基于光热效应的显微光谱技术在单粒子检测中应用和发展

高灵敏度的单粒子检测技术是纳米粒子在生物医学、化学、光电子等领域应用的前提条件。常见的单粒子检测技术主要包括基于粒子的荧光、拉曼、散射和吸收等信号而发展起来的光学显微成像及光谱技术。其中,拉曼光谱和荧光光谱技术主要适用于一些具有拉曼活性的分子/粒子或可发光的荧光分子或粒子,然而即使对于荧光效率高的有机染料分子和半导体纳米粒子,固有的光漂白和blinking现象也对单粒子探测形成了挑战。散射光谱测量是应用于单粒子检测的另外一种方法,从理论上讲,由于瑞利散射随着尺寸的减小而呈六次方减弱的趋势,在细胞或生物组织内,小尺寸粒子的散射信号很难从背景散射噪声中分离出来。众所周知,介质吸收激发光后会引起介质内的折射率变化,进而在光加热区附近出现折射率的梯度分布,称为光热效应(photothermal effect)。基于粒子光热效应的光学显微成像和光谱测量技术具有信号灵敏度高、无背景散射、原位和免标记等优点,在单粒子检测领域展现了良好的应用潜力。综述了近年来基于光热效应的显微光谱技术在单粒子检测中应用和研究发展,首先介绍了光热效应的测量原理;接着分别讨论了光热透镜测量技术、微分干涉相差测量技术和光热外差测量技术的实验装置,比较了各种测量技术的信噪比、灵敏度、分辨率等特点,并且介绍这些测量技术在单粒子检测中的应用研究进展;接着,论述了近年来研究人员在提高光热显微测量的信噪比、改善动态测量性能以及在红外波段拓展等方面的最新研究成果;最后,简单总结了光热测量技术在单粒子检测领域所面临的挑战。     

Efficient mid-IR spectral generation via spontaneous fifth-order cascaded-Raman amplification in silica fibers

Spontaneous cascaded Raman amplification is demonstrated as a practical and efficient means of power transfer from telecommunications wavelengths to mid-IR wavelength bands through use of conventional silica fibers and amplifiers. We show that silica fibers possessing normal dispersion over all near-IR and mid-IR wavelengths can facilitate 37% and 16% efficient Raman power conversion from 1.53 microm to 2.15 and 2.41 microm wavelength bands, respectively, using nanosecond pulses from an all-fiber laser source. In contrast to supercontinuum-based techniques for long-wavelength generation, the high levels of Raman gain generated at these wavelength bands could produce useful optical amplification necessary for the development of numerous mid-IR laser sources.     

Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack “fishnet” structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 μm. In the mid-IR range, the metamaterial was an ordered array of fourfold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 μm and 5.25 μm, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach. PACS 42.25.Bs; 81.16.Nd; 42.70.-a; 81.07.-b     

Generation of mid-IR radiation in near-IR semiconductor lasers based on low-dimensional heterostructures

A method for generating mid-IR radiation (λ ～ 10μm) in near-IR (λ ～1 μm) quantum-well semiconductor heterolasers has been proposed. This method is based on the formation of population inversion at a mid-IR intersubband transition as a result of the depletion of its lower level by a strong near-IR field. In contrast to the previous investigations of this problem, the inhomogeneous broadening of the noted transition (caused by the dependence of its frequency on the carrier energy) is taken into account, and it is proposed to invert it not in the entire spectral range but only in the region resonant with the mid-IR field. This approach makes it possible to significantly reduce (in comparison with the previous estimates) the threshold pump current density for initiating mid-IR generation and, as a result, hope to implement operation of the proposed laser at room temperature in the cw mode.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Imaging photothermal microscopy for absorption measurements of optical coatings

For absorption measurement of large-aperture optical coatings, a novel method of imaging photothermal microscopy based on image lock-in technique is presented. Detailed theoretical analysis and numerical calculation are made based on the image photothermal technique. The feasibility of this imaging method is proved through the coincidence between the theoretical results of single spot method and multi-channel method. The measuring speed of this imaging method can be increased hundreds of times compared with that of the raster scanning. This technique can expand the applications of photothermal technique.     

