Thermoreflectance Microscopy
The main focus of our research is on studying thermal effects at the micrometer size scales. Specifically, we are interested in exploring the temperature distribution in optoelectronic and electronic devices under operating conditions.
The devices we use as samples are mostly semiconductor devices such as laser diodes, LEDs, and Photonic Integrated Circuits (PICs). However, the versatility of our experimental technique extends its application to micro-scaled electronic devices and even passive devices such as polymer-based photonic wire bonds.
In all of these devices, injection of electrical current (or optical power) can lead to excess heat, hot spots and eventual device failure. That is why the temperature of many of these devices is actively controlled during operation. From a practical point of view, hot spot detection is very important in diagnosing potential sources of failure. This can eventually lead to better chip and packaging designs and improved thermal management.
From a more fundamental point of view, understanding temperature distribution and heat flow through the device can yield valuable information about the thermal properties of the structures from which the device is made. Physical parameters such as thermal conductivity, thermal diffusivity and thermal transient times can be extracted.
Experimental Technique
In order to study thermal effects in the devices we test, we use an all optical technique called Thermoreflectance Microscopy. This technique has a few advantages: it is non-contact, it has a high thermal and spatial resolution (higher resolution than IR thermography) and is an imaging technique, which means we can get two dimensional thermal maps (or profiles) of the surfaces of our devices.
The physical basis of thermoreflectance is that by changing the temperature of a device (T), its surface reflectivity (R) changes (because of the changes in refractive index which is temperature dependent). In theory, the relative change in reflectivity (ΔR/R) is proportional to the change in temperature ( ΔT ), such that
The wavelength- and material-dependent calibration factor, Cth, has to be determined in an independent experiment.
Experimental Setup
The setup of the thermoreflectance microscopy is shown in the schematic drawing below. Basically, the sample temperature (e.g., VCSEL in the diagram) is changed using a current modulation or pulsing. The device is under a microscope objective and is illuminated by visible light from an LED whose wavelength is chosen to result in the highest signal to noise ratio. The LED can also be pulsed with or without a delay relative to the heating pulse. The relative changes in the surface reflectivity are measured using a CCD camera and image processing. Using this setup, we can obtain thermal maps of the surfaces of the devices under test under operating condition. The maps will tell us about sources of heat and potential hot spots. Thermal properties of the devices can also be studied using these images. We are also capable of measuring thermal transient behavior of the devices in response to a heating pulse.
Optical Tweezers
Optical tweezers consist of a high power, highly focused laser beam which optically trap dielectric particles such as microspheres near the focus of an objective lens (see the diagram and the picture below). Optical tweezers have a wide range of applications in physics and biology. They have been used to manipulate and study objects such as microspheres, macromolecules such as DNA and RNA, and even live cells. A particular characteristic of the technique is the possibility to trap individual objects, which can then be studied alone, free from the influence of other objects in the sample.
We used this setup to optically trap fused silica beads with a diameter of 2 micrometers. We explored the Brownian motion of the beads, the viscosity of the bead-water solution, and measured the trapping force of our optical tweezers.
