Janice A. Hudgings

CCD-based thermoreflectance microscopy: principles and applications

CCD-based thermoreflectance microscopy has emerged as a high resolution, non-contact imaging technique for thermal profiling and performance and reliability analysis of numerous electronic and optoelectronic devices at the micro-scale. This thermography technique, which is based on measuring the relative change in reflectivity of the device surface as a function of change in temperature, provides high-resolution thermal images that are useful for hot spot detection and failure analysis, mapping of temperature distribution, measurement of thermal transient, optical characterization of photonic devices and measurement of thermal conductivity in thin films. In this paper we review the basic physical principle behind thermoreflectance as a thermography tool, discuss the experimental setup, resolutions achieved, signal processing procedures and calibration techniques, and review the current applications of CCD-based thermoreflectance microscopy in various devices.

Optical Measurement of Thermal Conductivity Using Fiber Aligned Frequency Domain Thermoreflectance

Fiber aligned frequency domain thermoreflectance (FAFDTR) is a simple noncontact optical technique for accurately measuring the thermal conductivity of thin films and bulk samples for a wide range of materials, including electrically conducting samples. FAFDTR is a single-sided measurement that requires minimal sample preparation and no microfabrication. Like existing thermoreflectance techniques, a modulated pump laser heats the sample surface, and a probe laser monitors the resultant thermal wave via the temperature dependent reflectance of the surface. Via the use of inexpensive fiber coupled diode lasers and common mode rejection, FAFDTR addresses three challenges of existing optical methods: complexity in setup, uncertainty in pump-probe alignment, and noise in the probe laser. FAFDTR was validated for thermal conductivities spanning three orders of magnitude (0.1-100 W/m K), and thin film thermal conductances greater than 10 W/m(2) K. Uncertainties of 10-15% were typical, and were dominated by uncertainties in the laser spot size. A parametric study of sensitivity for thin film samples shows that high thermal conductivity contrast between film and substrate is essential for making accurate measurements. DOI: 10.1115/1.4003545

Modulation of a vertical-cavity surface-emitting laser using an intracavity quantum-well absorber

We demonstrate a novel modulation technique with a vertical cavity surface-emitting laser (VCSEL) using an intracavity embedded voltage-biased quantum well absorber. We achieved a -3-dB small-signal bandwidth of 9 GHz and a response of 7 GHz//spl radic/(mA) by modulation of the absorber. Our calculations show that this technique introduces significantly less chirp at higher frequencies than direct current modulation.

Nanoscale thermoreflectance with 10mK temperature resolution using stochastic resonance

We present 2D temperature measurements with 250nm spatial and 10mK temperature resolution using thermoreflectance microscopy. We measure the temperature-induced reflectivity change with an accuracy better than /spl Delta/R/R=2/spl middot/10/sup -6/ using a 12bit CCD, which has a quantization limitation of /spl Delta/R/R=2.5/spl middot/10/sup -4/. The dynamic range is thus expanded from 72dB to 114dB, equivalent to more than 18 effective bits. We quantitatively explain this dramatic improvement using the concept of stochastic resonance. In addition, we optimize the thermoreflectance calibration coefficient K/spl equiv/R/sup -1//spl middot/ R/T by matching the illumination wavelength to a combination of the thermoreflectance coefficient spectrum R/T and the reflectivity spectrum R. For gold illuminated with a 467nm LED, we obtain the extraordinarily large value /spl kappa/ =3.3/spl middot/10/sup -4/ K/sup -1/. This calibration coefficient yields a temperature resolution of better than 10mK.

Thermal profiling for optical characterization of waveguide devices

We demonstrate a thermal profiling technique for wafer-scale testing of the optical power distribution in photonic integrated circuits. Radiative cooling of the lattice of a semiconductor optical amplifier is observed in response to an optical signal; likewise, lattice heating occurs in an optically injected absorber. We develop a total energy balance model that can be used to quantify modal gain or absorption within these devices, along with longitudinal and vertical thermal impedances and heat transfer coefficients. Spatially resolved thermal profiling in conjunction with our energy balance model accurately describes the optical power distribution inside optoelectronic devices, without recourse to direct optical measurement.

Temperature Profiling of VCSELs by Thermoreflectance Microscopy

Surface temperature measurements with submicron spatial resolution are reported for operating vertical-cavity surface-emitting lasers (VCSELs) by means of thermoreflectance microscopy. We measure increasingly convex radial temperature distributions with increasing bias power for three types of VCSELs. The corresponding convex refractive index profiles are consistent with previously observed thermal lensing; this effect is far more prominent for the oxide confined single-mode (SM) VCSEL than for the broader aperture devices. For all samples, the change in the average surface temperature varies linearly with the change in dissipated power. A comparison with the temperature of the top distributed Bragg reflector mirror of an oxide confined SM VCSEL, obtained from the wavelength shift of the spontaneous emission, shows that both methods yield comparable results

The physics of negative differential resistance of an intracavity voltage-controlled absorber in a vertical-cavity surface-emitting laser

We have constructed a vertical-cavity, surface-emitting laser with a voltage-controlled quantum well absorber in the upper mirror stack. If the lasing wavelength of this device is designed to be slightly longer than the absorber band edge, sharp negative differential resistance can be obtained in the absorber under lasing conditions. We present strong experimental evidence that this behavior arises from redshifting of the absorption excitonic peak due to the quantum confined Stark effect. Design criteria are proposed for applications including high speed modulation and self-pulsation.

Coherence collapse and redshifting in vertical-cavity surface-emitting lasers exposed to strong optical feedback

The response of a commercial-grade, single-mode vertical-cavity surface-emitting laser to very strong optical feedback is investigated. With increasing feedback magnitude, the lasing spectrum broadens dramatically, eventually resulting in coherence collapse, and the lasing peak shifts to longer wavelengths. These effects became less severe with the onset of higher-order modes. A theoretical model is provided to explain the redshifting of the lasing spectrum with feedback.

Design and modeling of passive optical switches and power dividers using non-planar coupled fiber arrays

Arrays of coupled waveguides such as the ubiquitous directional coupler are used extensively in optoelectronic devices, with demonstrated applications to optical communications networks, fiber interferometers, and optical homodyne receivers. In order to analyze the transmission characteristics of circular arrays of coupled optical fibers, we have developed a matrix representation of the coupled-mode formalism, allowing for varying fiber diameters and differing coupling strengths between the fibers in the array. The model is used to identify design criteria for application of such arrays as passive optical switches and power dividers.

Compact, integrated optical disk readout head using a novel bistable vertical-cavity surface-emitting laser

We demonstrate a novel integrated optical disk readout head using a vertical-cavity surface-emitting laser (VCSEL) with an intracavity quantum-well absorber. Optical pickup detection is performed by measuring the change in the absorber voltage as optical feedback into the VCSEL cavity is varied. We obtain a 0.22-V peak-to-peak response with a RC time constant of 20 /spl mu/s, indicating a rolloff frequency of 50 kHz. The unique design flexibility of this device allows amplification of the detection signal by operating the device in a bistable regime.