Frequency-comb-based absolute frequency measurements in the mid-infrared with a difference-frequency spectrometer

We demonstrate the possibility of extending the well-established metrological performance of optical frequency-comb synthesizers to the mid-IR region by phase locking the pump and signal lasers of a difference-frequency source to two near-IR teeth of an optical comb. An uncertainty of 800 Hz (1.1 x 10(-11)) in the absolute frequencies of CO2 transitions near 4.2 microm has been measured by cavity-enhanced saturated-absorption spectroscopy. Prospects for the creation of a new dense set of high-quality molecular frequency standards in the IR are discussed.     

A single molecule as a probe of optical intensity distribution

Single terrylene molecules embedded in microscopic p-terphenyl crystals are identified with the technique of fluorescence excitation spectroscopy. By use of the architecture of a scanning-probe microscope at T = 1.4 K , a single molecule is scanned through an excitation laser beam while the fluorescence signal is recorded. In this manner we have mapped the intensity distribution in a one-dimensional optical standing wave, demonstrating the potential of a single molecule as a nanometric probe. We discuss future experiments aimed at combining the high spatial and spectral sensitivity of a single molecule.     

Polymer ablation: From fundamentals of polymer design to laser plasma thruster

UV-Laser ablation of polymers is a well-established method to structure and deposit polymers, but the mechanisms of ablation are still controversial, i.e. photothermal or photochemical processes. An approach to probe the ablation mechanisms and to improve ablation is to incorporate photoactive groups into the polymer structure.The investigation of the ablation behavior of designed triazene polymers showed that the ablation mechanism is always a combination of both photothermal and photochemical processes, but the ratio can be changed by using different polymers and irradiation wavelengths. Also the quality of structures in the triazene polymers is superior at an irradiation wavelength of 308 nm compared to commercially available polymers.Polymers can be designed not only for UV irradiation, but also for applications in the IR range, but with different requirements. One application for designed polymers in the near-IR range is as fuel for the laser plasma thruster, which is used as propulsion system for small satellites. With commercially available polymers the necessary thrust could not be achieved. A specially designed polymer-absorber system for this application produce more energy in the form of thrust, than the laser delivered.     

Fourier transforms method for measuring thermal lens induced in diluted liquid samples

Drawing on interferometry and Fourier analysis, this paper describes the use of a two-beam thermal lens technique for measuring thermo-optical properties in optical materials. The procedure consists of yield interference patterns deformed by a localized photothermal effect. The photothermal phase shift is locally induced by the pump beam focused on a tested sample located on an on-axis probe beam, which is the first arm of a Mach-Zehnder interferometer. The plane where the effect is localized is imaged onto a CCD camera. Then two interferograms are recorded: one without effect and the other one with the induced photothermal phase. Fourier analysis performed on these interferograms allow us to plot the thermal lens map and, therefore, to estimate thermo-optic constant of Malachite Green in water solution. The method is applied to measure low linear absorptions of a diluted sample of Rhodamine B in water solution at 633 nm, showing that the proposed technique allows to measure photothermal phase shift as low as 3.1 mrad at 8 mW of input power in diluted materials.     

Heat-generating property of a local plasmon resonator under illumination

We have investigated the heat generation from gold nanoparticles resulting from their local plasma resonance. We have demonstrated the self-assembly of Au nanoparticle arrays/dielectric layer/Ag mirror sandwiches, i.e., a local plasmon resonator, using a dynamic oblique deposition technique. The thicknesses of the Au and dielectric layers were changed combinatorially on a single substrate. As a result, local plasmon resonator chips were successfully fabricated. Because of strong interference, their optical absorption can be controlled between 0.0% and 97% in the near-IR region, depending on the thickness of the dielectric layer. We evaluated the heat generation from Au nanoparticles by measuring the temperature of water with which a cell prepared on a chip is filled under laser illumination. The change in the water temperature is proportional to the optical absorption of the local plasmon resonator chips. This suggests that the photothermal conversion efficiency can be controlled by interference. These features make the application of the local plasmon resonator to nanoheaters, which can spatiotemporally control heat generation, suitable.     

Homodyne fiber optic backscatter dynamic light scattering

Optical homodyne detection in the backscatter direction is achieved through a pair of collinearly located fibers in a cylindrical probe body. One fiber illuminates the scattering solution while the other fiber provides optical mixing of the backscattered optical field with a local oscillator derived from the Fresnel reflections at the glass interfaces of the sample container. Homodyne detection is possible over a broad range of particle size and sample concentration with a single probe design.     

