In this work, we present the development of an infrared scanning near-field optical microscope (IR-SNOM) for thermal imaging. As an example, we explore thermal imaging of quantum cascade lasers (QCLs). QCLs are attractive infrared (IR) sources for chemical detection due to their tunability and wide emission range spanning from mid-wavelength to longwavelength infrared radiation (MWIR and LWIR). However, they require high performance cooling systems and have limited use at low power in continuous wave (CW) operation due to the potential for thermal failure of the device. Thermal imaging can help identify mechanisms and points of failure during laser operation. Because the size of the features of QCLs (~1 μm) are much smaller than the wavelength of the emitted thermal radiation, IR-SNOM is an ideal technique to image the spatial thermal profile of QCLs during operation to guide design improvement.
KEYWORDS: Thermography, Quantum cascade lasers, Near field scanning optical microscopy, Spatial resolution, Temperature metrology, Infrared imaging, Infrared radiation, Modulation, Near field
The fundamental optical diffraction in infrared microscopes limits their spatial resolution to about ~5μm and hinders the detailed observation of heat generation and dissipation behaviors in micrometer-sized optoelectronic and semiconductor devices, thus impeding the understanding of basic material properties, electrical shorts and structural defects at a micron and sub-micron scale. We report the recent development of a scanning near-field optical microscopy (SNOM) method for thermal imaging with subwavelength spatial resolution. The system implements infrared fiber-optic probes with subwavelength apertures at the apex of a tip for coupling to thermal radiation. Topographic imaging and tip-to-sample distance control are enabled by the implementation of a macroscopic aluminum tuning fork of centimeter size to support IR thermal macro-probes. The SNOM-on-a-fork system is developed as a capability primarily for the thermal profiling of MWIR quantum cascade lasers (QCLs) during pulsed and continuous wave (CW) operation, targeting QCL design optimization. Time-resolved thermal measurements with high spatial resolution will enable better understanding of thermal effects that can have a significant impact on a laser's optical performance and reliability, and furthermore, will serve as a tool to diagnose failure mechanisms.
Although Quantum cascade lasers (QCLs) are frequently used in sensing, spectroscopy, and free space communication applications, their poor thermal properties lead to high temperature gradients in the devices. To diagnose failure mechanisms of mid-wave infrared (MWIR) QCLs, it is critical to understand their thermal generation and transport characteristics. In this work, we use 3D anisotropic steady state heat transfer analysis to investigate the thermal behavior in lattice matched InP/InAlAs/InGaAs buried heterostructure (Bh) mounted epi-layer side down QCLs. We introduce anisotropic thermal conductivities in the in-plane and cross-plane directions in QCL’s superlattice active region, and study the temperature distribution inside the laser. We consider several configurations, including the overhanging of the laser chip on the submount by different amounts, the choice of front facet dielectric coating materials and their thicknesses, and the width of the active region. Combining these effects, we optimize QCL’s thermal performance. This work aims to provide guidelines for the design of durable QCLs as well as to help diagnose QCL failure mechanisms.
A simple rate-equation-based model for transport and gain in quantum cascade lasers is developed. According to the model, the IV characteristics of a quantum cascade laser can be well described by equivalent circuit containing a serial connection of a Schottky diode, a tunnel diode, and a field-effect transistor. As a consequence, a quantum cascade laser can be described in terms accessible to circuit designers.
It is well-known that many explosives have characteristic terahertz (THz) absorption features, and that THz waves can
penetrate many dielectric materials. However security applications generally prohibit using THz technology for
transmission measurements, either because of standoff distances, thick targets, or opaque targets (metals). As a result,
we focus our attention on THz reflection spectroscopy. We have measured the THz reflectivity signature of RDX
residues on smooth metal surfaces that contain about 0.4 mg of RDX. We discuss our efforts in detecting trace
explosives in reflection as well as our recent results including THz spectroscopy of four explosives from 1 to 6 THz,
and measurement of the absolute absorption cross-section of explosives.
We use femtosecond laser mass spectrometry (FLMS) to study the fragmentation patterns of solid phase explosive materials subjected to femtosecond laser pulse irradiation. In condensed phase FLMS a compound deposited on a solid substrate is desorbed into vacuum by femtosecond irradiation forming a plume of ionized and neutral species. Positive or negative ions are accelerated by an electric potential, allowed to drift in the field-free region of a time-of-flight (TOF) mass spectrometry instrument, and flight-times are recorded by a micro-channel plate detector and a digital oscilloscope. From the value of the accelerating field and the ion flight time, the mass-to-charge ratio of each ion is obtained. In this paper we report femtosecond laser mass spectra for the positive and negative ions formed by desorbing TNT and RDX with 150 fs pulses centered at 800 nm. The fragmentation pathways for the formation of the observed ions are described and are used to interpret femtosecond laser induced breakdown spectroscopy results.
We use femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of TNT and RDX. A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical emission from the plasma is spectrally analyzed to determine the material composition. Femtosecond LIBS results for TNT and RDX on aluminum substrates and glass slides are reported. Results are examined in terms of the optical properties of the substrate and the strong linear absorption for aluminum is contrasted with the weaker multiphoton absorption for glass. Optical microscope images of the ablated explosives are shown for femtosecond and nanosecond laser excitation. Fragmentation studies by femtosecond laser mass spectrometry are used to interpret LIBS results.
In this work, we describe femtosecond laser-induced breakdown spectroscopy (LIBS) to detect trace amounts of explosive-related compounds (ERCs). A high-power pulsed laser is used in LIBS to form a plasma on the material surface and the optical radiation from the plasma is spectrally analyzed to determine the material composition. LIBS is minimally destructive because only a minute amount of material is consumed in the process. LIBS also enables remote analysis because only optical access to the material is needed. Femtosecond LIBS results for TNT on brass and molybdenum substrates, and RDX on molybdenum substrates are reported. We will also show the effect of detection gate delay and gate width on the enhancement of spectral information provided by LIBS.
High-speed free-space optical communication systems have recently utilized components that have been developed for fiber-optic communication systems. The received laser beam in such a system must be coupled into a single-mode fiber at the input of a commercially available receiver module or a wavelength division demultiplexer. However, propagation through atmospheric turbulence destroys the spatial coherence of a laser beam and limits how much of the available power can be coupled into the single-mode fiber. In this paper, we numerically evaluate fiber coupling efficiency for laser light distorted by atmospheric turbulence. This involves the overlap integral of the optical field and the fiber mode and the calculation makes use of the mutual coherence function of the incident light. The results for weak fluctuation conditions indicate what level of coupling efficiency can be expected for a given turbulence strength. In addition, the results provide an estimate of how long the link distance can be before the coupling efficiency degrades to an unacceptable level.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.