dewar and a preamplifier is developed. An IR photodiode of the n +-p type is created on the basis of mercury-cadmium telluride (HgCdTe or MCT) heteroepitaxial structures grown by means of molecular beam epitaxy. A special profile of MCT structures throughout the MCT layer thickness ensures a low series resistance of less than 10 Ω and a high quantum efficiency at a level of 0.65 without any antireflection coating. For the spectral range of 8 to 12 μm, IR photodiodes are characterized by a low threshold power of 3.8·10−13 W·Hz−1/2 and 10−19 W·Hz−1 in the direct and heterodyne regimes, respectively. The cooled Dewar maintains the operating temperature of the IR photodiode in the interval of 77 to 80 K for 8. The preamplifier operates in a frequency range up to 1 GHz. With the use of this microwave detector, a method is developed for detection of intensity of a CO2 laser beam on plasma fluctuations caused by heating by a relativistic electron beam (REB). At high REB intensities, dips of the turbulence signal with a duration of several nanoseconds are observed, which is interpreted as a collapse of Langmuir waves.

Low-Temperature Growth of ZnO Thin Films by Atomic Layer Deposition

Zinc oxide (ZnO) has rapidly emerged as a promising optoelectronic material due to its large band gap of 3.37 eV, low power threshold for optical pumping at room temperature, and highly efficient UV emission resulting from a large exciton binding energy of 60 meV. Among the various deposition techniques, atomic layer deposition (ALD) has attracted much attention because high-quality films are attainable, especially at low temperature. In this study, we will report the self-limiting growth of ZnO films on 6-inch p-type (100) silicon wafers by using ALD at a low substrate temperature of 150 ◦C. Diethylzinc (DEZn) was used as the zinc precursor, H2O as the oxidant, and argon as the carrier and purge gas. The film thickness was measured by using cross-sectional images of field-emission scanning electron microscopy (FESEM). The crystal growth orientation was examined by using an X-ray diffractometer (XRD). The composition (O, Zn, etc.) was qualitatively investigated by using an Auger electron spectroscopy (AES) depth profile. Self-limiting growth of one monolayer of ZnO per cycle was successfully obtained. The average deposition rate was found to be 0.28 nm/cycle, and the film had high purity and was (100) oriented.

Low threshold characteristics of an L-band Brillouin-erbium comb fiber laser in a linear cavity

The L-band linear-cavity multiwavelength Brillouin-erbium fiber laser source has been experimentally investigated. The proposed L-band laser cavity exhibits a low threshold power of ∼23.4 mW due to the high efficiency of double-pass amplification in the erbium gain medium and also bidirectional generation of the Brillouin pump in the single-mode fiber. The effects of Brillouin pump power and the wavelength on the threshold pump power are also discussed. Consequently, it has been found that there is a limited range for the Brillouin pump wavelength to achieve a lasing condition in the laser cavity.

Spheromak Formation by Steady Inductive Helicity Injection

A spheromak is formed for the first time using a new steady state inductive helicity injection method. Using two inductive injectors with odd symmetry and oscillating at 5.8 kHz, a steady state spheromak with even symmetry is formed and sustained through nonlinear relaxation. A spheromak with about 13 kA of toroidal current is formed and sustained using about 3 MW of power. This is a much lower power threshold for spheromak production than required for electrode-based helicity injection. Internal magnetic probe data, including oscillations driven by the injectors, agree with the plasma being in the Taylor state. The agreement is remarkable considering the only fitting parameter is the amplitude of the spheromak component of the state.