High-average-power mid-infrared ZnGeP2 optical parametric oscillator with a wavelength-dependent polarization rotator

A wavelength-dependent polarization rotator is used to transform the orthogonal polarizations of the signal and idler of a near-degenerate type II KTP optical parametric oscillator (OPO) into a common polarization state. This common polarization allows a single ZnGeP2 OPO to fully utilize the 2 microm signal and idler KTP OPO outputs in a mid-IR conversion. The simple design of the wavelength-dependent polarization rotator yields a compact source that simultaneously generates four mid-JR wavelengths collinearly with a total mid-IR average power of 5.5 W at a >15 kHz pulse repetition rate.     

Nondestructive readout of a three-dimensional photochromic optical memory with a near-infrared differential phase-contrast microscope

A three-dimensional (3D) rewritable optical memory using photochromic material is described for high-density memory. The bits are recorded in a 3D volume of photochromic material. A transformation of the photochromic molecule between two isomers with different absorption spectra can be stimulated by irradiation with appropriate wavelengths. We show that a nondestructive readout of photochromic memory is possible by use of a small difference in the refractive index of the photochromic isomers in the near-IR range. For this purpose a near-IR laser-scan differential phase-contrast microscope is used. Experimental results of 3D recording and nondestructive reading are presented.     

Ultrabroadband tunable continuous-wave difference-frequency generation in periodically poled lithium niobate waveguides

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.     

光学薄膜热膨胀系数的研究

光热位移偏转技术结合横向光热偏转技术可用于研究薄膜样品的热膨胀系数.本文以SiO_2、TiO_2、ZrO_2、MgF_2、ThF4等单层光学薄膜为例.报道相关的实验方法及实验结果.     

Dark fringe operation and noise reduction with optical subtraction approach and power allocation

We demonstrate a technique to achieve the reduction of excess noise with dark fringe operation by optical subtraction and power allocation strategy. By performing the subtraction in optical way, the excess noise resulted from the photocurrent is eliminated and the phase information can be extracted with single photoreceiver. Meanwhile, by allocating more optical power to the probe beam, the DC background and the signal amplitude of the photocurrent become very small simultaneously so that the dark fringe operation is achieved. We compare the spectral distributions resulted from the proposed technique with that from the widely employed balanced detection and the modified balanced detection techniques which both are based on electronic subtraction. It is found that the experimental results closely agree to the theoretical predictions.     

Tunable third-harmonic generation in a solid-core tellurite glass fiber

A solid-core tellurite glass fiber with 1.8?dB/m loss at 1.55?μm was made by using the built-in casting preform fabrication method and rod-in-tube fiber drawing technique. Pumping a 10?cm fiber piece with picosecond pulses of 3-5×10(12)?W/cm(2), 0.1% of the fundamental power limited by the coherence length of 0.3-5?μm was converted into visible third-harmonic power tunable over a broad near-IR wavelength ranging from 1500 to 1680?nm. Frequency conversion from the mid-IR to near-IR was found to be even more efficient due to the longer coherence lengths of 12-20?μm in the wavelength range of 2200-2500?nm.     

Trace complex-molecule detection using near-IR diode lasers

The goal of the present paper was to develop an experimental technique to detect trace concentrations of complex molecules in the atmosphere using near-IR diode lasers. Ethanol and propane were chosen as model species. New optical, hardware, instrument-operation-mode and data-processing approaches for detection of trace complex molecules were developed. Different physical processes limiting the sensitivity of trace molecule detection were considered and solutions were developed to reduce the influence of these mechanisms on instrument operation. An absorption sensitivity of α_min=2.5×10^-5 for 4.5 ms measurement time was achieved, which is comparable with the sensitivity of “small” molecule detection. The first results utilizing this technique for in-field monitoring of trace complex molecules are presented. Received: 25 April 2002 / Revised version: 29 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-8281, E-mail: anber@nsc.gpi.ru     

Adiabatic frequency conversion of ultrafast pulses

A new method for efficient, broadband sum and difference frequency generation of ultrafast pulses is demonstrated. The principles of the method follow from an analogy between frequency conversion and coherent optical excitation of a two-level system. For conversion of ultrafast pulses, the concepts of adiabatic conversion are developed further in order to account for dispersion and group velocity mismatch. The scheme was implemented using aperiodically poled nonlinear crystals and a single step nonlinear mixing process, leading to conversion of near-IR (∼790 nm) ultrafast pulses into the blue (∼450 nm) and mid-IR (∼3.15 μm) spectral regions. Conversion bandwidths up to 15 THz FWHM and efficiencies up to 50% are reported.