The Objective

This microscope uses a Schwarzschild reflective objective (figure 2.7). Unlike a standard refractive objective, a reflective objective uses no glass optics and instead focuses light using a set of curved mirrors. This provides many benefits over refractive objectives - it is lightweight, has no chromatic aberration over a very large wavelength range, is vacuum compatible, and often has longer working distances. It does have some drawbacks, though. Reflective objectives transmit less light, can be

Figure 2.7: Schematic of a Schwarzschild reflective objective. The beam enters the aperture from the left and is reflected off of a small convex mirror. A larger concave mirror focuses the beam onto the sample. Some portion of the beam is blocked by the initial mirror. This both reduces the transmitted power and induces an Airy pattern at the focus.

tricky to align, and have lower numerical apertures than glass objectives. In this section I discuss the use of the reflective objective in our microscope, its alignment, optical properties, and power transmission.

Objective Alignment The reflective objective is far more sensitive to alignment issues than refractive objectives and our standard objective alignment procedure is not precise enough. Even a small beam misalignment can cause problems focusing, astigmatism, and beam asymmetry. We developed a new method for alignment which provides repeatably high quality images.

To align the beam correctly you will need a 1” iris to screw into the objective holder, a power meter, and a reflective surface such as a mirror or reflective sample. Use the probe to do the alignment rather than the pump. The probe carries the signal, so the microscope is more sensitive to its alignment. Place an iris before the beam splitter (BS2, figure 2.3) to spatially filter the probe so it looks circular. This will help improve the precision of your alignment. Screw the 1" iris into the objective mount so that you can see the probe beam on it with an IR viewer. Place the reflective surface under the objective so that the beam focuses on that surface. Put the power meter immediately before the balanced photodiode (BPD) to measure the light that reflects off of the mirror/sample. Adjust the power meter’s position to maximize the power reading. Make sure that the light you are measuring is from the objective and not a back reflection from one of the lenses! Using the IR viewer and the second mirror away from the objective, center the probe beam on the iris. Remove or open the iris and maximize reflected power by using the mirror immediately before the objective to adjust the angle of the beam into the objective. Iterate until your power

reading is maximized and the beam is entered on the iris. If the beam is properly aligned, an spatially-separated pump-probe (SSPP) image should show a symmetrical Airy pattern.

Depth of Field The focal plane of an objective is not infinitely small. For any objective there is a distance over which an object will remain in focus, called a depth of field (DoF) – the distance over which you can move a microscope objective and have an object remain in focus. The ideal DoF for a particular objective is given by equation 2.1.41

dof = λn

N A2 (2.1)

where λis the wavelength of light, NA is the objective’s numerical aperture, and n is the refractive index of the material you are focusing through (typically air, n = 1.00). For our objective this expression gives a DoF of 1.7µm for 425 nm light and 3.4µm for 850 nm light.

The DoF expression as written assumes the light is focusing through only one material. In this microscope the light focuses through air and then a 2 mm thick quartz window (n=1.45). Focusing through the window distorts the beam and prevents the focus at the sample from being as tight. To determine the DoF experimentally we collect images of a Si NW with the pump, probe, and spatially-overlapped pump-probe (SOPP) with the objective at various heights (Figure 2.8). The size and signal intensity of the NW remains constant over 4µm, 2-3 µm, and 3µm for 850 nm light, 425 nm light, and SOPP images, respectively. This tells us that the DoF of this system is 3-4µm depending on wavelength. These empirical DoFs are about 20% larger than the predicted value.

Power Transmission Reflective objectives occlude the center of the laser beam resulting in an intense Airy pattern and a loss of pulse energy. Our objective is specified to obscure the central 22% of the beam by area. Our beam is Gaussian shaped so the central portion of the beam is the most intense. Obscuring the central 22% of the area results in a reduction in power significantly larger than 22%. The beam also overfills the objective slightly, further reducing the transmitted power. We measure the beam power immediately before and after the objective and found that 29% of the probe and 24% of the pump is transmitted.

Figure 2.8: Depth of field: pump (left), probe (right) and SOPP (bottom) images of a SiNW collected by moving the objective in the z axis over 3 µm, 4µm, and 3µm, respectively. Over this range the wire just starts to lose resolution in the final image, indicating the edge of the objective’s depth of field. Scale bars are 500 nm.